denervated, and the contralateral sinus was perfused with arterial blood Rochester, Minnesota, U.S.A.

Transcription

1 J. Physiol. (1969), 205, pp With 6 text-figures Printed in Great Britain INFLUENCE OF CAROTID BARORECEPTORS ON DIFFERENT COMPONENTS OF THE VASCULAR SYSTEM BY DAVID BRENDER* AND MICHAEL M. WEBB-PEPLOEt From the Section of Physiology, Mayo Clinic and the Mayo Graduate School of Medicine, University of Minnesota, Rochester, Minnesota, U.S.A. (Received 11 November 1968) SUMMARY 1. Reflex changes in wall tension of the lateral saphenous vein of one hind limb, the splenic veins and capsule, and the resistance vessels of the other hind limb caused by changes in baroreceptor activity were measured in vagotomized dogs under thiopentone-chloralose anaesthesia. 2. Three different methods were used to alter pressure in one or both carotid sinuses. (1) Both carotid sinuses were vascularly isolated and filled with fully oxygenated Krebs-Ringer bicarbonate solution (ph 7.4) from a reservoir in which the pressure could be altered at will. (2) One sinus was denervated, and the contralateral sinus was perfused with arterial blood at different flow rates. (3) One sinus was denervated, and the innervated sinus was perfused with arterial blood at constant flow, the pressure being altered by changing the outflow resistance. 3. The left saphenous vein was perfused at constant flow with autologous blood; changes in perfusion pressure were used as a measure of changes in veno-motor activity. The right common iliac artery was perfused at constant flow to measure changes in resistance vessel activity. Blood flow through the spleen was temporarily arrested, trapping a fixed volume of blood in the organ. Under these conditions, changes in splenic vein pressure were a measure of changes in smooth-muscle tension in the splenic capsule and veins. 4. In order to assess the responses to baroreceptor stimulation in terms of alterations in sympathetic nerve traffic to different components of the * Research Assistant in Physiology, Mayo Graduate School of Medicine, and recipient of the Travelling Fellowship in Medicine or the Allied Sciences of the Royal Australasian College of Physicians. t Nuffield Foundation Fellow and Research Associate in Physiology, Mayo Graduate School of Medicine. 9 Pbyt. 205

2 258 DAVID BRENDER AND MICHAEL M. WEBB-PEPLOE peripheral vascular system, 'frequency-response curves' were constructed for spleen, saphenous vein, and limb resistance vessels by electrical stimulation of the splenic nerves and lumbar sympathetic chains. 5. The saphenous vein showed no consistent response to changes in baroreceptor activity. Reduction in carotid sinus pressure from 180 to 100 mm Hg caused an increase in venous pressure in the isovolumetric spleen and in the iliac artery perfusion pressure. These results were confirmed by electrical stimulation of the carotid sinus nerve. Whereas the peak responses of the limb resistance vessels corresponded to an increase in lumbar sympathetic nerve traffic of 6-10 c/s, the maximal splenic responses were equivalent to an increase in splenic nerve traffic of 1-4 c/s. These results are consistent with selective autonomic nervous control of different components of the peripheral vascular system. INTRODUCTION Since Hering (1924) first described the baroreceptor function of the carotid sinus, numerous studies have been conducted on the reflex connexions and functions of these receptors. Whereas there is general agreement that carotid sinus hypotension results in reflex tachycardia and constriction of the resistance vessels in muscle, skin, and splanchnic bed (Heymans & Neil, 1958), the role of the veins in the carotid sinus reflex is controversial. Previously it was thought that venoconstriction played an important part in the reflex response to carotid sinus hypotension by expressing blood from the systemic venous system, thereby increasing the cardiac output and so contributing to the increase in systemic arterial blood pressure (Heymans & Neil, 1958; Bartelstone, 1960; Ross, Frahm & Braunwald, 1961; Oberg, 1964). However, a number of investigators, using a variety of techniques, have been unable to demonstrate any significant change in cardiac output in dogs, cats, or rabbits during bilateral carotid occlusion and have concluded, therefore, that the rise in systemic blood pressure is caused solely by constriction of the resistance vessels (Edwards, Korner & Thorburn, 1959; Polosa & Rossi, 1961; Groom, Lofving, Rowlands & Thomas, 1962; Corcondilas, Donald & Shepherd, 1964). The possibility remains that the systemic veins do participate in carotid sinus reflexes, but that their role is a minor one in comparison to that of the resistance vessels. Constriction of the 'over-all venous bed' in the dog in response to a decrease in carotid sinus pressure has been documented (Bartelstone, 1960; Ross et al. 1961; Gaffhey, Bryant & Braunwald, 1962), but direct studies on specific venous beds have produced conflicting results (Alexander, 1954; Salzman, 1957; Browse, Donald & Shepherd, 1966). In man, the forearm veins do not appear to take part in the carotid sinus

