Relationship between Ca2+ uptake and catecholamine secretion in primary dissociated cultures of adrenal medulla

Carbachol or elevated K+ stimulated 45Ca2+ uptake into chromaffin cells two- to fourfold. The uptake was stimulated by cholinergic drugs with nicotinic activity, but not by those with only muscarinic activity. Ca2+ uptake and catecholamine secretion induced by the mixed nicotinic-muscarinic agonist carbachol were inhibited by the nicotinic antagonist mecamylamine, but not by the muscarinic antagonist atropine. Significant Ca2+ uptake occurred within 15 s of stimulation by carbachol or elevated K+ at a time before catecholamine secretion was readily detected. At later times the time course of secretion induced by carbachol or elevated K+ was similar to that of Ca2+ uptake. There was a close correlation between Ca2+ uptake and catecholamine secretion at various concentrations of Ca2+. The concentration dependencies for inhibition of both processes by Mg2+ or Cd2+ were similar. Ca2+ uptake saturated with increasing Ca2+ concentrations, with an apparent Km for both carbachol-induced and elevated K+-induced Ca2+ uptake of approximately 2 mM. The Ca2+ dependency, however, was different for the two stimuli. The studies provide strong support for the notion that Ca2+ entry and a presumed increase in cytosolic Ca2+ concentration respectively initiates and maintains secretion. They also provide evidence for the existence of saturable, intracellular, Ca2+-dependent processes associated with catecholamine secretion. Ca2+ entry may, in addition, enhance nicotinic receptor desensitization and may cause inactivation of voltage-sensitive Ca2+ channels.