C-peptide stimulates glucose transport in isolated human skeletal muscle independent of insulin receptor and tyrosine kinase activation

We have previously demonstrated that C-peptide stimulates glucose transport in skeletal muscle from non-diabetic subjects in a dose-dependent manner. To further elucidate the mechanism by which C-peptide activates glucose transport, we investigated the influence of human recombinant C-peptide on receptor and post-receptor events involved in the glucose transport process. Human skeletal muscle specimens were obtained from the vastus lateralis by means of an open biopsy procedure. Stimulation of isolated muscle strips from healthy control subjects with supra-physiological concentrations of insulin (6,000 pmol/l) and C-peptide (2,500 pmol/l), did not further augment the twofold increase in the rate of 3-o-methylglucose transport induced by either stimulus alone. C-peptide did not displace I-125-insulin binding from partially purified receptors, nor did it activate receptor tyrosine kinase activity. Tyrosine-labelled I-125-C-peptide did not bind specifically to crude membranes prepared from skeletal muscle, or to any serum protein other than albumin. The beta-adrenergic receptor stimulation with isoproterenol inhibited insulin- but not C-peptide-mediated 3-o-methylglucose transport by 63 +/- 18% (p < 0.01), whereas the cyclic AMP analogue, Bt2cAMP, abolished the insulin- and C-peptide-stimulated 3-o-methylglucose transport. C-peptide (600 pmol/l) increased 3-o-methylglucose transport 1.8 +/- 0.2-fold in skeletal muscle specimens from patients with insulin-dependent diabetes mellitus. In conclusion, C-peptide stimulates glucose transport by a mechanism independent of insulin receptor and tyrosine kinase activation. In contrast to the effect on insulin-stimulated glucose transport, catecholamines do not appear to have a counter regulatory action on C-peptide-mediated glucose transport.