Intracellular ionic activities in frog skin

Summary

Intracellular Na⁺, K⁺, and Cl⁻ activities (a ⁱ_Na ,a ⁱ_K ,a ⁱ_Cl ) 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 ⁱ_Na =14±3mm,a ⁱ_K =132±10mm anda ⁱ_Cl =18±3mm (sd). The value ofa ⁱ_Cl 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 ⁱ_K ,a ⁱ_Na , anda ⁱ_Cl were essentially unchanged althoughV _o had hyperpolarized by about 30–40 mV. During long-term exposure to amiloridea ⁱ_K anda ⁱ_Cl did not change significantly,V _o depolarized by about 16 mV from the maximal value anda ⁱ_Na 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 ⁱ_Na , 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.