Intracellular Na+ Spatially Controls Ca2+ Signaling during Acidosis in the Ventricular Myocyte

Abstract

Myocardial intracellular pH (pHi) exerts major effects on cardiac function. Decreased pHi can depress Ca2+-transients (CaTs) via effects on SR CaATPase and ryanodine-receptors. The inhibition is typically offset by low pHi stimulation of sarcolemmal Na/H exchange (NHE). This raises [Na+]i sufficiently to increase SR Ca2+-loading via t-tubular Na/Ca exchange, thus increasing CaT-amplitude, which helps to protect contraction. We find that NHE activity (rat ventricular myocytes) also affects Ca2+-signalling spatially. A longitudinal pHi non-uniformity can be induced by microperfusion of the weak acid, acetate (80mM), over one end of the cell. pHi falls in the acetate-exposed region, resulting in a stable pHi-gradient of ∼0.6 units. With NHE inhibited (30μM cariporide), this procedure persistently reduced CaT amplitude, but only in the acidic region. With functional NHE (no cariporide), the same stably-imposed pHi-gradient increased CaT-amplitude uniformly throughout the myocyte. This occurred despite NHE being stimulated only at the acidic end of the cell. One explanation is that local Na+-influx during regional NHE-stimulation uniformly elevates [Na+]i, thus increasing SR Ca2+-loading in both acidic and non-acidic regions. This requires that cytoplasmic Na+ be readily diffusible, as NHE protein is mainly expressed at surface sarcolemma and intercalated-disks (not in transverse-tubules). We therefore imaged [Na+]i (AM-loaded SBFI) during localised NHE-stimulation (local acetate microperfusion, rest of cell exposed to Na-free solution; 10−6 M strophanthidin throughout to inhibit Na/K-transporters). This induced a local NHE-mediated [Na+]i-rise, which dissipated longitudinally, with an apparent diffusion-coefficient of 680μm2/s (∼50% of that in pure water), consistent with rapid Na+i-mobility. We therefore propose that high Na+i-mobility permits local NHE activity to unify CaT-amplitude spatially, even during pHi non-uniformity. NHE activity in the ventricular myocyte thus not only enhances Ca2+-signalling during acidosis, to protect contraction, but also spatially unifies Ca2+-signalling within the cell.