The Eeffect of Sodium Nitroprusside on Muscle Tension in Guinea-pig Ileum. Seong Chun Kwon， Si Yeon Kim， Eun Ju Kim， and Bok Soon Kang Department of Physiology， Yonsei University College of Medicine， Seoul 120 -752， Korea Nitric oxide (NO) has been known as a mediator of nonadrenergic， noncholinergic inhibitory neurotransmitter in intestinal smooth musc1es. It has been suggested that NO donor such as sodium nitroprusside (SNP) produces relaxation of smooth musc1e via activation of guanylate cyc1ase and elevation of cGMP levels. We have therefore investigated the effects of NO， using SNP， on musc1e tension in the longitudinal smooth musc1e of guinea-pig + ileum. The possible role of cGMP was also investigated as well as the involvement of K. channel on SNP-induced inhibitory effect. The resu1ts are summarized as follows; high KCl-or CCh-activated contractions were inhibited by SNP in a concentration-dependent manner. 8-Br‘-cGMP also showed a similar effect in that of SNP. TEA (1 mM) significantly reduced the SNP-induced inhibitory effect. SNP-induced effect was further reduced by the presence of 10 mM TEA. On the other hand， 4-AP (0.1 mM)， glibenc1amide (10 μM) and apamin (0.1 μM) showed little effects on SNP-induced re1axation. Zaprinast significantly potentiated the SNP-induced i띠ubitory effect in all ranges. ODQ also significant1y decreased the SNP-induced inhibitory effect. Pretreatment with CPA (10 μM) slightly reduced the SNP-induced inhibitory effect. From the above resu1ts， both effect mediated by 2+ 十NO and cGMP might be responsible for the activation of Ca