Intracellular calcium and hormone secretion in clonal [formula omitted] anterior pituitary cells

Abstract

Intracellular ionized Ca 2+ concentration was measured in clonal mouse anterior pituitary tumor cells with the fluorescent Ca 2+ indicator Quin-2. In control physiological solution, free cytoplasmic Ca 2+ concentration was found to be 139 ± 11 nM. Replacement of 50 mM NaCl by 50 mM KCl in the extracellular fluid caused a 29 mV depolarization and a 4.2-fold increase in the concentration of free cytoplasmic Ca 2+. Under comparable depolarizing conditions, a specific influx of 2.66 nmole of 45Ca 2+ per mg protein was detected 1 min after addition of high K +, accompanied by a marked increase in the initial rate of β-endorphin secretion. In the absence of external Ca 2+, depolarization by K + produced little or no increase in either intracellular free Ca 2+ or hormone release. Incubation of AtT-20 D16-16 cells in the secretagogue norepinephrine led to a depolarization accompanied by an increase in spontaneous action potential frequency and a marked elevation in cytosolic Ca 2+ concentration. Exposure of cells to somatostatin, an inhibitor of normone release, led to only transient decreases in burst frequency and no significant reduction in intracellular Ca 2+ levels. These results indicate that in addition to intracellular Ca 2+, other factors also control secretory activity in AtT -20 D16-16 anterlor pituitary cells.