Decrease in beta-cell mass leads to impaired pulsatile insulin secretion, reduced postprandial hepatic insulin clearance, and relative hyperglucagonemia in the minipig

Abstract

Most insulin is secreted in discrete pulses at an interval of approximately 6 min. Increased insulin secretion after meal ingestion is achieved through the mechanism of amplification of the burst mass. Conversely, in type 2 diabetes, insulin secretion is impaired as a consequence of decreased insulin pulse mass. beta-cell mass is reported to be deficient in type 2 diabetes. We tested the hypothesis that decreased beta-cell mass leads to decreased insulin pulse mass. Insulin secretion was examined before and after an approximately 60% decrease in beta-cell mass achieved by a single injection of alloxan in a porcine model. Alloxan injection resulted in stable diabetes (fasting plasma glucose 7.4 +/- 1.1 vs. 4.4 +/- 0.1 mmol/l; P < 0.01) with impaired insulin secretion in the fasting and fed states and during a hyperglycemic clamp (decreased by 54, 80, and 90%, respectively). Deconvolution analysis revealed a selective decrease in insulin pulse mass (by 54, 60, and 90%) with no change in pulse frequency. Rhythm analysis revealed no change in the periodicity of regular oscillations after alloxan administration in the fasting state but was unable to detect stable rhythms reliably after enteric or intravenous glucose stimulation. After alloxan administration, insulin secretion and insulin pulse mass (but not insulin pulse interval) decreased in relation to beta-cell mass. However, the decreased pulse mass (and pulse amplitude delivered to the liver) was associated with a decrease in hepatic insulin clearance, which partially offset the decreased insulin secretion. Despite hyperglycemia, postprandial glucagon concentrations were increased after alloxan administration (103.4 +/- 6.3 vs. 92.2 +/- 2.5 pg/ml; P < 0.01). We conclude that an alloxan-induced selective decrease in beta-cell mass leads to deficient insulin secretion by attenuating insulin pulse mass, and that the latter is associated with decreased hepatic insulin clearance and relative hyperglucagonemia, thereby emulating the pattern of islet dysfunction observed in type 2 diabetes.