Desensitization of Insulin Secretion by Depolarizing Insulin Secretagogues

Abstract

Prolonged stimulation of insulin secretion by depolarization and Ca²⁺ influx regularly leads to a reversible state of decreased secretory responsiveness to nutrient and nonnutrient stimuli. This state is termed “desensitization.” The onset of desensitization may occur within 1 h of exposure to depolarizing stimuli. Desensitization by exposure to sulfonylureas, imidazolines, or quinine produces a marked cross-desensitization against other ATP-sensitive K⁺ channel (K_ATP channel)-blocking secretagogues. However, desensitized β-cells do not necessarily show changes in K_ATP channel activity or Ca²⁺ handling. Care has to be taken to distinguish desensitization-induced changes in signaling from effects due to the persisting presence of secretagogues. The desensitization by depolarizing secretagogues is mostly accompanied by a reduced content of immunoreactive insulin and a marked reduction of secretory granules in the β-cells. In vitro recovery from a desensitization by the imidazoline efaroxan was nearly complete after 4 h. At this time point the depletion of the granule content was partially reversed. Apparently, recovery from desensitization affects the whole lifespan of a granule from biogenesis to exocytosis. There is, however, no direct relation between the β-cell granule content and the secretory responsiveness. Even though a prolonged exposure of isolated islets to depolarizing secretagogues is often associated with the occurrence of ultrastructural damage to β-cells, we could not find a cogent link between depolarization and Ca²⁺ influx and apoptotic or necrotic β-cell death.