PHORBOL DIBUTYRATE INDUCES CONTRACTIONS IN BOVINE CEREBRAL-ARTERIES BY AN EXTRACELLULAR CALCIUM-INDEPENDENT MECHANISM

The aim of the present study was to analyse the ability of phorbol 12, 13-dibutyrate (PDB) to activate protein kinase C (PKC), measured by it s capacity to translocate the enzyme from the cytosol to the membrane fraction, as well as to induce vasconstrictive responses in segments f rom branches of bovine cerebral arteries. PDB (0.1 muM) produced a mar ked translocation of PKC activity from the cytosolic to the membranous fraction. This drug induced concentration-dependent contractions whic h were slow in onset. The contraction elicited by PDB was reduced by t he PKC inhibitor, staurosporine (1 and 10 nm), but unaltered by both C a2+-free medium containing 3 mm EGTA and the Ca2+-channel antagonist, nifedipine (1 muM). Preincubation of segments with PDB (10 and 30 nm) reduced the vasoconstriction elicited by 5-hydroxytryptamine (5-HT) in a concentration- and preincubation time-dependent manner. These data indicate that bovine cerebral arteries possess cytosolic and membranou s PKC activities, that the vasoconstrictive responses induced by PDB w ere independent of extracellular Ca2+, that cytosolic C-kinase is tran slocated to the membrane and probably down-regulated by PDB, and that this enzyme is not involved in 5-HT responses, but is down-regulated b y PDB.