Levels of ATP, phosphocreatine, glycogen and glucose of individual pancreatic islets were measured by quantitative histochemical technics. The islets were obtained from frozen-dried sections and were analyzed with enzymatic fluorometric methods coupled to enzymatic cycling and using an oil well technic. The rate of energy consumption in the islets was estimated by the rate of change in energy stores under complete ischemia. The energy use rate of the pancreatic islet was 38 mmoles of ∼ P per kg. dry tissue per min. which is approximately 50 per cent of the rate in brain. Tolbutamide (5 mg./kg. or more in vivo) resulted in a peak of insulin release one minute after administration. Regardless of the dose employed, ATP and phosphocreatine were unchanged one minute after tolbutamide, when insulin output was maximal or thirty minutes after the drug, when serum insulin levels were falling. In addition, ATP and P-creatine levels were unchanged in islets from the perfused pancreas thirty seconds after eliciting at least a sixfold increase in insulin secretion by perfusing with media containing 50 jug./ml. tolbutamide, whether glucose was present or absent in the medium. Since the readily available energy reserves were not affected by exposure to tolbutamide, the metabolic rate of the islets was either not influenced by the drug or the effect was small or well compensated.

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