The Effect of NO Donor on Contraction, Cytosolic Ca2+ Level and Ionic Currents in Guinea-pig Ileal Smooth Muscle

Abstract

This study was designed to clarify the mechanism of the inhibitory action of a nitric oxide (NO) donor, 3-morpholino-sydnonimine (SIN-1), on contraction, cytosolic Ca2+ level ([Ca2+]i), and ionic currents in guinea-pig ileum. SIN-1 (0.01~100 uM) inhibited 25 mM KC1- or histamine (10 uM)-induced contraction in a concentration-dependent manner. SIN-1 reduced both the 25 mM KC1- and the histamine-stimulated increases in muscle tension in parallel with decreased [Ca2+]i. Using the patch clamp technique with a holding potential of ᅳ 60 mV, SIN-1 (10 uM) decreased peak Ba currents (IBa) by 30.9 ± 5.4% (n=6) when voltage was stepped from — 60 mV to +10 mV and this effect was blocked by ODQ (1 uM), a soluble guanylyl cyclase inhibitor. Cu/Zn SOD (100 U/ml), the free radical scavenger, had little effect on basal IBa，and SIN-1 (10 uM) inhibited peak IBa by 32.4±5.8% (n=5) in the presence of Cu/Zn SOD. In a cell clamped at a holding-potential of 一40 mV, application of 10 uM histamine induced an inward current. The histamine-induced inward current was markedly and reversibly inhibited by 10 uM SIN-1, and this effect was abolished by ODQ (1 /uM). In addition, SIN-1 markedly increased the depolarization-activated outward K+ currents in the all potential ranges. We concluded that SIN-1 inhibits smooth muscle contraction mainly by decreasing [Ca2+]； resulted from the inhibition of L-type Ca2+ channels and the inhibition of nonselective cation currents and/ot by the activation of K+ currents via a cGMP-dependent pathway.

Key Words: Intestinal smooth muscle relaxation, SIN-1, [Ca2+]i, Ionic currents, Guinea-pig ileum