Feedback inhibition of insulin in healthy and overweight subjects studied by use of an artificial endocrine pancreas.

Insulin self-inhibition is still a controversial subject. The majority of data is in favour of an inhibition; however, whether this mechanism is of physiologic relevance in the regulation of insulin secretion is open to discussion. We examined the effect of exogenous insulin on beta cell secretion in 16 volunteers, including 3 who were overweight. Blood glucose (BG) was clamped by means of the dextrose infusion unit of the artificial beta cell, and the secretion of the pancreatic beta cell was monitored by immunomeasureable C-peptide (IMCP) before, during, and after infusion of insulin. The subjects were divided into 3 experimental groups. The inhibition of the basal insulin secretion was examined in group I by clamping BG at the fasting level. The inhibition of the glucose-stimulated insulin secretion was examined in group II by clamping BG at a raised level. The stimulation of insulin secretion by glucose during insulin infusion was examined in group III by stepwise BG rises from the fasting level. An inhibition of the basal insulin secretion was observed in all volunteers examined according to the protocol for group I (n = 9, including 3 overweight volunteers). The lowest insulin infusion rate applied was 1.75 U/h. An inhibition occurred at this low infusion rate corresponding to 44 mu U immunomeasurable insulin (IMI) per ml of serum. However the inhibition was impaired in the overweight participants, who, in addition, were the only ones showing a rebound rise of IMCP after stopping the insulin infusion. An inhibiting effect of exogenous insulin appeared as likely in only 1 of 5 participants examined according to the protocol for group II. Sudden rises of BG abolished the inhibition, while a total or partial inhibition was found at a constantly raised BG level in both participants examined according to protocol III. We conclude that exogenous insulin inhibits beta cell secretion, depending on the BG level, the mode of glucose stimulation, and the time relation between glucose and insulin application. Physiologically occuring IMI levels in peripheral serum were sufficient to cause an inhibition. A disturbance of the negative feedback inhibition of insulin should be discussed as a pathogenetic factor of adipositas.