Effects of Palytoxin on Na, K and ATP Contents of Vascular Smooth Muscle of Rabbit Aorta

Abstract

Paiytoxin (PTX) at concentrations higher than 10−9 M increased tissue Na and decreased tissue K contents in the smooth muscle of rabbit aorta. The decrease in the tissue K content induced by PTX (10−8 M) was complete within 1 hr. Saponin (1 mg/ml) and Triton X-100 (0.1% wt./vol.) also rapidly decreased the tissue K content. On the other hand, a high concentration of ouabain (10−3 M) did not change the tissue K content within 1 hr of application, and the maximum loss of K was obtained after 6 hr. Loss of tissue Na into Na- and K-free solution from Na loaded muscle was accelerated by PTX (10−8 M). The PTX-induced increase in loss of Na was inhibited in proportion to the decrease in the temperature from 37 to 10°C, while the loss of Na in the absence of PTX was almost completely inhibited at 24°C. Decrease in the wet weight of the muscle induced by hyperosmotic solution was inhibited by pretreatment with PTX (10−8 M) or saponin (1mg/ml) for 1 hr. PTX (10−9 and 10−8 M) had no effect on the ATP content of the muscle. However, PTX at concentrations above 10−7 M reduced the ATP content, and a significant amount of ATP was detected in the incubation medium. Saponin (1 mg/ml) and Triton X-100 (0.2% wt./vol.) induced a release of Na from liposomes prepared with synthesized lecithin or total lipids of rabbit red blood cells. However, no Na leak was induced by PTX (10−8-10−6 M) in these liposomes. These results suggest that PTX at low concentrations (10−9-10−8 M) increases the membrane permeability of vascular smooth muscle cells to Na and K ions. At higher concentrations (10−7-10−6 M), PTX seems to form pores which are permeable to a larger molecule like ATP. The results further suggest that the mode of action of PTX is different from that of saponin or detergent.