Role of PKC in the attenuation of the cGMP-mediated relaxation of skinned resistance artery smooth muscle seen in glyceryl-trinitrate-tolerant rabbit.

We examined whether 10 days' in vivo treatment with glyceryl trinitrate (GTN) might reduce cGMP-induced relaxation in the smooth muscle of rabbit mesenteric resistance arteries and, if so, whether protein kinase C (PKC) plays a role in this downregulation. The relaxation responses to GTN and the nitric oxide donor NOC-7 were significantly reduced in endothelium-denuded strips from GTN-treated rabbits. In beta-escin-skinned smooth muscle, the ability of 8-bromoguanosine 3',5' cyclic monophosphate (8-Br-cGMP, a phosphodiesterase-resistant cGMP analogue) to relax the contraction induced by 0.3 microM Ca2+ was significantly reduced in GTN-treated rabbits. In beta-escin-skinned smooth muscle, an inhibitor of conventional and/or novel PKCs, GF109203X (0.6 microM), inhibited the Ca2+ -induced contraction and enhanced the 8-Br-cGMP-induced relaxation. However, since the relaxing ability of 8-Br-cGMP was found to be unchanged by GF109203X when contractions were amplitude-matched (0.2 microM Ca2+ alone vs 0.3 microm Ca2+ + GF109203X), the increase in the 8-Br-cGMP-response seen with GF109203X was probably due to its inhibitory action on the Ca2+ -induced contraction. Furthermore, although the PKC activator phorbol 12,13-dibutyrate (PDBu, 0.1 microM) decreased the 8-Br-cGMP-induced relaxation of the Ca2+ (0.3 microM) contraction, this was probably due to its enhancement of the Ca2+ -induced contraction since no such effect of PDBu was seen when the Ca2+ -induced contractions were amplitude-matched (0.2 microM Ca2+ + PDBu vs 0.3 microM Ca2+ alone). These results suggest that the relaxing response to cGMP is reduced in the smooth muscle of mesenteric resistance arteries in GTN-treated rabbits but that conventional and/or novel PKCs do not play a major role in maintaining this downregulation. British Journal of Pharmacology (2004) 141, 391-398. doi:10.1038/sj.bjp.0705625