Insulin response to a spontaneously ingested standard meal during the development of obesity in GTG-injected mice

(1) To determine glucose and insulin levels in response to ingestion of a standard meal during the development of gold-thioglucose (GTG)-induced obesity. (2) To examine whether the pancreatic beta-cells of GTG-injected mice possess sufficient insulin secretory capacity to compensate for the increasing tissue insulin resistance that occurs with the development of this obesity.

The insulin secretory response to a standard meal of chow was examined in chronically catheterised conscious mice 2, 5 and 10 weeks after induction of obesity by a single injection of GTG.

At 2 weeks after administration of GTG both the basal insulinaemia and the incremental area under the curve (iAUC) of insulin release after a chow meal were increased compared with age-matched lean control mice (2 week control: 1004 +/- 316 min/microU/ml; 2 week GTG: 1968 +/- 300 min/microU/ml; P < 0.05). By 5 weeks, the GTG-injected mice were approximately 42% heavier than their lean controls and showed a marked glucose intolerance. This was accompanied by hyperinsulinaemia in both the basal state and also in response to ingestion of the chow meal as indicated by the increase in the iAUC of insulin (5 week control: 1113 +/- 331 min/microU/ml; 5 week GTG: 2682 +/- 295 min/microU/ml; P < 0.05). At 10 weeks after GTG administration body weight was further increased, as was the degree of glucose intolerance. Plasma insulin levels, in both the basal state and in response to the ingestion of chow, were also further elevated by 10 weeks following GTG injection (10 week control: 1234 +/- 311 min/microU/ml; 10 week GTG: 6640 +/- 1198 min/microU/ml; P < 0.05).

It is apparent that the secretion of insulin in response to a standard chow meal increases progressively with the development of obesity. This finding, in conjunction with an earlier study showing that the insulin secretory response to intravenously administered glucose becomes impaired in the latter stages of the development of obesity in GTG-injected mice [Blair SC, Caterson ID, Cooney GJ. Diabetes 1993; 42: 1153-1158], suggests that the ability of beta-cells of GTG-obese animals to produce and secrete insulin is not impaired but that the beta-cells may become insensitive to glucose within the circulation.