Birmingham B15 2TJ. fi-adrenoreceptor influence of circulating catecholamines.

Transcription

1 Journal of Physiology (1991), 436, pp With 2 figures Printed in Great Britain THE ROLES OF CATECHOLAMINES IN RESPONSES EVOKED IN ARTERIOLES AND VENULES OF RAT SKELETAL MUSCLE BY SYSTEMIC HYPOXIA BY RUBINA MIAN AND JANICE M. MARSHALL From the Department of Physiology, The Medical School, Vincent Drive, Birmingham B15 2TJ (Received 10 July 1990) SUMMARY 1. Studies have been made in the anaesthetized rat of the roles played by a- and /8-adrenoreceptor stimulation in determining diameter changes induced in individual arterioles and venules of the spinotrapezius muscle during systemic hypoxia (breathing 6% 02 for 3 min). 2. Topical application to the spinotrapezius of phentolamine, the cz-adrenoreceptor antagonist, or sotalol, the,3-adrenoreceptor antagonist, had no effect on the fall in systemic arterial pressure and tachyeardia induced by hypoxia. 3. All arterioles and venules showed a decrease in diameter in response to topical application of noradrenaline (10-6 g ml-1): these responses were abolished by topical application of phentolamine. Moreover, those arterioles and venules that showed a decrease in diameter during hypoxia before phentolamine, showed a significantly smaller decrease, or an increase in diameter after phentolamine. This effect was most marked in primary and secondary arterioles ( am diameter). 4. All arterioles and venules showed an increase in diameter in response to topical application of isoprenaline (10-6 g ml-1); these responses were abolished by topical application of sotalol. Moreover, these arterioles and venules that showed an increase in diameter during hypoxia before sotalol, showed a significantly smaller increase or even a decrease in diameter after sotalol. 5. These results suggest that during hypoxia the arterioles of skeletal muscle, especially primary and secondary arterioles, are under the constrictor influence of a reflex increase in sympathetic nerve activity while the venules, which have no sympathetic innervation, are under the constrictor influence of circulating catecholamines. They also suggest that in individual arterioles and venules, these constrictor influences may be overcome by dilatation mediated by the fi-adrenoreceptor influence of circulating catecholamines. 6. Since some arterioles and venules still showed constriction during hypoxia after phentolamine and some still showed dilatation during hypoxia after sotalol, it seems that factors other than catecholamines contribute to the diameter changes. It is suggested that locally released metabolites exert a substantial dilator influence, particularly on terminal arterioles and collecting venules, those vessels nearest to the capillary bed. MS 8638

2 500 R. MIAN AND J. M. MARSHALL 7. These findings are discussed in relation to responses recorded during hypoxia in gross flow studies on hindlimb muscles and in view of the functional roles of the consecutive sections of muscle microcirculation. INTRODUCTION Our direct observations on the rat spinotrapezius revealed that during a 3 min period of systemic hypoxia some arterioles from each section of the arterial tree showed an increase in diameter, while others showed a decrease. By contrast, the venules predominantly showed an increase in diameter. Concomitant with these changes, systemic arterial pressure fell (Mian & Marshall, 1991). Studies on the regional blood flow changes induced in the rat by systemic hypoxia, showed that simultaneous with the fall in systemic arterial pressure there was a mean increase in the vascular conductance of hindlimb muscles, indicating net vasodilatation. However, in some individual animals, hindlimb vascular conductance fell, indicating vasoconstriction (Marshall & Metcalfe, 1988a, b, 1989). We concluded that reflex vasodilatation evoked by lung stretch receptors as a consequence of the hypoxia-induced hyperventilation made no significant contribution to the vascular conductance change in hindlimb muscle (Marshall & Metcalfe, 1988a, b). Accordingly, dilator responses induced in arterioles of the spinotrapezius were not affected by vagotomy which would have severed the afferent fibres from lung stretch receptors (Booth & Marshall, 1987). However, pharmacological studies indicated that the change in hindlimb conductance reflected competition between (i) constriction induced by a reflex increase in sympathetic activity and (ii) dilatation induced by metabolites released from the skeletal muscle fibres as a consequence of tissue hypoxia (Marshall & Metcalfe, 1988b) and by the influence of circulating catecholamines upon 8-adrenoreceptors (Mian, Marshall & Kumar, 1990). Thus, it was a reasonable hypothesis that a- and 8-adrenoreceptor stimulation play important roles in determining the responses of individual arterioles of the spinotrapezius muscle during systemic hypoxia. Accordingly, we have now observed the responses induced by hypoxia in individual arterioles of the consecutive sections of the arterial tree before and after topical application of a- or,-adrenoreceptor antagonists to the spinotrapezius and have also examined the effects of these antagonists upon responses induced in individual sections of the venular tree. As diameter changes observed in any individual arteriole or venule were comparable in the first and second periods of hypoxia, but not in subsequent periods (Mian & Marshall, 1991), the antagonist was applied between the first two periods. Some of these findings have already been reported in brief (Marshall & Mian, 1989, 1990). METHODS Experiments were performed on seventy-four of the male Sprague-Dawley and Wistar rats whose responses to systemic hypoxia were described in the preceding paper (Mian & Marshall, 1991), plus an additional six rats. The details of the anaesthesia, surgical preparation, arrangement of the spinotrapezius muscle for in vivo microscopy and the methods of recording and administering hypoxic mixtures were as described previously (Mian & Marshall, 1991). Vessel diameters were

