The Cardiac Na+−Ca2+ Exchanger: Relative Rates of Calcium and Sodium Movements and Their Modulation by Protonation-Deprotonation of the Carrier

Daniel Khananshvili*, Evelyne Weil-Maslansky

*Corresponding author for this work

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Abstract

The exchange cycle of the cardiac Na+−Ca2+ exchanger can be described as separate steps of Ca2+ and Na+ transport [Khananshvili, D. (1990) Biochemistry 29, 2437–2442]. In order to determine the relative rates of Na+ and Ca2+ movement during the Na+−Ca2+ and Ca2+−Ca2+ exchange modes, the ratios (R) of Na+−Ca2+/Ca2+−Ca2+ exchanges were estimated with saturating concentrations of ions at both sides of the membrane. The effect of extravesicular pH and voltage (potassium valinomycin) on the initial rates (t = 1 s) of Na+−Ca2+ and Ca2+−Ca2+ exchange were investigated by assuming that, under the conditions tested, the intravesicular pH (pH 7.4) is not affected. Na+- or Ca2+-preloaded sarcolemma vesicles were diluted rapidly in assay medium containing 45Ca and buffer (pH 5.0–10.9), and the reaction of 45Ca uptake was quenched by using a semi-rapid-mixing device. Under conditions in which [45Ca]0 = [Ca]i = 250 µM, the pH-dependent curve of Ca2+−Ca2+ exchange shows a bell shape in the acidic range (pKai = 5.1 ± 0.1 and pKa2 = 6.5 ± 0.2) followed by activation of the exchange in the alkaline range (pKa3 = 10.0±0.2). With [45Ca]0 = 250 µM and [Na];= 160 mM, the Na+−Ca2+ exchange increases monotonically from pH 5.0 to 9.5 (pKai = 5.1 ± 0.1, pKa2 = 7.2 ± 0.2, and pKa3 = 9.1 ± 0.2). At pH <6.1, the ratio of Na+−Ca2+/Ca2+−Ca2+ exchange is close to unity (R≈1), while it increases to R = 3–4 in the range of pH 7.1–9.3. These data suggest that, in the absence of monovalent ions and at pH >6.5, Ca2+ moves severalfold faster from the extravesicular to the intravesicular side than in the opposite direction, meaning that the bidirectional movement of Ca2+ is an intrinsically asymmetric process. The pH-voltage curve analysis shows that at pH >6.5 the inside positive potential accelerates Na+−Ca2+ exchange by 2–3-fold. However, at pH <6.1 the voltage response of Na+−Ca2+ exchange is lost, suggesting that the deprotonation of the carrier determines a voltage response of Na+−Ca2+ exchange. In the range of pH 5.0–10.8 and Δψp = 0–200 mV, the Ca2+–Ca2+ exchange is voltage-insensitive, indicating that the Ca2+ transport is not affected by membrane potential. The data can be interpreted as follows: (a) When the carrier is protonated (pH <6.1), the “electroneutral” movement of Ca2+ from the extravesicular (intracellular) side to the intravesicular (extracellular) side limits the rates of both Na+−Ca2+ and Ca2+−Ca2+ exchanges, (b) In the deprotonated carrier (pH >6.5), the ion movements in the opposite direction (from the intravesicular side to the extravesicular side) become rate-limiting: the voltage-sensitive Na+ movement controls the rate of Na+−Ca2+ exchange, and a Ca2+ transport limits the Ca2+−Ca2+exchange rate, (c) Deprotonation of the carrier (pH >6.5) has opposite effects on Na+ and Ca2+ movements: it accelerates the voltage-sensitive and rate-limiting transport of Na+ during the Na+−Ca2+ exchange, while it decelerates the rate-limiting Ca2+ transport during the Ca2+−Ca2+ exchange.

Original languageEnglish
Pages (from-to)312-319
Number of pages8
JournalBiochemistry
Volume33
Issue number1
DOIs
StatePublished - 1 Jan 1994

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