Inhibition by protein kinase C of the 86Rb+ efflux and vasorelaxation induced by P1075, a KATP channel opener, in rat isolated aorta

Abstract

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±2nM. Phorbol 12-myristate 13-acetate (PMA), another stimulator of protein kinase C, was 150 times weaker in this respect whereas 4α-PDBu, the inactive stereoisomer of PDBu, was ineffective. Staurosporine (300nM), 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 (100nM), 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.