3 PERIPHERAL CIRCULATION AND BARORECEPTORS 259 reflex (Bevegard & Shepherd, 1966; Samueloff, Browse & Shepherd, 1966; Epstein, Beiser, Stampfer & Braunwald, 1968). In the present study, reflex changes in wall tension of the lateral saphenous vein of one hind limb, the splenic veins and capsule, and the resistance vessels of the other hind limb caused by changes in baroreceptor activity were measured in vagotomized dogs by methods that have been described previously (Browse, Lorenz & Shepherd, 1966; Webb-Peploe, 1969a; Webb-Peploe & Shepherd, 1968d). In each animal frequencyresponse curves were constructed for spleen, saphenous vein, and limb resistance vessels by electrical stimulation of the splenic nerves and lumbar sympathetic chains. Comparison of the responses to baroreceptor stimulation with these frequency-response curves permitted an assessment of the effect of such stimulation on the superficial limb veins, splenic capacity elements, and limb resistance vessels in terms of change in sympathetic nerve discharge to these different components of the vascular system. The saphenous vein showed no consistent response to changes in baroreceptor activity. Progressive reduction in carotid sinus pressure in the range mm Hg caused a progressive constriction of both limb resistance vessels and splenic capsule and veins. The peak responses of limb resistance vessels corresponded to an increase in lumbar sympathetic nerve traffic of 6-10 c/s, and the maximal splenic responses were equivalent to an increase in splenic nerve traffic of 1-4 c/s. METHODS Mongrel dogs, kg in body weight, were anaesthetized with thiopentone and chloralose (20 and 80 mg/kg body weight, respectively) and artificially respired with oxygen by means of a Harvard respirator. Carotid 8ifnlU 8timulation The activity of the baroreceptors of the carotid sinus was changed in the following three ways: 1. I[n four dogs both carotid sinuses were isolated (Moissejeff, 1927) by ligating the common carotid arteries proximal to the bifurcation, the internal carotid arteries distal to the bifurcation, and the external carotid arteries and their branches. Cannulae were then inserted into the common carotid arteries above the ligatures, and the sinuses were filled from a pressurized reservoir containing fully oxygenated Krebs-Ringer bicarbonate solution at ph 7-4. The pressure in this system was measured at the Y junction connecting both common carotid artery cannulae to the reservoir. Both vagi were divided at the level of the cricoid cartilage. With this method, changes in carotid sinus pressure in the range of mm Hg (mean values) were achieved. 2. In four dogs, one carotid sinus (the right sinus in two dogs, and the left sinus in the other two animals) was perfused with each dog's own blood withdrawn from the cardiac end of the divided common carotid artery and returned to the cephalic end of the same artery by means of a roller pump and depulsator. The outflow 9-2

4 260 DAVID BRENDER AND MICHAEL M. WEBB-PEPLOE resistance of the carotid bifurcation was increased by ligating the external carotid and lingual arteries. Sinus pressure was measured through a catheter inserted into the lingual artery. The pressure in the sinus was varied by altering the speed of the pump. The contralateral carotid sinus was denervated by dividing the sinus nerve, and the completeness of denervation was demonstrated by temporary occlusion of the common carotid artery, which, in every case, failed to produce any change in blood pressure. Both vagi were divided as before. With this method, changes in carotid sinus pressure in the range of mm Hg (mean values) were achieved. 3. Vascular isolation of the right carotid artery bifurcation was carried out in two dogs as described by Heymans & Bouckaert (1930). The right common carotid artery was cannulated proximal to its bifurcation, and the right external carotid artery was cannulated distally. The right internal carotid artery was ligated distal to the bulb, as were all other large branches between the cannulae. The carotid sinus was perfused at constant flow (roller pump) with blood taken from the cardiac end of the divided right common carotid artery. This blood was returned via the external carotid artery cannula and a short length of Silastic tubing to the right external jugular vein. Pressure in the sinus (measured through a catheter inserted into the lingual artery) was altered (within a range of mm Hg) by means of a screw clamp on the tubing joining the external carotid artery and jugular vein cannulae. The left carotid sinus nerve and both vagi were divided. Carotid sinus nerve stimulation In three dogs the central end of the divided carotid sinus nerve was stimulated by means of a platinum bipolar electrode using monophasic shocks of 10 V for 1 msec at a frequency of 10 c/s. The responses to changes in carotid sinus activity of aortic pressure, resistance vessels of one hind limb, the lateral saphenous vein of the other hind limb, and the capacity elements of the spleen were studied simultaneously by the following techniques: Aortic pressure. Pressure was measured through a catheter inserted via the left brachial artery. Hind limb resistance vessel8. The right common iliac artery was cannulated and perfused at constant flow using a roller pump. The blood feeding the pump was taken from a cannula placed in the median sacral artery. A depulsator and heat exchanger were interposed between the pump and the iliac artery cannula, and perfusion pressure was measured through the side arm of a T junction placed immediately upstream to the cannula. Previous studies (Webb-Peploe & Shepherd, 1968d) have shown that after occlusion of the median sacral and common iliac arteries, little blood enters the hind limbs. Cannulation of the median sacral and right common iliac arteries in the present experiments thus ensured that the blood perfusing the limb came almost entirely from the constant-flow roller pump. Under these conditions, changes in common iliac artery perfusion pressure reflect changes in the activity of the resistance vessels of the limb. The pump speed was adjusted to give an initial perfusion pressure approximately equal to the mean aortic pressure. The temperature of the perfusate was maintained at 370 C. Superficial limb vein. The left lateral saphenous vein was cannulated at the ankle, and, by means of a roller pump, was perfused at constant flow with blood taken from the median sacral artery. Perfusion and femoral vein pressures were measured (the former from the side arm of a T junction placed immediately upstream to the cannula in the saphenous vein and the latter through a catheter inserted via the medial saphenous vein). A depulsator and heat exchanger were placed in the pump outflow line and, during experiments, arterial inflow to the left leg was temporarily