3 HYPOXIA AND CATECHOLAMINES recorded before hypoxia and at 1, 2, 3, 5, 7, 9 and 10 min after the onset of hypoxia as described previously (Mian & Marshall, 1991), the maximum change in diameter at these times being used for statistical analyses. The values of arterial pressure and heart rate that were used for analysis were taken before hypoxia and at the end of the 2nd min of hypoxia (cf. Marshall & Metcalfe, 1988a, b; Mian & Marshall, 1991). In experiments designed to test the effect of a-adrenoreceptor blockade, the Saran wrap was briefly removed from the spinotrapezius after the first 3 min periods of 6% 02 to allow noradrenaline (10-6 g ml-') to be topically applied. It was left on for 1 min, during which time peak changes in vessel diameter were measured, then after removing the Saran wrap it was washed off with Krebs solution. The cx-receptor antagonist phentolamine 10-6 g ml- was then applied topically and the Saran wrap was replaced. A period of 30 min was allowed to elapse and then a second period of 6% 02 was administered as described above. Following this the response to noradrenaline was retested. The same protocol was adopted in order to test the effect of,-adrenoreceptor blockade except that the,-receptor agonist isoprenaline (10-6 g ml-') and the,-receptor antagonist sotalol (6 x 10-6 g ml-1) were used. In six rats, combined a- and fl-blockade was attempted. Sotalol was applied first as above, then the responses evoked by a first exposure to hypoxia was tested. Phentolamine was then applied and the response evoked by a second exposure to hypoxia was tested. The blockade was checked by topical application of noradrenaline and isoprenaline at the beginning and end of the experiment as described above. Statistical analyses. All results are given as a means +S.E.M. Changes in vessel diameter are expressed as a percentage of the control diameter, the latter being a mean of the measurements made before the onset of the hypoxia stimulus (see above). Comparisons within and between groups were made using Student's paired and unpaired t tests respectively, P < 0-05 being taken to indicate a significant difference. 501 RESULTS Administration of 6% 02 for 3 min produced a gradual fall in arterial pressure and increase in heart rate, both of which reached a maximum by the end of the 2nd min of hypoxia. During this time some arterioles and venules showed an increase in diameter and others, a decrease in diameter as described previously (see Figs 1 and 2 and Mian & Marshall, 1991). Effects of the ac-receptor antagonist phentolamine These experiments were performed on fifty-three animals. During the first period of hypoxia, arterial pressure fell by % from a control level of mmhg, while heart rate increased from to beats min-' (Fig. 1). Topical application of noradrenaline (10-6 g ml-') to the spinotrapezius muscle evoked a decrease in diameter of between 5 and 15 % from control diameter in each type of arterial and venous vessel although in many of the terminal, secondary and primary arterioles there was a secondary relaxation towards the control diameter (Marshall, 1982). When the noradrenaline had been washed off and the vessels had regained their control diameter, topical application of phentolamine caused a transient increase in the diameter of the majority of arterioles and some venules. However, 95% of all vessels studied returned to within 5% of their original control diameters within the following 20 min. Those that did not return are excluded from the data presented below. In order to limit the length of the experiment, the effect of noradrenaline was not retested immediately after phentolamine, but only after the second period of hypoxia. At this time noradrenaline produced no measurable

