Interaction between norepinephrine and hypoxia on carotid body chemoreception in rabbits.

The effects of both bolus injection and constant infusion of various levels of norepinephrine (NE) on the activity of single-carotid chemoreceptor nerve fibers was studied in pentobarbital-anaesthetized rabbits under normoxic and hypoxic conditions. All animals were paralyzed and artificially ventilated. The dominant effect of NE was to excite carotid body chemoreceptor fibers in a dose-dependent fashion. There was an inverse relationship between the effect of NE on afferent fiber discharge and arterial O2 tension at all levels of exogenous NE above threshold levels. Propranolol (PROP) in sufficient dosage abolished the chemoreceptor response to NE and also markedly reduced the chemoreceptor response to hypoxia. These results support suggestions that beta-adrenergic receptors may be an integral part of the O2 chemosensory mechanism. They also demonstrate a nonadditive interaction between the effects of NE and hypoxia on the chemoreceptor afferent fibers. A reduction in resting discharge levels after infusion of PROP further suggests that endogenous catecholamines may contribute to carotid body chemoreceptor activity even under normoxic normocapnic conditions.