5 PERIPHERAL CIRCULATION AND BARORECEPTORS 261 arrested by tightening a snare around the left common iliac artery. Changes in saphenous vein perfusion pressure were a measure of changes in smooth muscle tension in the wall of the vein, since femoral vein pressure and blood flow through the vein were constant (Webb-Peploe & Shepherd, 1968d). The temperature of the venous perfusate was maintained at 370 C. Capacity elements of spleen. A fine catheter was inserted into a left gastroepiploic vein and advanced until its tip lay in the splenic hilum. All vessels entering or leaving the spleen, except for the splenic artery and vein, were ligated or divided. Snares were placed around the splenic artery and vein after dissecting them free from the splenic nerves and, during experiments, blood flow through the spleen was temporarily arrested by tightening these snares. With total arrest of the splenic circulation, the blood volume within the spleen remained constant, and changes in venous pressure in the isovolumetric spleen were a measure of the reflex changes in tension in the smooth muscle of the splenic capsule and veins (Webb-Peploe, 1969 a). All pressures were measured by means of strain-gauge transducers (Statham Model P23 De) and were recorded on a U.V. Honeywell 1508 Visicorder. In all except the two dogs in which the isolated right carotid sinus was perfused at constant flow (that is, in eight of the ten animals), frequency-response curves for limb resistance vessels, superficial limb vein, and splenic capacity elements were constructed at the end of each study by electrical stimulation of the lumbar sympathetic chains and splenic nerves as follows: The lumbar sympathetic trunks were divided at the level of the second or third lumbar vertebral bodies and dissected free down to the level of the fifth lumbar vertebral body. The distal ends of the divided sympathetic trunks were stimulated at the level of the fourth lumbar vertebral body via a platinum bipolar electrode using a Grass Stimulator (Model S 4). A stimulus of 10 V (supramaximal) was applied for 1 msec with no delay at frequencies varying from 2 to 10 c/s. The splenic nerves lying between the splenic artery and vein and entwining the artery were dissected free at a point distal to the snares on the artery and vein, divided centrally, and stimulated via a platinum bipolar electrode using a stimulus of 25 V (supramaximal) for 1 msec with no delay at frequencies varying from 0 5 to 10 c/s. RESULTS Bilateral isolated carotid sinuses: changes in carotid sinus pressure achieved by alterations in reservoir pressure Stepwise changes in sinus pressure. In ten experiments in four dogs, the carotid sinus pressure was decreased in steps of approximately 50 mm Hg from 242 to 30 mm Hg (mean values). This resulted in increases in mean aortic pressure, iliac artery perfusion pressure, and venous pressure in the isovolumetric spleen that averaged 40, 73, and 7 mm Hg respectively. The saphenous vein did not show consistent responses, dilating slightly in two dogs, constricting slightly in another, and showing no response in the fourth animal. The average change in saphenous perfusion pressure was a decrease of 9 mm Hg. In seven further experiments in the same four animals the carotid sinus pressure was increased, again in steps of about 50 mm Hg, from 18 to 250 mm Hg (mean values), causing decreases in mean aortic, iliac artery perfusion, and splenic venous pressures that averaged 71, 79, and 5 mm Hg respectively. Again the saphenous vein did not show a con-

6 262 DAVID BRENDER AND MICHAEL M. WEBB-PEPLOE (mm Hg) Carotid sinus 200 Iliac artery Ara{ ki ( m Hg) Splenic vein Femoral vein... O Fig. 1. Dog 1. Thiopentone-chloralose. Responses to stepwise changes in carotid sinus pressure (bilateral isolated carotid sinus). In these and subsequent tracings, carotid sinus = pressure within sinus; iliac artery = pressure in right common iliac artery perfused at constant flow; aorta = aortic pressure; saphenous perfusion = perfusion pressure of the left lateral saphenous vein perfused at constant flow; splenic vein = venous pressure in the isovolumetric spleen. The right-hand scale in each panel refers to the splenic vein pressure only; the left-hand scale refers to pressures in aorta, carotid sinus, leg veins, and iliac artery. Note that alterations in carotid sinus pressure resulted in directionally opposite changes in aortic, iliac perfusion, and splenic venous pressures but had little effect on the saphenous vein. O EIO bo _ 0'v -O o 0 E J ~~~~~~~~0 C 0 4~~~~~~~~~~) 0 0 0~~~~~~~~ *: Carotid.g scinu(mhg o /odog NZ Dogs.~~~~~~~~ 6 o4 2. hoetn-hoaoe feto tpiesnspesr changes (bilateral isolated sinuses), one representative experiment fromn each dog. With decreases in carotid sinus pressure, aortic, iliac artery, and splenic venous pressures increased, but saphenous vein perfusion pressure (upper right panel) did not show consistent change.

