Aims/hypothesis  

Proinsulin C-peptide has been implicated in reducing complications associated with diabetes and also in improving blood flow. We hypothesised that incubation of erythrocytes with C-peptide would improve the ability of these cells to release ATP, a stimulus for nitric oxide production.

Methods  

Erythrocytes obtained from rabbits (n = 11) and both healthy and type 2 diabetic humans (n = 7) were incubated with C-peptide in the absence and presence of Fe²⁺ and Cr³⁺, and the resulting ATP release was measured via chemiluminescence. This release was also measured in the presence and absence of phloretin, an inhibitor of GLUT1, and also of mannose, a glycolysis inhibitor. To determine glucose transport, ¹⁴C-labelled glucose was added to erythrocytes in the presence and absence of the C-peptide–metal complex and the aforementioned inhibitors.

Results  

The release of ATP from the erythrocytes of patients with diabetes increased from 64 ± 13 to 260 ± 39 nmol/l upon incubation of the cells in C-peptide. The C-peptide activity was dependent upon binding to Fe²⁺, which was extended upon binding to Cr³⁺. The increase in ATP release from the erythrocytes is due to metal-activated C-peptide stimulation of glucose transfer into the erythrocytes via the GLUT1 transporter. In the presence of C-peptide complexed to Cr³⁺, the amount of glucose transferred into the erythrocyte increased by 31%.

Conclusions/interpretation  

When complexed to Fe²⁺ or Cr³⁺, C-peptide has the ability to promote ATP release from erythrocytes. This release is due to an increase in glucose transport through GLUT1.