Relationship between insulin secretion and rat islet plasma membrane Ca2+-ATPase during short-term fasting

A simple procedure to measure plasma-membrane Ca2+-ATPase in rat islet homogenate, in a medium containing low Ca2+ and Mg2+ concentrations and in the absence of K+ ions, is described. The Ca2+-ATPase activity detected is inhibited with high affinity by vanadate and compound 48/80 (a specific anticalmodulin drug). The curve of Ca2+ -ATPase activity as a function of Ca2+ concentration shows two components, one with high apparent affinity and low maximum velocity and the other with low apparent affinity and high maximum velocity. The kinetic parameters for these two components are similar to those measured in partially purified islet-membranes. These results suggest that the main Ca2+-ATPase activity detected in these conditions belongs to the islet plasma-membrane. The islets obtained from 48 h fasted rats show a simultaneous and significant decrease both in their insulin response to glucose and in their Ca2+ -ATPase activity. These results suggest that Ca2+-ATPase activity of the islet plasma membrane might participate in the regulatory process of insulin secretion.