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Carotid chemoreceptor reflexes in normotensive and spontaneously hypertensive rats
Gen. Pharmac. Vol. 12, pp. 459 to 464. 1981
0306-3623/81/D~459-06$02.00/0 Pergamon Press Lid
Printed in Great Britain
CAROTID CHEMORECEPTOR REFLEXES IN NORMOTENSIVE AND SPONTANEOUSLY HYPERTENSIVE RATS O. A. SOFOLA a n d P. E. EGBE Department of Physiology, College of Medicine, P.M.B. 12003, Lagos, Nigeria
(Received 14 April 1981) Abstract--1. Carotid body chemoreceptors were stimulated in normotensive and spontaneously hypertensive rats by intracarotid injection of nicotine or sodium dithionite. 2. Stimulation of the chemoreceptors resulted in hyperventilation and a rise in blood pressure in both groups of rats. However the pressor response was much less in the hypertensive rats. 3. When ventilation was controlled by artificial ventilation, carotid chemoreceptor stimulation resulted in a significant reduction in heart rate in both the normotensive and hypertensive rats. 4. The observed responses were abolished by intracarotid injection of acetic acid suggesting that they are of carotid chemoreceptor origin.
INTRODUCTION
Stimulation of the carotid body chemoreceptors, a p a r t from the expected hyperventilation, results in an increase in the systemic b l o o d pressure (Daly, 1971). However, the changes in h e a r t rate are variable in spontaneously breathing dogs (Daly & Scott, 1962), cats (Macleod & Scott, 1964) a n d rabbits (Scott, 1966), b u t when the ventilatory response is prevented by constant rate ventilation, the response is invariably a bradycardia. In contrast to the response in the above species studied, carotid chemoreceptor stimulation in the rat has been reported as not inducing any significant cardiovascular responses (Sapru & Krieger, 1977). In addition, there was also no study on the effect of controlled ventilation in these rats. The cardiovascular responses to carotid chemoreceptor stimulation has also been shown to be attenuated or abolished by an increase in carotid sinus pressure (Mancia, 1975; Hainsworth et al., 1979) a n d by systemic hypertension induced by occlusion of the descending a o r t a (Heistad et al., 1974). In the present paper, we are reporting cardiovascular responses to carotid chemoreceptor stimulation in normotensive a n d spontaneously hypertensive rats. W e have also attempted to find out if the magnitude of the responses are affected in the hypertensive rats, since they are expected to have a high carotid sinus pressure. The effect of controlled ventillation, especially on the heart rate response, is also reported. MATERIALS AND METHODS
Experiments were carried out on 10 normotensive albino Wistar rats and 7 spontaneously hypertensive rats of the Okimoto-Aoki strain (Okimoto & Aoki, 1963). The rats were of either sex and were matched for weight and they weighed between 180 and 220 g. The animals were anaesthetized with urethane (BDH Chemicals Ltd., Poole) at a dosage of 1.5 g/kg body weight administered intraperitoneally. After induction of anaesthesia, a mid-line incision was made in the neck and the trachea and one common artery were isolated. The trachea was cannulated using a 459
polyethylene cannula with an internal diameter similar to that of the trachea. The trachea cannula was connected to a Fleish pneumotachograph and then to a Grass Integrator via a volumetric transducer (Model PT5A) for the recording of tidal volume. The recordings were made using a Grass Polygraph-Model 7D (Grass Instruments Ltd., Quincy, Mass.). The pneumotachograph was calibrated using a 5 c m 3 syringe. Respiratory frequency was counted from the number of recorded tidal volumes per rain. The common carotid artery was ligated and then cannulated peripherally for the recording of