Differentiation of the active sodium transport system during metamorphosis in Rana catesbeiana skin in relation to cadmium- and amiloride-induced responses.

The differentiation of the sodium active transport system across Rana catesbeiana skin during metamorphosis was investigated and the system was analyzed by the effects of Cd2+ and amiloride. Active transport of Na in the frog skin first appeared at stage XXI of the tadpole, indicated by the appearance of a potential difference (PD) and a short circuit current (SCC) across the skin. The effects of epidermal application of Cd2+ and amiloride on the various indicators of active Na transport were as follows: (1) Cd2+ increased PD and SCC after stage XXII; (2) Cd2+ also increased the skin resistance (RM) from stage B to XXIII, but decreased it after stage XXV; and (3) amiloride decreased PD and SCC but increased RM after stage XXI. The adult frog skin can be expressed as an equivalent circuit by three parameters: ENa, the electromotive force of the active Na current, and RNa and R sigma, corresponding to the resistance to the active Na current and the resistance of the shunt pathway, respectively. These three parameters were calculated from the amiloride effect on PD and SCC. ENa was almost null and RNa was infinite until stage XX. ENa then gradually increased, while RNa was fairly constant between 10 and 20 k omega . cm2 after stage XXI. R sigma gradually increased during metamorphosis. These data suggest that there is not an active Na pathway but a passive one in the early stage of metamorphosis of the tadpole, and that the active Na transport system suddenly appears at stage XXI and gradually develops thereafter.