4 502 R. MIAN AND J. M. MARSHALL decrease in diameter in any type of vessel, indicating that a-receptor blockade had been achieved and maintained. Topical application of phentolamine had no effect on the increase in heart rate nor on the fall in arterial pressure evoked by 6% 02 (Fig. 1 C). However, phentolamine A MA VA 2 A TA CV 20V 1 V MV 40 - ((1) (12) -o 30 - * I63 (5) (6) E E * vf 1 0 (6 0, -C J a All b All B arterioles ii venules ii 30 (32) (13) (19) i (21) ~- 20 I(28) 10 E ~ CD~~~ a, C 6c ,- fulgrupf al ateral essls(53) and4al veou vesl b n nsugop fatra Fig. 1. Effect of topical application of phentolamine to spinotrapezius on responses evoked by 6% 02. A, percentage changes in diameter in individual sections of arterial and venous trees. significant13difrne Columns fro show mean coto change CDpos (+(P S.E.M.) in < 0) diameter in main arteries (MA), primary arterioles (PA), secondary arterioles (20A), terminal arterioles (TA), collecting venules (CV), secondary venules (20V), primary venules (10V) and main veins (MV). Numbers in parentheses indicate number of vessels. B, percentage changes in diameter of full group of all arterial vessels (a) and all venous vessels (b) and in subgroups of arterial and venous vessels whose response before phentolamine was constriction (i) or dilatation (ii); numbers in parentheses indicate number of vessels in group. C, percentage changes in arterial pressure (ABP) and changes in heart rate (HR, beats min-') recorded during changes in vessel diameter, numbers in parentheses indicating number of animals. Open and hatched columns, before and after phentolamine respectively. Asterisks indicate significant differences from control response (P < 0-05). 0 After phentolamine

5 HYPOXIA AND CATECHOLAMINES did significantly affect the responses seen in individual vessels. In the main arteries and primary and secondary arterioles changes in diameter tended towards larger increases, this effect being particularly pronounced in the primary and secondary arterioles (Fig. IA). But there was no consistent effect on the responses of terminal arterioles. In fact, of the two terminal arterioles that showed a decrease in diameter before phentolamine, one showed a smaller decrease and the other, an increase in diameter after phentolamine. Of the nine terminal arterioles that showed an increase in diameter before phentolamine, four showed a larger increase after phentolamine. However, the remaining five that had shown a diameter increase showed a smaller increase or even a decrease in diameter after phentolamine, thus going against the trend for the larger arterioles. In accord with the last group of terminal arterioles, in the collecting venules the mean increase in diameter evoked by hypoxia was significantly smaller after phentolamine. However, in the remaining venous vessels the trend was similar to that seen in the primary and secondary arterioles (Fig. IA). The trends just described are also apparent when the vessels are grouped according to whether they initially showed a decrease or an increase in diameter in response to hypoxia (Fig. IB). But this analysis also revealed that amongst the arterial and venous vessels it was those that initially showed a decrease in diameter that were most affected by phentolamine, these responses being converted to a mean increase (Fig. IB). Despite these general trends, hypoxia still evoked decreases in diameter in many vessels after phentolamine, i.e. in 20% of all arterial vessels and in 14% of all venous vessels as compared with 37 and 28 % respectively before phentolamine. Before application of phentolamine the mean time taken for all vessels to return to their control diameter after hypoxia was min. After phentolamine it was I1 min, the difference between the two values almost reaching significance (P < 0 06). There was no correlation either before or after phentolamine between the percentage c-hanges in diameter of individual vessels and the percentage change in arterial pressure in any of the types of vessels, nor in arterial or in venous vessels, when considered as grouped populations (Pearson coefficient < 0 05 in each case). 503 Effects of the /3-receptor antagonist sotalol These experiments were carried out on twenty-one rats. During the first period of 6% 02, arterial pressure fell by % from a control level of mmhg while heart rate increased from to beats min-' (Fig. 2C). These changes were not significantly different from those recorded in the experiments involving a- blockade. The mean changes in vessel diameter induced by the first period of 6% 02 are shown in Fig. 2A (open columns). These are somewhat different from the changes shown in Fig. IA in that the mean change in the primary and terminal arterioles was a decrease rather than an increase, and the mean change in the secondary arterioles was an increase rather than a decrease. For the primary and terminal arterioles, this discrepancy reflects the fact that in the 'phentolamine experiments', 100 and 80% respectively of these vessels showed increases in diameter as compared with 35 and 50% in the 'sotalol experiments'. For the secondary arterioles, 50% showed increases in diameter in both groups of experiments, so the discrepancy between the

