Glucose Stimulus Insulin Secretion Coupling in the Pancreatic Islets of Lean and Obese Mice

Abstract

The mechanism by which the B-cell recognises glucose as a stimulus for the release of insulin has been studied using pancreatic islets isolated by collagenase digestion from lean and obese hyperglycaemic mice. 2-desoxy-D-altromethylose (digitoxose) specifically and competitively inhibited glucose-stimulated insulin secretion from lean and obese mouse islets without affecting the rates of glucose oxidation or oxygen consumption. Obese mouse islets were marginally more resistant to the inhibitory effect of digitoxose than lean mouse islets. Digitoxose thus provided a means for the dissociation of metabolic and secretory roles of glucose. A novel perifusion system involving a vibrating reed electrometer and ionization chamber was designed for the simultaneous measurement of the rates of glucose oxidation and insulin release by lean and obese mouse islets. Perifused lean mouse islets responded rapidly to increasing glucose concentrations in terms of insulin release and glucose oxidation rates. However, insulin release was sluggish from obese mouse islets, whilst glucose oxidation remained unimpaired. This reduced secretory response to glucose in obese mouse islets suggests a glucoreceptor abnormality. Digitoxose had no effect on glucose oxidation but simultaneously inhibited insulin release. Mannoheptulose, on the other hand, inhibited both glucose oxidation and insulin release. These observations confirm that glucose metabolism per se is not a necessary prerequisite for the initiation of release but is essential for sustaining the secretory process. The interaction between Ca²⁺-calmodulin and cAMP in the process of insulin release was studied using lean mouse islets. When cAMP levels were raised by forskolin or theophylline, the calmodulin inhibitors trifluoperazine and MB17108 were found to have no significant effect on glucose-stimulated insulin release. This suggested that high intracellular cAMP levels were capable of stimulating insulin release independently of any involvement by Ca²⁺-calmodulin.