Interaction of Cadmium, Calcium, and Amiloride in the Kinetics of Active Sodium Transport through Frog Skin

Abstract

The epidermal effects of activator (cadmium) or inhibitors (calcium and amiloride) as well as their interactions in active Na trans-port through bullfrog abdorminal skin were analyzed kinetically.Cad-mium at 1 mM increased the short circuit current (SCC) to 126%.The Hill coefficient (n) for the relation between [Cd²⁺] and SCC was O. 737, which indicated negative cooperation.Calcium at over 8 mM decreased the SCC to 80% at its maximum effect.Calcium inhibited the Cd²⁺-induced SCC at lower [Cd²⁺], and facilitated it at higher [Cd²⁺].Calcium acted uncompetitively on the binding reaction of Na⁺with a Na entry channel. Cd²⁺alone or Cd²⁺ under the influence of calcium also acted uncom-petitively on the reaction. Amiloride at 5×10^-5M inhibited the SCC almost completely.The Hill coefficient in the relation between amilo-ride and SCC was 0.757, indicating the presence of negative cooperation. In contrast to the effect of calcium, the Cd²⁺-induced SCC was much reduced at higher [amiloride].Amiloride inhibited the channel-Na binding reaction in a mixed manner (competitive+noncompetitive), while Cd²⁺ activated the reaction under the influence of amiloride noncomoeti-tively. From the SCC model based on the hypothesis that the kinetic constants are invariable even under interaction of the activator and inhibitors, the percentage of the various forms of channel ion complex were calculated. The SCC was then calculated, assuming that it was directly proportional to the sum of the rate of decomposition of chan-nel·Na complex and the higher rate of decomposition of chan-nel·Na·Cdⁿ complex.The above assumption is supported by the fact that there is good agreement between the calculated SCC and experimentally ob-tained SCC in the mixture of the activator and the inhibitor.