vasoconstriction) were mediated for the most part through the Sydney, Australia circulatory response was determined largely by the opposing effects of

Transcription

1 J. Phy8iol. (1967), 188, pp With 4 text-ftgurew Printed in Great Britain THE RELATIVE ROLES OF THE AORTIC AND CAROTID SINUS NERVES IN THE RABBIT IN THE CONTROL OF RESPIRATION AND CIRCULATION DURING ARTERIAL HYPOXIA AND HYPERCAPNIA By J. P. CHALMERS,* P. I. KORNER AND S. W. WHITE* From the School of Physiology, University of New South Wales, Sydney, Australia (Received 16 August 1966) SUMMARY 1. The respiratory and circulatory effects of graded arterial hypoxia, alone or with superadded hypercapnia, were studied in four groups of unanaesthetized rabbits including normal animals and those with selective section of the aortic nerves, selective section of the carotid sinus nerves and section of both sets of nerves. 2. When measured 2-4 days after selective section of the carotid sinus nerves the resting respiratory minute volume and arterial Po, were lower and the Pco, higher than normal. These effects were not observed after selective section of the aortic nerves. Selective aortic nerve section, and selective carotid sinus nerve section each produced a similar increase in the resting arterial pressure and heart rate, but were without effect on the resting cardiac output.. During arterial hypoxia reflex respiratory and circulatory effects ascribable to arterial chemoreceptor stimulation (hyperventilation, bradycardia, vasoconstriction) were mediated for the most part through the carotid sinus nerve. In animals with only the aortic nerves intact the circulatory response was determined largely by the opposing effects of aortic baroreceptor reflexes and the local peripheral dilator action of hypoxia. 4. The circulatory effects of hyperventilation induced by hypercapnia during arterial hypoxia, in animals with both aortic and carotid sinus nerves cut were small. 5. The results suggest that relatively few chemoreceptor fibres originate from the aortic region in the rabbit, though the carotid sinus and aortic nerves both contain baroreceptor fibres. * Fellow of the National Heart Foundation of Australia.

2 46 J. P. CHALMERS, P. I. KORNER AND S. W. WHITE INTRODUCTION In intact, unanaesthetized rabbits there are qualitative differences between the circulatory responses evoked by arterial hypoxia of different degrees of severity (Korner, 1965a, b; Chalmers, Isbister, Korner & Mok, 1965; Chalmers, Korner & White, 1966). With milder grades of hypoxia there is elevation of cardiac output and heart rate, with some peripheral vasodilatation. These effects can be partly attributed to secondary reflexes associated with chemoreceptor-induced hyperventilation (cf. Daly & Scott, 1962 a, b; Korner, 1965 a), and partly to the local, peripheral dilator effects of hypoxia (Chalmers et al. 1965, 1966 a; Korner & White, 1966). With more severe arterial hypoxia there is reduction in cardiac output, bradycardia, and a rise in total peripheral resistance, which can be ascribed to the direct circulatory reflex effects of strong chemoreceptor stimulation (Korner, 1965a, b). In analysing these responses in the rabbit the relative contributions of the aortic and carotid reflex zones have not so far been determined. In the rabbit the arterial chemoreceptor afferents appear to travel entirely in the carotid sinus nerve, since, for example, electrical stimulation of the aortic nerve does not produce hyperpnoea (Schmidt, 192; Gernandt, 1946; Neil, Redwood & Schweitzer, 1949; Douglas & Ritchie, 1956; Douglas, Ritchie & Schaumann, 1956; Mott, 196). This contrasts with the results obtained in the dog and cat, where the aortic and carotid reflex zones each give rise to chemoreceptor fibres as well as baroreceptor fibres (Comroe, 199; Neil et al. 1949; Douglas & Schaumann, 1956; Howe, 1956; Heymans & Neil, 1958; Daly, Hazzledine & Howe, 1965; Paintal & Riley, 1966). In these species reflex circulatory chemoreceptor responses can be elicited from both aortic and carotid zones, but the dominant respiratory drive is mediated through the carotid sinus nerve, suggesting differences in these species in the central connexions of the chemoreceptor fibres carried in each nerve (Comroe, 199; Bernthal & Schwind, 1945; Bernthal, Greene & Revzin, 1951; Daly & Scott, 1958, 1962; Comroe & Mortimer, 1965; Daly, Hazzledine & Howe, 1965; Daly & Ungar, 1966). In the present experiments the effects of three levels of severity of arterial hypoxia have been examined in unanaesthetized rabbits, with selective section of the carotid sinus or aortic nerves, and the results have been compared with those of normal animals on the one hand, and with those of section of both sets of afferent nerves on the other. At each level of hypoxia the effects of superimposed hypercapnia have also been studied. The combination of hypercapnia with hypoxia potentiates reflex arterial chemoreceptor responses when these are present, as a result of increased chemoreceptor discharge (Bartels & Witzleb, 1956; Joels & Neil, 1961; Neil & Joels, 196; Eyzaguirre & Lewin, 1961; Eyzaguirre & Koyano,

