The role of dihydropydine-sensitive Ca2 + channels in stimulus-evoked catecholamine release from chemoreceptor cells of the carotid body

Abstract

Ahatraet-The present study utilized an in vitro preparation of the rabbit carotid body, with tissue catecholamine stores labeled by incubation with ‘H-tyrosine. The goal was to characterize pharmacologically the vol~g~~n&nt Ca*+ channels present in the type I (glomus) cells of this arterial chemoreceptor organ, and to elucidate their role as pathways for Ca2+ entry. We found that release of ‘H-dopamine induced by high external potassium was over 95% dependent on external cakium concentration and that this release was 9&100% inhibited by the dihydropy~~ne antagonists, nisoldipine and nitrendipine, and was potentiated by the dihydropyridine agonist, BayK 8444. Therefore, any stimulus-induced, cakiumdependent release of 3H-dopamine that was inhibited by nisoldipine and potentiated by BayK 8644, was considered to be supported by Ca2+ entry into the cells via voltage-dependent Ca2+ channels. Significant differences were observed in the release of ‘H-dopamine induced by 75 vs 25mM K+. On prolonged stimulation, release induced by 75 mM K+ was large and transient, whilst that induced by 25 mM K+, although more moderate, was sustained. The release elicited by 75 mM K+ was inhibited approximately 90% by 1.5 mM Co2+ or 625 nM nisoldipine, while release by 25 mM K+ was completely blocked by 0.6 mM Co*+ or 125 nM nisoldipine. Low PO,-induced release of 3H-dopamine was 95% dependent on Ca*+, and was inhibited by nisoldipine (625 nM) in a manner inversely proportional to the intensity of hypoxic stimulation, i.e. 79% inhibition at a PO, of 49 Torr, and 20% inhibition at PO2 of 0 Torr. BayK 8644 potentiatcd the release induced by moderate hypoxic stimuli. Release elicited by high PCOJlow pH, or by Na+-propionate or dinitrophenol~n~ining solutions, was approximately 80% Ca’+-dependent, and the ~hyd~y~din~ failed to modify this release. It is concluded that type I mlls possess vol~~de~nd~t Ca ‘+ channels sensitive to the dihydropy~dines, which in agreement with previous el~trophysiolo~~l data should be defined as L-type Ca*+ channels. Calcium entry which supports the release of 3H-dopamine elicited by moderate hypoxia should occur mainly through these channels while the release induced by strong hypoxic stimuli will be SetNed by Ca2+ entry which occurs in part via voltage-dependent Ca2+ channels, and in part through an additional pathway, probably a Na+/Ca2+ exchanger. The insensitivity to dihydropyridines of the release of )H-dopamine induced by high 1DC02/low pH, Na+-propionate and dinitrophenol may indicate a complete loss of efficacy of the drugs to modulate Ca 2+ channels under these conditions or more likely, that other mechanisms are activated, probably the Na+-Ca’+ exchanger. Carotid body (CB) chemoreceptors are thought to be composite receptors in which the type I (glomus) cells detect changes in blood PO,, PCO, and pH and respond with the release of neurotransmitt~ to activate the closely apposed chemosensory nerve terminals.~** One such neurotransmitter that has received considerable attention in recent years and is known to be released by the type I cells is dopamine (DA). This biogenic amine has been shown to be released in proportion to both the intensity of stimulation and the resultant sensory discharge recorded from the carotid sinus nerve $To whom correspondence should be addressed. Abbr~~~~~~ CB, carotid body; CSN, carotid sinus nerve; DA, dopamine; DHMA, dihydrox~~delic acid, DOPAC, dihydroxyphenyl acetic acid; NE, norepinephrine. (CSN). This relationship between stimulus