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

Isolated skin of the clawed frogXenopus laevis was mounted in an Ussing-chamber. The transcellular sodiumcurrent (INa) was identified either as amiloride-blockable (10−3 mol/l) short-circuit current (ISC), or by correctingISC 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+ stimulatesINa. Cd2+ intefered with the Na+-current self-inhibition, and therefore with the saturation ofINa by increasing the apparent Michaelis constant (KNa) of this process. The “INa 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+ increaseINa by about 100%. The half-maximal effect was obtained with 4.4 mmol/l Ca2+. The mechanism ofINa-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.