3 AORTIC AND CAROTID REFLEXES IN RABBITS ; Korner, 1956a; Chalmers et al. 1966; Daly & Ungar, 1966). In addition, owing to the effects of raised Pco2 on the central chemoreceptors, its use in animals with section of both carotid sinus and aortic nerves permits assessment of the circulatory effects of hyperventilation in the absence of peripheral chemoreceptor activity in unanaesthetized animals. METHODS Animals. New Zealand White rabbits, cross-bred with the New Zealand Giant strain were used in these experiments (mean weight 2-55 kg; range 22-0 kg). Operative procedures. A thermistor catheter was inserted into the upper abdominal aorta and the trachea transposed into a subcutaneous position, at a preliminary operation under anaesthesia. In some of these rabbits section of the aortic nerves, the carotid sinus nerves or both nerves was carried out as well. Animals with section of both these nerves were given mg of strophanthin G (Ouabain, Arnaud) i.v. at the time of the operation to avoid development of left ventricular failure. This was not necessary in animals with section of only one set of buffer nerves. On the day of the experiment, which was always 2-4 days after the preliminary operation, catheterization of the central ear artery and right atrium, and cannulation of the trachea, were carried out using local anaesthesia. All these procedures have been fully described elsewhere (Korner, 1965c). Measurement of circutlatory and respiratory parameters. Cardiac output was measured by the thermodilution technique (Fegler, 194; Korner, 1965c), and mean ear artery pressure, mean right artial pressure and heart rate were determined as described previously (Korner, 1965c). V-entilation was measured by collecting the expired air and was expressed as 1./min of dry gas at s.t.p. (Edwards, Korner & Thorburn, 1959). Respiratory rate was obtained using a small mercury-in-silastic strain gauge placed round the animal's chest. Arterial ph (to an accuracy of units), Po, and PCO2 (to an accuracy of + 1 mm Hg) were determined using a Mlodel 11 Instrumentation Laboratory Inc. Blood Gas Analyser and ph Meter (Chalmers & Korner, 1966). Coniduct of experiments. Following completion of the minor operative procedures on the day of the experiment, the rabbit was placed inside a large box where it sat comfortably without restraint. Recording commenced 1 hr later. Each experiment consisted of a control period of 20 min breathing room air, a treatment period of 40 min breathing the test gas, and a recovery period of 20 min, again breathing room air. In most animals two experiments were carried out in random order with an interval of 60 min between them. In one experiment the rabbit breathed an O2-N2 gas mixture adjusted so as to reduce the arterial P02 to one of three grades of hypoxia (6-45 mm Hg; 0-5 mm Hg; mm Hg), whilst in the other experiment it was given a CO2 02 N'2 mixture adjusted to produce mild hypercapnia but a similar degree of reduction in the arterial P 02 as observed in the experiment with arterial hypoxia alone. The 02-N2 mixtures administered to the various preparations were adjusted so as to produce comparable reduction in arterial PF0 despite variation in respiratory response. The timing of the various measurements was always similar in different experiments, and 1- measurements of arterial pressure, cardiac output, right atrial pressure, heart rate, ventilation and respiratory rate were obtained and averaged for each animal at a number of selected time intervals as shown in Fig. 1. In a given set of experiments the mean value for each time interval was determined for each parameter for all the animals in the group. In order to facilitate comparisons of the responses of different groups of animals with different initial control values the changes in ventilation, cardiac output, arterial pressure and heart rate were expressed as percentages of the initial control value (see Figs. 1-4). The standard error of a single time interval was calculated for each parameter by analysis of variance (Mather, 1949).

4 48 J. P. CHALMERS, P. I. KORNER AND S. W. WHITE I./min 2-50F r 200 F *00 * 700 o 600 C) r $S L 270 mi./min Beats/mi n mm Hg m120_ L Time (mm) 150 L 100 _ F 100 L 90 _ F 100 F 90 F 80 L 110 _ 100 _ go L I Fig. 1. Effects of arterial hypoxia (PO2 7 mm Hg) in an individual experiment. In the left-hand panel results are presented in absolute values-ventilation (1./min at s.t.p.); cardiac output (ml./min); heart rate (beats/min); and arterial pressure (mm Hg). Each masurement taken is shown individually. During the control period (0-20 min) and the recovery period (60-80 min) the animal breathed room air, and during the treatment period (20-60 min), which is indicated by a thick black bar on the abscissa, it was given the test gas. In the right-hand panel the results from the same experiment are expressed as a percentage of initial control values for the same parameters. In this case, however, the individual measurements taken have been grouped into selected time intervals. The average of all the measurements taken in the 20 min control period gives the 100% 'control' value, while 1- individual points are averaged for the subsequent time intervals. The real time scale now starts from the beginning of the treatment period (0-40 min). Air breathing resumed from min.

