The Intracellular pH of Frog Skeletal Muscle: Its Regulation in Isotonic Solutions

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1. The behaviour of intracellular pH (pHi) was studied with micro-electrodes in frog semitendinosus muscle which was superfused with Ringer solution and with depolarizing solutions. The electrodes were introduced into the depolarized muscle about 40 min after contracture had subsided. All studies were done at external pH (pH0) of 7:35 and at 22 °C.

2. The pHi in normal Ringer solution buffered with HEPES was 7.18 ± 0.03 (S.E. of mean) (n = 10); the membrane potential, Vm, was –88 ± 1.8 mV. When pHi was lowered to about 6.8 by replacing the HEPES by 5% C02, 24 mM-HCO3 (constant pH0), it recovered at a very slow rate of 0.025 ± 0.011 ΔpHi h-1 (n = 6). When all the Na was replaced by N-methyl-D-glucamine (initial pHi 7.20 ± 0.04, initial Vm –89 ± 1.5 mV, n = 8), this slow alkalinization was converted into a slow acidification at a rate of 0.069 ± 0.024 ΔpHi h-1.

3. In muscle depolarized in 15 mM-K (Vm ~ 50 mV), the rate of recovery from CO2 acidification was not increased above that in normal Ringer solution (2.5 mM-K). When, however, the muscle was depolarized in 50 mM-K to about -20 mV, the rate of recovery increased to 0.33 ± 0.07 ΔpHi h-1 (n = 6) when external Cl was kept constant, or to 0.21 ± 0.03 (n = 9) when [K]. [Cl] product was kept constant. In the absence of Na, pHi recovery rate in 50 mM-K was reduced by at least 90%.

4. Enhanced recovery from C02-induced acidification was also observed in 2.5 mM-K when the fibres were depolarized to about –20 mV in one of two ways: (a) by previous exposure for 60 min to 50 mM-K at constant Cl, or (b) by reduction of external Cl to 5-9 mM in the presence of 0.5 mM-Ba.

5. When pHi of depolarized fibres (50 mM-K) was lowered to about 6.8 by the weak acid dimethyl-2,4-oxazolidinedione (DMO), it recovered at a rate of 0.12 ΔpHi h-1 in two experiments.

6. In fibres depolarized in 50 mM-K and constant Cl, either 0.1 mM-SITS or 0.5 mM-amiloride slowed pHi recovery from CO2 exposure by about 50 %. When the depolarization was achieved at constant [K]. [Cl] product, amiloride slowed pHi recovery by about 50%, while SITS had, at most, only a slight effect.

7. It can be concluded that, in frog muscle, depolarization to about –20 mV accomplished in a number of ways enhances pHi recovery from CO2 acidification. Part of this recovery most probably represents the exchange of external Na for internal H. When depolarization is achieved by 50 mM-K at constant Cl, an Na-dependent Cl-HCO3 exchange may also play a role.