Ryanodine Inhibits Caffeine‐Evoked Ca2+ Mobilization and Catecholamine Secretion from Cultured Bovine Adrenal Chromaffin Cells

Abstract

The effects of ryanodine, a selective inhibitor of the Ca²⁺‐induced Ca²⁺ release mechanism, on caffeine‐evoked changes in cytosolic Ca²⁺ concentration ([Ca²⁺]_i) and cate‐cholamine secretion were investigated using cultured bovine adrenal chromaffin cells. Caffeine (5–40 mM) caused a concentration‐dependent transient rise in [Ca²⁺]_i and catecholamine secretion in Ca²⁺/Mg²⁺‐free medium containing 0.2 mM EGTA. Ryanodine (5 × 10^–5M) alone had no effect on either [Ca²⁺]_i or catecholamine secretion. Although the application of ryanodine plus caffeine caused the same increase in both [Ca²⁺]_i and catecholamine secretion as those induced by caffeine alone, ryanodine (4 × 10^–7–5 × 10^–5M) irreversibly prevented the increase in both [Ca²⁺]_i and catecholamine secretion resulting from subsequent caffeine application over a range of concentrations. The secretory response to caffeine was markedly enhanced by replacement of Na⁺ with sucrose in Ca²/Mg²⁺‐free medium, and this enhanced response was also blocked by ryanodine. Caffeine was found to decrease the susceptibility of the secretory apparatus to Ca²⁺ in digitonin‐permeabilized cells. These results indicate that caffeine mobilizes Ca²⁺ from intracellular stores, the function of which is irreversibly blocked by ryanodine, resulting in the increase in catecholamine secretion in the bovine adrenal chromaffin cell.