Effects of okadaic acid on cytosolic calcium concentration and contraction of porcine coronary artery.

We simultaneously measured cytosolic Ca concentrations ([Ca]i) and tension in fura-2-loaded porcine coronary artery and evaluated the effects of okadaic acid (OA), C44H66O13, a potent phosphatase inhibitor, on [Ca]i and tension. OA over 10(-6) M induced dose-dependent contractions. There were no differences in the time course and the extent of developed tension between the presence and absence of extracellular Ca. Only in the presence of extracellular Ca did OA induce increase in [Ca]i. During contractions induced by cumulative applications of extracellular Ca in high K(+)-depolarizing solution, OA 10(-6) M decreased the maximum tension development during [Ca]i increase, with no effects on the Kd value and Hill coefficient. OA seems to inhibit Ca-dependent contractions by reducing Ca-effectiveness but without affecting Ca-sensitivity. OA-induced contraction are independent of extracellular Ca and [Ca]i.