arterial blood pressure and centrally for injection of drugs to stimulate the carotid body chemoreceptors. The pressure cannula was connected via a Statham pressure transducer to the polygraph. Chemoreceptor stimulation was accomplished in the first series by intra-carotid injection of 20/~g Nicotine in 0.2 cm 3 volume of injectate and in the second series by using the reducing agent--sodium dithionite (Critchley & Ungar, 1974; Henderson & Ungar 1978). Sodium dithionite, 0.05 molar solution, was freshly prepared each time and stored under liquid paraffin. The injectate volume was 0.2 cm a. The injectates were warmed to 39~C and administered over a period of 5 sec. Experiments with nicotine injection were carried out on 9 normotensive and 7 hypertensive rats while those with sodium dithionite were carried out on 4 normotensive and 3 hypertensive rats. Heart rate (HR) and respiratory frequency (F) were determined by counting the number of blood pressure pulses and tidal volumes respectively over 15 sec and converted to rate per min. Mean blood pressure (BP) was calculated from diastolic pressure + one-third pulse pressure. The paper speed used was 5 or 10 mm per sec. Control records of arterial blood pressure, heart rate, tidal volume (TV) and respiratory frequency were taken. Then either nicotine or sodium dithionite was injected and the changes in the measured variables were determined. Ventilation per min (l?t~) was determined in the immediate control period from the tidal volume and frequency and 15 sec following chemoreceptor stimulation. Arterial Po,, Pco: and pH were analysed before and after injection of the drugs using a Coming Blood-Gas analyser--Model 164. Arterial blood, about 0.5 cm 3 was withdrawn each time for analysis and not more than 3 samples were taken from each animal. In some experiments, the responses to chemoreceptor stimulation during spontaneous ventilation were compared
460
O.A. SOFOLA and P. E. EGBE
Table 1. The effects of carotid chemoreceptor stimulation with nicotine and sodium dithionite on blood pressure, heart rate and minute ventilation in normotensive and hypertensive rats Normotensive rats
Nicotine BP (mmHg) HR (beats/min) l?E (mls/min) Sodium dithionite BP (mmHg) HR (beats/min) f'r (mls/min)
Hypertensive rats
C
T
o,~,Change
P
C
T
o~ Change
P
115 + 3 357 + 8 641 + 38 n=9
152 + 6 351 + 7 1034 + 147
32.2 - 1.7 61.3
<0.001 NS <0.001
150 + 4 368 +__14 530 + 93
168 + 5 361 + 11 753 +_ 91 n=7
12.0 -1.9 42.1
<0.001 NS <0.001
101 + 4 341 + 18 545 + 95 n=8
146 + 6 338 + 16 906 + 212
34.7 -0.9 66.9
<0.00l NS <0.025
154 + 4 388 +__6 594 + 55
167 + 7 384 + 8 940 _ 80 n=6
8.4 - 1.0 58.2
<0.005 NS <0.001
C = control, T = test period, 12E = minute ventilation. Values stated are means +_ standard error of the mean. P values from paired t-test. NS = not significant. n = number of experiments.
with those during controlled ventilation, using sodium dithionite as the stimulus. Artificially controlled ventilation was induced using a Respirator (C.F. Palmer Ltd., London) at a tidal volume similar to that during spontaneous ventilation. In 3 rats of each group, the effect of inactivation of the chemoreceptors by administration of 0.2 cm 3 of a 0.5 N acetic acid solution (Kenney & Neil, 1951) was also determined. Tests of significance were carried out using the paired t-test for the difference between control and test responses in each group of rats. In addition the t-test was used to compare the percentage difference between the 2 groups.
RESULTS
1. Blood--Gas Analysis The control Po2 Pco~ a n d p H in the rats were 97 5- 4 m m H g , 38.5 5- 2 m m H g a n d 7.38 5- 0.02 units respectively. After sodium dithionite there were no appreciable changes in these parameters, Po~ was 98+3mmHg; Pco2 36.7 5 - 2 m m H g and pH 7.39 5- 0.02 units.
