Bicarbonate and fast-twitch muscle: Evidence for a major role in pH regulation

Summary Internal pH (pH_i) was analyzed in rat extensor digitorum longus (Edl) muscle at 30°C with single-barrel liquid ionselective electrodes. Average pH_i in 284 cells was 7.197±0.006. Increases in CO₂ from nominally 0 to 5% produced an acidification from which recovery took place. In different groups of cells, recovery from the 5% CO₂ acidification was significantly inhibited by 100 m 4,4 diisothiocyanatostilbene 2,2 disulfonic acid (DIDS), Cl removal, Na removal and 2mm amiloride. Prepulsing with 20mm NH₄ in the presence of CO₂/HCO₃ typically reduced pH_i to only about neutral, whereas 50mm reduced pH_i to 6.7–6.8. In the nominal absence of CO₂/HCO₃, 20mm NH₄ reduced pH_i to about 6.7 from which recovery took place at about 58% of the rate seen in different cells in the presence of CO₂/HCO₃. In the presence of CO₂/HCO₃, cells prepulsed with 50mm NH₄ had fully recovered to an average pH_i of 7.22±0.04 about 90 min after removal of NH₄. However, 90 min after removal of 20mm NH₄ in the absence of CO₂/HCO₃, average pH_i was significantly less (7.05±0.03). Intrinsic buffering capacity ( _i ) was obtained during pulses of CO₂, acetic acid or after an NH₄ pulse, _i was significantly reduced in the absence of HCO₃, Cl or Na and HCO₃. The data provide significant support for an important role of HCO₃ in the control of pH_i in fast-twitch muscle.