5 AORTIC AND CAROTID REFLEXES IN RABBITS 49 RESULTS Resting values in different preparations. In the groups of animals in which the carotid sinus nerve had been cut there was reduction in resting arterial Po2 and elevation in Pco2 (Table 1). The blood gas findings were similar in animals with selective section of the carotid sinus nerve and in animals with combined aortic + carotid nerve section. The respiratory minute volumes were also somewhat lower in these two groups of animals than in normal rabbits, or rabbits with selective aortic nerve section, suggesting that there is resting alveolar hypoventilation in the rabbit 2-4 days after selective section of the carotid sinus nerve. TABLE 1. Initial control values of the various parameters measured 2-4 days after the preliminary operations in the four groups of rabbits. The results are given as means and standard error of the mean. Aortic nerve Carotid sinus Aortic + carotid Normal section nerve section nerve section Number of animals Body weight (kg) Ventilation (1./min stp) *02 Respiratory rate (/min) Arterial P02 (mm Hg) l14 Arterial Pco, (mm Hg) F Arterial ph Cardiac output (ml./min) Heart rate (/min) Arterial pressure (mmhg) Rt. atrial pressure (mm Hg) The resting arterial blood pressure values of animals, subjected to selective aortic and selective carotid nerve section, were about 20 mm Hg above those of normal rabbits (Table 1). Animals with combined section of both sets of nerves had even more elevated resting arterial pressures than selectively denervated animals. The resting heart rates were raised to about the same extent in animals with either selective or 'complete' de-afferentation, but the cardiac output was not significantly different from normal in any of these three groups. The latter finding in animals with combined aortic and carotid nerve sections differs from previous observations (Korner, 1965c), but the reason for this is not clear. Section of the carotid sinus nerve or aortic nerves thus produces circulatory effects of approximately similar magnitude in the rabbit. Effects of hypoxia in different preparations. The reflex effects of arterial hypoxia have been examined at three levels of arterial Po2: (1) 6-45 mm Hg; (2) 0-5 mm Hg; () mm Hg, since previous work has shown that in arterial hypoxia there is only a small increase in orthosympathetic activity above a Po, of 5 mm Hg, but progressive increase in this activity below this level (Chalmers et al. 1965). The results obtained in animals with

6 440 J. P. CHALMERS, P. I. KORNER AND S. W. WHITE selective nerve section have been compared with those of the normal and ' completely' de-afferented groups at each level of hypoxia. Where possible within-animal comparisons ofthe effects of hypoxia alone, and of hypoxia + hypercapnia, were carried out. Arterial Po mm Hg. The circulatory and respiratory findings are shown in Fig. 2, and the arterial blood gas and ph changes in Table 2. Changes in right atrial pressures and in respiration rate are given in Table. TABLE 2. Arterial blood gas tensions and ph in experiments with hypoxia alone, and with hypoxia+hypercapnia. C = control period breathing room air; T = test period breathing low 02 mixtures alone or with the addition of C02 in the same animals. S.E. = standard error of difference between control and treatment means calculated from within animal comparisons* P02 (mm Hg) PCO2 (mm Hg) ph A Group No. C T S.E. C T S.E. C T S.E. Normal rabbits 'Mild' hypoxia * * *02 'Mild' hypoxia + C * 'Moderate' hypoxia * 'Moderate' hypoxia + C02* * *06 Severe hypoxia * * 7* *04 'Mild' hypoxia 'Mild' hypoxia+ C02 'Moderate' hypoxia 'Moderate ' hypoxia + C02 Severe hypoxia 'Mild' hypoxia 'Mild' hypoxia+c02 'Moderate' hypoxia 'Moderate' hypoxia + Co2 Severe hypoxia Severe hypoxia+c02 'Mild' hypoxia 'Mild' hypoxia + C02 'Moderate' hypoxia 'Moderate' hypoxia+ C02 Severe hypoxia Severe hypoxia + C Selective aortic nerve section * * *47 7* * *60 Selective carotid sinus nerve section * Section of carotid sinus and aortic nerves * * * F9 7* * * *06 + *02 + * *02 + * * * This is the only group of rabbits where the effects of superimposing hypercapnia to hypoxia were not studied in the same animal as the effects of hypoxia alone. During arterial hypoxia alone there was an approximately similar increase in respiratory minute volume in normal animals and in animals with selective aortic nerve section. On the other hand in animals with selective section of the carotid sinus nerve, and in 'completely' de-afferented rabbits there was no change in ventilation. Hypoxia plus hyper-