2. Responses to injection of Nicotine in spontaneously breathing rats In the normotensive rats, there were significant increases in b l o o d pressure (P < 0.001) a n d minute ventilation (P < 0.005). T h e b l o o d pressure changes ranged between 13.6 a n d 45.8~o (average 32.2%) while those for minute ventilation ranged from 18.2 to 144.0% (average 61.3%). Changes in heart rate in these rats were variable a n d overall, there was n o significant change in heart rate ( - 1.7%), see Table 1. In the hypertensive rats, there was also a significant increase in b l o o d pressure in response to carotid chemoreceptor stimulation (P < 0.001). However, the percentage increase in B P in these rats (12.0%) was smaller than that in the normotensive rats (32.2%). This difference in the pressor response between the two groups of rats was statistically significant (P < 0.005). M i n u t e ventilation was also significantly increased in the hypertensive rats (P < 0.001), b u t there was n o significant change in heart rate. These results are summarised in Table 1.
3. Responses to sodium dithionite in spontaneously breathing rats Eight experiments were performed in 4 n o r m o t e n sive rats a n d 6 experiments in 3 hypertensive rats. The pattern of responses in normotensive and hypertensive rats in response to stimulation of the carotid chemoreceptors with sodium dithionite was similar to that of nicotine injection (Table 1). Similarly the pressor response in the hypertensive rats, was also significantly lower than in the normotensive rats (P < 0.001). An example of the record of the responses in a normotensive and a hypertensive rat are shown in Fig. 1.
4. Effect of controlled ventilation on the responses During controlled ventilation, the pressor response to carotid chemoreceptor stimulation with sodium dithionite was not different from that in the spontaneously breathing rats. However, in b o t h groups of rats, there was a significant decrease in heart rate; 7.9To decrease in the normotensive rats (P < 0.001) a n d 16.0~o decrease in the hypertensive rats (P < 0.025). The results of this series of experiments • are listed in Table 2.
5. Effect of intracarotid injection of acetic acid The pressor and ventilatory responses to stimulation of the carotid chemoreceptors b o t h in the normotensive a n d hypertensive rats were abolished after intracarotid injection of acetic acid. In the majority of cases administration of nicotine or sodium dithionite invariably produced a fall in b l o o d pressure and tidal volume. An example of such a response is shown in Fig. 2. DISCUSSION
Nicotine has been shown to stimulate chemoreceptors a n d electrophysiological studies have confirmed the increase in impulse traffic in carotid chemoreceptor fibres during nicotine injection (Jacobs et al. 1971). Similarly sodium dithionite stimulates the chemoreceptors because, as a reducing agent, it acts
Carotid chemoreflex in rats
461
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Fig. 1. The effect of intracarotid injection of sodium dithionite (DITH) on respiration and blood pressure in a normotensive and a hypertensive rat. Note the reduced pressor response in the hypertensive rat despite the ventilatory stimulation. BP = Blood pressure; TV = tidal volume.
by lowering arterial oxygen tension (Henderson & Ungar, 1978). Thus both agents used in the present experiments are expected to provide adequate stimuli to the carotid body chemoreceptors of the rat. The hyperventilation in response to either nicotine or sodium dithionite injection confirms that the chemoreceptor are being stimulated. Intracarotid sodium dithionite did not alter the systemic Po, indicatirl'g that the stimulus was confined mainly to the carotid chemoreceptors. Furthermore, the ventilatory and
cardiovascular responses to stimulation of the chemoreceptors were abolished by intracarotid injection of acetic acid. Since acetic acid has been shown to inactivate the chemoreceptors (Kenney & Neil, 1951), it therefore confirms that the observed responses in this series are due to stimulation of the carotid body chemoreceptors. Stimulation of the chemoreceptors in the present report has been shown to result in an increase in blood pressure in both the normotensive and hyper-
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Carotid chemoreflex in rats Table 2. The effect of constant artificial ventilation on the cardiovascular responses to intracarotid injection of sodium dithionite in rats Hypertensive rats
Normotensive rats Rat No.
BP (mmHg)
HR (beats/min)
N8a N8b N9 Nl0a Nl0b
C 88 97 95 80 90
T 152 140 120 120 130
C 384 374 384 360 336
T 360 350 354 318 312
Mean SEM *P o~ Change
90 133 3 6 < 0.005 47.8
368 8
339 9 < 0.001 - 7.9
Rat No.
