Synergistic insulin release induced by glucose and carbachol is not associated with an increase in cytoplasmic free calcium levels.

Synergistic insulin release is observed in combination with muscarinic agonist and glucose. In order to define the role of changes in the cytoplasmic free Ca2+ level ([Ca2+]i), the effect of combination with carbachol and glucose on both phosphoinositide breakdown in rat pancreatic islets and [Ca2+]i dynamics in single fura 2-loaded B-cells was investigated. When the islets were prelabeled with myo-[2-3H]inositol, phosphoinositide breakdown was obtained and inositol trisphosphate levels were increased in the presence of carbachol (0.5 mM). Inositol trisphosphate levels induced by carbachol were further increased by co-stimulation with 16.7 mM glucose. [Ca2+]i in single-B-cells was increased by 16.7 mM glucose. A prompt, transient rise in [Ca2+]i was induced by carbachol. However, carbachol failed to augment the [Ca2+]i response in the presence of 16.7 mM glucose. These data suggest that a synergistic release of insulin occurs without an amplification of [Ca2+]i regardless of an increase in inositol trisphosphate.