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NEUROREGULATION AND MOTILITY

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

Department of Physiology and Cell Biology, Center of Biomedical Research Excellence, University of Nevada School of Medicine, Reno, Nevada

Submitted 10 May 2006 ; accepted in final form 19 December 2006

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 SNP-induced 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]quinoxaline-1-one (ODQ) and inhibited by cyclopiazonic acid (CPA) or KN-93. SNP also increased PLBSer¹⁶ and PLBThr¹⁷ phosphorylation. Both PLBSer¹⁶ and Thr¹⁷ phosphorylation were ODQ sensitive. However, only PLBThr¹⁷ 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.

Ca²⁺/calmodulin-dependent protein kinase II; phospholamban; nitric oxide

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