Leakage of the fluorescent Ca2+ indicator fura-2 in smooth muscle

The movement of a fluorescent intracellular Ca2+ indicator, fura-2, in smooth muscle was examined. Strips of rat and rabbit aortas and bovine trachea were loaded with the acetoxymethyl ester of fura-2 (fura-2/AM), followed by washing with normal physiological solution. Not only fura-2/AM but also fura-2 was detected in the washout solution. The amount of fura-2 in the cells, measured fluorometrically, decreased gradually during the washout. The decrease was fastest in rat aorta followed by rabbit aorta > bovine trachea. In rat aorta, fura-2 leakage was inhibited by an inhibitor of anion transport, probenecid, or by a decrease in bath temperature. The Ca2+ ionophore ionomycin (10 microM) increased the leakage of fura-2, which was not inhibited by probenecid, possibly because a high concentration of ionomycin nonselectively increased membrane permeability. These results suggest that fura-2/AM is cleaved to fura-2 in the cell which gradually leaked out of the cell mainly by an anion transport system. The amount of fura-2 in the cell seemed to be determined mainly by the rate of leakage of fura-2, which is the largest in rat aorta followed by rabbit aorta and bovine trachea.