7 PERIPHERAL CIRCULATION AND BARORECEPTORS 263 sistent response, the over-all result for the group being a decrease in perfusion pressure of 2 mm Hg. Figure 1 shows the results of a representative experiment, and in Fig. 2 mean aortic pressure, iliac artery perfusion pressure, venous pressure in the isovolumetric spleen, and saphenous vein perfusion pressure (ordinates) have been plotted against carotid sinus pressure (abscissae) for a representative experiment from each of the four dogs. In each case the saphenous vein perfusion pressure showed no consistent response to changes in carotid sinus pressure. By contrast, decreases in carotid sinus pressure caused increases in mean aortic pressure, iliac artery perfusion pressure, andvenous pressure inthe isovolumetric spleen, decreases in sinus pressure having the opposite effect. The most marked responses were obtained with changes in sinus pressure in the rangel mm Hg, and in this range sudden decreases in sinus pressure often produced 'overshoots' in aortic, iliac perfusion, and splenic venous pressures (Fig. 1), while sudden increases in sinus pressure resulted in 'undershoots' in these three pressures. In Fig. 2, the pressures plotted are those obtained at the end ofthe overshoot or undershoot, when a relatively steady state had been achieved. In comparing the changes in iliac artery perfusion and splenic vein pressures induced by alterations in sinus pressure with those caused by electrical stimulation of the appropriate sympathetic nerves, the hind limb resistance vessels and splenic capacity elements have been assumed to possess no sympathetically mediated constrictor tone at the upper extreme of sinus pressure. Iliac artery perfusion pressures and venous pressures in the isovolumetric spleen obtained at the highest sinus pressure in each experiment thus became zero reference points comparable to the pressures obtained after section and before stimulation of the right lumbar sympathetic trunk and splenic nerves, respectively. In the case of the lateral, saphenous vein, which did not respond to changes in carotid sinus pressure, such an assumption obviously was not justified, and the perfusion pressure obtained when the vein was fully dilated by warming the venous perfusate to 450 C (Webb-Peploe & Shepherd, 1968 a) was taken as the zero reference point for this vessel. Comparison of the results of changes im carotid sinus pressure with those caused by nerve stimulation demonstrated that reduction in carotid sinus pressure over the full range resulted in an average rise in iliac artery perfusion pressure that was equivalent to an increase in sympathetic nerve discharge of 6-10 c/s (Fig. 3, right panel). The rise in venous pressure in the isovolumetric spleen corresponded to an increase in splenic nerve discharge of 2-4 c/s. Although there was no consistent response of the lateral saphenous vein perfusion pressure to changes in carotid sinus pressure in any of the dogs studied, stimulation of the lumbar sympathetic trunks in these animals produced increases in saphenous vein perfusion

8 264 DAVID BRENDER AND MICHAEL M. WEBB-PEPLOE pressure that averaged 13 mm Hg at a stimulation frequency of 2 c/s, 59 mm Hg at 6 c/s, and 138 mm Hg at 10 c/s. Similar results for the frequency-response curve of the cutaneous vein have been reported previously (Webb-Peploe & Shepherd, 1968d). Gradual changes in sinus pressure. In three of the four dogs in this group, the sinus pressure not only was changed in steps of 50 mm Hg but also was altered gradually over the pressure range of mm Hg at a rate (mm Hg) c/s Unilateral perfused sinus (n=15) 122 Bilateral isolated sinus (n=17) co 40 ~20? 0 1 *(mm Hg)10 11 #E 'J U -.. Sinus pressure range: Fig. 3. Dogs 1 to 8. Thiopentone-chloralose. Comparison of the responses (mean + 1 s.e.) of hind limb resistance vessels and splenic capacity elements to maximal carotid baroreceptor stimulation using the unilateral perfused sinus (dogs 5 to 8) and the bilateral isolated sinus (dogs 1 to 4). The responses to electrical stimulation of the lumbar sympathetic and splenic nerves are showni also (left-hand bar graphs). of 100 mm Hg/min. In all three animals, iliac artery perfusion pressure, splenic venous pressure, and aortic pressure increased as the carotid sinus pressure was reduced. These changes were reversed by increasing the sinus pressure. With slow changes in carotid sinus pressure made at a constant rate, it was possible to gain a more accurate idea of the range of sinus pressure affecting carotid sinus baroreceptor discharge by averaging the lower and upper sinus pressures at which changes in aortic pressure, iliac artery perfusion pressure, and splenic venous pressure began or ended

9 PERIPHERAL CIRCULATION AND BARORECEPTORS 265 during both gradual increases and decreases in sinus pressure. This range was mm Hg in the first dog (one experiment), mm Hg in the second dog (two experiments), and mm Hg in the third dog (two experiments). In all three animals the lateral saphenous vein failed to respond to changes in baroreceptor discharge. That this lack of response was not due to impaired reactivity of the venous smooth muscle was demonstrated by experiments such as the one shown in Fig. 4. In this dog, sudden changes in carotid sinus pressure from 15 to 250 mm Hg had no effect on the saphenous vein, although the expected responses were seen in aortic pressure, iliac artery perfusion pressure, and venous pressure in the isovolumetric spleen. The saphenous vein, however, did respond vigorously to changes in the temperature of the blood perfusing it, dilating when the perfusate was warmed and constricting when the perfusate was cooled. Comparison of the responses of the hind limb resistance vessels and splenic capacity elements to changes in carotid sinus pressure in the range of mm Hg in steps in 50 mm Hg, and gradually at a rate of 100 mm Hg/min in the same three dogs, showed no significant difference. Unilateral perfused carotid sinus: changes in sinus pressure achieved by alterations in rate of sinus perfusion In four dogs, the carotid sinus pressure was progressively increased or decreased in steps of approximately 25 mm Hg by appropriate alterations in the speed of the pump used to perfuse the sinus. Individual variations in the size of the carotid artery and in the collateral arterial supply to the carotid bifurcation in different dogs resulted in considerable variation in the sinus pressures obtained with maximal and minimal pump flows in the four animals. In nine experiments the carotid sinus pressure was increased in steps of 25 mm Hg from 90 to 190 mm Hg (mean values). This resulted in total decreases in mean aortic pressure, iliac artery perfusion pressure, and venous pressure in the isovolumetric spleen that averaged 68, 80, and 7-5 mm Hg respectively. The saphenous vein perfusion pressure again showed no consistent response, increasing in some experiments and decreasing in others, the average change for all nine experiments being + 1 mm Hg. In six further experiments, the carotid sinus pressure was decreased in steps of 25 mm Hg from 210 to 108 mm Hg (mean values), and the resultant increases in mean aortic pressure, iliac artery perfusion pressure, and venous pressure in the isovolumetric spleen averaged 55, 101, and 7x5 mm Hg respectively. Again no consistent change was seen in the superficial limb vein, the average change in saphenous vein perfusion pressure being -1 mm Hg.

