An investigation of the effects of external acidification on sodium transport,internal pH and membrane potential in barnacle muscle fibers

Summary Radiosodium efflux from barnacle muscle fibers is a function of pH_e, the threshold pH_e for stimulation of Na efflux into HCO₃^–-artificial sea water (ASW) being 6.8 and the fixed thresholdpCO₂ (in an open CO₂ system) being approximately 30 mm Hg. Acidification of ASW containing non-HCO₃^– buffer is without effect on the Na efflux. The Na efflux following stimulation by reducing the pH of 10mM HCO₃^–-ASW from 7.8 to 6.3 is reduced by 17.3% as the result of microinjecting 100mM EGTA, and increased by 32.6% as the result of microinjecting 0.5M ATP. The Na efflux into K-free HCO₃^–-ASW is markedly stimulated by external acidification. Ouabain-poisoned fibers are more responsive to a low pH_e than unpoisoned fibers. Applying the 2-¹⁴C-DMO technique, it is found that fibers bathed in 10mM HCO₃^–-ASW at pH 7.8 have an internal pH of 7.09±0.106 (mean±SD), whereas fibers bathed in 25mM TRIS-ASW at pH 7.8 have a pH_i of 7.28±0.112. The relationship between pH_i and pH_e as external pH is varied by adding H⁺ is linear. Measurements of the resting membrane potential indicate that external acidification in the presence of HCO₃^– as buffer is accompanied by a fall inE_m, the threshold pH_e being 7.3 both at 24 and 0°C. This sensitivity amounts to 8.2 mV per pH unit (at 24°C) over a wide range of pH_e. Membrane resistance following external acidification remains unchanged. Microinjection of the protein inhibitor of Walsh before external acidification fails to stop depolarization from occurring. Cooling to 0°C also fails to abolish depolarization following acidification. Whereas external ouabain and ethacrynic acid reduceE_m in the absence or presence of acidification, DPH hyperpolarizes the membrane or arrests depolarization both at 24 and 0°C. This effect of DPH at 0°C is seen in the absence or presence of acidification. It is suggested that depolarization following acidification of a HCO₃^–-containing medium is due to activation of a Cl^–-and/or HCO₃^–-pump and that ouabain and ethacrynic acid reducesE_m by abolishing uncoupled Na transport.