The effect of arginine vasopressin,lysine vasopressin or oxytocin on ADP or arginine vasopressin-induced Ca2+ increase in human platelets

Oxytocin (OT), lysine vasopressin (L⁸VP), arginine vasopressin (A⁸VP) or arginine vasotocin (A⁸VT) are found in plasma from several species and stimulate various cell types by activation of the polyphosphoinositide metabolism and mobilization of intracellular calcium. We therefore studied the effects of the nonapeptides OT, A⁸VT, L⁸VP or A⁸VP on cytoplasmic calcium (Ca²⁺) and the effect of the nonapeptides on A⁸VP-induced Ca²⁺ mobilization. The preincubation of platelets with ‘physiological’ concentrations of A⁸VP (2–4 pmol l^–1) did not enhance the intracellular calcium increase caused by the agonists used. However, the ADP-induced calcium increase was enhanced by prior addition of subthreshold concentrations of A⁸VP (less than 1 nmol l^–1) to the platelet suspension (by 10%, P=0·027, n=12). Neither OT nor A⁸VT in concentrations from 10^–5 μmol l^–1 to 1 μmol l^–1 increased the cytoplasmic calcium concentration. We found that both OT and A⁸VT blocked the effect of subsequent exposure to A⁸VP. ADP (0·4 μmol l^–1) did not block the effect of A⁸VP.