Palytoxin induces an increase in the cation conductance of red cells

Abstract

Palytoxin (PTX), isolated from the marine soft coral Palythoa tuberculosa, increases the cation conductance of human red cell membranes. In the presence of 10-10 M PTX and 10-5 M DIDS, the membrane potential approximates the equilibrium potential for Na+ or K+ rather than Cl-. Even in the absence of DIDS, the Na+ and K+ conductances were greater than the Cl- conductance. The selectivity of the PTX-induced cation conductance is K+ > Rb+ > Cs+ > Na+ > Li+ ≫ choline+ > TEA+ ≫ Mg2t. Measurements of K+ efflux revealed two apparent sites for activation by PTX, one with a Ka1 of 0.05 nM and a maximum flux, vmax1, of 1.4 mol/liter of cells per h and another with a Ka2 of 98 nM and a vmax2 of 24 mol/liter of cells per h. These effects of PTX are completely blocked by external ouabain (300 μM) and prevented by internal vanadate (100 μM). When the PTX channels are open, the Na,K pumps do not catalyze ATP hydrolysis. Upon thorough washout of cells exposed to about five molecules of PTX/pump, the Na,K pump of these cells operates normally. Blockage of the positively charged NH2 terminus of PTX with a p-bromobenzoyl group reduces the potency of the compound to induce Na and K fluxes by at least a factor of 100, and to compete with the binding of [3H]ouabain by at least a factor of 10. These data are consistent with the conclusion that PTX binds reversibly to the Na,K pumps in the red cell membrane and opens a (10-pS) channel equally permeable to Na and K at or near each pump site.