A novel synergistic stimulation of Na⁺-transport across frog skin (Xenopus laevis) by external Cd²⁺- and Ca²⁺-ions

Abstract

Isolated skin of the clawed frogXenopus laevis was mounted in an Ussing-chamber. The transcellular sodiumcurrent (I _Na) was identified either as amiloride-blockable (10⁻³ mol/l) short-circuit current (I _SC), or by correctingI _SC for the shunt-current obtained with mucosal Tris. A dose of 10 mmol/l Cd²⁺ applied to the mucosal side increased the current by about 70%. The half-maximal effect was reached at a Cd²⁺-concentration of 2,6 mmol/l (in NaCl-Ringer). The quick and fully reversible effect of Cd²⁺ could not be seen when 10⁻³ mol/l amiloride was placed in the outer, Na⁺-containing solution, nor when Na⁺ was replaced by Tris. This suggests that Cd²⁺ stimulatesI _Na. Cd²⁺ intefered with the Na⁺-current self-inhibition, and therefore with the saturation ofI _Na by increasing the apparent Michaelis constant (K _Na) of this process. The “I _Na recline” after stepping up mucosal [Na⁺] was much reduced in presence of Cd²⁺. Ca²⁺-ions on the mucosal side had an identical effect to Cd²⁺, and 10 mmol/l Ca²⁺ increaseI _Na by about 100%. The half-maximal effect was obtained with 4.4 mmol/l Ca²⁺. The mechanism ofI _Na-stimulation by Ca²⁺ did not seem to differ from that of Cd²⁺. Thus, although of low Na⁺-transport capacity,Xenopus skin appears to be as good a model for Na⁺-transporting epithelia asRanidae skin, with the exception of the calcium effect which, so far, has not been reported forRanidae.