Insulin modulates rat intestinal glucose transport: effect of hypoinsulinemia and hyperinsulinemia.

The current studies were designed to evaluate the role of plasma insulin and glucose as regulators of intestinal glucose transport in vivo. Initially, rats received either intravenous glucose infusion or intraperitoneal streptozotocin to induce sustained hyperglycemic hyperinsulinemia or hyperglycemic hypoinsulinemia. Net jejunal uptake rates of glucose were measured in vivo at several perfusate concentrations, and the kinetic constants, corrected for diffusion barrier resistance, were derived. Maximal velocity (Jmax) was increased 1.8-fold by hyperglycemic hyperinsulinemia and 2.6-fold by hyperglycemic hypoinsulinemia compared with controls, whereas Km and passive permeability of glucose were unchanged. The rate of L-proline uptake at saturation conditions was not increased by these experimental interventions. The corrected kinetic constants for jejunal glucose transport in streptozotocin diabetic rats kept normoglycemic and normoinsulinemic with insulin were similar to controls. Finally, induction of normoglycemic hyperinsulinemia by intravenous glucose-insulin infusion increased Jmax 1.8-fold, similar to hyperglycemic hyperinsulinemia, but induction of hyperglycemic normoinsulinemia by intravenous glucose-somatostatin infusion did not change Jmax. In conclusion, changes in plasma insulin but not glucose concentration cause a specific and reversible increase in Jmax of intestinal glucose transport. Hypoinsulinemia is a more potent signal than hyperinsulinemia. The membrane level, microvillus or basolateral, at which insulin induces an increased number of glucose transporters in intestinal epithelial cells remains to be defined.