Synthesis and Release of Catecholamines by the Cat Carotid Body in Vitro: Effects of Hypoxic Stimulation

Abstract

The role of catecholamines (CAs) in cat carotid body chemoreception has been controversial. On the basis of pharmacological experiments, it would appear that endogenous dopamine (DA) may act either as an inhibitory or excitatory transmitter. Neurochemical studies on the effects of natural stimulation on the release of carotid body CAs in the cat have also been inconclusive. In the present study, we have characterized the synthesis and release of CAs in the in vitro cat carotid body preparation in response to different levels of hypoxic stimulation and have correlated these measures with the chemosensory activity of the carotid sinus nerve. The synthesis of [3H]DA and [3H]norepinephrine was linear for at least 4 h in carotid bodies incubated with their natural precursor [~H]tyrosine. Synthesis of both [3H]CAs plateaued when the [3H]tyrosine concentration in the media reached 40 uM, which is a concentration similar to that found in cat plasma. Exposure of the animals to an atmosphere of 10% 02 in N~ for 3 h prior to removal and incubation of the carotid bodies with [3H]tyrosine resulted in an approximately 100% increase in the rate of [3H]DA synthesis but no change in [3H]norepinephrine synthesis. This selective increase in [3H]DA synthesis was not detected when [3H]dihydroxyphenylalanine was used as precursor. Carotid bodies first incubated with [3H]tyrosine and later superfused with solutions equilibrated with different gas mixtures (0 100% 0 2 in N2) exhibited an increase in [3H]DA release and carotid sinus nerve discharge which were inversely related to the oxygen concentration. This relationship was strongest for the weaker stimuli (between 50% and 20% O 2 in N2), where both nerve activity and [3H]DA release increased almost in parallel. With lower oxygen concentrations (10% O 2 and 0% 02 equilibrated solutions), the increase in the release of [3H]DA was proportionally greater than the increase in carotid sinus nerve discharge. Our results demonstrate that hypoxic stimulation increases both the rate of synthesis and release of DA in the cat carotid body. Although the precise role of DA in this chemoreceptor organ is presently unknown, our findings suggest that this biogenic amine plays a direct role in generating or controlling the electrical activity in the carotid sinus nerve. INTRODUCTION The mammalian carotid body is an arterial chemoreceptor organ activated by low paO2, low pH and high p~CO2 (ref. 21). Structurally, the receptor complex is formed by clusters of two types of cells, the type I and type II cells; the clusters lie within a supporting connective tissue matrix containing a dense capillary net24, 41. Sensory fibers of the carotid sinus nerve (CSN) penetrate these cell clusters to end in synaptic apposition with type I cells, which are considered to be preneural (receptor) elements. Two perennial issues pertaining to arterial chemoreception, concern first, whether the type 1 cells are in fact