7 AORTIC AND CAROTID REFLEXES IN RABBITS 441 capnia (interrupted lines in Fig. 2) produced a marked hyperpnoea in all preparations and intensified the ventilatory response seen with hypoxia alone in the normal group and in animals with selective aortic nerve section. % % Normal Aortic nerve section Carotid nerve section Aortic+carotid nerve section 250 r 200_, I I 150 L YN I I% II 10 L EI L ,e E100 ; ; -*a <ci 50 L l ' ' II Time (min) Fig. 2. Mean effects of 'mild' arterial hypoxia alone (continuous lines) and of hypoxia plus hypercapnia (interrupted lines), on respiratory minute volume (V8rt 11 -, and dry), cardiac output, heart rate and arterial pressure of four groups of rabbits-from left to right, seven normal animals, three with selective aortic nerve section, three with selective carotid sinus nerve section and three with section ofboth nerves. Changes are expressed as percentages of initial control values. The solid black bar on the abscissa of each panel represents the treatment period, during which the animal breathed the test gas. The height of the symbols on the left in each panel is twice the standard error of the mean of a single time interval; closed circle -hypoxia alone; open circle-hypoxia+hypercapnia. Arterial Po mm Hg. In normal animals the circulatory findings during hypoxia alone were similar to those described previously (Korner, 1965a) and consisted of tachycardia of gradual onset, an increase in cardiac output, but no significant change in arterial blood pressure (Fig. 2). In animals with selective aortic nerve section a similar rise in heart rate above control values was observed after a brief initial phase of bradycardia. In these animals the rise in cardiac output during hypoxia was similar to normal, but there was a significant fall in arterial blood pressure (Fig. 2, second panel). In the group with selective section of the carotid sinus nerve there was more rapidly commencing tachycardia than in the normal animals and those

8 442 J. P. CHALMERS, P. I. KORNER AND S. W. WHITE with selective aortic section, little change in cardiac output, and a fall in arterial pressure comparable to that seen in animals with selective aortic nerve section. In the 'completely' de-afferented animals heart rate changes were abolished but the cardiac output increased despite marked reduction in arterial blood pressure. TABLE. Mean right atrial pressure and respiratory rate in the various groups. C = control period of 20 min breathing room air. The treatment period of 40 min breathing the test gas has been divided into an initial period of 5 min (T1) and a subsequent period of 5 min (T2). R = recovery period of 20 min breathing room air. S.E. is the standard error of the mean of a single treatment or recovery time interval, based on within animal comparisons* Group 'Mild' hypoxia 'Mild' hypoxia + C02 'Moderate' hypoxia 'Moderate' hypoxia + C02* Severe hypoxia 'Mild' hypoxia 'Mild' hypoxia + C02 'Moderate' hypoxia 'Moderate' hypoxia + C02 Severe hypoxia 'Mild' hypoxia 'Mild' hypoxia + C02 'Moderate' hypoxia 'Moderate' hypoxia+ C02 Severe hypoxia Severe hypoxia+c02 'Mild' hypoxia 'Mild' hypoxia + C02 'Moderate' hypoxia 'Moderate' hypoxia + C02 Severe hypoxia Severe hypoxia+c02 Right atrial pressure Respiratory rate A,A No. C T1 T2 R S.E. C T1 T2 R S.E. Normal rabbits Selective aortic nerve section Selective carotid sinus nerve section Section of carotid sinus +aortic nerves * This is the only group of rabbits where the effects of superimposing hypercapnia to hypoxia were not studied in the same animal as the effects of hypoxia alone. When hypercapnia was superimposed on hypoxia, sustained bradycardia, together with some decrease in cardiac output and maintenance of blood pressure, was observed in normal rabbits and in animals with selective aortic nerve section, suggesting more marked degrees of arterial chemoreceptor stimulation (Korner, 1965a; cf. Figs., 4). On the other hand in the other 2 groups of rabbits with section of the carotid sinus nerves, which lacked carotid chemoreceptors, no such potentiating effects were observed. Although hyperventilation due to hypercapnia was

9 AORTIC AND CAROTID REFLEXES IN RABBITS present in these animals the circulatory findings differed minimally from the effects of hypoxia alone. The results show that in unanaesthetized rabbits during hypoxia alone respiration is stimulated only through carotid chemoreceptors. Above a Po2 of 5 mm Hg circulatory chemoreceptor effects were not evident in any group during hypoxia alone, but addition of hypercapnia did evoke such effects in animals with carotid sinus nerves intact (i.e. normal rabbits and rabbits with selective aortic nerve section). Both aortic and carotid nerves however contribute to the maintenance of the arterial pressure at this level of hypoxia. 44 o '200 - Normal 100 1! 120 _ 100 F <N 0 80 L- - m Aortic nerve section Carotid nerve section 12 --z AL ,- I % I % I if --r'- 4,-, --, Aortic+carotid nerve section IE 11 In I II I I2.. ;50 -- v I( L.-> Time (min) Fig.. Mean effects of 'moderate' arterial hypoxia alone (continuous lines) and combined with hypercapnia (interrupted lines) on respiratory minute volume, cardiac output, heart rate and arterial pressure of 4 groups of rabbits-from left to right, seven normal animals, six with selective aortic nerve section, three with selective section of carotid sinus nerve and five with section of both nerves. In these experiments the effects of hypoxia+hypercapnia were tested in the same animals as hypoxia alone in all groups, except the 'normal' group where they were compared with another group of four rabbits. Notation as in Fig. 2. Arterial P mm Hg. Arterial PO, 0-5 mm Hg. The results are shown in Fig., and Tables 2 and. At this level, hypoxia alone again only produced hyperpnoea in animals with the carotid sinus nerve intact. The augmentation of the hyperpnoea by hypercapnia in these animals was somewhat less marked at this level of hypoxia (two left panels, Fig. ) than with milder arterial

