Re-evaluation of the 'contralateral' effects of sodium and potassium on sodium transport through toad skin.

1. Changes in the concentration of Na in the outer bathing solution, [Na]o, or of K in the inner bathing solution, [K]i, alter the electrical responses of the isolated toad skin to changes in ionic concentrations in the contralateral solutions. The mechanism(s) of these apparently contralateral effects remain(s) unknown. 2. The phenomenon has been investigated here in the isolated abdominal skin of Xenopus laevis. Each skin was exposed to multiple levels of [Na]o and [K]i, of between 5 and 112 m-mole 1.(-1) The p.d. and short-circuit current (s.c.c.) responses were analysed both in terms of kinetics and in terms of changes in the equivalent electrical circuit of the Na transport mechanism. 3. Kinetic analysis revealed that the relationship between [Na]o and s.c.c., at any level of [K]i, followed Michaelis-Menten kinetics. Increasing levels of [K]i reduced the s.c.c. response to changes in [Na]o, conforming with the algebraic descriptions of 'slope-parabolic competitive inhibition'. High levels of [Na]o (of 60-112 m-mole 1.(-1)) occasionally reduced the s.c.c. in a manner reminiscent of 'substrate inhibition'; this effect was independent of the level of [K]i. At high [K]i and low [Na]o, s.c.c was again often less than that predicted by Michaelis-Menten kinetics. 4. In terms of the equivalent electrical circuit, increasing [Na]o produced a fall in Rseries; in the presence of 'substrate inhibition', however, Rseries rose on increasing [Na]o; in either case, ENa and Rsh remained unchanged. Increasing [K]i lowered both ENa and Rsh; Rseries fell with modest increments in [K]i, but increased at higher levels of [K]i. 5. These results can be interpreted without invoking unknown contralateral effects. Thus the changes in s.c.c., as induced by changes in [Na]o or [K]i, are consistent with homolaterally mediated effects on an enzymic mechanism of transepithelial Na transport; the changes in p.d., given the [K]i-dependent changes in Rsh, are similarly explicable.