Abstract: AimTo study the effects of protease inhibitors on the large and small intestinal absorption of insulin in rats and to explore the mechanism of various protease inhibitors in different intestinal regions. MethodsThe intestinal absorption of insulin was evaluated by its hypoglycemic effect and serum insulin level using an in situ loop method with the washing treatment. ResultsAdministration of insulin alone did not decrease the glucose level at either intestinal region with or without the washing treatment. With the unwashing treatment, there were no hypoglycemic effects in small intestinal loop when coadministration of insulin with protease inhibitors. With the washing treatment, the biological effects of insulin were amplified a little in small intestinal loop; obvious hypoglycemic effects were found in large intestinal loop with or without the washing treatment. The effectiveness of protease inhibitors was susceptible to their categories, concentrations and activities of proteolytic enzymes in different regions. The efficacy order of various protease inhibitors for enhancing hypoglycemic response of insulin was: leupeptin > sodium glycocholate > bacitracin > bestatin > cystatin; the percutaneous enhancement effects were observed in the presence of either sodium glycocholate or bacitracin. ConclusionCoadministration of protease inhibitors could increase the insulin efficacy more effectively in the large intestine than in the small intestine.