Regulation of Cyclic AMP Levels by Calcium in Bovine Adrenal Medullary Cells

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Abstract
Both nicotine and histamine have been reported to increase cyclic AMP levels in chromaffin cells by Ca(2+)-dependent mechanisms. The present study investigated whether Ca2+ was an adequate and sufficient signal for increasing cyclic AMP in cultured bovine adrenal medullary cells. Depolarization with 50 mM K+ caused a two- to three-fold increase in cellular cyclic AMP levels over 5 min, with no change in extracellular cyclic AMP. This response was abolished by omission of extracellular Ca2+ and by 100 microM methoxyverapamil, and was unaffected by 1 microM tetrodotoxin and by 1 mM isobutylmethylxanthine. Veratridine (40 microM) also increased cellular cyclic AMP levels by two- to fourfold. This response was abolished by either methoxyverapamil or tetrodotoxin. The Ca2+ ionophore A23187 (10-50 microM) had little or no effect on cellular cyclic AMP levels. When the concentration of K+ used to depolarize the cells was reduced to 12-15 mM, the catecholamine release was similar to that induced by 50 microM A23187, and the cyclic AMP response was almost abolished. The results suggest that Ca2+ entry into chromaffin cells is a sufficient stimulus for increasing cellular cyclic AMP production. The possible involvement of a Ca2+/calmodulin-dependent isozyme of adenylate cyclase is discussed.