Sodium Nitroprusside Inhibits Detrusor Muscle Contraction through G-protein Mediated Inhibition of Intracellular Ca2+ Release

Abstract

Background: Nitric oxide (NO) physiology is very complex in detrusor smooth muscle (DSM). While NO relaxes detrusor in some species, in contrary, it produces contraction in other. NO donor sodium nitroprusside (SNP) was found to inhibit CCh-induced contraction in goat DSM in earlier studies, but it is not known how SNP produces inhibitory action on goat DSM. Therefore, the aim of this in vitro study was to investigate the mechanism of inhibitory action of SNP pertaining to involvement of G- protein and other second messenger on goat DSM. Methods: Goat detrusor muscle strips collected from local abattoir were mounted in a thermostatically controlled (37o±0.5oC) organ bath (20 ml capacity) containing physiological solution. Following 1 hr of equilibration, carbachol (CCh) (10-5 M) induced sub-maximal contraction was elicited both in the absence and presence of SNP (10-5 M). Involvement of NO and guanylyl cyclase was assessed using nitric oxide synthase inhibitor L-NAME (10-5 M) and guanylyl cyclase inhibitor ODQ (10-5 M). Experiments were also carried out in the presence of G-protein activator aluminium fluoride (AlF) (10-5 M), protein kinase C (PKC) activator phorbol-12-myristate (PMA) (10-5 M), nifedipine, low Ca2+ PSS, theophylline and Zero Ca2+ PSS. Result: Inhibitory effect of SNP (10-5 M) on CCh-induced contraction on goat DSM was reversed by nitric oxide synthase inhibitor L-name (10-5 M) and guanylyl cyclase inhibitor ODQ (10-5 M). Prior incubation of the tissues with AlF and PMA also reversed the SNP mediated inhibition. On the other hand, nifedipine and low Ca2+ PSS; and theophylline and zero Ca2+ PSS potentiated the SNP-mediated inhibition. Thus, the present study shows that SNP elicited inhibitory effect on goat DSM is both NO-dependent and cGMP-dependent. In addition, the result shows that SNP inhibits G-protein coupled PKC leading to inhibition of both Ca2+ entry and intracellular Ca2+ release with a resultant decrease in intracellular Ca2+.