Inhibitory effect of CO on internal anal sphincter: heme oxygenase inhibitor inhibits NANC relaxation.

We examined the effect and role of CO in opossum internal anal sphincter (IAS) relaxation in response to nonadrenergic noncholinergic (NANC) nerve stimulation. Effects of NANC nerve stimulation on the IAS tension and second messengers (cAMP and cGMP) were examined before and after the selective heme oxygenase (HO) inhibitor zinc protoporphyrin IX (Zn PP-IX). The HO activity of the IAS smooth muscle was determined before and after NANC nerve stimulation. CO caused a concentration-dependent and tetrodotoxin-resistant fall in the resting tension of the IAS. The direct action of CO was confirmed by its relaxant action on the isolated smooth muscle cells. Furthermore, CO caused an increase in the tissue cGMP levels comparable to that observed with nerve stimulation. Zn PP-IX caused suppression of IAS relaxation caused by NANC nerve stimulation and vasoactive intestinal polypeptide (VIP) but not by peptide histidine-isoleucine and suppression of the increase in cGMP in response to NANC nerve stimulation. Zn PP-IX had no significant effect on the IAS responses to CO, nitric oxide (NO), and the beta-adrenoceptor agonist isoproterenol. The IAS responses to CO were not modified by the NO synthase inhibitor NG-nitro-L-arginine. Significant HO activity was detected in the IAS, which increased further in response to NANC nerve stimulation and VIP. The direct relaxant actions of CO and the suppression of NANC-mediated relaxation of the IAS by the HO inhibitor suggest the involvement of CO in the neurally mediated IAS relaxation.