Front-surface fluorometry with fura-2 and effects of nitroglycerin on cytosolic calcium concentrations and on tension in the coronary artery of the pig

Abstract

1. By use of front-surface fluorometry and fura-2-loaded strips of the coronary artery of the pig, the effects of nitroglycerin (NG) on cytosolic Ca2+ concentrations ([Ca2+](i)) and on tension development were measured simultaneously. 2. Both high K+ depolarization and histamine increased [Ca2+](i) and tension in a concentration-dependent manner. However, the tension development in relation to the [Ca2+](i) increase ([Ca2+](i)-tension relation) observed with histamine was much greater than that observed with K+ depolarization. 3. NG reduced in a concentration-dependent manner both [Ca2+](i) and tension, irrespective of whether the vascular strips were in a resting state or during exposure to high K+ or to histamine stimulation. However, the extent of reduction in tension (relaxation) was greater than that expected from the reduction in [Ca2+](i) based on the [Ca2+](i)-tension relationship observed with K+-depolarization. 4. In the absence of extracellular Ca2+, NG depleted stored Ca2+ and also inhibited Ca2+ release from histamine-sensitive stores, but had no effect on the caffeine-sensitive stores. NG inhibited the caffeine-induced tension development with no change in [Ca2+](i). 5. We suggest that NG relaxes the coronary artery of the pig by reducing [Ca2+](i) and also by directly controlling contractile elements through second messengers not related to changes in [Ca2+](i).