BP (mmHg)
H5a H5b H6a H6b H7
C 133 133 160 155 160
HR (beats/min)
T 150 160 190 200 165
C 384 396 360 396 396
T 360 296 3J8 334 294
148 173 6 8 < 0.025 16.9
386 6
324 11 < 0.025 - 16.0
Legends as in Table 1. SEM = standard error of the mean. * From paired t-test.
tensive rats, but this rise was lower in the hypertensive rats. This however is different from the earlier observation in normotensive rats where no significant pressor effects were elicited (Sapru & Kriger, 1977). One reason for the discrepancy may be due to the fact that in the present experiments, the injectate was warmed to 39°C since the temperature of the carotid perfusate has been shown to influence reflex responses to chemoreceptor stimulation (McQueen & Ezyaguirre, 1974). However, this cannot be the complete explanation as Sapru & Krieger (1977) still obtained ventilatory responses in their study. The attenuation of the pressor response in the hypertensive rats may be explained on the basis of some earlier reports where either a high carotid sinus pressure (Mancia, 1975; Hainsworth et al. 1979) or induced systemic hypertension (Heistad et al. 1974) attenuates the pressor response to carotid chemoreceptor stimulation in dogs. One of the suggestions is that of a possible interaction between baroreceptor and chemoreceptor afferents at the central nervous system such that a higher baroreceptor input would tend to inhibit the chemoreceptor input (Mancia, 1975). The present experiments therefore indicate that chemoreceptor stimulation in rats results in a pressor re-" sponse, a finding similar to that in other species. The heart rate response in the spontaneously breathing rats was variable and the changes insignificant. However, during constant artificial ventilation carotid chemoreceptor stimulation both in the normotensive and hypertensive rats resulted in significant reductions in the heart rate. The experiments of Sapru & Krieger (1977) showed no heart rate changes. In their experiment, ventilation was not controlled and since changes in ventilation can affect the heart rate responses to chemoreceptor stimulation (Daly, 1971), one may therefore explain their findings on this basis. The results of the present experiments have therefore shown that in rats, as it is the case in other species, when changes in ventilation are prevented, carotid chemoreceptor stimulation also results in a reduction in heart rate. In conclusion, our results have established that the cardiovascular and ventilatory responses to carotid
chemoreceptor stimulation in rats are identical with those of other species. In addition, it shows that hypertensive rats exhibit an attenuated pressor response when compared with normotensive rats. Acknowledgements--We wish to acknowledge with thanks, the gift of the hypertensive rats from Professor (Mrs.) O. Elebute and Dr F. Tayo. We are also grateful to Mr Albert Adjenu and Miss G. Bamishile for technical assistance; Messrs F. O. Oderinde and C. O. Owunna for the typescript and the Biomedical Communications department for the illustration.
REFERENCES
CRITCHLEY J. A. J. H. & UNGAR A. (1974) A chemical method of lowering the Po2 of blood in experimental studies of arterial chemoreceptor reflexes. J. Physiol. 244, 12-13P. DALY M. de B. (1972) Interaction of cardiovascular re~ flexes. Sei Basis Med. Ann. Rer. Ch. XVII, 307-332. DALY M. de B. & SCOTT,M. J. (1962) An analysis of the primary cardiovascular reflex effects of stimulation of the carotid body chemoreceptors in the dog. J. Physiol. 162, 555-573. HAINSWORTH R., KARIM, F. & SOFOLA,O. A. (1979) Left Ventricular inotropic responses to stimulation of carotid body chemoreceptors in anaesthetized dogs. J. Physiol. 2117, 455-466. HEISTAD D. D., ABBOUD F. M., MARK R. C. & SCHMID P. G. (1974) Interaction of baroreceptor and chemoreceptor reflexes. Modulation of chemoreceptor reflex by changes in baroreceptor activity. J. Clin. Incest. 53, 1226-1236. HENDERSONC. G. & UNGAR A. (1978) Effect of cholinergic antagonists on sympathetic ganglionic transmission of vasomotor reflexes from the carotid baroreceptors and chemoreceptors of the dog. J. Physiol. 277, 379-385. JACOSS L., SAMPSON,S. R. & COMROEJ. H. Jr (1971) Carotid sinus versus carotid body origin of nicotine and cyanide bradycardia in the dog. Am. J. Physiol. 220, 472-476. KENNEY R. A. & NElL E. (1951) The contribution of aortic chemoreceptor mechanisms to the maintenance of arterial blood pressure of cats and dogs after haemorrhage. J. Physiol. !12, 223-228.
