Glucose metabolism and pulsatile insulin release from isolated islets

The effects of metabolic inhibition on insulin release and the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) were studied in individually perifused pancreatic islets from ob/ob mice. The modest basal secretion in the presence of 3 mmol/l glucose was pulsatile with a frequency of approximately 0.2/min, although [Ca(2+)](i) was stable at approximately 100 nmol/l. Introduction of 11 mmol/l glucose resulted in large amplitude oscillations of [Ca(2+)](i) and almost 20-fold stimulation of average secretion manifested as increased amplitude of the insulin pulses without change in frequency. Inhibition of glycolysis with iodoacetamide or mitochondrial metabolism with dinitrophenol or antimycin A reduced glucose-stimulated secretion back to basal levels with maintained pulsatility. The [Ca(2+)](i) responses to the metabolic inhibitors were more complex, but in general there was an initial peak and eventually sustained elevation without oscillations. When introduced in the presence of 3 mmol/l glucose, the metabolic inhibitors tended to increase the amplitude of the insulin pulses, although the simultaneous elevation in [Ca(2+)](i) occurred without oscillations. The data indicate that pulsatile secretion is regulated by factors other than [Ca(2+)](i) under basal conditions and after metabolic inhibition. Although pulsatile secretion can be driven by oscillations in metabolism when [Ca(2+)](i) is stable, it was not possible from the present data to determine whether insulin pulses have a glycolytic or mitochondrial origin.