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 °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 Pi·h-1·mg-1 in the basal state to 287±5 nmol of Pi·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.

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