464
O. A. SOFOLAand P. E. EGBE
MACLEOD R. D. M. & SCOTT M. J. (1964) The heart rate responses to carotid body chemoreceptor stimulation in the cat. J. Physiol. 175, 193-202. MANCIA G. (1975) Influence of carotid baroreceptors on vascular responses to chemoreceptor stimulation in the dog. Circulation Res. 36, 270-276. McQuv~N K. & EYZAGUmREC. (1974) Effects of temperature on carotid chemoreceptor and baroreceptor activity. J. Neurophysiol. 37, 1287-1296.
OKIMOTO K. & AOKI K. (1963) Development of a strain of spontaneously hypertensive rats. Jap. Circul. J. 27, 282-293. SAPRU H. N. & KRmGER A. J. (1977) Carotid and aortic chemoreceptor function in the rat. J. appl. Physiol. 42, 344-348. ScoTr M. J. (1966) Reflex effects of carotid body chemoreceptor stimulation on the heart rate of the rabbit. Aust. J. exp. Biol. reed. Sci. 44, 393-404.
0306-3623/81/D~459-06$02.00/0 Pergamon Press Lid
Printed in Great Britain
CAROTID CHEMORECEPTOR REFLEXES IN NORMOTENSIVE AND SPONTANEOUSLY HYPERTENSIVE RATS O. A. SOFOLA a n d P. E. EGBE Department of Physiology, College of Medicine, P.M.B. 12003, Lagos, Nigeria
(Received 14 April 1981) Abstract--1. Carotid body chemoreceptors were stimulated in normotensive and spontaneously hypertensive rats by intracarotid injection of nicotine or sodium dithionite. 2. Stimulation of the chemoreceptors resulted in hyperventilation and a rise in blood pressure in both groups of rats. However the pressor response was much less in the hypertensive rats. 3. When ventilation was controlled by artificial ventilation, carotid chemoreceptor stimulation resulted in a significant reduction in heart rate in both the normotensive and hypertensive rats. 4. The observed responses were abolished by intracarotid injection of acetic acid suggesting that they are of carotid chemoreceptor origin.
INTRODUCTION
Stimulation of the carotid body chemoreceptors, a p a r t from the expected hyperventilation, results in an increase in the systemic b l o o d pressure (Daly, 1971). However, the changes in h e a r t rate are variable in spontaneously breathing dogs (Daly & Scott, 1962), cats (Macleod & Scott, 1964) a n d rabbits (Scott, 1966), b u t when the ventilatory response is prevented by constant rate ventilation, the response is invariably a bradycardia. In contrast to the response in the above species studied, carotid chemoreceptor stimulation in the rat has been reported as not inducing any significant cardiovascular responses (Sapru & Krieger, 1977). In addition, there was also no study on the effect of controlled ventilation in these rats. The cardiovascular responses to carotid chemoreceptor stimulation has also been shown to be attenuated or abolished by an increase in carotid sinus pressure (Mancia, 1975; Hainsworth et al., 1979) a n d by systemic hypertension induced by occlusion of the descending a o r t a (Heistad et al., 1974). In the present paper, we are reporting cardiovascular responses to carotid chemoreceptor stimulation in normotensive a n d spontaneously hypertensive rats. W e have also attempted to find out if the magnitude of the responses are affected in the hypertensive rats, since they are expected to have a high carotid sinus pressure. The effect of controlled ventillation, especially on the heart rate response, is also reported. MATERIALS AND METHODS
Experiments were carried out on 10 normotensive albino Wistar rats and 7 spontaneously hypertensive rats of the Okimoto-Aoki strain (Okimoto & Aoki, 1963). The rats were of either sex and were matched for weight and they weighed between 180 and 220 g. The animals were anaesthetized with urethane (BDH Chemicals Ltd., Poole) at a dosage of 1.5 g/kg body weight administered intraperitoneally. After induction of anaesthesia, a mid-line incision was made in the neck and the trachea and one common artery were isolated. The trachea was cannulated using a 459
