Effects of proinsulin C-peptide on nitric oxide, microvascular blood flow and erythrocyte Na+,K+-ATPase activity in diabetes mellitus type I

This study was conducted to evaluate the influence of proinsulin C-peptide on erythrocyte Na+,K+-ATPase and endothelial nitric oxide synthase activities in patients with type I diabetes. In a randomized double-blind study design, ten patients with type I diabetes received intravenous infusions of either human C-peptide or physiological saline on two different occasions. C-peptide was infused at a rate of 3 pmol . min(-1) . kg(-1) for 60 min, and thereafter at 10 pmol . min(-1) . kg(-1) for 60 min. At baseline and after 60 and 120 min, laser Doppler flow (LDF) was measured following acetylcholine iontophoresis or mild thermal stimulation (44 degrees C), and venous blood samples were collected to determine plasma cGMP levels and erythrocyte membrane Na+,K+-ATPase activity. The LDF response to acetylcholine increased during C-peptide infusion and decreased during saline infusion [18.6 +/- 19.2 and -13.2 +/- 9.4 arbitrary units respectively; mean +/- S.E.M.; P < 0.05). No significant change in LDF was observed after thermal stimulation. The baseline plasma concentration of cGMP was 5.5 +/- 0.6 nmol . l(-1); this rose to 6.8 +/- 0.9 nmol . l(-1) during C-peptide infusion (P < 0.05). Erythrocyte Na+,K+-ATPase activity increased from 140 +/- 29 nmol of P-i . h(-1) . mg(-1) in the basal state to 287 +/- 5 nmol of P-i . h(-1) . mg(-1) during C-peptide infusion (P < 0.01). There was a significant linear relationship between plasma C-peptide levels and erythrocyte Na+,K+-ATPase activity during the C-peptide infusion (r = 0.46, P < 0.01). No significant changes in plasma cGMP levels or Na+,K+-ATPase activity were observed during saline infusion. This study demonstrates an effect of human proinsulin C-peptide on microvascular function, which might be mediated by an increase in NO production and an activation of the erythrocyte Na+,K+-ATPase. These mechanisms are compatible with the previous observed microvascular effects of C-peptide in patients with type I diabetes.