Mode of activation of salt secretion by C-type natriuretic peptide in the shark rectal gland

We studied the modes of activation of the salt-secreting rectal gland of th e spiny dogfish, Squalus acanthias, by the native cardiac peptide CNP. The stimulatory action of CNP in isolated perfused glands is inhibited by 10 mM procaine, presumably by blocking release of vasoactive intestinal peptide (VIP) from nerves. Procaine reduces the slope of the dose-response curve of human CNP and that of shark CNP teach P < 0.0001). CNP increases short-cir cuit current in cultured rectal gland cells from 4.8 +/- 1.6 to 27.0 +/- 7. 8 mu A/cm(2). it also stimulates the secretion of chloride in isolated perf used glands in the presence of 10 mM procaine from 72 +/- 31 to 652 +/- 173 mu eq . h(-1) . g(-1). These results suggest that CNP has a direct cellula r action not mediated by the neural release of Vip. The residual stimulatio n of perfused glands in the presence of procaine was almost completely inhi bited by staurosporine [10 nM; an inhibitor of protein kinase C (PKC)] from 652 +/- 173 to 237 +/- 61 mu eq . h(-1) . g(-1). Although CNP stimulates g uanylyl cyclase in shark rectal gland, chloride secretion of perfused gland s was not elicited by 8-bromoadenosine-cGMP (8-BrcGMP) alone nor by the act ivator of PKC phorbol ester. The combination of PKC activation and 8-BrcGMP infusion, however, stimulated chloride secretion in perfused glands from 9 4 +/- 30 to 506 +/- 61 . mu eq . h(-1) . g(-1), a level comparable to that observed in glands blocked with procaine. Several parallel pathways appear to be synergistic in activating chloride secretion stimulated by CNP in the rectal gland.