6 t~~~ ~E 504 R. MIAN AND J. M. MARSHALL mean responses reflects differences in the relative magnitudes of increases and decreases in diameter. Topical application of isoprenaline (10-6 g ml-') produced increases in the diameter of all arterioles and all venules ranging from (n = 8) in the main arteries to A 40 MA 1DA 20A TA CV 25V 1 0V MV 30 g ~~~~~~~(8) J (9) 10 (8) O 0) A (8) 14 A S 8 0)~~~~~~~~~~~~~~~~~~~~8 -c- C) B a All b art :erioles ii All venul1le Bs ii 307 (14) 20-0 (22) r- E 10 - (33) 0. (34) (20) (1 1) 0- C,_ CD c C- 20 C 21)C5,Q110 cc~~~~~~)c0m 0 c-c -20; ~ - UO Xc_ -20 z v After sotalol Fig. 2. Effect of topical application of sotalol to spinotrapezius on responses evoked by 6% 02 A, B and C arranged as in Fig. 1. Abbreviations as in Fig. 1. Asterisks indicate significant difference from control response (P < 0-05) , and % in primary, secondary and terminal arterioles (n = 10, 8, 8) respectively and from , and % in collecting, secondary and primary venules respectively (n 8, 8, 9), to % = in the main veins (n 8 respectively). Subsequently, topical application of sotalol = causes a transient decrease in diameter of many vessels, but 95 % returned to within 5 % of their resting diameter within 20 min; the remainder were excluded from the study. When the effect of isoprenaline was retested after the second exposure to

7 HYPOXIA AND CATECHOLAMINES hypoxia, there was no measurable change in diameter in any vessel indicating that effective blockade of a-receptors had been achieved and maintained throughout the 2nd period of hypoxia. Topical application of sotalol had no effect on the changes in arterial pressure and heart rate induced by 6% 02 (Fig. 2 C). By contrast, sotalol considerably changed the responses seen in individual vessels (Fig. 2A). In both arterial and venous vessels the changes in vessel diameter tended towards larger decreases, this effect being significant in main arteries, secondary arterioles and secondary venules and main veins (Fig. 2A). However, considering the responses seen in individual vessels there was no apparent trend for the effect of sotalol on terminal arterioles. Of the four terminal arterioles that showed an increase in diameter before sotalol, one showed a decrease, but three showed a larger increase in diameter after sotalol. Moreover, of the four terminal arterioles that showed a decrease in diameter before sotalol, three showed a larger decrease, but one, an increase in diameter after sotalol. Grouping the vessels according to the direction of their diameter change during hypoxia before sotalol, reveals that sotalol preferentially affected those whose control response was an increase in diameter (Fig. 2B). However, it should be noted that even after sotalol hypoxia still evoked increases in diameter of many vessels, i.e. in 29 % of all arterial vessels and 57 % of all venous vessels, as compared with 44 % and 63 % respectively before sotalol. Sotalol had no effect on the mean time taken for vessels to return to their control diameter after hypoxia. Before sotalol it was min for arterioles and min for venules, while after sotalol it was min for arterioles and 7* min for venules. Combined effects of phentolamine and sotalol In six rats, after sotalol, the 1st exposure to 6 % 02 evoked changes in diameter of , + 68, , -8-0, and % in each of six secondary arterioles. After the addition of phentolamine the 2nd exposure to 6% 02 evoked changes in diameter of , -5 5, + 4.5, , and -4 0 % respectively. At the beginning of these experiments topical application of noradrenaline and isoprenaline produced decreases and increases in diameter respectively. At the end of the experiments neither produced any measurable change in diameter, indicating that effective oc- and f-receptor blockade had been achieved and maintained throughout. 505 DISCUSSION In the present experiments on anaesthetized rats, administration of 6% 02 for 3 min induced a tachyeardia and fall in arterial pressure, while some arterioles and venules of the spinotrapezius muscle showed an increase and others, a decrease in diameter (cf. Mian & Marshall, 1991). In the previous study, we demonstrated that for individual arterioles and venules the responses evoked by two successive hypoxic periods were consistent in direction and magnitude. Further, we argued that the diameter change in systemic arterial pressure, rather than myogenic or passive responses to a change in intravascular pressure (Mian & Marshall, 1991). The present study supports that view in that the diameter changes were significantly altered after

