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

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 Ca 2+ concentrations ([Ca 2+ ] i ) and on tension development were measured simultaneously. Both high K + depolarization and histamine increased [Ca 2+ ] i and tension in a concentration‐dependent manner. However, the tension development in relation to the [Ca 2+ ] i increase ([Ca 2+ ] i ‐tension relation) observed with histamine was much greater than that observed with K + depolarization. NG reduced in a concentration‐dependent manner both [Ca 2+ ] 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 [Ca 2+ ] i based on the [Ca 2+ ] i ‐tension relationship observed with K + ‐depolarization. In the absence of extracellular Ca 2+ , NG depleted stored Ca 2+ and also inhibited Ca 2+ 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 [Ca 2+ ] i . We suggest that NG relaxes the coronary artery of the pig by reducing [Ca 2+ ] i and also by directly controlling contractile elements through second messengers not related to changes in [Ca 2+ ] i .