10 266 DAVID BRENDER AND MICHAEL M. WEBB-PEPLOE %II E 0o > 72~004 0 o o W 4. &4 0 B4 ; o ri - A c 0 0 U E o p44 0 * 34O C4-4 W o = A_ C4.4 bn ~ 0 0 p4 go 0-0.X. o l-0 -Q Ca 0 - w co C w e*

11 PERIPHERAL CIRCULATION AND BARORECEPTORS 267 Figure 5 (plotted in the same way as Fig. 2) shows the results of a representative experiment from each of the four dogs in this group. In two of the four dogs the lateral saphenous vein possessed considerable venomotor tone as judged by the high perfusion pressures (Fig. 5, upper right panel), while in the other two dogs the vein was less constricted. Whether the vein was constricted initially or not, it failed to show any consistent change in tone in response to alterations in carotid sinus pressure. (mm Hg) 0 (mm Hg) t 100, 100LAA o C I) Dogi - Dog < Dog 3 _ D~~~~~~~~~~~~~og2 u -A ADog L Carotid sinus (mm Hg) Fig. 5. Dogs 5 to 8. Thiopentone-chloralose. Effect of stepwise sinus pressure changes (unilateral perfused sinus). One representative experiment from each dog. With decreases in carotid sinus pressure, aortic, iliac artery, and splenic venous pressures increased, but the saphenous vein perfusion pressure (upper right panel) showed little change whether the initial venomotor tone was high (dogs 7 and 8) or low (dogs 5 and 6). In these animals the responses of the iliac artery perfusion pressure and splenic venous pressure to maximal decreases in carotid sinus pressure also were compared to the results of electrical stimulation of the lumbar sympathetic and splenic nerves, and corresponded to an increase in efferent nerve discharge of 10 or more c/s and of less than 1 c/s, respectively (Fig. 3, left panels). Again, although there was no response of the saphenous vein to baroreceptor stimulation, electrical stimulation of the lumbar

12 268 DAVID BRENDER AND MICHAEL M. WEBB-PEPLOE sympathetic trunks produced rises in saphenous perfusion pressure that averaged 17 mm Hg (2 c/s), 70 mm Hg (6 c/s), and 131 mm Hg (10 c/s). Unilateral perfused carotid sinus: changes in sinus pressure achieved by alterations in outflow resistance In the two dogs in which the carotid sinus was perfused at constant flow, aortic pressure, venous pressure in the isovolumetric spleen, and saphenous vein perfusion and femoral vein pressures (but not iliac artery perfusion (mm Hg) 200 Iliac artery Wnl lejr u.n.111s1 x 370 C saphenous perfusate temperature Aorta a\, Saphenous perfusion 130 sec 10 Splenic vein LAk.A.t.AAl...4k 0 O jw_ WW W W Femorai vein _Stimulus tirnulus Fig. 6. Dog 7. Thiopentone-chloralose. Electrical stimulation of carotid sinus nerve (black bar) caused a fall in aortic, iliac artery perfusion, and splenic vein pressures. The saphenous perfusion pressure increased slightly. pressure) were measured. In both animals, aortic and splenic venous pressures increased with a decrease in carotid sinus pressure and vice versa. In one dog the saphenous vein relaxed slightly with a decrease in sinus pressure, whereas the cutaneous vein of the other animal constricted slightly with a similar decrease in sinus pressure. Carotid sinus nerve stimulation In three of the four dogs in which one carotid sinus was perfused at varying flow rates, the contralateral sinus nerve was identified and stimulated. Figure 6 shows a typical example of the response obtained: nerve

