CALCIUM INDUCED GLUCAGON-RELEASE IN MONOLAYER-CULTURE OF ENDOCRINE PANCREAS - STUDIES WITH IONOPHORE A23187

The possible role of Ca2+ in glucagon release was investigated by the use of ionophore A23187 [2-[(3.beta.,9.alpha.,11.beta.-trimethyl)-8-(2-pyrrolecarboxymethyl)-1,7-dioxaspiro[6,6]undecyl-2.beta.-methyl]-5-methylaminobenzoxazole-4-carboxylic acid]. This ionophore permits Ca2+ entry down a suitable concentration gradient by complexing and releasing Ca2+, thereby acting as a carrier in plasma membranes. Cultured cells obtained by enzymatic digestion of pancreases from newborn rats were studied on the 3rd day of culture. The effects of the ionophore were dependent upon the presence of Ca2+ in the medium. Either stimulation or inhibition of glucagon release resulted when different concentrations of ionophore and Ca2+ were used. With 1.0 mM Ca2+ in the medium, glucagon release was stimulated in the presence of 0.01 and 0.1 .mu.g/ml ionophore but inhibited in the presence of 3.0 and 10.0 .mu.g/ml. With 0.1 .mu.g/ml ionophore, glucagon release was stimulated by 0.3 and 1.0 mM Ca2+ but not by 2.5 mM Ca2+. With 10 .mu.g/ml ionophore glucagon release was stimulated by 0.03, 0.1 and 0.3 mM Ca2+, whereas at 1.0 mM, glucagon release was depressed. By increasing Ca2+, glucagon is probably released from the .alpha.-cells, whereas as too large an increase in Ca2+ is probably inhibitory. The effect to stimulate release was not completely specific for Ca2+ in that while the ionophore did not stimulate release in the presence of either Mg2+ or Sr2+ in the absence of Ca2+, it did stimulate release when Ba2+ was tested. Ba2+ at 0.3 mM was stimulatory even in the absence of ionophore. Glucagon release in the absence of ionophore was also enhanced by addition of 30 mM Ca2+ or by omission of Ca2+ from the medium. Ca2+, which plays an essential role in the stimulus-secretion coupling in several different cell types, may be involved in the stimulation of glucagon release from the .alpha.-cells of the pancreas.