Na/K Pump Current and [Na]i in Rabbit Ventricular Myocytes: Local [Na]i Depletion and Na Buffering

AbstractNa/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 10–12min 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]=15mM, but prevented by [Na]i ∼ 100mM. 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±0.2mM), whereas that based on d[Na]i/dt matched peak Ipump (11.6±0.3mM). [Na]SL depletion (vs. [Na]i) could be as high as 3mM for [Na]i ∼ 18–20mM. 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)