Effects of pH and ionic strength on the potassium system in the internally perfused giant barnacle muscle fibre

Membrane potentials in single barnacle muscle fibres of Megabalanus psittacus, internally perfused with 200 mM K-acetate (KAc) solution of pH 7.5 were in the range --53 to --60 mV. These were followed as a function of pH of the external chloride saline. Decrease of pH of the Cl-saline from 8.5 to 3.5 hyperpolarized the membrane reversibly by about 8 mV. On further decrease of pH to 3.0, a transient hyperpolarization (from --60 to --65 mV) followed by a sudden and irreversible drop of potential to --40 mV was observed. Replacement of the external Cl-saline by acetate saline in the pH range 8.5 to 6.5 had no effect on the membrane potential. Further decrease of pH to 5.0 brought about an irreversible reduction of membrane potential. For fibres bathing in Cl- or Ac-saline at pH 7.5 when the internal pH was changed to 5.5, a transient hyperpolarization in Cl-saline and a sustained hyperpolarization in Ac-saline were observed. Further studies of membrane potential changing the concentration of external K, keeping the concentration of Cl or Ac constant, and changing the concentration of Cl or Ac, keeping the concentration of K constant, at different internal and external pHs showed that as the pH was reduced the membrane became more permeable to anions, Cl being more permeable than Ac. Experiments in which the membrane potential was controlled showed that when the internal pH of 200 mM KAc solution was reduced from 7.5 to 6.5 or raised to 8.5, the changes in outward K currents at various depolarizations were negligible. However, when the pH was reduced to 5.5 or 5.0, there was a progressive decrease in the outward K currents. The leakage currents in all these cases were relatively small. Use of high ionic strength solution of 580 mM KAc internally protected the K system in that when the pH was lowered the depression of the outward K current was smaller than that observed at a corresponding pH when the internal solution was 200 mM KAc. Use of low ionic strength solution of 50 mM KAc had the opposite effect. The results have been explained in terms of the ion exchange theory by postulating that the membrane has fixed amphoteric groups.