Pharmacological Study on Mechanism of Arterial Smooth Muscle Relaxation by Organic Nitrates, Nitroso-compound and Nitroprusside

Abstract

The purpose of the present study was to determine the involvement of Ca²⁺ (Mg²⁺)-ATPase, guanylate cyclase, PGI₂ synthesis and/or glutathion in mechanism of vascular smooth muscle relaxation by nitro- and nitroso-vasodilator drugs. Vasodilators used in the experiment were nitroglycerin (GTN), isosorbide dinitrate (ISD), N-nitroso-morpholino aminoacetonitrile (SIN-1A, an active metabolite of molsidomine) and sodium nitroprusside (SN P).
Guinea-pig thoracic aorta strips were suspended in tissue bath which permitt cotinuous recording of isometric tension. Relaxation was measured in submaximally contracted strips with noradrenaline (NA,30 μM) or PGF₂α (5 μM) and dose-relaxation curves were constructed to show the effects or IC₅₀ ovalues of vasodilators.
The cumulative concentrations of GTN from 0.001 to 100.0 μM and ISD from 0.01 to 100.0 μM relaxed the contracted strips in a concentration dependent manner. IC₅₀values in GTN and ISD were 0.26 μM and 4.8 μM, respectively in the NA-contracted preparation. When tranylcypromine (TC), indomethacin (IDM) and/or 15 HPETE, each 10 μM which inhibit the production of PGI₂, were added to the bath 20 min before NA application, the relaxation by GTN or ISD was attenuated to cause a significant shift of dose-relaxation curves to the right and IC₅₀ values obtained from the curves was larger than the control one in each case. On the other hand, TC, IDM and 15 HPETE failed to alter the enhancement by GTN and ISD in Ca²⁺ (Mg²⁺)-ATPase activity in the microsome fraction from guinea pig thoracic aorta smooth muscle.
The tension develope d by NA or PGF₂α was decreased by the cumulative concentrations of SIN-1A from 0.001 to 100.0 μM or SNP 0.001 to 100.0 μM in a concentration dependen t manner. The relaxation was attenuated in the presence of methylene blue (MB) or 6-anilino-5,8-quinolinedione (AQD), which is reported as a guanylate cyclase inhibitor. SIN-1A was found to stimulate the activity of Ca²⁺ (Mg²⁺)-ATPase in the microsome fraction from guinea pig aorta. Inorganic nitroso complex salt SNP in the low concentration exerted a similar effect on Ca²⁺ (Mg²⁺)-ATPase activity to SIN-1A, but not in the high concentration of the drug. MB inhibited the stimulatory effect of SIN-1A in the Ca²⁺ (Mg²⁺) -ATPase activity, while AQD had no effect on the ATPase activity. The addition of SNP to the strips in the high concentration caused a depression of contractions obtained with NA even after removal of SNP from the bath.
This depression was completely reversible upon the addition of glutathion 100 μM or MB 1.0 μM. The effects of SNP on vascular tissue may be, in part, account for the decrease in glutathion content in the smooth muscle of the tissue.
These results suggest that GTN and ISD elicit vasodilation, partially through the stimulation of Ca²⁺ pump ATPase activity and also through the accelerated production of PGI₂ in arterial blood vessel, and that SIN-1A and SNP dilate the blood vessel by the stimulatory effect on the Ca²⁺ pump ATPase as well as guanylate cyclase activity, although the glutathion content in the vascular tissue may decrease after the application of SNP because of a possible interaction between Fe³⁺-prussiate and SH-group of this active peptide.