Microelectrode study of intracellular pH in frog skin: Dependence on serosal chloride

Replacement of external chloride has been known to reduce Na⁺ transport across whole frog skin. However, the sidedness, and mechanism of the phenomenon have been unclear. In the present study, transepithelial current (I(T)), transepithelial resistance (R(T)), and basolateral membrane potential measured both with reference micropipettes (ψ(sc)) and pH-selective microelectrodes (E(H)(sc)) were monitored in isolated epithelial sheets from frog skin; removal of the underlying dermis facilitates ionic exchange across the basolateral membranes. The intracellular hydronium ion activity (a(H)(c)) was 58 ± 4 nM (means ± SE) when the extracellular hydronium activity was 25 ± 1 nM under base-line conditions. This measurement is equivalent to an intracellular pH (pH(c)) of 7.24 ± 0.03 at an extracellular pH of 7.60 ± 0.01, in reasonable agreement with estimates obtained by ³¹P- and ¹⁹F-nuclear magnetic resonance (NMR) analyses of frog skin. Complete replacement of mucosal Cl^- by gluconate had effects on tissue current and resistance from preparation to preparation. The same ionic substitution on the serosal side uniformly produced a prompt reversible decrease in I(T), increase in R(T), and a substantial membrane depolarization of the short-circuited skins. In most of the preparations, the depolarization was preceded by a small hyperpolarization of 0.5-3.5 mV. The replacement of serosal Cl^- also produced a fall in intracellular hydronium ion activity of 33 ± 10 nM. The present data are consistent with the concept that serosal replacement of Cl^- alkalinizes the cells by either favoring HCO₃^- entry or blocking HCO₃^- exit through a Cl-HCO₃ antiport at the basolateral membrane. The membrane depolarization and fall in short-circuit current may reflect (at least in part) inhibition of apical sodium entry produced indirectly by membrane depolarization, or a primary reduction in rate of Na⁺ translocation by the Na⁺-K⁺ exchange pump.