Potential utility of various protease inhibitors for improving the intestinal absorption of insulin in rats.

The aim of the investigation was to study the effects of protease inhibitors on the absorption of insulin in-situ from closed small and large intestinal loops in rats and to investigate the mechanism of various protease inhibitors in different intestinal loops. The intestinal absorption of insulin was evaluated by its hypoglycaemic effect and serum insulin level in the presence or absence of luminal contents. No marked hypoglycaemic effect was observed after administration of insulin alone in either region in the presence or absence of luminal contents. A significant hypoglycaemic effect of insulin was obtained in the large intestinal loop in the presence or absence of luminal contents when insulin was co-administered with bacitracin (20, 30 mM), sodium glycocholate (20, 40 mM), bestatin (29 mM), leupeptin (21 mM) and cystatin (0.8 mM). In contrast, there was no hypoglycaemic effect in the small intestinal loop in the presence of luminal contents following small intestinal co-administration of insulin with these protease inhibitors. The effectiveness of protease inhibitors was susceptible to their categories, concentrations and activity of proteolytic enzymes in different regions. The degree of improving insulin absorption in intestine was in the order of leupeptin>sodium glycocholate>bacitracin>bestatin>cystatin. At the same time, the percutaneous enhancement effect was observed in the presence of either sodium glycocholate or bacitracin. These results suggest that protease inhibitors could increase the insulin efficacy more effectively in the large intestine than in the small intestine.