The effects ex vivo and in vitro of insulin and C-peptide on Na/K adenosine triphosphatase activity in red blood cell membranes of type 1 diabetic patients

The decrease in Na/K adenosine triphosphatase (ATPase) activity observed in several tissues of type 1 diabetic patients is thought to play a role in the development of long-term complications. Infusion of insulin may restore this enzyme activity in red blood cells (RBCs), and recent arguments have been developed for a similar role of C-peptide. The aims of this study were to determine whether insulin acts directly on the RBC enzyme and to evaluate the effect of C-peptide on Na/K ATPase activity. Thirty-nine C-peptide—negative type 1 diabetic patients were studied (blood glucose, 11.2 ± 1.49 mmol/L; hemoglobin A1c [HbA1c], 8.9 ± 0.1%, mean ± SEM). Blood samples were obtained in the morning, before breakfast and insulin injection. Intact and living RBCs were resuspended in their own plasma and incubated with or without insulin (50 μU/mL) or C-peptide (6 nmol/L). Ex vivo by microcalorimetry, the heat produced after 1 hour by the enzyme-induced hydrolysis of adenosine triphosphate (ATP), was measured in a thermostated microcalorimeter at 37°C. The results showed that Na/K ATPase activity was significantly increased by insulin (12.4 ± 0.5 v 15.4 ± 0.9 mW/L RBCs, P < .05, n = 23) but not by C-peptide (11.9 ± 0.7 v 12.9 ± 0.9 mW/L RBCs, NS, P = .26, n = 12). In another experiment, RBC suspensions were incubated at 37°C in a water bath with or without insulin (50 μU/mL) or C-peptide (6 nmol/L) for 10 minutes. RBC membranes were isolated and Na/K ATPase activity was assessed by measuring inorganic phosphate release at saturating concentrations of all substrates. The results showed that insulin and C-peptide significantly increased RBC Na/K ATPase activity (342 ± 25, P < .005 and 363 ± 30, P < .005, respectively v 255 ± 22 nmol Pi·mg protein−1 · −1, n = 14). We conclude that insulin and C-peptide act directly on RBC Na/K ATPase, thus restoring this activity in type 1 diabetic patients. The stimulatory effect of C-peptide observed in vitro on RBC Na/K ATPase activity confirms that C-peptide plays a physiological role.