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1. The effects of ouabain and diphenylhydantoin (DPH) to inhibit and stimulate, respectively, the Na⁺—K⁺ pump were used to correlate transmembrane resting potentials (RP), ionic gradients, and cell pH (DMO method) in rat muscle and liver in vivo.

2. Ouabain effects included a rise in K⁺ and fall in Na⁺ concentration in plasma, a rise in intracellular Na⁺ and Cl^- and a fall in K⁺ concentration and pH_i in muscle, and a rise in intracellular K⁺ concentration in liver.

3. Measured muscle RP was decreased from -90 to -65 mV by ouabain with the RP predictable from the Goldman equation for Na⁺ and K⁺ with P_Na/P_K = 0·01.

4. Measured hepatic RP was increased from -44 to -48 mV by ouabain, whereas the Goldman equation predicts the potential should decrease. A change in permeability of some ion or activation of an electrogenic pump component is necessary to explain this result.

5. DPH produced no significant effect on muscle electrolytes or RP and failed to reverse the effect of ouabain at the time measured and in the doses used.

6. DPH produced a slight rise in hepatic cell K⁺ and a rise from -42 to -47 mV in hepatic RP. This hyperpolarization also cannot be explained without invoking a permeability change or activation of an electrogenic pump.

In all cases intracellular Cl^- in both muscle and liver changed in the direction expected from the change in the RP. Muscle Cl^- appears passively distributed if a constant amount of extra or bound Cl^- is first subtracted from each group. Hepatic intracellular Cl^- is always less than expected on the basis of passive distribution, although errors in determination do not allow elimination of the possibility that Cl^- distribution is determined only by the RP.

8. Cell pH and RP data were used to calculate H⁺ gradients. DPH had no effect on cell pH and only slightly increased the H⁺ gradient in liver. Ouabain produced a slight fall in muscle cell pH but reduced the H⁺ gradient by half. In liver only the H⁺ gradient was increased slightly. The data support the concept of a loose coupling between active H⁺ and Na⁺—K⁺ transport.

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