Changes of cytoplasmic pH in frog nerve fibers during K +-induced membrane depolarization

Abstract

Frog sciatic nerve and its thin bundles were loaded with fluorescein diacetate in order to monitor changes in cytoplasmic pH (pH i) caused by high K + depolarization. Isosmotic substitution of external Na + by K + at pH o 7.3 led to a steady concentration-dependent (20–120 mM K +) decrease in pH i. Elevation of pH o from 7.3 to 8.5 prevented or even reversed these pH i changes, indicating their strong dependence on transmembrane H + fluxes. The depolarization-induced intracellular acidification could not be prevented or decreased by any of the following treatments: removal of external Ca 2+; application of the Cat 2+ antagonists Ni 2+ and Co 2+; blockade of + channels by TEA; addition to the external solution of Zn 2+, a blocker of putative voltage-sensitive H + channels. By contrast, blockade of Na + channels by 1–3 μM TTX prevented the effect of high K + concentrations on pH i. It is concluded that the decrease in pH i induced by a prolonged membrane depolarization in frog nerve fibers is mainly due to an enhanced H + influx through non-inactivating Na + channels.