The effect of intracellular acidification on the relationship between cell volume and membrane potential in amphibian skeletal muscle

The relationship between cell volume ( V c ) and membrane potential ( E m ) in Rana temporaria striated muscle fibres was investigated under different conditions of intracellular acidification . Confocal microscope xz ‐scanning monitored the changes in V c , whilst conventional KCl and pH‐sensitive microelectrodes measured E m and intracellular pH (pH i ), respectively. Applications of Ringer solutions with added NH 4 Cl induced rapid reductions in V c that rapidly reversed upon their withdrawal. These could be directly attributed to the related alterations in extracellular tonicity. However: (1) a slower and persistent decrease in V c followed the NH 4 Cl withdrawal, leaving V c up to 10% below its resting value; (2) similar sustained decreases in resting V c were produced by the addition and subsequent withdrawal of extracellular solutions in which NaCl was isosmotically replaced with NH 4 Cl; (3) the same manoeuvres also produced a marked intracellular acidification, that depended upon the duration of the preceding exposure to NH 4 Cl, of up to 0.53 ± 0.10 pH units; and (4) the corresponding reductions in V c similarly increased with this exposure time. These reductions in V c persisted and became more rapid with Cl − deprivation, thus excluding mechanisms involving either direct or indirect actions of pH i upon Cl − ‐dependent membrane transport. However they were abolished by the Na + ,K + ‐ATPase inhibitor ouabain. The E m changes that accompanied the addition and withdrawal of NH 4 + conformed to a Nernst equation modified to include realistic NH 4 + permeability terms, and thus the withdrawal of NH 4 + restored E m to close to control values despite a persistent change in V c . Finally these E m changes persisted and assumed faster kinetics with Cl − deprivation. The relative changes in V c , E m and pH i were compared to predictions from the recent model of Fraser and Huang published in 2004 that related steady‐state values of V c and E m to the mean charge valency ( z x ) of intracellular membrane‐impermeant anions, X − i . By assuming accepted values of intracellular buffering capacity ( β i ), intracellular acidification was shown to produce quantitatively predictable decreases in V c . These findings thus provide experimental evidence that titration of the anionic z x by increased intracellular [H + ] causes cellular volume decrease in the presence of normal Na + ,K + ‐ ATPase activity, with Cl − ‐dependent membrane fluxes only influencing the kinetics of such changes.