EFFECTS OF PKC DOWN-REGULATION ON NOREPINEPHRINE AND PROSTAGLANDIN F-2-ALPHA-INDUCED CONTRACTION IN RAT AORTA

The purpose of this study was to investigate, through phorbol ester-in duced protein kinase C (PKC) downregulation, the role of PKC in the re gulation of alpha-adrenergic agonist- and prostaglandin (PG) F-2 alpha -induced contraction in vascular smooth muscle. In rat aorta, long-ter m phorbol ester exposure (10 mu M phorbol dibutyrate for 17 h), a proc edure that decreased PKC activity by >95%, and maximal phorbol myrista te acetate (PMA)-induced contraction by similar to 75%, decreased tiss ue sensitivity to norepinephrine (NE) and PGF(2 alpha) 2.8- and 4.6-fo ld, respectively, while maximal contraction was not significantly decr eased. In contrast, long-term phorbol ester exposure did not alter tis sue sensitivity to KCl, while maximal KCl contraction was decreased by 40%. Long-term phorbol ester exposure, as well as short-term phorbol ester exposure (1 mu M PMA for 1 h), abolished the initial transient N E contraction elicited in Ca2+-free solution. In contrast, long-term p horbol ester exposure did not alter the plateau NE or PGF(2 alpha) con traction elicited in Ca2+-free solution. Short-term phorbol ester expo sure of PKC-downregulated tissues potentiated the plateau PGF(2 alpha) contraction elicited in Ca2+-free solution, although the magnitude of potentiation was less than that observed in non-PKC downregulated tis sues. These results suggest that PKC that is subject to phorbol ester- induced downregulation 1) serves as a positive modulator of the extrac ellular Ca2+-dependent component of the NE- and PGF(2 alpha)-induced c ontraction; 2) serves as a negative modulator of the component of the NE-induced contraction dependent on intracellular Ca2+ release, i.e., the transient NE contraction elicited in the absence of extracellular Ca2+; and 3) does not modulate the component of the NE- and PGF(2 alph a)-induced contraction independent of both extracellular Ca2+ and intr acellular Ca2+ release, i.e., the plateau contraction to PGF(2 alpha) and NE elicited in the absence of extracellular Ca2+.