Actions downstream of cyclic GMP/protein kinase G can reverse protein kinase C-mediated phosphorylation of CPI-17 and Ca²⁺ sensitization in smooth muscle.

Ca(2+) sensitivity of smooth muscle contraction is modulated by several systems converging on myosin light chain phosphatase (MLCP). Rho-Rho kinase is considered to inhibit MLCP via phosphorylation, whereas protein kinase C (PKC) induced sensitization has been shown to be dependent on phosphorylation of the inhibitory protein CPI-17. We have explored the interaction of cGMP-dependent protein kinase (PKG) with Ca(2+) sensitization pathways using permeabilized mouse smooth muscle. Three conditions giving approximately 50% of maximal active force were compared in small intestinal preparations: 1). Ca(2+)-activated unsensitized muscle (pCa 5.9 with Rho kinase inhibitor Y27632); 2). Rho-Rho kinase-sensitized muscle (pCa 6.1 with guanosine 5'-3-O-(thio)triphosphate); and 3). PKC-sensitized muscle (pCa 6.0 with Y27632 and PKC activator phorbol 12,13-dibutyrate). 8-Br-cGMP relaxed the sensitized muscles but had marginal effects on unsensitized preparations, showing that PKG reverses both PKC and Rho-mediated Ca(2+) sensitization. CPI-17 was present in permeabilized intestinal tissue. In PKC-sensitized preparations, CPI-17 phosphorylation decreased in response to 8-Br-cGMP. The rate of PKC-mediated phosphorylation in the presence of the MLCP inhibitor microcystin-LR was not influenced by 8-Br-cGMP. PKC-induced Ca(2+) sensitization also was reversed in vascular smooth muscle tissues (portal vein and femoral artery). We conclude that actions downstream of cGMP/PKG can reverse PKC-mediated phosphorylation of CPI-17 and Ca(2+) sensitization in smooth muscle.