13 PERIPHERAL CIRCULATION AND BARORECEPTORS 269 stimulation caused considerable decreases in aortic and iliac artery perfusion pressures and a slight decrease in venous pressure in the isovolumetric spleen. By contrast, the perfusion pressure of the saphenous vein showed little change. DISCUSSION In 1921, Donegan observed that stimulation of the central ends of the vagal or sciatic nerves, or asphyxia, all of which caused a rise in blood pressure, also caused a corresponding decrease in the calibre of the mesenteric veins. The superficial limb veins did not appear to be involved in these reflex changes, and Donegan suggested that they might take part in temperature reflexes. This suggestion has been borne out by recent studies (Webb-Peploe & Shepherd, 1968 a-c) demonstrating that the lateral saphenous vein of the dog constricts in response to a decrease in either local or central body temperature. The superficial limb veins are capable of extremely powerful constriction, whereas the deep veins do not respond to sympathetic nerve stimulation (Webb-Peploe & Shepherd, 1968d). The superficial veins probably play an important role in regulation of temperature, directing venous blood preferentially either through the deep venae comitantes effecting a preservation of heat by countercurrent exchange with arterial blood, or through the superficial veins resulting in heat dissipation. These vessels thus have a 'resistance' in addition to a ' capacity' function, and it is therefore not inappropriate to measure their responses by perfusing them at constant flow and recording changes in perfusion pressure. The participation of the superficial limb veins in thermoregulation appears to be a specialized function of these vessels, since tone in the splenic capsule and veins is unaffected by changes in central body temperature (Webb-Peploe, 1969 b). In the present study, Donegan's (1921) early observation that the mesenteric veins but not the superficial limb veins take part in reflexes that involve the resistance vessels is shown to apply also to the reflex responses to carotid baroreceptor stimulation. Changes in venous pressure in the isovolumetric spleen were used to measure reflex changes in tension in the smooth muscle of the splenic capsule and veins, and a previous study has shown that these reflex changes in capacity function of the spleen can be used as an index to the reflex responses of capacity vessels throughout the splanchnic bed (Webb-Peploe, 1969a). The chief disadvantage of the Moissejeff (1927) preparation is that the sinuses are not perfused, so that experiments in which this preparation is employed may well be conducted against a background of chemoreceptor discharge. The temperature of the fluid in both carotid segments is also less than 380 C, and this reduces the activity of the nerve endings

14 270 DAVID BRENDER AND MICHAEL M. WEBB-PEPLOE (Diamond, 1955). These two factors may account for the slightly smaller responses of the hind limb resistance vessels and splenic capacity elements to the full range of carotid sinus pressure variation achieved with the Moissejeff technique as compared to the single perfused sinus method (Fig. 3), a result that was unexpected in view of the evidence that there is a 'summation by simple addition' of bilateral carotid sinus signals in the carotid sinus reflex (Sagawa & Watanabe, 1965). The two other methods used for changing carotid sinus pressure preserved the normal carotid blood flow on the denervated side. Since all the dogs were respired with oxygen, it is unlikely that cerebral hypoxia occurred. Thus the failure of the lateral saphenous vein to constrict with a reduction in sinus pressure cannot be attributed to masking of a constrictor response by partial inhibition of thermoregulatory tone consequent on cerebral hypoxia (Webb-Peploe & Shepherd, 1968a). The fact that the lateral saphenous vein showed no consistent response to changes in carotid sinus pressure is in keeping with the observations of Browse, Donald & Shepherd (1966). Similarly, the superficial limb veins in man play little part in baroreceptor reflexes, since no consistent changes in tone of the veins of the forearm and hand are seen in response to sudden changes in posture (Samueloff et al. 1966), to transfer of blood to the legs by suction in the lower part of the body (Samueloff et al. 1966; Epstein et al. 1968), or to an increase in carotid sinus-transmural pressure achieved by applying suction to the neck (Bevegard & Shepherd, 1966). Because of the different methods used, it is difficult to compare our findings with those of Salzman (1957) or of Zingher & Grodins (1964); in both of these studies the limb veins of the dog appeared to respond in a manner similar to that of the resistance vessels to changes in carotid sinus pressure. Neil, Redwood & Schweitzer (1949), who investigated the blood pressure responses of the dog to electrical stimulation of the carotid sinus nerve with pulses of varying voltage, frequency, and duration, concluded that 'in all circumstances the effects of baroreceptor stimulation are predominant over those of chemoreceptor fibre stimulation' in this species, since they invariably obtained a depressor response to stimulation. Thus, the failure of the lateral saphenous vein to show any response to carotid sinus nerve stimulation that produced significant dilatation of the hind limb resistance vessels and of the splenic capacity elements (Fig. 6) is further confirmation that changes in baroreceptor discharge, while influencing the tone of both the resistance vessels of the hind limb and the capacity elements of the spleen, have no effect on the superficial limb veins. That this lack of response was not due to impaired reactivity of the venous smooth muscle was shown in experiments (Fig. 4) in which the saphenous vein, though unresponsive to changes in carotid sinus pressure, constricted

