To investigate the effect of hypoglycemia on the regulation of muscle-derived insulin receptor tyrosine kinase activity, four groups of Sprague-Dawley rats were studied: two groups in which either insulin (4 mU/kg.min) or phloridzin (3 mg/kg.min) was infused to acutely reach hypoglycemia (mean, 3.2-3.5 mM); and two control groups in which either saline or phloridzin (3 mg/kg.min) was infused, while maintaining euglycemia. Plasma glucose was maintained constant for 40 min in the hypoglycemic group and for 60 min in the phloridzin-infused euglycemic groups by a variable glucose infusion. Insulin receptors were isolated under conditions designed to preserve their in vivo phosphorylation state, and their tyrosine kinase activity toward poly(Glu-Tyr) was measured in the absence and presence of in vitro exposure to insulin. Insulin infusion resulted in an enhanced in vivo tyrosine kinase activity. Surprising was the finding of a slight increase of the in vivo tyrosine kinase activity in the phloridzin-infused hypoglycemic rats. The in vitro insulin dose-response curves of tyrosine kinase activity showed no significant differences between insulin-infused and control rats. In contrast, there was a marked increase of the insulin-stimulated kinase activity in phloridzin-infused hypoglycemic rats; at 100 nM insulin, tyrosine kinase activity was 1.8-fold more responsive when compared with either insulin-infused rats or control groups. Moreover, in phloridzin-infused hypoglycemic rats, the half-maximal stimulation of tyrosine kinase activity was greater than 10-fold (0.36 +/- 0.01 nM) more sensitive to insulin than both insulin-infused (3.8 +/- 0.03 nM, mean +/- SE) and control groups (4.2 +/- 0.05 and 4.1 +/- 0.04 nM in saline- and phloridzin-infused euglycemic rats, respectively, mean +/- SE). In conclusion, hypoglycemia associated with low plasma insulin concentrations determines a hypersensitization of the intrinsic tyrosine kinase of the insulin receptor.