polyethylene cannula with an internal diameter similar to that of the trachea. The trachea cannula was connected to a Fleish pneumotachograph and then to a Grass Integrator via a volumetric transducer (Model PT5A) for the recording of tidal volume. The recordings were made using a Grass Polygraph-Model 7D (Grass Instruments Ltd., Quincy, Mass.). The pneumotachograph was calibrated using a 5 c m 3 syringe. Respiratory frequency was counted from the number of recorded tidal volumes per rain. The common carotid artery was ligated and then cannulated peripherally for the recording of arterial blood pressure and centrally for injection of drugs to stimulate the carotid body chemoreceptors. The pressure cannula was connected via a Statham pressure transducer to the polygraph. Chemoreceptor stimulation was accomplished in the first series by intra-carotid injection of 20/~g Nicotine in 0.2 cm 3 volume of injectate and in the second series by using the reducing agent--sodium dithionite (Critchley & Ungar, 1974; Henderson & Ungar 1978). Sodium dithionite, 0.05 molar solution, was freshly prepared each time and stored under liquid paraffin. The injectate volume was 0.2 cm a. The injectates were warmed to 39~C and administered over a period of 5 sec. Experiments with nicotine injection were carried out on 9 normotensive and 7 hypertensive rats while those with sodium dithionite were carried out on 4 normotensive and 3 hypertensive rats. Heart rate (HR) and respiratory frequency (F) were determined by counting the number of blood pressure pulses and tidal volumes respectively over 15 sec and converted to rate per min. Mean blood pressure (BP) was calculated from diastolic pressure + one-third pulse pressure. The paper speed used was 5 or 10 mm per sec. Control records of arterial blood pressure, heart rate, tidal volume (TV) and respiratory frequency were taken. Then either nicotine or sodium dithionite was injected and the changes in the measured variables were determined. Ventilation per min (l?t~) was determined in the immediate control period from the tidal volume and frequency and 15 sec following chemoreceptor stimulation. Arterial Po,, Pco: and pH were analysed before and after injection of the drugs using a Coming Blood-Gas analyser--Model 164. Arterial blood, about 0.5 cm 3 was withdrawn each time for analysis and not more than 3 samples were taken from each animal. In some experiments, the responses to chemoreceptor stimulation during spontaneous ventilation were compared
460
O.A. SOFOLA and P. E. EGBE
Table 1. The effects of carotid chemoreceptor stimulation with nicotine and sodium dithionite on blood pressure, heart rate and minute ventilation in normotensive and hypertensive rats Normotensive rats
Nicotine BP (mmHg) HR (beats/min) l?E (mls/min) Sodium dithionite BP (mmHg) HR (beats/min) f'r (mls/min)
Hypertensive rats
C
T
o,~,Change
P
C
T
o~ Change
P
115 + 3 357 + 8 641 + 38 n=9
152 + 6 351 + 7 1034 + 147
32.2 - 1.7 61.3
<0.001 NS <0.001
150 + 4 368 +__14 530 + 93
168 + 5 361 + 11 753 +_ 91 n=7
12.0 -1.9 42.1
<0.001 NS <0.001
101 + 4 341 + 18 545 + 95 n=8
146 + 6 338 + 16 906 + 212
34.7 -0.9 66.9
<0.00l NS <0.025
154 + 4 388 +__6 594 + 55
167 + 7 384 + 8 940 _ 80 n=6
8.4 - 1.0 58.2
<0.005 NS <0.001
C = control, T = test period, 12E = minute ventilation. Values stated are means +_ standard error of the mean. P values from paired t-test. NS = not significant. n = number of experiments.
with those during controlled ventilation, using sodium dithionite as the stimulus. Artificially controlled ventilation was induced using a Respirator (C.F. Palmer Ltd., London) at a tidal volume similar to that during spontaneous ventilation. In 3 rats of each group, the effect of inactivation of the chemoreceptors by administration of 0.2 cm 3 of a 0.5 N acetic acid solution (Kenney & Neil, 1951) was also determined. Tests of significance were carried out using the paired t-test for the difference between control and test responses in each group of rats. In addition the t-test was used to compare the percentage difference between the 2 groups.
