Inhibition by protein kinase C of the 86Rb+ efflux and vasorelaxation induced by P1075, a K(ATP) channel opener, in rat isolated aorta.

In rat aortic rings, P1075, an opener of ATP-dependent potassium channels (K(ATP) channels), produces relaxation and 86Rb+ efflux from preloaded tissues; the increase in 86Rb+ efflux qualitatively reflects K(ATP) channel opening. In this study we have investigated the effects of protein kinase C modulation on the 86Rb+ efflux stimulating, the vasorelaxant and the binding properties of P1075. Phorbol 12,13-dibutyrate (PDBu), a direct activator of protein kinase C, inhibited the 86Rb+ efflux produced by P1075 with an IC50 value of 20+/-2 nM. Phorbol 12-myristate 13-acetate (PMA), another stimulator of protein kinase C, was 150 times weaker in this respect whereas 4alpha-PDBu, the inactive stereoisomer of PDBu, was ineffective. Staurosporine (300 nM), an inhibitor of protein kinase C, induced a small but significant increase of P1075-induced tracer efflux and partially reversed the inhibitory effect of PDBu on P1075-stimulated tracer efflux. The vasorelaxant effect of P1075 was inhibited only to a moderate degree by PDBu at concentrations which inhibited P1075-induced 86Rb+ efflux to >90%; however, in the presence of PDBu, the relaxation kinetics of P1075 were increasingly slowed. The vasorelaxant effect of P1075 in the presence of PDBu was still sensitive to inhibition by glibenclamide (100 nM), the standard inhibitor of the K(ATP) channel openers. Specific binding of [3H]-P1075 to rat aortic rings was unaffected by PDBu and PMA even in the micromolar concentration range. The data show that stimulation of protein kinase C inhibits the K+ channel opening effect of P1075 in rat aorta and suggest that protein kinase C may exert a weak tonic inhibition on the K(ATP) channels in this vessel under quasiphysiological conditions. At concentrations of PDBu which essentially abolished P1075-induced tracer efflux, the glibenclamide-sensitive vasorelaxant effect of P1075 was slowed down but not prevented; this supports earlier suggestions that K+ channel openers are also able to relax smooth muscle cells by a mechanism independent of K(ATP) channel opening.