Intracellular ionic activities in frog skin

Abstract

Intracellular Na⁺, K⁺, and Cl⁻ activities (a _Na ⁱ ,a _K ⁱ ,a _Cl ⁱ ) and transapical membrane potentials (V_o) were measured with liquid ion-exchanger and open-tip microelectrodes in isolated short-circuited frog skins (R. pipiens) incubated at 23°C in normal amphibian Ringer's solution. Under control conditionsa _Na ⁱ =14±3mm,a _K ⁱ =132±10mm anda _Cl ⁱ =18±3mm (sd). The value ofa _Cl ⁱ is 4.4 times the value corresponding to electrochemical equilibrium for this ion. Thus, Cl⁻ is actively accumulated by epithelial cells of the frog skin. Shortly after addition of amiloride (2–5 μm) to the apical bathing medium,a _K ⁱ ,a _Na ⁱ , anda _Cl ⁱ were essentially unchanged althoughV _o had hyperpolarized by about 30–40 mV. During long-term exposure to amiloridea _K ⁱ anda _Cl ⁱ did not change significantly,V _o depolarized by about 16 mV from the maximal value anda _Na ⁱ decreased to 8±3mm. Immediately after exposure to amiloride the transmembrane driving force for Na⁺ increased from 124 to 154 mV. During further exposure to amiloride, despite changes in bothV _o anda _Na ⁱ , this driving force remained virtually constant. SinceI_sc during this period was close to zero, it is suggested that the observed driving force for Na⁺ under these conditions approximates the maximal driving force generated by the Na⁺−K⁺ ATP-ase pump in the basolateral cell membrane.