Palytoxin-induced K+ efflux from ileal longitudinal smooth muscle of the guinea-pig.

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Abstract
In guinea-pig ileal longitudinal smooth muscle, both palytoxin (PTX) and carbachol (CCh) increased K+ efflux with an EC50 of 1.8×10-10 M and 4.1×10-7 M, respectively. Atropine (10-6 M) did not inhibit the K+ efflux due to PTX (3×10-9 M), but completely inhibited the efflux due to CCh (10-5 M). External Ca2+ removal and verapamil (10-5 M) did not change the PTX-induced K+ efflux, although the CCh-induced K+ efflux was inhibited about 77% and 71%, respectively. PTX-induced K+ efflux was reduced to 31% by a depletion of intracellular Ca2+. Tetraethylammonium (15 mM) inhibited the K+ efflux due to PTX or CCh to 61% or 75%, respectively. The PTX-induced K+ efflux was also inhibited by cymarin (3×10-8 M), ouabain (10-5 M) and digitoxin (10-5 M). These results suggest that the PTX-induced K+ efflux is less dependent on Ca2+ influx than that due to CCh. Furthermore, the binding sites for PTX in the ileal muscle of guinea-pig may be Na+, K+-ATPase, as has been suggested in other types of cells.