Na/K pump current and [Na]_i in rabbit ventricular myocytes: Local [Na]_i depletion and Na buffering

Na/K pump current (I_pump) and intracellular Na concentration ([Na]_i) were measured simultaneously in voltageclamped rabbit ventricular myocytes, under conditions where [Na]_i is controlled mainly by membrane transport. Upon abrupt pump reactivation (after 10-12 min blockade), I_pump decays in two phases. Initially, I_pump declines with little [Na]_i change, whereas the second phase is accompanied by [Na]_i decline. Initial I_pump sag was still present at external [K] = 15 mM, but prevented by [Na]_i ∼ 100 mM. Initial I_pump sag might be explained by subsarcolemmal [Na]_i ([Na]_SL) depletion produced by rapid Na extrusion and I_pump. Brief episodes of pump blockade allowed [Na]_SL repletion, since peak postblockade I_pump exceeded I_pump at the end of previous activation (without appreciably altered global [Na]_i). The apparent K_m for [Na]_i was higher for continuous I_pump activation than peak I_pump (14.1 ± 0.2 vs. 11.2 ± 0.2 mM), whereas that based on d[Na]_i/dt matched peak I_pump (11.6 ± 0.3 mM). [Na]_SL depletion (vs. [Na]_i) could be as high as 3 mM for [Na]_i ∼ 18-20 mM. A simple diffusion model indicates that such [Na]_SL depletion requires a Na diffusion coefficient 10³- to 10⁴-fold below that expected in bulk cytoplasm (although this could be subsarcolemmal only). I_pump integrals and [Na]_i decline were used to estimate intracellular Na buffering, which is slight (1.39 ± 0.09).