Beta cell desensitization to glucose induced by hyperglycemia is augmented by increased calcium

Chronic hyperglycemia has been shown to induce a decrease in beta cell sensitivity to a subsequent glucose challenge. Calcium is a necessary cofactor in the insulin secretory process and glucose elevates cytoplasmic levels. This study was designed to study whether chronic exposure to different extracellular calcium and glucose concentrations would affect the islets' subsequent response to regulatory stimuli. Islets were isolated and cultured in TC 199 plus 10% beta calf serum, glucose (5.5 or 27.5 mM) and calcium (0.5, 2.5 or 4.0 mM) for 48 h. Following culture, the islets were harvested and incubated a second time in the presence of glucose and/or arginine, theophylline, and trifluoperazine (TFP). Some islets were used for insulin content, protein synthesis studies and/or CO2 production from labelled glucose. Islets cultured in a normal glucose environment with low or normal calcium concentration maintained the capacity to respond to a subsequent glucose or arginine challenge. However, islets cultured in a high glucose or high calcium medium failed to respond to a second glucose or arginine stimulus. Theophylline stimulated insulin secretion from both glucose-sensitive and non-sensitive islets, while trifluoperazine inhibited glucose-stimulated insulin secretion in previously sensitive islets and increased insulin secretion in previously non-sensitive islets. The different culture conditions did not alter insulin content, protein synthesis or glucose conversion to labelled CO2. We conclude that chronic exposure to high glucose decreases beta cell responsiveness to glucose and amino acids. Increased extracellular calcium augmented this response. However, the beta cell remained sensitive to theophylline-induced insulin secretion, while TFP paradoxically increased insulin secretion in the glucose-insensitive beta cells. Protein synthesis and glucose oxidation were not affected by culture conditions. Thus we suggest that the glucose-induced desensitization of the beta cells may be due to alterations in the calcium-dependent release mechanism.