Modulation of smooth muscle contraction by cyclic nucleotides in mice lacking cGMP-dependent-kinase I.

The cGMP-dependent protein kinase (PKG) is the main mediator of nitric oxide-induced relaxation of smooth muscle. Although this pathway is well established, the cellular action of PKG, nitric oxide, and cGMP is complex and not fully understood. A cross-talk between the cGMP-PKG and other pathways (e.g. cAMP-protein kinase A) seems to exist. We have explored cGMP- and cAMP-dependent relaxation of smooth muscle using PKG-deficient mice (cGKI–/–). In intact ileum strips of wild type mice (cGKI+/+), 8-Br-cGMP inhibited the sustained phase of carbachol contractions by ~80%. The initial peak was less inhibited (~30%). This relaxation was associated with a reduction in intracellular [Ca2+] and decreased Ca2+ sensitivity. Contractions of cGKI–/– ileum were not influenced by 8-Br-cGMP. EC50 for 8-Br-cGMP for PKG was estimated to be 10 nM. PKG-independent relaxation by 8-Br-cGMP had an EC50 of 10 µM. Relaxation by cAMP (~50% at 100 µM), Ca2+ sensitivity of force, and force potentiation by GTP{gamma}S were similar in cGKI+/+ and cGKI–/– tissues. The results show that PKG is the main target for cGMP-induced relaxation in intestinal smooth muscle. cGMP desensitize the contractile system to Ca2+ via PKG. PKG-independent pathways are activated at 1000-fold higher cGMP concentrations. Relaxation by cAMP can occur independently of PKG. Long term deficiency of PKG does not lead to an apparent up-regulation of the cAMP-dependent pathways or changes in Ca2+ sensitivity.