15 PERIPHERAL CIRCULATION AND BARORECEPTORS 271 vigorously in response to cooling the blood perfusing it and dilated when the perfusate was warmed. Local cooling enhances the response of the venous smooth muscle to arriving sympathetic nerve impulses, and local warming has the opposite effect; in the absence of sympathetic constrictor tone, the vein is unresponsive to changes in local temperature (Webb- Peploe & Shepherd, 1968c). A local temperature of 440 C or more is required to abolish the venous response to sympathetic nerve stimulation. These experiments thus confirmed that although the saphenous vein possessed sympathetic constrictor tone, the level of that tone was unaffected by changes in carotid sinus pressure. Koch (1931) was the first to plot changes in systemic arterial pressure against stepwise increases in carotid sinus pressure over a range of mm Hg. He found that in the dog the maximal sensitivity of the reflex was about 120 mm Hg, and that below mm Hg and above 210 mm Hg there was no response to a fall or rise in pressure, respectively. In the present study, aortic pressure, iliac artery perfusion pressure, and venous pressure in the isovolumetric spleen all showed maximal responses in the same range of carotid sinus pressures. In three dogs, it was possible to estimate this range accurately by averaging the lower and upper sinus pressures at which changes in aortic pressure, iliac artery perfusion pressure, and splenic venous pressure began or ended during both gradual increases and decreases in sinus pressure made at a rate of 100 mm Hg/min. The results suggested that there may be considerable individual variation in the 'effective' carotid sinus pressure- range in different animals. Within this effective carotid sinus pressure range, overshoots and undershoots in aortic, iliac artery perfusion, and splenic venous pressures occurred with stepwise decreases and increases, respectively, in carotid sinus pressure. This phenomenon, which was not seen when the carotid sinus pressure was changed gradually, only lasted approximately 30 sec, and thus occurred too rapidly to have been caused by autoregulation in the vascular sections under study. Furthermore, it was seen in the splenic venous pressure, and there is evidence (at least in striated muscle) that autoregulation is 'essentially concentrated to the precapillary resistance vessels' (Folkow & Oberg, 1961). The baroreceptors are known to be sensitive not only to the magnitude of the mechanical stimulus but also to the velocity of its application (Green, 1967), and it seems likely, therefore, that the overshoots and undershoots originated at the baroreceptors and were not caused by an autoregulatory phenomenon in the peripheral vascular sections under study. Fleisch (1931), Gollwitzer-Meier & Schulte (1931), and Alexander (1954) have demonstrated an increase in wall tension in mesenteric and colic veins in response to bilateral carotid occlusion. In the present study,

16 272 DAVID BRENDER AND MICHAEL M. WEBB-PEPLOE comparison of the changes in venous pressure in the isovolumetric spleen produced by the full range of carotid sinus pressure variation with the responses obtained by electrical stimulation of the splenic nerves at the end of each study suggested that the responses of the capacity elements of the spleen to maximal changes in baroreceptor discharge corresponded to a change in splenic nerve discharge of 1-4 c/s in most animals (Fig. 3). In making this comparison, the splenic nerve traffic was assumed to be zero at peak carotid sinus pressures, an assumption supported by the observation that an increase in blood pressure of about 100 mm Hg above normal induced by infusion of adrenaline results in the disappearance of action potentials recorded in the splanchnic nerve of the cat (Gernandt, Liljestrand & Zotterman, 1946; Dontas, 1955). A similar comparison for the resistance vessels of the hind limb suggested that the responses of these vessels to the full range of baroreceptor discharge corresponded to an alteration in lumbar sympathetic nerve impulses of 6-10 c/s, which is in agreement with the results of Browse, Donald & Shepherd (1966). As with the splenic capacity elements, the nerve discharge to the resistance vessels of the hind limb was assumed to be zero at peak carotid sinus pressures, an assumption that ignores the possibility of active vasodilatation in response to baroreceptor stimulation. Parasympathetic and sympathetic cholinergic vasodilator fibres do not participate actively in baroreceptor reflexes (Heymans & Neil, 1958), but recent evidence points to a sympathetic histaminergic dilator component in the responses of the resistance vessels of the canine hind limb to baroreceptor stimulation (Brody, 1966). In the present study, the iliac artery perfusion pressures obtained at peak carotid sinus pressures immediately before lumbar sympathectomy averaged (S.E.) mm Hg, whereas after sympathectomy they averaged (S.E.) mm Hg in the same eight dogs. These results suggest that in most cases an active dilator response to carotid sinus hypertension, if present, was small. The authors wish to thank Robert R. Lorenz and Roger L. Ready for their technical assistance and Dr John T. Shepherd for helpful criticism and advice. The investigation was supported in part by Research Grant HE-5883 from the National Institutes of Health, Public Health Service. REFERENCES ALEXANDER, R. S. (1954). The participation of the venomotor system in pressor reflexes. Circulation Res. 2, BARTELSTONE, H. J. (1960). Role of the veins in venous return. Circulation Res. 8, BEVEGARD, B. S. & SHEPHERD, J. T. (1966). Circulatory effects of stimulating the carotid arterial stretch receptors in man at rest and during exercise..j. clin. Invest. 45,

