Delayed insulin transport across endothelium in insulin-resistant JCR:LA-cp rats

Capillary endothelial cells are thought to limit the transport of insulin across the endothelium, resulting in attenuated insulin action at target sites. Whether endothelial insulin transport is altered in dysglycemic insulin-resistant states is not clear and was therefore investigated in the JCR:LA-cp corpulent male rat, which exhibits the metabolic syndrome of obesity, insulin resistance, hyperlipidemia, and hyperinsulinemia. Lean littermates that did not develop these alterations served as controls. Animals of both groups mere normotensive (mean arterial pressure 136 +/- 2 mmHg). Hearts from obese and lean rats aged 7 (n = 6) or 18 (n = 8) weeks were perfused in vitro at 10 ml/min per gram wet wt over 51 min with Krebs-Henseleit buffer containing 0.1 or 0.5 U human insulin/l (equivalent to 0.6 and 3 nmol/l), Interstitial fluid was collected using a validated method, and interstitial insulin was determined with a radioimmunoassay. At 0.1 U/l, insulin transfer velocity was similar in both experimental groups (half-times of transfer: 11 +/- 0.2 min in obese and 18 +/- 4 min in lean rats; NS), but at 0.5 U/l, the respective half-times were 7 +/- 1 min in lean and 13 +/- 2 min in obese rats (P < 0.05). The steady-state level of insulin in the interstitium was 34 +/- 1% of the vascular level at 0.1 U/l and reached the vascular level (102 +/- 2%) at 0.5 U/l in both lean and obese rats. In rats aged 18 weeks, the half-times of insulin transfer mere 31 +/- 2 and 14 +/- 1 min in obese rats and 10 +/- 0.3 and 7 +/- 0.3 min in lean rats (P < 0.05). Again, interstitial steady-state levels were similar in both groups. Finally postprandial insulin dynamics were simulated over a period of 120 min with a peak concentration of 0.8 U/l in rats aged 27 weeks (n = 4). The maximal interstitial level mas 0.38 +/- 0.02 U/l in lean rats and 0.24 +/- 0.02 U/l in obese rats (P < 0.05), and a similar difference was noted throughout insulin infusion (areas under the transudate concentration-time curves: 17 and 11 U/min per l, respectively). These data show, for the first time in a genetic animal model of insulin resistance, that transfer of insulin across the endothelium is substantially delayed in obese insulin-resistant rats and that it likely contributes to the postprandial alterations of glucose metabolism observed in the metabolic syndrome.