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

Oxytocin (OT), lysine vasopressin ((LVP)-V-8), arginine vasopressin (A(8)VP ) or arginine vasotocin (A(8)VT) are found in plasma from several species a nd stimulate various cell types by activation of the polyphosphoinositide m etabolism and mobilization of intracellular calcium. We therefore studied t he effects of the nonapeptides OT, A(8)VT, (LVP)-V-8 or A(8)VP on cytoplasm ic Calcium (Ca2+) and the effect of the nonapeptides on A(8)VP-induced Ca2 mobilization. The preincubation of platelets with 'physiological' concentr ations of A8VP (2-4 pmol l(-1)) did not enhance. the intracellular calcium increase caused by the:agonists used. However, the ADP-induced calcium incr ease 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 A(8)VT in concentrations from 10(-5) mu mol l(-1) to 1 mu mol l(-1) increased the cytoplasmic calcium concentration. We found tha t both OT and A8VT blocked the effect of subsequent:exposure to A(8)VP. ADP (0.4 mu mol l(-1)) did not block the effect of A(8)VP.