Relationship Between Ca 2+ Uptake and Catecholamine Secretion in Primary Dissociated Cultures of Adrenal Medulla

Abstract

Carbachol or elevated K + stimulated 45 Ca 2+ 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. Ca 2+ 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 Ca 2+ 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 Ca 2+ uptake. There was a close correlation between Ca 2+ uptake and catecholamine secretion at various concentrations of Ca 2+ . The concentration dependencies for inhibition of both processes by Mg 2+ or Cd 2+ were similar. Ca 2+ uptake saturated with increasing Ca 2+ concentrations, with an apparent K m for both carbachol-induced and elevated K + -induced Ca 2+ uptake of approximately 2 m M . The Ca 2+ dependency, however, was different for the two stimuli. The studies provide strong support for the notion that Ca 2+ entry and a presumed increase in cytosolic Ca 2+ concentration respectively initiates and maintains secretion. They also provide evidence for the existence of saturable, intracellular, Ca 2+ - dependent processes associated with catecholamine secretion. Ca 2+ entry may, in addition, enhance nicotinic receptor desensitization and may cause inactivation of voltage-sensitive Ca 2+ channels.