The role of dihydropyridine-sensitive Ca 2+ channels in stimulus-evoked catecholamine release from chemoreceptor cells of the carotid body

Abstract

The present study utilized an in vitro preparation of the rabbit carotid body, with tissue catecholamine stores labeled by incubation with 3H-tyrosine. The goal was to characterize pharmacologically the voltage-dependent Ca 2+ channels present in the type I (glomus) cells of this arterial chemoreceptor organ, and to elucidate their role as pathways for Ca 2+ entry. We found that release of 3H-dopamine induced by high external potassium was over 95% dependent on external calcium concentration and that this release was 90–100% inhibited by the dihydropyridine antagonists, nisoldipine and nitrendipine, and was potentiated by the dihydropyridine agonist, BayK 8644. Therefore, any stimulus-induced, calcium-dependent release of 3H-dopamine that was inhibited by nisoldipine and potentiated by BayK 8644, was considered to be supported by Ca 2+ entry into the cells via voltage-dependent Ca 2+ channels. Significant differences were observed in the release of 3H-dopamine induced by 75 vs 25 mM 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 Co 2+ or 625 nM nisoldipine, while release by 25 mM K + was completely blocked by 0.6 mM Co 2+ or 125 nM nisoldipine. Low PO 2-induced release of 3H-dopamine was 95% dependent on Ca 2+, 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 2 of 49 Torr, and 20% inhibition at PO 2 of 0 Torr. BayK 8644 potentiated the release induced by moderate hypoxic stimuli. Release elicited by high PCO 2/low pH, or by Na +-propionate or dinitrophenol-containing solutions, was approximately 80% Ca 2+-dependent, and the dihydropyridines failed to modify this release. It is concluded that type I cells possess voltage-dependent Ca 2+ channels sensitive to the dihydropyridines, which in agreement with previous electrophysiological data should be defined as L-type Ca 2+ 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 served by Ca 2+ entry which occurs in part via voltage-dependent Ca 2+ channels, and in part through an additional pathway, probably a Na +/Ca 2+ exchanger. The insensitivity to dihydropyridines of the release of 3H-dopamine induced by high PCO 2/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 2+ exchanger.