Involvement of protein kinase C in the first step of vasopressin sensitive phospholipase C homologous desensitization

In WRK1 cells, a rat mammary tumor cell line, arginine vasopressin (AVP) stimulates phosphoinositide (PI) turnover and calcium mobilization. We wxamined the influence of phorbol ester PDBu (a protein kinase C activator) on the AVP-sensitive accumulation of inositol phosphate and thus studied the possible involvement of protein kinase C in the homologous mechanisms of desensitization of vasopressin receptor.
PDBu was shown to inhibit vasopressin-stimulated inositol phosphate accumulation in a dosedependent manner (ED₅₀ around 5nM). High pressure liquid chromatography (HPLC) experiments demonstrated that Ins1P, Ins(1,4)P2, Ins(1,3,4)P3 and Ins(1,4,5)P3 accumulations were reduced. This inhibiroty effect was specific to active phorbol esters like PDBu, since it was not observed with the inactive phorbol ester 4 αPDD. For short period of incubation, PDBu had no effect on the specific binding of [³H]-AVP to its specific receptors and weakly inhibited basal and sodium fluoride-stimulated phospholipase C activities.These results wuggest that protein kinase C activation may alter the coupling between the vasopressin receptor and the G protein which coulpled the hormonal receptor to the phospholipase C. Such mechanisms were probably involved in the homologous desensitization of vasopressin receptor, since staurosporine (a PKC inhibitor) both reversed PDBu inhibitory effects and the first step of vasopressin homologous desensitization.