10 444 J. P. CHALMERS, P. I. KORNER AND S. W. WHITE hypoxia, probably because the increase in ventilation with hypoxia alone was already nearly maximal. In the normal animals the heart rate fell during the early phase of hypoxia, returning gradually towards normal. There was a transient reduction in cardiac output followed by an increase, whilst the arterial pressure was well maintained throughout the treatment period (cf. Korner, 1965a; Chalmers et al. 1966a). In animals with selective aortic nerve section the changes in heart rate and cardiac output during hypoxia were similar, but the arterial pressure was slightly less well maintained than in normal rabbits. In the group with selective section of the carotid sinus nerve the heart rate did not change during hypoxia. There was a gradual increase in cardiac output, but at this level the fall in arterial pressure was considerably greater than in animals with selective aortic section. In 'completely' de-afferented animals there was a small fall in heart rate during the treatment period, an increase in cardiac output, and reduction in arterial pressure which was greater than in any other group. Hypercapnia plus hypoxia produced a greater fall in heart rate in normal animals and in animals with selective aortic nerve section (two left panels, Fig. ). Compared to the effects of hypoxia alone, the cardiac output was reduced further in these groups, and the total peripheral resistance increased further. In the two groups of animals with the carotid sinus nerves cut the circulatory effects resulting from the addition of hypercapnia were similar to the effects of hypoxia alone in the same animals, except for the occurrence of a slight degree of bradyeardia in animals with section of the carotid sinus nerve only. Reducing the arterial Po2 to 0-55 mm Hg thus only evoked definite reflex respiratory and circulatory chemoreceptor effects in animals with carotid sinus nerves intact and these were accentuated by superimposing hypercapnia. However, both aortic nerves and carotid sinus nerves contributed to the maintenance of blood pressure though to an unequal degree at this level of hypoxia. Arterial Po mm Hg. The results are summarized in Fig. 4, and Tables 2 and. Reduction of the arterial Po2 to mm Hg produced about the same effects on respiration in normal animals and in those with section of the aortic nerves as observed after reducing the arterial Po2 to 0-5 mm Hg. However, in the most severe grade of hypoxia there was a slight increase in ventilation (P = 0.05) and respiration rate in animals with selective carotid nerve section. In 'competely' de-afferented animals the changes in ventilation were not statistically significant. Below an arterial Po2 of 0 mm Hg animals with carotid sinus nerves intact would not tolerate the combination of hypoxia and hypercapnia. This was in

11 AORTIC AND CAROTID REFLEXES IN RABBITS 445 contrast with results obtained in rabbits with section of the carotid sinus nerve (two right panels, Fig. 4) which tolerated the additional hypercapnia with no obvious discomfort. In normal animals the circulatory findings with hypoxia alone were similar to those found at a PO, of 0-5 mm Hg with bradyeardia, transient reduction in output and somewhat greater elevation of arterial Normal Aortic nerve Carotid nerve Aortic+carotid % section section nerve section v 150 L 0, 120 r "1040_ I L 120 r ;-Q 100 _- I } 60[ 60 _ I-_ 5o 0-1t I III i. A<>, I Time (min) Fig. 4. Mean effects of 'severe' arterial hypoxia, alone (continuous lines) and combined with hypercapnia (interrupted lines) on the respiratory mixjlte volume, cardiac output, heart rate, and arterial pressure of four groups of rabbits-from left to right, six normal animals, four with selective section of aortic nerve, four with selective section of carotid sinus nerve and six with section of both nerves. Notation as in Fig. 2. Arterial P mm Hg. pressure during the treatment period (Fig. 4). In animals with aortic nerve section, changes in heart rate and cardiac output were similar to those in normal rabbits. The arterial pressure increased during the first 5 min of hypoxia in this group but then fell slightly during the next 5 min. Thus at this level of hypoxia arterial pressure was better maintained than with the milder grades (cf. Fig. 4) with Figs. 2 and. In animals with selective carotid nerve section there was no significant change in heart rate. There was a rise in cardiac output and a large fall in arterial pressure. In 'completely' deafferented animals there was a small but significant reduction in the heart rate (P = 0.02), the usual rise in cardiac output and large reduction in arterial pressure. In the two groups with carotid sinus nerve section (Fig. 4, two right panels) hypercapnia plus hypoxia produced 29 Physiol. i88 I II III liz I I

