The intracellular pH of frog skeletal muscle: its regulation in isotonic solutions.

Abstract

The behavior 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 .apprx. 40 min after contracture had subsided. All studies were done at external pH (pHo) of 7.35 and at 22.degree. C. The pHi in normal Ringer solution buffered with HEPES [N-2-hydroxyethyl piperazine-N-2-ethanesulfonic acid] was 7.18 .+-. 0.03 (SE of mean) (n = 10); the membrane potential, Vm, was -88 .+-. 1.8 mV. When pHi was lowered to .apprx. 6.8 by replacing the HEPES by 5% CO2, 24 mM-HCO3 (constant pHo), it recovered at a very slow rate of 0.025 .+-. 0.011 .DELTA.pHi h-1 (n = 6). When all the Na was replaced by N-methyl-D-glucamine (initial pH1 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 .DELTA.pHi h-1. In muscle depolarized in 15 mM-K (Vm .apprx. -50 mV), the rate of recovery from CO2 acidification was not increased above that in normal Ringer solution (2.5 mM-K). When the muscle was depolarized in 50 mM-K to about -20 mV, the rate of recovery increased to 0.33 .+-. 0.07 .DELTA.pHi h-1 (n = 6) when external Cl was kept constant, or to 0.21 .+-. 0.03 (n = 9) when [K] .cntdot. [Cl] product was kept constant. In the absence of Na, pHi recovery rate in 50 mM-K was reduced by at least 90%. Enhanced recovery from CO2-induced acidification was also observed in 2.5 mM-K when the fibers were depolarized to about -20 mV in one of 2 ways: by previous exposure for 60 min to 50 mM-K at constant Cl or by reduction of external Cl to 5.9 mM in the presence of 0.5 mM-Ba. When pHi of depolarized fibers (50 mM-K) was lowered to .apprx. 6.8 by the weak acid dimethyl-2,4-oxazolidinedione (DMO), it recovered at a rate of 0.12 .DELTA.pHi h-1 in 2 experiments. In fibers depolarized in 50 mM-K and constant Cl, either 0.1 mM-SITS [4-acetamido-4''-isothiocyanatostilbene-2,2''-disulfonic acid] or 0.5 mM-amiloride slowed pHi recovery from CO2 exposure by .apprx. 50%. When the depolarization was achieved at constant [K] .cntdot. [Cl] product, amiloride slowed pHi recovery by .apprx. 50%, while SITS had, at most, only a slight effect. 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.