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 β‐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 μM Ca2+ was significantly reduced in GTN‐treated rabbits. In β‐escin‐skinned smooth muscle, an inhibitor of conventional and/or novel PKCs, GF109203X (0.6 μM), 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 μM Ca2+ alone vs 0.3 μM 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 μM) decreased the 8‐Br‐cGMP‐induced relaxation of the Ca2+ (0.3 μM) 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 μM Ca2++PDBu vs 0.3 μM 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