Effects of caffeine on cytosolic Ca2+ level ([Ca2+]cyt), measured simultaneously with muscle tension using fura-2-Ca2+ fluorescence, were examined in isolated smooth muscle of rat aorta. 2. Caffeine (20 mmol/l) induced a large transient increase in [Ca2+]cyt followed by a plateau which was higher than resting level. However, muscle tension showed a transient increase followed by a decrease to or below the resting level. In Ca2+-free solution, caffeine induced only a transient increase in both [Ca2+]cyt and muscle tension. 3. At low temperature (22 degrees C), high K+ (72.7 mmol/l) induced sustained increase in both [Ca2+]cyt and muscle tension which were smaller than those observed at 37 degrees C. At 22 degrees C, however, caffeine-induced transient changes were greater than those observed at 37 degrees C. 4. Ryanodine (10 mumol/l) inhibited the transient changes due to caffeine but showed little effects on the sustained changes due to high K+. 5. During the sustained increase in [Ca2+]cyt induced by noradrenaline (10 mumol/l) or high K+ (140 mmol/l), addition of caffeine transiently increased [Ca2+]cyt followed by a decrease to a level slightly lower than that before the addition of caffeine. In contrast to this, muscle tension transiently increased and then decreased to or below the resting level. 6. These results suggest that caffeine-induced contraction is due to the release of Ca2+ from cellular store. Caffeine also has an inhibitory effect which is partly attributable to decrease in [Ca2+]cyt and partly to the decrease in the sensitivity to Ca2+ of the contractile elements.