RESULTS
1. Blood--Gas Analysis The control Po2 Pco~ a n d p H in the rats were 97 5- 4 m m H g , 38.5 5- 2 m m H g a n d 7.38 5- 0.02 units respectively. After sodium dithionite there were no appreciable changes in these parameters, Po~ was 98+3mmHg; Pco2 36.7 5 - 2 m m H g and pH 7.39 5- 0.02 units.
2. Responses to injection of Nicotine in spontaneously breathing rats In the normotensive rats, there were significant increases in b l o o d pressure (P < 0.001) a n d minute ventilation (P < 0.005). T h e b l o o d pressure changes ranged between 13.6 a n d 45.8~o (average 32.2%) while those for minute ventilation ranged from 18.2 to 144.0% (average 61.3%). Changes in heart rate in these rats were variable a n d overall, there was n o significant change in heart rate ( - 1.7%), see Table 1. In the hypertensive rats, there was also a significant increase in b l o o d pressure in response to carotid chemoreceptor stimulation (P < 0.001). However, the percentage increase in B P in these rats (12.0%) was smaller than that in the normotensive rats (32.2%). This difference in the pressor response between the two groups of rats was statistically significant (P < 0.005). M i n u t e ventilation was also significantly increased in the hypertensive rats (P < 0.001), b u t there was n o significant change in heart rate. These results are summarised in Table 1.
3. Responses to sodium dithionite in spontaneously breathing rats Eight experiments were performed in 4 n o r m o t e n sive rats a n d 6 experiments in 3 hypertensive rats. The pattern of responses in normotensive and hypertensive rats in response to stimulation of the carotid chemoreceptors with sodium dithionite was similar to that of nicotine injection (Table 1). Similarly the pressor response in the hypertensive rats, was also significantly lower than in the normotensive rats (P < 0.001). An example of the record of the responses in a normotensive and a hypertensive rat are shown in Fig. 1.
4. Effect of controlled ventilation on the responses During controlled ventilation, the pressor response to carotid chemoreceptor stimulation with sodium dithionite was not different from that in the spontaneously breathing rats. However, in b o t h groups of rats, there was a significant decrease in heart rate; 7.9To decrease in the normotensive rats (P < 0.001) a n d 16.0~o decrease in the hypertensive rats (P < 0.025). The results of this series of experiments • are listed in Table 2.
5. Effect of intracarotid injection of acetic acid The pressor and ventilatory responses to stimulation of the carotid chemoreceptors b o t h in the normotensive a n d hypertensive rats were abolished after intracarotid injection of acetic acid. In the majority of cases administration of nicotine or sodium dithionite invariably produced a fall in b l o o d pressure and tidal volume. An example of such a response is shown in Fig. 2. DISCUSSION
Nicotine has been shown to stimulate chemoreceptors a n d electrophysiological studies have confirmed the increase in impulse traffic in carotid chemoreceptor fibres during nicotine injection (Jacobs et al. 1971). Similarly sodium dithionite stimulates the chemoreceptors because, as a reducing agent, it acts
Carotid chemoreflex in rats
461
NORMOTEN$1VE RAT
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lhlm.
