Abstract

We investigated the mechanisms involved in palytoxin (PTX)-induced cytosolic Ca⁺⁺ ([Ca⁺⁺]_i) mobilization and contraction in porcine coronary arteries using a fluorescent Ca⁺⁺ indicator fura-PE3. PTX (1 pM-10 nM) induced concentration-dependent and sustained increases in [Ca⁺⁺]_i and tension, both of which were partially inhibited by 10 μM verapamil or 1 μM nicardipine. In Ca⁺⁺-free solution containing 1 mM EGTA, PTX did not increase [Ca⁺⁺]_i. In nominally Ca⁺⁺-free solution (no EGTA), however, PTX increased [Ca⁺⁺]_i, which was presumed to be due to release of Ca⁺⁺ from intracellular stores. PTX-induced rise in [Ca⁺⁺]_i was dependent on external Na⁺ because it did not increase [Ca⁺⁺]_i in Na⁺-free solutions containing verapamil. An increase in [Ca⁺⁺]_iin response to 65.4 mM KCl also involved a verapamil-resistant but external Na⁺-dependent component. After blockage of voltage-dependent Ca⁺⁺ channels with verapamil, elevation of external K⁺ to 65.4 mM enhanced the responses of [Ca⁺⁺]_i and tension to PTX. PTX at 10 and 100 pM depolarized the membrane by 4.5 ± 0.8 and 18.6 ± 1.7 mV, respectively. Because PTX is known to increase membrane Na⁺permeability, our results suggest that an increase in cytosolic Na⁺ and the depolarization were primary events required for the PTX-induced Ca⁺⁺ mobilization and that Ca⁺⁺influxes through voltage-dependent Ca⁺⁺ channels and Na⁺-Ca⁺⁺ exchange and Ca⁺⁺ release from Ca⁺⁺ stores, which was triggered by increased Ca⁺⁺ entry, were responsible for the PTX-induced increase in [Ca⁺⁺]_i.