8 506 R. MIAN AND J. M. MARSHALL topical application of a- or /3-adrenoreceptor antagonists to the spinotrapezius between the first and second hypoxic periods even though this had no effect on the change in arterial pressure. The role of ac-adrenoreceptors It was reported previously that topical application of noradrenaline (> 10-8 g ml-') constricts all arterial and venous vessels of the spinotrapezius and that such responses were abolished by topical application of phentolamine (Marshall, 1982). This was confirmed in the present study. As the arterioles showed shortlasting dilatation immediately after phentolamine this indicates they were under a tonic a-receptor-mediated constrictor influence of sympathetic nerve activity. That they subsequently returned to their original control diameter can be explained by generation of myogenic tone and possibly by the action of circulating hormones. As phentolamine application to the muscle had no effect on the hypoxia-induced fall in arterial pressure and tachycardia, it can be concluded that we successfully achieved a local a-receptor blockade. As phentolamine especially attenuated the constrictor responses induced in arterioles by hypoxia, this indicates that they were mediated by cz-adrenoreceptors. This could be attributed to the effect of an increase in sympathetic nerve activity to the spinotrapezius initiated as part of the primary cardiovascular response to peripheral chemoreceptor stimulation (Marshall, 1987; Marshall & Metcalfe, 1988b). Circulating catecholamines may also have contributed, for catecholamine levels in plasma rise substantially in the rat during ventilation with 10% 02 (Biesold, Kurosawa, Sato & Trzebski, 1990). Since the primary and secondary arterioles were most affected by phentolamine, their response being converted from net constriction to dilatation, this accords with the fact that these arterioles showed the strongest vasoconstrictor responses to sympathetic stimulation and to noradrenaline (see Marshall, 1982). On the other hand, as the responses of terminal arterioles were not consistently changed by phentolamine this accords with the observation that although they constricted initially during sympathetic stimulation, they then relaxed, probably under the influence of accumulating vasodilator metabolites to which they are particularly sensitive (see Zweifach, 1961; Marshall, 1982; Hebert & Marshall, 1986, 1988). Accordingly those terminal arterioles whose dilator response was enhanced or whose constrictor response was attenuated by phentolamine may have been dominated by the constrictor effect of sympathetic activity. By contrast, those whose dilator response was attenuated by phentolamine may have been responding predominantly to vasodilator metabolites. For, after phentolamine, the generally greater dilatation of the primary and secondary arterioles presumably led to a greater blood flow through the spinotrapezius during hypoxia and improved wash-out of metabolites from the interstitial space around the arterioles. This explanation can also be offered for the fact that the dilatation of the collecting venules was significantly reduced by phentolamine, if their dilatation was caused mainly by local metabolites (see Mian & Marshall, 1991). Since the venous vessels of the spinotrapezius have no sympathetic nerve supply and show no change in diameter when the sympathetic supply to the muscle is activated (Marshall, 1982), the fact that phentolamine tended to reduce constrictor