12 446 J. P. CHALMERS, P. 1. KORNER AND S. W. WHITE circulatory effects similar to those of hypoxia alone; however, in the hyperventilating animals (especially in the 'completely' deafferented group) heart rates during hypoxia were somewhat higher and the cardiac output rose more rapidly than with hypoxia alone (Fig. 4, fourth panel). The findings show that when the arterial Po2 is reduced below 0 mm Hg, to levels producing intense reflex respiratory and circulatory chemoreceptor effects in animals with intact carotid receptors, there is only a small amount of respiratory stimulation and still no evidence of circulatory chemoreceptor effects in animals with only the aortic nerves preserved. DISCUSSION The present findings support the view that in the rabbit almost all chemoreceptor fibres run in the carotid sinus nerve, and that the aortic nerve consists almost exclusively of baroreceptor fibres (Schmidt, 192; Gernandt, 1946; Neil et al. 1949; Douglas & Ritchie, 1956; Douglas et al. 1956). There is thus a species difference between rabbits on the one hand, and the dog and cat on the other (Comroe, 199; Daly & Ungar, 1966). In the unanaesthetized rabbit the arterial chemoreceptors exert tonic effects on the resting respiration (Euler & Liljestrand, 1941, 1942; Hejneman, 1944; Dejours, 1962), and their loss is clearly evident -4 days after section of the carotid sinus nerve when signs of alveolar hypoventilation are still present. The arterial chemoreceptors did not apparently exert tonic effects on the resting circulation, since changes in heart rate, blood pressure and cardiac output were similar in the groups of animals subjected to selective aortic and carotid nerve section. The tonic circulatory effects seem thus to be entirely mediated through the arterial baroreceptors. In the present series there were no significant differences in the resting cardiac output values of the four groups of animals. This result differs from previous observations in unanaesthetized rabbits (Korner, 1965c), where combined section of aortic and carotid sinus nerves produced a % increase in cardiac output. In the previous study there was considerable overlap in the cardiac output values of normal and de-afferented animals, and the effect was most clearly seen within animal comparisons. During arterial hypoxia alone hyperventilation was only observed in animals with the carotid sinus nerves intact, except for the group of rabbits with selective section of the carotid sinus nerves exposed to the most severe grade of hypoxia (Po mm Hg). In this group the ventilation increased by about 15 % during the first 15 min of hypoxia, and this effect probably results from stimulation of the aortic chemoreceptors. Milder grades of hypoxia (Po mm Hg) did not stimulate breathing in

13 AORTIC AND CAROTID REFLEXES IN RABBITS 447 other rabbits with selective section of the carotid sinus nerve. These findings indicate that very severe hypoxic stimulation is necessary to produce a small effect in this group of animals, suggesting that there are very few chemoreceptor fibres of aortic origin in the rabbit. In this species the aortic zone is functionally a much less important source of peripheral chemoreceptor drive than the carotid region. The circulatory findings in normal animals in this study are in agreement with previous observations (Korner, 1965a). At arterial Po2 greater than 5 mm Hg hypoxia produced an increase in heart rate and cardiac output, with maintenance of the arterial pressure effects ascribable to the action of hyperventilation and hypocapnia (Daly & Scott, 1958, 1962, 196b; Korner, 1965 a, b). Reduction of the arterial Po, below 5 mm Hg produced reflex circulatory chemoreceptor effects with an initial reduction in heart rate and cardiac output, whilst the blood pressure remained unchanged or rose. Similar effects have been obtained in the dog by hypoxic stimulation of the isolated carotid as well as the isolated aortic chemoreceptors during controlled ventilation (Daly et al. 1965; Daly & Ungar, 1966). However, Comroe (199) and Comroe & Mortimer (1964) using pharmacological stimulants obtained tachycardia and marked hypertension from selective stimulation of the aortic regions. In the present study hypoxia never produced hypertension or a rise in total peripheral resistance in rabbits with only the aortic nerves intact even when associated with hypercapnia, and in addition in five out of the six groups of experiments in these animals no bradyeardia was observed (Figs. 2-4). The tachycardia observed in these rabbits (i.e. Fig. 2, animals with carotid nerve section) with arterial Po, 6-45 mm Hg can be ascribed to baroreceptor reflexes elicited by the fall in arterial pressure (see below). Thus the findings suggest that in the rabbit practically all the reflex circulatory effects of strong chemoreceptor stimulation produced by hypoxia are mediated through the carotid sinus nerve, and that the contribution through aortic nerves or vagus is very small. The results in 'completely' de-afferented animals were similar to those observed previously (Korner, 1965a). In the absence of both carotid sinus and aortic nerves there is no reflex circulatory control in the rabbit in this type of hypoxia (Chalmers et al. 1965; Korner & White, 1966). Lowering of the arterial Po2 produced increasing reduction in arterial pressure and total peripheral resistance probably due to the local effects of hypoxia on the resistance vessels (Guyton, 196; Chalmers et al. 1966). The changes in heart rate were minimal at Po2 above 5 mm Hg, but a slight bradycardia occurred during more severe hypoxia. This fall in heart rate is probably due to the local effects of hypoxia on the heart accentuated by poor coronary perfusion as a result of hypotension. Similar effects have 29-2

