The Stimulus-Secretion Coupling of Glucose-Induced Insulin Release LIII. Calcium-Dependency of the Cyclic AMP Response to Nutrient Secretagogues

Abstract

Above a threshold value in excess of 5.6 mM, D-glucose increases the amount of cyclic AMP measured by radioimmunoassay in pancreatic rat islets and their surrounding incubation medium. As judged from the cyclic AMP content of islets exposed to isobutylmethylxanthine (1.0 mM), the glucose-induced increment in the rate of cyclic AMP generation represents a rapid and sustained phenomenon. The stimulant action of glucose on cyclic AMP accumulation is mimicked by L-leucine, and L-glutamine, these amino acids acting synergistically to one another. Trifluoperazine slightly decreases but fails to abolish the effect of glucose. In the absence of extracellular Ca²⁺, however, the cyclic AMP response to D-glucose, L-leucine and/or L-glutamine is severely impaired. These findings are compatible with the view that an increase in the generation rate of cyclic AMP participates in the process of nutrient-stimulated insulin release. This increase could be secondary to the nutrient-induced accumulation of Ca²⁺ in the islet cells leading to activation of adenylate cyclase by calmodulin.