C-peptide stimulates rat renal tubular Na+, K+-ATPase activity in synergism with neuropeptide Y

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Abstract
This study was performed in order to test the hypothesis that the connecting peptide of proinsulin, C-peptide, might in itself possess biological activity. Renal tubular Na+, K+-ATPase, which is a well-established target for many peptide hormones, was chosen as a model. Rat C-peptide (I) was found to stimulate Na+, K+-ATPase activity in single, proximal convoluted tubules dissected from rat kidneys. C-peptide increased the Na+ affinity of the enzyme and all subsequent studies were performed at non-saturating Na+ concentrations. C-peptide stimulation of Na+, K+-ATPase activity occurred in a concentration-dependent manner in the dose range 10−8–10−6 mol/l. The presence of neuropeptide Y, 5×10−9 mol/l, enhanced this effect and stimulation of Na+, K+-ATPase activity then occurred in the C-peptide dose range 10−11–10−8 mol/l. C-peptide stimulation of Na+, K+-ATPase activity was abolished in tubules pretreated with pertussis toxin. It was also abolished in the presence of FK 506, a specific inhibitor of the Ca2+-calmodulin-dependent protein phosphatase 2B. These results indicate that C-peptide stimulates Na+, K+-ATPase activity, probably by activating a receptor coupled to a pertussis toxin-sensitive G-protein with subsequent activation of Ca2+-dependent intracellular signalling pathways.