Differential stimulation of Na+ pump activity by insulin and nitric oxide in rabbit aorta

The effect of insulin on Na+ pump activity, measured as ouabain-sensitive (OS) Rb-86 uptake, was studied in the rabbit aorta. In the absence of insulin, incubation of endothelium-intact rings for 3 h in a medium containing a high concentration of glucose (44 mM) decreased OS Rb-86 uptake by 42% compared with that observed at 5.5 mM glucose. Addition of insulin (0.1-10 mU/ml) increased OS Rb-86 uptake at both glucose concentrations and eliminated the differences between the groups. Insulin also increased OS Rb-86 uptake in endothelium-intact and -denuded (ED) rings in the presence of the nitric oxide (NO) synthase inhibitor N-G-monomethyl-L-arginine. Removal of the endothelium before the incubations did not diminish the insulin-induced increase in OS Rb-86 uptake, which was concentration dependent. The NO donor sodium nitroprusside increased OS Rb-86 uptake in ED rings, and its effect and that of insulin were additive. Phorbol 12,13-dibutyrate, a direct activator of protein kinase C (PKC), also increased OS Rb-86 uptake in ED rings; however, its effect and that of insulin were not additive. The PKC inhibitor bisindolylmaleimide totally inhibited insulin-induced, but not sodium nitroprusside-induced, increases in OS Rb-86 uptake. The results suggest that insulin activates the Na+ pump in the aorta and reverses the inhibition of the pump caused by hyperglycemia. This effect of insulin can occur at physiological concentrations, is independent of endothelium-derived NO, and is presumably mediated by an increase in PKC activity. In contrast, activation of the Na+ pump by NO appears to be independent of PKC.