9 HYPOXIA AND CATECHOLAMINES responses induced in venous vessels by hypoxia may be attributed to blockade of constriction mediated by circulating catecholamines (cf. Marshall, 1982; Biesold et al. 1989). Clearly, when the constrictor influence of catecholamines was abolished, vasodilator responses to hypoxia predominated in all sections of the arterial and venous trees. This is fully consistent with our observations on the total vascular conductance of limb muscle (Marshall & Metcalfe, 1988 b). Judging from the effect of phentolamine on the duration of the diameter changes, those that persisted were longer lasting, as would be consistent with their being mediated by vasodilator metabolites and circulating hormones (see below). The role of f-adrenoreceptors Previous studies on the spinotrapezius, using the mixed a- and f-agonists, noradrenaline and adrenaline indicated that at least some of the arterial and venous vessels have f-receptors that mediate dilatation (Marshall, 1982; Hebert & Marshall, 1986). The present experiments involving isoprenaline indicated that all sections of the arterial and venous trees of the spinotrapezius have,-receptors whose effects can be blocked with sotalol. In some contrast, Borgstrom, Lindbom, Arfors & Intaglietta (1988), in studies by in vivo microscopy on the rabbit teniussimus muscle, reported that isoprenaline dilated the transverse arterioles (18,sm diameter) that supplied both the muscle and adherent connective tissue, but not the terminal arterioles (49,um diameter) which supplied only muscle capillaries. However, our results accord with evidence from gross flow studies that local infusion of isoprenaline or circulating catecholamines can produce a substantial increase in the vascular conductance of limb muscles of rat, rabbit and cat (Uther, Hunyor, Shaw & Korner, 1970; Lundvall, Hillman & Gustafsson, 1982; Yardley & Hilton, 1987). Our finding that immediately after sotalol application some vessels showed a short-lasting constriction suggests that they were under a tonic dilator influence of circulating catecholamines. Moreover, the effect that sotalol had on the responses induced by hypoxia - preferentially reducing the dilator responses - suggests that increased levels of circulating catecholamines had an important dilator influence on both arterial and venous vessels. The effect on the arterioles, notably on the secondary arterioles, is consistent with our recent demonstration that sotalol substantially reduced the increase in limb muscle vascular conductance evoked by severe hypoxia (Mian et al. 1990). In the rabbit, systemic fl-receptor blockade with propranolol converted the dilatation evoked in limb muscle by severe hypoxia, to constriction (Uther et al. 1970). Thus, these results all indicate that circulating catecholamines exert a dilator influence in muscles during hypoxia. This accords with the evidence that circulating levels of catecholamines are increased, as could be explained by peripheral chemoreceptor stimulation and by defence area activation (Marshall, 1987; Yardley & Hilton, 1987). The fact that the responses of primary and terminal arterioles and of collecting and primary venules were not significantly affected by sotalol (Fig. 2) and yet they did show substantial dilator responses to topically applied' isoprenaline may indicate that during hypoxia the f-mediated dilator influence upon these vessels is dominated by other factors. However, this result may simply reflect the small sample size for each vessel group and the fact that, by chance, the vessels selected for study within these groups tended to show 507

10 508 R. MIAN AND J. M. MARSHALL constrictor responses to hypoxia. In fact, the results presented indicate that whether or not sotalol had an effect on the response of a particular vessel to hypoxia was more dependent on whether the response was a dilatation than on the position of that vessel in the vascular tree. Further experiments would be required to clarify this. Other influences on microcirculation Clearly, after a-receptor blockade, many arterial and venous vessels showed constrictor responses during hypoxia, while after,-receptor blockade many showed dilatation. Moreover, substantial constrictor and dilator responses occurred in the few secondary arterioles that were tested after dual x- and fl-receptor blockade. These results are compatible with those obtained on the vascular conductance of limb muscle (Marshall & Metcalfe, 1988b; Mian et al. 1990). They demonstrate that catecholamines were not the only factors contributing to the constrictor and dilator responses. A similar conclusion could be drawn from the studies of Hutchins, Bond & Green (1974) and Morff, Harris, Weigman & Miller (1981) on the rat cremaster muscle, who showed, respectively, that diameter increases induced in arterioles of < 50 #tm during administration of 18 % 02 for 1-2 min were not affected by dual a- and fl-adrenoceptor blockade, and that diameter decreases induced in arterioles and venules of > 100,um during 10% 02 for 1 h were not affected by local a-adrenoceptor blockade. As discussed before (Marshall & Metcalfe, 1988b; Mian & Marshall, 1991) and indicated above, our findings are compatible with the view that locally released metabolites make an important contribution to the dilator responses of both arterial and venous vessels, particularly terminal arterioles and collecting venules. These could include any factor that is a product of muscle metabolism when oxygen supply is limited. Adenosine and K+ are possibilities for which there is supportive evidence (Mian et al. 1990; Marshall & Neylon, 1990). In addition, vasodilator factors may be released by the action of hypoxia on vascular endothelium (Busse, Pohl, Kellner & Klem, 1983), although arteriolar smooth muscle itself may not be sensitive to hypoxia (Jackson, 1987). Vasoconstrictor substances whose release by endothelium is triggered by hypoxia may have contributed to the non-adrenergic vasoconstrictor responses (Rubanyi & Vanhoutte, 1985). However, circulating hormones may also have been involved: the constrictor responses evoked in spinotrapezius arterioles by hypoxia were reduced by blockade of vasopressin V1 receptors (Lloyd & Marshall, 1987). Functional implications Previous comparisons between the responses of individual vessels of the spinotrapezius and the results obtained in whole-organ studies on skeletal muscle have indicated the functional roles of the different sections of muscle microcirculation (Marshall, 1982; Marshall & Tandon, 1984; Hebert & Marshall, 1988). On this basis, the present results lead us to propose that the arterioles, particularly those comparable to the primary and secondary arterioles make the major contribution to the change in total vascular conductance of skeletal muscle during systemic hypoxia. We suggest that they show reflex vasoconstriction in response to peripheral chemoreceptor stimulation, but that this may be overcome in individual vessels by

