Involvement of transport of H+ equivalents in activation-induced alkaline shift in frog muscle.

Previous studies have shown that "activation" of frog skeletal muscle by low concentrations of caffeine (less than 2.5 mM) and K+ (5-20 mM) causes a steady-state alkaline shift and the appearance of H+ in the medium. Both responses are sensitive to blockers of calcium release from sarcoplasmic reticulum. A variety of conditions that inhibit H+-equivalent transport processes in several different tissues were tested for effects on the activation-induced alkaline shift. Blockers of anion transport (4-acetoamido-4'-isothiocyanostilbene-2,2'-disulfonic acid, furosemide, acetazolamide, and Cl- replacement) had no effect on the alkaline shift. Replacement of Na+ by Mg2+, tris(hydroxymethyl)aminomethane, or choline decreased the alkaline shift. Replacement of Na+ by Li+ had no effect. As little as 5 mM external Na+ gave 50% of the alkaline shift. In 15 mM sodium Ringer solution amiloride inhibited the alkaline shift. Lowering extracellular pH (pHe) inhibited the alkaline shift. At pHe less than 7.0 there was a small constant alkaline shift, whereas at pHe greater than 7.0 the shift was dependent on pHe. This pHe sensitivity was independent of changes of intracellular Ca2+ as determined by twitch potentiation or stimulation of Ca2+ flux. It was concluded that the alkaline shift is probably due to activation of a "H+" transport process requiring Na+ and sensitive to external or internal pH.