CaM kinase II activation and phospholamban phosphorylation by SNP in murine gastric antrum smooth muscles

Elevations in the intracellular Ca2+ concentration activate the serine/ threonine protein kinase Ca2+/calmodulin- dependent protein kinase II ( CaM kinase II). We tested the hypothesis that increased sarco( endo) plasmic reticulum Ca2+- ATPase activity by phospholamban ( PLB) phosphorylation contributes to smooth muscle relaxation by elevating the sarcoplasmic reticulum ( SR) Ca2+ load and increasing the frequency of Ca2+ release events from the SR. We have previously shown that caffeine or sodium nitroprusside ( SNP) relaxes murine gastric fundus smooth muscles and increases PLB phosphorylation by CaM kinase II. These findings suggest that an increased SR Ca2+ load increases the frequency of Ca2+ transients from the SR and results in PLB phosphorylation by CaM kinase II, contributing to caffeine- or SNPinduced relaxation. The aim of the present study was to investigate the effects of SNP on CaM kinase II and PLB phosphorylation in gastric antrum smooth muscles. SNP or 8- bromo- cGMP decreased the basal tone and amplitudes of spontaneous phasic contractions and activated CaM kinase II. SNP- induced relaxation and CaM kinase II activation were blocked by [ 1,2,4] oxadizolo-[ 4,3 alpha] quinoxaline- 1- one ( ODQ) and inhibited by cyclopiazonic acid ( CPA) or KN- 93. SNP also increased PLBSer(16) and PLBThr(17) phosphorylation. Both PLBSer(16) and Thr(17) phosphorylation were ODQ sensitive. However, only PLBThr(17) phosphorylation was inhibited by CPA or KN- 93. These results suggest that CaM kinase II activation and PLB phosphorylation participate in the relaxant effect of SNP on murine gastric antrum smooth muscles through a nitric oxide/ guanylyl cyclase/ cGMP pathway.