The mechanism of contractile action of palytoxin on vascular smooth muscle of guinea-pig aorta.

The mode of action of palytoxin (PTX) on the contractile responses and ion movements in guinea-pig aorta was investigated. PTX (10(-8) M) induced a contraction with a latency period of 0.5-1 min, and it reached maximum after about 20 min. This contraction was not affected by phentolamine (10(-6) M). The contraction induced by PTX was rapidly abolished by removal of external Ca. Readdition of Ca restored the contraction. Verapamil, which markedly antagonized the contraction induced by some depolarizing agents (K, Ba and tetraethylammonium), decreased the rate of rise of the PTX-induced contraction, but did not affect the sustained tension level. Removal of external Na markedly inhibited the contraction induced by PTX (10(-10) - 10(-8) M), while it had no effect on the contraction induced by histamine (10(-7) - 10(-5) M). PTX rapidly decreased tissue K content with a similar time course to that of the increase in tension. Ouabain (10(-4) M) also caused contraction and decreased tissue K content in the muscle, but the rate of these changes was much slower than the PTX-induced ones. PTX increased cellular 45Ca content in normal solution, but not in Na deficient solution. These results suggest that the PTX-induced contraction in guinea-pig aorta is due to an increase in membrane Na permeability.