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1 Physiology & Cell Biology, University of Nevada School of Medicine, Reno, Nevada, United States
* To whom correspondence should be addressed. E-mail: perrinob{at}unr.edu.
Elevations in [Ca2+]i activate the Ser/Thr protein kinase Ca2+/calmodulin-dependent protein kinase II (CaM kinase II). We tested the hypothesis that increased SERCA activity by phospholamban (PLB) phosphorylation contributes to smooth muscle relaxation by elevating the SR Ca2+ load and increasing the frequency of Ca2+ release events from the SR. We have previously shown that caffeine or 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 the amplitudes of spontaneous phasic contractions and activated CaM kinase II. The SNP-induced relaxation and CaM kinase II activation were blocked by ODQ, and inhibited by 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 NO/guanylyl cyclase/cGMP pathway.
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