COMPARISON OF THE VASOCONSTRICTOR RESPONSES INDUCED BY ENDOTHELIN AND PHORBOL 12,13-DIBUTYRATE IN BOVINE CEREBRAL-ARTERIES

The vascular effects of endothelin-1 (ET-1) were compared with those elicited by phorbol 12,13-dibutyrate (PDB), an activator of the protein kinase C (PKC), to analyze the involvement of this enzyme on ET-1 responses. PDB and ET-1 caused slow-developing contractions (sustained and transient, respectively), which were reduced by the PKC inhibitor, staurosporine (1 and 10 nM). Only the contractile effects evoked by ET-1 were reduced in Ca-free medium and by the Ca channel antagonist, nifedipine (1 muM), and increased by the Ca channel agonist, BAY K 8644 (10 nM). PDB (10 and 30 nM) preincubation reduced the vasoconstriction elicited by 5-hydroxytryptamine (5-HT; 0.01, 0.1 and 1 muM) in a way dependent on phorbol concentration and preincubation time, whereas ET-1 (1 nM) increased the contractile response to 5-HT (0.1 muM). Furthermore, PDB (0.1 muM) also reduced the responses elicited by ET-1 (30 muM) and vice versa. ET-1 (0.1 muM) induced transient translocation of PKC activity from the cytosol to the membrane, which was less than that produced by PDB (0.1 muM). Electrical stimulation induced [H-3]noradrenaline (NA) release, which was increased by PDB (10 and 100 nM) and not affected by ET-1 (10 nM). These results indicate: (1) the responses induced by PDB and ET-1 were independent and dependent on extracellular Ca, respectively; (2) PKC is involved in NA release and 5-HT responses, but mainly in desensitization of these responses, and (3) PKC is activated by ET-1 and is implicated in vascular actions of ET-1, but other mechanisms, such as the activation of ET-1 receptors and opening of dihydropyridine-sensitive Ca channels also appear to be involved.