Active proton transport stimulated by CO₂/HCO₃⁻, blocked by cyanide

Abstract

EXPOSURE of a cell to increased CO₂ levels causes a drop in the intracellular pH (pH_i) (ref. 1). The time development of this acidification has been monitored in snail giant neurones² and squid giant axons³ by means of glass microelectrodes. It was found that pH_i does not simply fall to a new steady value during CO₂ exposure, but, given sufficient time, will slowly recover; and that, on return to a CO₂-free solution, a higher pH_i value (more alkaline) than the initial one is attained. The slow, secondary alkalinisation observed during exposure to CO₂ cannot be accounted for by any passive mechanism, since the electrochemical gradients for H⁺, OH⁻ and HCO₃⁻ all favour further acidification. For example, during the alkalinisation, the external pH (pH_o) in squid axons is typically 7.7 and pH_i 7.0; hence, the Nernst potentials for H⁺ (E_H), OH⁻, and HCO₃⁻ are all ∼ −40 mV, compared with a membrane potential (Km) of −60 mV. Thus, a proton extruding pump, or its equivalent, must be postulated. We now report that the rate of alkalinisation is at least partially dependent on the CO₂ and/or HCO₃⁻ content of the bathing medium, that it is reversibly blocked by metabolic inhibitors and that an extracellular acidification accompanies the intracellular alkalinisation.