Tadayoshi Takeuchi1,*, Keisuke Sugimoto1, Hajime Morimoto1, Akikazu Fujita1,2 and Fumiaki Hata1,2
1Department of Veterinary Pharmacology, Graduate School of Agriculture and Life Science and 2Department of Molecular Physiology and Biochemistry, Research Institute for Advanced Science and Technology, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan
*Corresponding author. FAX: +81-722-54-9480, E-mail: takeuchi@vet.osakafu-u.ac.jp
Abstract: We previously suggested that nitric oxide (NO)-mediated relaxation of the rat proximal colon is not associated with change in cyclic GMP content. We further studied the intracellular mechanism of NO-induced relaxation by measuring changes in tension and intracellular Ca2+ concentration ([Ca2+]i), simultaneously. NOR 1, NO donor, relaxed the longitudinal muscle of the rat proximal colon, which was precontracted by carbachol, with a concomitant decrease in [Ca2+]i. ODQ, an inhibitor of soluble guanylate cyclase, partially inhibited the relaxant effect of only higher concentrations of NOR 1, but Rp-8-Br-cGMPS, an inhibitor of cyclic GMP-dependent protein kinase (PKG), did not have any effects on the relaxant effect of NOR 1. When the preparations were transferred to normal solution after the treatment with thapsigargin, an inhibitor of sarcoplasmic reticulum (SR) Ca2+-ATPase, in the absence of Ca2+, contraction with a concomitant increase in [Ca2+]i occurred. NOR 1 did not show significant effects on the tension and [Ca2+]i in thapsigargin-treated preparations. In high K+-precontracted preparations, NOR 1 relaxed the preparations with a slight change in [Ca2+]i. The relaxant effect was significantly inhibited by ODQ and Rp-8-Br-cGMPS. These results suggest that NO induces the relaxation preferentially by acting thapsigargin-sensitive function of SR and in turn decreasing [Ca2+]i, although a cyclic GMP-PKG pathway is suggested under the experimental conditions of a high K+ concentration.
Keywords: Nitric oxide-induced relaxation, NOR 1, Rat proximal colon, Decrease in intracellular Ca2+ concentration, Sarcoplasmic reticulum
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