14 448 J. P. CHALMERS, P. 1. KORNER AND S. W. WHITE been observed after post-haemorrhagic hypotension in rabbits without functioning autonomic effectors (Chalmers, Korner & White, 1967). Assessment of the effects of hyperventilation in the present study was carried out by comparing the effects of hypoxia alone and those of hypoxia +hypercapnia (i.e. hyperventilation) in animals with section of both aortic and carotid nerves but intact vagal afferents from the lungs (Daly & Scott, 196 b). In these animals hyperventilation did not modify the effects of hypoxia alone on the cardiac output and arterial pressure. This could be due to the marked hypoxic peripheral vasodilatation which would obscure any tendency towards a further fall in total peripheral resistance due to hyperventilation. Such a vasodilatation has been observed in man and the dog during hyperventilation with associated hypocapnia (Mc- Gregor, Donevan & Anderson 1962; Daly & Scott, 1962). The only evidence of reflex effects of hyperventilation due to hypercapnia observed in the ' completely' de-afferented rabbit was a demonstration of a slightly higher heart rate than with hypoxia alone at all three levels of arterial Po2. In rabbits with only aortic nerves intact the role of the aortic baroreceptors counteracting the local tissue effects of hypoxia can be assessed in the absence of significant chemoreceptor drive. Comparison of the results of this group with those of 'completely' de-afferented animals shows that the aortic baroreceptors minimize the fall in arterial pressure at all three levels of hypoxia, but most effectively when the arterial Po2 is, above 5 mm Hg. In these animals the heart rate either rose or remained unchanged at each level of hypoxia and was always higher than in the 'completely' de-afferented group at similar oxygen pressures. The reflex effects on heart rate and blood pressure are similar to those observed during tissue hypoxia with normal arterial Po2 produced by the inhalation of carbon monoxide-air mixture where reflex circulatory control occurs. also through the baroreceptor reflexes. Local arteriolar dilatation seems to. be the main factor involved in the rise in cardiac output rather than reflex control mechanisms, since the rise in output was as great or greater in 'completely' de-afferented animals as in any other group. There was little difference between the circulatory findings in normal animals and in animals with only carotid sinus nerve intact at all levels of arterial Po2. However, the arterial pressure was better maintained in normal animals particularly when the arterial Po0 was 6-45 mm Hg. This suggests that at these levels when the chemoreceptors are mainly concerned with increasing ventilation and their reflex circulatory effects are relatively small, the maintenance of the arterial pressure close to control values in normal animals is accomplished by the interaction of the chemoreceptor reflexes with the baroreceptor input from both carotid and aortic zones. However, with more severe degrees of arterial hypoxia.

15 AORTIC AND CAROTID REFLEXES IN RABBITS 449 the arterial pressure is as well maintained in animals without aortic nerves as in normal animals, suggesting that at these levels the chemoreceptor reflexes are by themselves powerful enough to maintain or elevate the arterial blood pressure. We are indebted to Professor M. de B. Daly for reading the manuscript and for his valuable comments. We are grateful to Mrs A. Edwards for her technical assistance with the experiments. This work was supported by grants-in-aid from the National Heart Foundation of Australia, the Life Insurance Medical Research Fund of Australia and New Zealand, and by research grants from the Australian Universities Research Grants Committee. REFERENCES BARTELS, H. & WITZLEB, E. (1956). Der Einfluss des arteriellen C02-Druckes auf die chemorezeptorischen Aktionspotentiale im Carotissinusnerven. Pfluger8 Arch. ges. Phy8iol. 262, BERNTHAL, T. G., GREENE, W. Jr. & REVZIN, A. M. (1951). Role of the carotid chemoreceptors in hypoxic cardiac acceleration. Proc. Soc. exp. Biol. Med. 76, BERNTHAL, T. G. & ScHwIND, F. J. (1945). A comparison in intestine and leg of the reflex vascular response to carotid-aortic chemoreceptor stimulation. Am. J. Phy&iol. 14, CHALMERS, J. P., ISBISTER, J. P., KORNER, P. I. & MOK, H. Y. I. (1965). The role of the sympathetic nervous system in the circulatory response of the rabbit to arterial hypoxia. J. Phy8iol. 181, CHAIMERS, J. P. & KORNER, P. I. (1966). Effects of arterial hypoxia on the cutaneous circulation of the rabbit. J. Phy8iol. 184, CHALMERS, J. P., KORNER, P. I. & WHITE, S. W. (1966). The control of the circulation in skeletal muscle during arterial hypoxia in the rabbit. J. Physiol. 184, CHALMERS, J. P., KORNER, P. I. & WHITE, S. W. (1967). Effects of haemorrhage in the unanaesthetised rabbit J. Phy8iol. (In the Press.) COMROE, J. H. Jr. (199). The location and function of the chemoreceptors of the aorta. Am. J. Phys-ol. 127, COMROE, J. H. & MORTIMER, L. (1964). The respiratory and cardio-vascular responses of temporally separated aortic and carotid bodies to cyanide, nicotine, phenyldiguanide, and serotonin. J. Pharmac. exp. Ther. 146, -41. DALY, M. DE B. & ScoTT, M. J. (1958). The effects of stimulation of the carotid body chemoreceptors on the heart rate in the dog. J. Phy8iol. 144, DALY, M. DE B. & ScoTT, M. J. (1962). An analysis of the primary cardiovascular reflex effects of the stimulation of the carotid body chemoreceptors of the dog. J. Physiol. 162, DALY, M. DE B. & ScoTT, M. J. (196a). The effects of changes in respiration on the cardiovascular responses to stimulation of the carotid body chemoreceptors. In The Regu?ation of Human Respiration. John Scott Haldane Centenary Volume, ed. CUNNINGHAM, D. J. C. & LLOYD, B. B. pp , Oxford: Blackwell. DALY, M. DE B. & ScoTT, M. J. (196b). The cardiovascular responses to stimulation of the carotid body chemoreceptors in the dog. J. Phy8iol. 165, DALY, M. DE B., HAZ7ZTLDINE, J. L. & HOWE, A. (1965). Reflex respiratory and peripheral vascular responses to stimulation of the isolated perfused aortic arch chemoreceptors of the dog. J. Physiol. 177, DALY, M. DE B. & UNGAR, A. (1966). Comparison of the reflex responses elicited by stimulation of the separately perfused carotid and aortic body chemoreceptors in the dog. J. Physiol. 182, DEJOURs, P. (1962). Chemoreflexes in breathing. Phy&iol. Rev. 42, DOUGLAS, W. W. & RITCHIE, J. M. (1956). Cardiovascular reflexes produced by electrical excitation of non-medullated afferents in the vagus, carotid sinus and aortic nerves. J. Phy8iol. 14, DouGLAs, W. W., RITCHn1, J. M. & ScHiE.AuANN, W. (1956). Depressor reflexes form medullated and non-medullated fibres in the rabbit's aortic nerve. J. Phy8iol. 12,

