Major mechanism of action of nitrates in acute attack of angina is:-

Aim for the treatment of classical angina is decrease in work of hea. It can be accomplished by decrease in preload, decrease in afterload or decrease in hea rate. Nitrates act by production of nitric oxide (NO). NO stimulates guanylate cyclase in smooth muscles which leads to formation of cGMP. Later acts on smooth muscles to cause vasodilation. Nitrates mainly produce NO in veins leading to its predominant action as venodilation. Dilation of veins results in decrease in pre-load. Therefore major mechanism of action of nitrates in angina is decrease in preload. However, in variant angina these drugs benefit by causing coronary vasodilation as variant angina occurs due to vasospasm of coronary aery.