.! DITH
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•
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m aL l
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Fig. 1. The effect of intracarotid injection of sodium dithionite (DITH) on respiration and blood pressure in a normotensive and a hypertensive rat. Note the reduced pressor response in the hypertensive rat despite the ventilatory stimulation. BP = Blood pressure; TV = tidal volume.
by lowering arterial oxygen tension (Henderson & Ungar, 1978). Thus both agents used in the present experiments are expected to provide adequate stimuli to the carotid body chemoreceptors of the rat. The hyperventilation in response to either nicotine or sodium dithionite injection confirms that the chemoreceptor are being stimulated. Intracarotid sodium dithionite did not alter the systemic Po, indicatirl'g that the stimulus was confined mainly to the carotid chemoreceptors. Furthermore, the ventilatory and
cardiovascular responses to stimulation of the chemoreceptors were abolished by intracarotid injection of acetic acid. Since acetic acid has been shown to inactivate the chemoreceptors (Kenney & Neil, 1951), it therefore confirms that the observed responses in this series are due to stimulation of the carotid body chemoreceptors. Stimulation of the chemoreceptors in the present report has been shown to result in an increase in blood pressure in both the normotensive and hyper-
462
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Carotid chemoreflex in rats Table 2. The effect of constant artificial ventilation on the cardiovascular responses to intracarotid injection of sodium dithionite in rats Hypertensive rats
Normotensive rats Rat No.
BP (mmHg)
HR (beats/min)
N8a N8b N9 Nl0a Nl0b
C 88 97 95 80 90
T 152 140 120 120 130
C 384 374 384 360 336
T 360 350 354 318 312
Mean SEM *P o~ Change
90 133 3 6 < 0.005 47.8
368 8
339 9 < 0.001 - 7.9
Rat No.
BP (mmHg)
H5a H5b H6a H6b H7
C 133 133 160 155 160
HR (beats/min)
T 150 160 190 200 165
C 384 396 360 396 396
T 360 296 3J8 334 294
148 173 6 8 < 0.025 16.9
386 6
324 11 < 0.025 - 16.0
Legends as in Table 1. SEM = standard error of the mean. * From paired t-test.
tensive rats, but this rise was lower in the hypertensive rats. This however is different from the earlier observation in normotensive rats where no significant pressor effects were elicited (Sapru & Kriger, 1977). One reason for the discrepancy may be due to the fact that in the present experiments, the injectate was warmed to 39°C since the temperature of the carotid perfusate has been shown to influence reflex responses to chemoreceptor stimulation (McQueen & Ezyaguirre, 1974). However, this cannot be the complete explanation as Sapru & Krieger (1977) still obtained ventilatory responses in their study. The attenuation of the pressor response in the hypertensive rats may be explained on the basis of some earlier reports where either a high carotid sinus pressure (Mancia, 1975; Hainsworth et al. 1979) or induced systemic hypertension (Heistad et al. 1974) attenuates the pressor response to carotid chemoreceptor stimulation in dogs. One of the suggestions is that of a possible interaction between baroreceptor and chemoreceptor afferents at the central nervous system such that a higher baroreceptor input would tend to inhibit the chemoreceptor input (Mancia, 1975). The present experiments therefore indicate that chemoreceptor stimulation in rats results in a pressor re-" sponse, a finding similar to that in other species. The heart rate response in the spontaneously breathing rats was variable and the changes insignificant. However, during constant artificial ventilation carotid chemoreceptor stimulation both in the normotensive and hypertensive rats resulted in significant reductions in the heart rate. The experiments of Sapru & Krieger (1977) showed no heart rate changes. In their experiment, ventilation was not controlled and since changes in ventilation can affect the heart rate responses to chemoreceptor stimulation (Daly, 1971), one may therefore explain their findings on this basis. The results of the present experiments have therefore shown that in rats, as it is the case in other species, when changes in ventilation are prevented, carotid chemoreceptor stimulation also results in a reduction in heart rate. In conclusion, our results have established that the cardiovascular and ventilatory responses to carotid
chemoreceptor stimulation in rats are identical with those of other species. In addition, it shows that hypertensive rats exhibit an attenuated pressor response when compared with normotensive rats. Acknowledgements--We wish to acknowledge with thanks, the gift of the hypertensive rats from Professor (Mrs.) O. Elebute and Dr F. Tayo. We are also grateful to Mr Albert Adjenu and Miss G. Bamishile for technical assistance; Messrs F. O. Oderinde and C. O. Owunna for the typescript and the Biomedical Communications department for the illustration.
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