16 450 J. P. CHALMERS, P. I. KORNER AND S. W. WHITE DOUGLAS, W. W. & SCHAUMANN, W. (1956). A study of the depressor and pressor components of the cat's carotid sinus and aortic nerves using electrical stimuli of different intensities and frequencies. J. Physiol. 12, 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. Ja exp. Physiol. 44, EULER, U. S. v. & LILJESTRAND, G. (1941). The effect of carotid sinus denervation on respiration during rest. Acta physiol. scand. 1, EuLER, U. S. v. & LILJESTRAND, G. (1942). Influence of oxygen inhalation on the chemoreceptor activity of the sinus region. Acta physiol. scand. 4, EYZAGUIRRE, C. & LEWIN, J. (1961). Chemoreceptor activity of the carotid body of the cat. J. Physiol. 159, EYZAGUIRRE, C. & KoYANo, H. (1965). Effects of hypoxia, hypercapnia and ph on the chemoreceptor activity of the carotid body in vitro. J. Physiol. 178, FEGLER, G. (1954). Measurement of cardiac output in anaesthetized animals by a thermodilution method. Q. Jl exp. Physiol. 9, GERNANDT, B. E. (1946). A study of the respiratory reflexes elicited from the aortic and carotid bodies. Acta physiol. scand. 11, Suppl. 5, GUYTON, A. C. (196). Circulatory Physiology: Cardiac Output and its Regulation. Philadelphia: W. B. Saunders. HEJNEMAN, E. (1944). Influence of oxygen inhalation on the chemoreceptor activity of thesinus region in rabbits. Acta physiol. scand. 6, -5. HEYMANS, C. & NEIL, E. (1958). Refiexogenic Areas of the Cardiovascular System. London: Churchill. HOWE, A. (1956). The vasculature of the aortic bodies in the cat. J. Physiol. 14, JOELS, N. & NEIL, E. (1961). Carotid chemoreceptor impulse activity during inhalation of carbon monoxide mixtures. J. Physiol. 156, 5P. KORNER, P. I. (1965a). The role of the arterial chemoreceptors and baroreceptors in the circulatory response to hypoxia of the rabbit. J. Physiol. 180, KORNER, P. I. (1965b). Control of the systemic circulation in hypoxia. Proc. XXIII int. Congr. Physiol, Sci. Excerpta Medica Intemational Congress Series no. 87, pp KORNER, P. I. (1965c). The effect of section of the carotid sinus and aortic nerves on thecardiac output of the rabbit. J. Physiol. 180, KORNER, P. I. & WHITE, S. W. (1966). Circulatory control in hypoxia by the sympathetic nerves and adrenal medulla. J. Physiol. 184, MATHER, K. (1949). Statistical Analysis in Biology. London: Methuen. MCGREGOR, M., DoNEvAN, R. E. & ANDERSON, N. M. (1962). Influence of carbon dioxide and hyperventilation on cardiac output in man. J. appl. Physiol. 17, MoTT, J. C. (196). The effects of baroreceptor and chemoreceptor stimulation on shivering. J. Physiol. 166, NEIL, E. & JOELS, N. (196). The carotid glomus sensory mechanisms. In The Regulation of Human Respiration. John Scott Haldane Centenary Volume, ed. CUNNINGHAM, D. J. C. & LLOYD, B. B., pp Oxford: Blackwell. NEIL, E., REDWOOD, C. R. M. & SCHWEITZER, A. (1949). Effects of electrical stimulation of the aortic nerve on blood pressure and respiration in cats and rabbits under chloralose and nembutal anaesthesia. J. Physiol. 109, PAINTAL, A. S. & RILEY, R. L. (1966). Responses of aortic chemoreceptors. J. appl. Physiol. 21, SCHMIDT, C. F. (192). Carotid sinus reflexes to the respiratory centre. Am. J. Physiol. 102,