11 HYPOXIA AND CATECHOLAMINES dilatation mediated by circulating catecholamines and locally released metabolites. As the terminal arterioles are thought to be particularly sensitive to local factors, we suggest that they dilate in areas where vasodilator metabolites accumulate as a result of tissue hypoxia and that this helps to improve local distribution of blood flow and thereby 02. The net dilatation of venules near to the capillary bed would be expected to decrease post-capillary resistance. If, as we suggest, their dilatation is mainly due to metabolites, then the venules may contribute to the matching of capillary flow with local metabolic demands. A decrease in post-capillary resistance would also help to offset any outward capillary filtration caused by arteriolar dilatation. Our evidence that during hypoxia, venular dilatation is augmented by the fl-effect of circulating catecholamines accords with previous evidence from studies on cat hindlimb muscles that, during haemorrhage, dilatation of post-capillary vessels induced by circulating catecholamines plays a major role in encouraging fluid reabsorption from tissue space (Hillman & Lundvall, 1981). As the venous vessels of muscle are regarded as capacitance vessels, our results also suggest that the regional blood volume of muscle is increased during systemic hypoxia. This work was supported by the Wellcome Trust. 509 REFERENCES BIESOLD, D., KUROSAWA, M., SATO, A. & TRZEBSKI, A. (1990). Responses of sympatho-adrenal medullary system to hypoxia and hypercapnia in anaesthetised artificially ventilated rats. In Proceedings: Chemoreceptors and Chemoreceptor Reflexes (Satellite of XXXI International Congress of Physiological Sciences), ed. ACKER, H., TREBSKI, A. & O'REGAN, R. G., pp Plenum Press, New York and London. BOOTH, A. P. & MARSHALL, J. M. (1987). Responses induced in muscle microcirculation of the rat by systemic hypoxia. Journal of Physiology 387, 67P. BORGSTROM, P., LINDBOM, L., ARFORS, K. E. & INTAGLIETTA, M. (1988). fl-adrenergic control of resistance in individual vessels of rabbit tenuissimus muscle. American Journal ofphysiology 254, H BUSSE, R., POHL, H., KELLNER, C. & KLEM, U. (1983). Endothelial cells are involved in the vasodilator response to hypoxia. Pflugers Archiv 397, HEBERT, M. T. & MARSHALL, J. M. (1986). Responses of skeletal muscle microcirculation to circulating noradrenaline in the rat. Journal of Physiology 373, 56P. HABERT, M. T. & MARSHALL, J. M. (1988). Direct observations on the effects of baroreceptor stimulation on skeletal muscle circulation of the rat. Journal of Physiology 400, HILLMAN, J. & LUNDVALL, J. (1981). Hormonal and neurogenic adrenergic control of the fluid transfer from skeletal muscle to blood during hemorrhage. Acta Physiologica Scandinavica 112, HUTCHINS, P. M., BOND, R. C. & GREEN, H. D. (1974). Participation of oxygen in the local control of skeletal muscle microvasculature. Circulation Research 34, JACKSON, W. F. (1987). Arteriolar oxygen reactivity: where is the sensor? American Journal of Physiology 253, H LLOYD, J. & MARSHALL, J. M. (1987). The influence of vasopressin on muscle microcirculation during systemic hypoxia in the rat. Journal of Physiology 396, 88P. LUNDVALL, J., HILLMAN, J. & GUSTAFSSON, D. (1982). fl-adrenergic dilator effects in consecutive vascular sections of skeletal muscle. American Journal of Physiology 243, H MARSHALL, J. M. (1982). The influence of the sympathetic nervous system on individual vessels of the microcirculation of skeletal muscle of the rat. Journal of Physiology 332, MARSHALL, J. M. (1987). Analysis of cardiovascular responses evoked following changes in peripheral chemoreceptor activity in the rat. Journal of Physiology 394,

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