Effects of sodium nitroprusside on potassium induced contracture in isolated vascular smooth muscle.

The present study investigates the effects of Sodium Nitroprusside (SNP) on isometric developed tension (IDT) and tonic component of K+ induced contracture of rat portal vein. The availability of activator calcium for the contraction was modified by changing the ionic composition of the Krebs Ringer bicarbonate medium (KRB) or by using pharmacological agents such as nifedipine or KCl. In control conditions (KRB media) magnitude of IDT was 10.61 +/- 0.56 mN (n = 13). Addition of increasing concentrations of SNP (non cumulative) diminished IDT in a dose related manner. Tonic component of 60 mM KCl evoked contracture decayed 27.04 +/- 10.9% (n = 6) after 10(-4)M SNP incubation. Relaxant effect of SNP was more pronounced in the presence of nifedipine 10(-10)M (72.35 +/- 7.5% of inhibition, n = 6). The sensitivity of the preparation to SNP was strongly reduced in conditions of diminished Ca+2 efflux (slow sodium media = KRB sucrose). Experiments here presented support the hypothesis that the vasodilator effect of SNP could be attributed to an enhanced Ca+2 efflux from the vessel.