Sodium flux ratio in Na/K pump-channels opened by palytoxin

Palytoxin binds to Na(+)/K(+) pumps in the plasma membrane of animal cells and opens an electrodiffusive cation pathway through the pumps. We investigated properties of the palytoxin-opened channels by recording macroscopic and microscopic currents in cell bodies of neurons from the giant fiber lobe, and by simultaneously measuring net current and (22)Na(+) efflux in voltage-clamped, internally dialyzed giant axons of the squid Loligo pealei. The conductance of single palytoxin-bound "pump-channels" in outside-out patches was approximately 7 pS in symmetrical 500 mM [Na(+)], comparable to findings in other cells. In these high-[Na(+)], K(+)-free solutions, with 5 mM cytoplasmic [ATP], the K(0.5) for palytoxin action was approximately 70 pM. The pump-channels were approximately 40-50 times less permeable to N-methyl-d-glucamine (NMG(+)) than to Na(+). The reversal potential of palytoxin-elicited current under biionic conditions, with the same concentration of a different permeant cation on each side of the membrane, was independent of the concentration of those ions over the range 55-550 mM. In giant axons, the Ussing flux ratio exponent (n') for Na(+) movements through palytoxin-bound pump-channels, over a 100-400 mM range of external [Na(+)] and 0 to -40 mV range of membrane potentials, averaged 1.05 +/- 0.02 (n = 28). These findings are consistent with occupancy of palytoxin-bound Na(+)/K(+) pump-channels either by a single Na(+) ion or by two Na(+) ions as might be anticipated from other work; idiosyncratic constraints are needed if the two Na(+) ions occupy a single-file pore, but not if they occupy side-by-side binding sites, as observed in related structures, and if only one of the sites is readily accessible from both sides of the membrane.