BRADYKININ AND ANGIOTENSIN-II - ACTIVATION OF PROTEIN-KINASE-C IN ARTERIAL SMOOTH-MUSCLE

The effects of bradykinin (BK) and angiotensin II (ANG II) were compar ed in cultured rat mesenteric arterial smooth muscle cells. BK and ANG II activated a phosphoinositide-specific phospholipase C, leading to the rapid release of [H-3]inositol phosphates, an increase in intracel lular calcium, and formation of sn-1,2-diacylglycero1 (DAG). DAG forma tion was biphasic with a transient peak at 5 s followed by a sustained increase from 60 to 600 s. The BK-mediated increases in inositol tris phosphate and DAG were dose dependent with half-maximal increases at c oncentrations of 5 and 2 nM, respectively. Both hormones were found to activate protein kinase C (PKC) as assessed by phosphorylation of the 68- to 72-kDa intracellular PKC substrate myristoylated alanine-rich C kinase substrate. However, despite similar phosphorylation of this s ubstrate, only ANG II produced a significant increase in membrane-boun d PKC activity. The mechanism accounting for the inability of BK to in crease membrane-bound PKC activity is unclear. Our studies excluded di fferential translocation of PKC to the nuclear membrane, production of an inhibitor of membrane-bound PKC activity, and expression of BK and ANG II receptors on different cells as the mechanism. Vascular smooth muscle cells were found to express at least four different PKC isozym es: alpha, delta, zeta, and a faint band for epsilon. All of the isozy mes except zeta-PKC were translocated by treatment with the phorbol es ter 4 beta-phorbol 12-myristate 13-acetate. However, neither ANG II no r BK produced significant translocation of any measured isozyme; there fore, we could not exclude the possibility that ANG II and BK activate different isozymes of PKC. Both hormones were found to have a similar small and inconsistent effect in stimulating [H-3]thymidine incorpora tion. These observations demonstrate that BK and ANG II have similar b iochemical effects on vascular smooth muscle cells and imply that, in selected vessels, the vasodilatory effects of BK mediated by the endot helium may be partially counterbalanced by a vasoconstrictor effect on the underlying vascular smooth muscle cells.