Calcium induced glucagon release in monolayer culture of the endocrine pancreas. Studies with ionophore A23187

Abstract

The possible role of Ca²⁺ in glucagon release has been investigated by the use of ionophore A23187. 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 third day of culture. As expected the effects of the ionophore were dependent upon the presence of Ca²⁺ in the medium. However, 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 μg/ml ionophore, but inhibited in the presence of 3.0 and 10.0 μg/ml. With 0.1 μg/ml ionophore, glucagon release was stimulated by 0.3 and 1.0 mM Ca²⁺ but not by 2.5 mM Ca²⁺. With 10 μg/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. These findings suggest that by increasing Ca²⁺, glucagon is released from the A-cells, whereas too large an increase in Ca²⁺ is 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. Furthermore 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. It is concluded that 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 A-cells of the pancreas.