Key Points
- To characterize the mechanism of sodium inhibition on calcium transport mediated by the cardiac sodium-calcium exchange system.
- Measured the initial velocity of calcium uptake via sodium-calcium exchange in cardiac sarcolemmal vesicles across varying extravesicular sodium concentrations.
- Constructed Hill plots to evaluate multisite competitive inhibition kinetics and determine the inhibition constant (Ki).
- Extravesicular sodium competitively inhibited calcium uptake with a Ki of approximately 16 mM.
- Hill plots were nonlinear, displaying a slope of 1 at sodium concentrations below 20 mM and increasing to 1.6–2.0 at higher concentrations.
- Data indicate the carrier possesses a divalent site binding either 1 calcium or 1–2 sodium ions (where 1 sodium is sufficient to block calcium), alongside a distinct site binding at least one additional sodium ion.
Structured PICO
PPopulationCardiac sarcolemmal vesicles
IInterventionExtravesicular Na+ (sodium)
OOutcomeInitial velocity of Ca2+ uptake via the Na-Ca exchange systemsurrogate
This basic science study elucidates the competitive interaction between sodium and calcium at the sodium-calcium exchange carrier, providing a mechanistic explanation for the dependence of cardiac contractility on the extracellular calcium-to-sodium ratio.