Carotid body chemoreceptor response to prolonged hypoxia in the rabbit: effects of domperidone and propranolol.

1. The discharge of single afferent chemoreceptor fibres was recorded from the cut sinus nerve over periods of 60 or 90 min of constant, isocapnic hypoxia (arterial O2 pressure, Pa,O2, 3.13-5.25 kPa), in twenty anaesthetized rabbits, after dividing the sympathetic supply to the carotid body. 2. Under control conditions, discharge after 60 min of hypoxia adapted to a mean (S.E.M.) of 71.95 (2.75)% of that attained at 5 min of hypoxia in twenty-three hypoxic experiments. This adaptation was more pronounced when Pa,O2 was lower than 4 kPa (30 Torr). 3. Domperidone (1 mg kg-1 bolus + 1 mg kg-1 h-1 infusion I.V.), increased normoxic afferent discharge by a mean of 142%. In ten experiments, discharge after 60 min of hypoxia adapted to a mean (S.E.M.) of 56.22 (+/- 3.40)% of that attained at 5 min of hypoxia which was significantly different from control hypoxic runs (P = 0.006). 4. In seven experiments propranolol (1 mg kg-1 bolus + 1 mg kg-1 h-1 infusion I.V.) did not affect the normoxic discharge. The mean adaptation of discharge after 60 min of hypoxia was to 77.43 (3.97)% of discharge attained at 5 min of hypoxia, which was not significantly different from control hypoxic runs (P = 0.34). 5. Under control conditions plasma [K+] increased steadily during 60 min of hypoxia, in fourteen experiments, from a mean of 2.76 (0.14) to 2.85 (0.12) mmol l-1 but this was not significant (P = 0.21). Domperidone (n = 6) did not affect plasma [K+] at any time, but after propranolol (n = 6) it increased by a mean (S.E.M.) of 0.39 (0.09) mmol l-1 (P = 0.01) in normoxia and by a further 0.62 (0.28) mmol l-1 (P = 0.08) at 60 min of hypoxia. 6. The results suggest that the adaptation of chemoreceptor discharge to hypoxia in the rabbit is not mediated by changes in plasma [K+]; in addition, endogenous dopamine, but not noradrenaline, contributes to the maintenance of chemoreceptor discharge in prolonged hypoxia.