Differentiation of the Active Sodium Transport System during Metamorphosis in Rana catesbeiana Skin in Relation to Cadmium- and Amiloride-induced Responses

Abstract

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 (R_M) from stage B to XXIII, but decreased it after stage XXV; and (3) amiloride decreased PD and SCC but increased R_M after stage XXI. The adult frog skin can be expressed as an equivalent circuit by three parameters: E_Na, the electromotive force of the active Na current, and R_Na and R_Σ, 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. E_Na was almost null and R_Na was infinite until stage XX. E_Na then gradually increased, while R_Na was fairly constant between 10 and 20KΩ•cm² after stage XXI. R_Σ 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.