The mode of contractile action of palytoxin on vascular smooth muscle

Experiments were designed to assess the mode of action of palytoxin (PTX), isolated from Palythoa tuberculosa, on mechanically denervated rabbit aortic strips. PTX induced a sustained contraction in the muscle dose dependently. The contraction was irreversible. In the depolarized aorta, PTX did not induce a contraction whereas norepinephrine (NE) did. Removal of calcium from the bathing medium prevented PTX and high K+ contractions, whereas phasic responses were elicited by NE. D600 also inhibited the contraction induced by PTX or high K+ but had a lesser effect on that induced by NE. Sodium nitroprusside inhibited only the effect of NE. PTX increased dose dependently the 45Ca uptake of a fraction not removable by La3+ treatment and the increase was inhibited by D600. High K+ also increased the 45Ca uptake although NE did not. It is suggested that PTX increased Ca2+ influx into the smooth muscle cell to cause a contraction, which may be analogous to the action of high K+.