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An electrogenic component of resting potential in rabbit ventricular muscle?

The resting potential and the intracellular Na and K concentrations (Nai, Ki) were determined at several extracellular K concentrations (Ko) between 0.5 and 18 mM and after inhibition of the sodium pump with 0.5 microM ouabain. Exposure to low Ko (0.5 mM) produced a transient hyperpolarization (from -80 to -100 mV) followed by a depolarization that led to a stable potential of -60 mV within 25 min. Similar potential levels were observed in the presence of ouabain regardless of the Ko/Ki ratio. Intracellular sodium increased at Ko < 5 mM, whereas Ki rose at Ko less than or equal to 1.0 mM. Because of the large decrease of Ki at Ko = 0.5 mM, Ko/Ki was the same at 0.5 and 1 mM. However, the resting potentials at the steady state differed by 50 mV at these concentrations. A PNa/PK of 0.032 for the control conditions was obtained with the Mullins-Noda equation using 2.5 as the Na-K coupling ratio. This PNa/PK value yielded a Goldman potential of -69 mV; so we estimated that electrogenic sodium extrusion contributed -10 mV to the resting potential. The size of the electrogenic potential increased as Ko was lowered from 5 to 1 mM. This finding suggests that the control of the Na-K coupling ratio may be independent of the mechanism that controls the pumping rate.