17 PERIPHERAL CIRCULATION AND BARORECEPTORS 273 BRODY, M. J. (1966). Neurohumoral mediation of active reflex vasodilatation. Fedn Proc. 25, BROWSE, N. L., DONALD, D. E. & SHEPHERD, J. T. (1966). Role of the veins in the carotid sinus reflex. Am. J. Physiol. 210, BROWSE, N. L., LORENZ, R. R. & SHEPHERD, J. T. (1966). Response of capacity and resistance vessels of dog's limb to sympathetic nerve stimulation. Am. J. Phy8iol. 210, CORCONDILAS, A., DONALD, D. E. & SHEPHERD, J. T. (1964). Assessment by two independent methods of the role of cardiac output in the pressor response to carotid occlusion. J. Physiol. 170, DIAMOND, J. (1955). Observations on the excitation by acetylcholine and by pressure of sensory receptors in the cat's carotid sinus. J. Phy8iol. 130, DONEGAN, J. F. (1921). The physiology of the veins. J. Phy8iol. 55, DONTAS, A. S. (1955). Effects of protoveratrine, serotonin and ATP on afferent and splanchnic nerve activity. Circulation Res. 3, EDWARDS, A. W. T., KORNER, P. I. & THORBURN, G. D. (1959). The cardiac output of the unanaesthetized rabbit, and the effects of preliminary anaesthesia, environmental temperature and carotid occlusion. Q. Jl exp. Phy8iol. 44, EPSTEIN, S. E., BEISER, G. D., STAMPFER, M. & BRAUNWALD, E. (1968). Role of the venous system in baroreceptor-mediated reflexes in man. J. clin. Inve8t. 47, FLEISCH, A. (1931). Venomotorenzentrum und Venenreflexe. II. Mitteilung. Blutdruckzugler und Venenreflexe. Pfluger8 Arch. ge8. Phy8iol. 226, FOLKOW, B. & OBERG, B. (1961). Autoregulation and basal tone in consecutive vascular sections of the skeletal muscles in reserpine-treated cats. Acta physiol. 8cand. 53, GAFFNEY, T. E., BRYANT, W. M. & BRAUNWALD, E. (1962). Effects of reserpine and guanethidine on venous reflexes. Circulation Re8. 11, GERNANDT, B., LILJESTRAND, G. & ZOTTERMAN, Y. (1946). Efferent impulses in the splanchnic nerve. Acta phy8iol. 8cand. 11, GOLLWITZER-MEIER, K. & SCHuLTE, H. (1931). Der Einfluss der Sinusnerven auf Venensystem und Herzminutenvolumen. Pflugerm Arch. ges. Physiol. 229, GREEN, J. H. (1967). Physiology of baroreceptor function: Mechanism of receptor stimulation. In Baroreceptors and Hypertension: Proceedings of an International Symposium, ed. KEZDI, P., pp New York: Pergamon Press. GROOM, A. C., LOFVING, B. M. A., ROWLANDS, S. & THOMAS, H. W. (1962). The effect of lowering the pulse pressure in the carotid arteries on the cardiac output in the cat. Acta physiol. 8cand. 54, HERING, H. E. (1924). Der Sinus caroticus an der Ursprungsstelle der Carotis interna als Ausgangsort eines hemmenden Herzreflexes und eines depressorischen Gefassreflexes. Munch. med. Wschr. 1, HEYMANS, C. & BOUCKAERT, J. J. (1930). Perfusion des sinus carotidiens isoles avec la pompe de Dale-Schuster: R6flexes vasomoteurs. C. r. Seanc. Soc. Biol. 103, HEYMANS, C. & NEIL, E. (1958). Reflexogenic Areas of the Cardiovascular System. Boston: Little, Brown & Company. KOCH, E. (1931). Die Refilektorische Selbsteurung des Krei8laufes. Dresden: Steinkopff. MOISSEJEFF, E. (1927). Zur Kenntnis des Carotissinusreflexes. Z. ges. exp. Med. 53, NEIL, E., REDWOOD, C. R. M. & SCHWEITZER, A. (1949). Blood pressure responses to electrical stimulation of the carotid sinus nerve in dogs and rabbits. J. Physiol. 109,

18 274 DAVID BRENDER AND MICHAEL M. WEBB-PEPLOE OBERG, B. (1964). Effects of cardiovascular reflexes on net capillary fluid transfer. Acta physiol. 8cand. 62, suppl. 229, POLOSA, C. & Rossi, G. (1961). Cardiac output and peripheral blood flow during occlusion of carotid arteries. Am. J. Phy&iol. 200, Ross, J., Jr., FiAHM, C. J. & BRAUNWALD, E. (1961). Influence of carotid baroreceptors and vasoactive drugs on systemic vascular volume and venous distensibility. Circulation Res. 9, SAGAWA, K. & WATANABE, K. (1965). Summation of bilateral carotid sinus signals in the barostatic reflex. Am. J. Phy8iol. 209, SATZ1MAN, E. W. (1957). Reflex peripheral venoconstriction induced by carotid occlusion. Circulation Rew. 5, SAMUELOFF, S. L., BROWSE, N. L. & SHEPHERD, J. T. (1966). Response of capacity vessels in human limbs to head-up tilt and suction on lower body. J. appl. Physiol. 21, WEBB-PEPLOE, M. M. (1969a). The isovolumetric spleen: index of reflex changes in splanchnic vascular capacity. Am. J. Physiol. 216, WEBB-PEPLOE, M. M. (1969b). Effect of changes in central body temperature on capacity elements of limb and spleen. Am. J. Physiot. 216, WEBB-PEPLOE, M. M. & SHEPHERD, J. T. (1968a). Response of the superficial limb veins of the dog to changes in temperature. Circulation Res. 22, WEBB-PEPLOE, M. M. & SHEPHERD, J. T. (1968b). Response of dogs' cutaneous veins to local and central temperature changes. Circulation Res. 23, WEBB-PEPLOE, M. M. & SHEPHERD, J. T. (1968c). Peripheral mechanism involved in response of dogs' cutaneous veins to local temperature change. Circulation Res. 23, WEBB-PEPLOE, M. M. & SHEPHERD, J. T. (1968d). Response of large hindlimb veins of the dog to sympathetic nerve stimulation. Am. J. Physiol. 215, ZINGHER, DIANA & GRODINS, F. S. (1964). Effect of carotid baroceptor stimulation upon the forelimb vascular bed of the dog. Circulation Res. 14,