PHORBOL ESTER PROMOTES A SUSTAINED DOWN-REGULATION OF ENDOTHELIN RECEPTORS AND CELLULAR-RESPONSES TO ENDOTHELIN IN HUMAN VASCULAR SMOOTH-MUSCLE CELLS

The effect of phorbol ester pretreatment of human vascular smooth muscle cells (hVSMC) was studied with respect to regulation of endothelin (ET)-receptor bindind and cellular responses to ET. The capacity of hVSMC to bind ET was decreased (by ∼50% at maximum) after phorbol exposure, and this reductive effect was both rapid (t 1 2 ∼10 min.) and sustained (for up to 24 hrs. of chronic phorbol exposure). Phorbol pretreatment inhibited both inositol phosphate and diacylclycerol production responses of hVSMC to ET in a manner that was time-dependent and sustained. Phorbol pretreatment also produced a persistent reduction in the ability of ET to release isotopically-labelled arachidonic and/or its metabolites from hVSMC, but importantly ionomycin-stimulated release was similarly negatively affected. Furthermore, ET-induced accumulation of the phospholipase A2/phospholipase B-derived inositol phospholipid metabolite, glycerophosphoinositol, was not different between control and phorbol-treated hVMSC. The mechanism whereby phorbol exerts differential, but notably sustained inhibitory effects on ET-promoted signal transduction pathways are thus complex and illustrative of the selectivity of protein kinase C in regulating cellular responses. © 1990.