Acetylcholine release from cat carotid bodies

Abstract

Hypoxia, hypercapnia and acidosis stimulate the carotid body (CB) sending increased neural activity via a branch of the glossopharyngeal nerve to nucleus tractus solitarius; this precipitates an impressive array of cardiopulmonary, endocrine and renal reflex responses. However, the cellular mechanisms by which these stimuli generate the increased CB neural output are only poorly understood. Central to the understanding of these mechanisms is the determination of which agents are released within the CB in response to hypoxia, and serve as the stimulating transmitter(s) for chemosensory nerve endings. Acetylcholine (ACh) has been proposed as such an agent from the outset, but this proposal has been, and remains, controversial. The present study tests two hypotheses: (1) The CB releases ACh under normoxic/normocapnic conditions; and (2) The amount released increases during hypoxia and other conditions known to increase neural output from the CB. These hypotheses were tested in 12 experiments in which both CBs were removed from the anesthetized cat and incubated at 37°C in a physiological salt solution while the solution was bubbled with four different concentrations of oxygen and carbon dioxide. The incubation medium was exchanged at 10 min intervals for 30 min (three periods of incubation). The medium was analyzed with high performance liquid chromatography-electrochemical detection for ACh content. Normoxic//normocapnic conditions (21% O2/6% CO2) produced a total of 0.639 ± 0.106 pmol/150 μl (mean ± S.E.M.; n = 12). All stimulating conditions produced larger total outputs: 4% O2/2% CO2 produced 1.773 ± 0.46 pmol/150 μl; 0% O2/5% CO2, 0.868 ± 0.13 pmol/150 μl; 4% O2/10% CO2, 1.077 ± 0.21 pmol/150 μl. These three amounts were significantly greater than the normoxic/normocapnic condition, but indistinguishable among themselves. Further, the amount of ACh released did not diminish over the 30 min of stimulation. These data support the concept that during hypoxia ACh functions as a stimulating transmitter in the CB, and are consistent with the earlier reports of cholinergic enzymes and receptors found in the CB.