Short-term hormonal control of fat metabolism in the liver [proceedings].

incubated in the presence of an excess of insulin (approx. 3pg of insulin/ml). Glucosestimulated membranes can release approx. 1500ng of insulin/ml before saturation of glucose-generated 8-granule-binding sites takes place at 60min incubation. When glucose 6-phosphate was added no further release of insulin took place despite the availability of 1.5pg of insulin/ml in 8-granules in the supernatant. It was possible that glucose 6-phosphate could not cause release under these circumstances because effector sites for glucose 6-phosphate had been diluted as a result of granule-membrane fusion with plasma membrane. It could also be that the membrane was not capable of responding t o any other secretogogue. To test these propositions, IOmM-tolbutamide, a known insulin-releasing agent, was added to a membrane/granule incubat ion that had been similarly exposed t o glucose. The system responded t o tolbutamide. Furthermore the amount of insulin released was equal to that found when membranes were treated with tolbutamide in the absence of glucose, i.e. approx. 0.5pg of insulin/ml. Glucose and glucose 6-phosphate utilize different pathways in the generation of p-granule-binding sites on the plasma membrane. The fact that the release system in vitro becomes refractory t o glucose 6-phosphate, but not t o another secretogogue, tolbutamide, when the glucose-generated 8-granule-binding sites are saturated, indicates that glucose and glucose 6-phosphate probably share a common final path. Tolbutamide must release insulin via a glucose-independent pathway.