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Department of Physiology, Loyola University Chicago, Maywood, Illinois
Correspondence: Address reprint requests to Donald M. Bers, Ph.D., Dept. of Physiology, Loyola University Chicago, 2160 S. First Ave., Maywood, IL 60153. Tel.: 708-216-1018; Fax: 708-216-6308; E-mail: dbers{at}lumc.edu.
Na/K pump current (Ipump) and intracellular Na concentration ([Na]i) were measured simultaneously in voltage-clamped rabbit ventricular myocytes, under conditions where [Na]i is controlled mainly by membrane transport. Upon abrupt pump reactivation (after 10–12 min blockade), Ipump decays in two phases. Initially, Ipump declines with little [Na]i change, whereas the second phase is accompanied by [Na]i decline. Initial Ipump sag was still present at external [K] = 15 mM, but prevented by [Na]i 
100 mM. Initial Ipump sag might be explained by subsarcolemmal [Na]i ([Na]SL) depletion produced by rapid Na extrusion and Ipump. Brief episodes of pump blockade allowed [Na]SL repletion, since peak postblockade Ipump exceeded Ipump at the end of previous activation (without appreciably altered global [Na]i). The apparent Km for [Na]i was higher for continuous Ipump activation than peak Ipump (14.1 ± 0.2 vs. 11.2 ± 0.2 mM), whereas that based on d[Na]i/dt matched peak Ipump (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). Ipump integrals and [Na]i decline were used to estimate intracellular Na buffering, which is slight (1.39 ± 0.09).
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