Which ion exchange is facilitated by the Na+/Ca2+ exchanger (NCX)?
Na+/Ca2+ exchanger NCX, SLC8 family with isoforms NCX1 broadly expressed in heart, brain, kidney and NCX2, NCX3 in brain and muscle, is electrogenic antiporter crucial for calcium extrusion after excitation. It operates generally with stoichiometry three Na+ imported downhill into cytoplasm in exchange for one Ca2+ exported against its gradient, generating net inward positive charge. In forward mode driven by sodium gradient, NCX rapidly lowers cytosolic Ca2+ after cardiac action potential and muscle contraction preventing overload. When intracellular Na+ rises or membrane depolarizes as during ischemia, exchanger reverses bringing Ca2+ inward contributing to contractility and excitotoxicity. Ca2+ affinity is lower than SERCA pump but capacity high, handling bulk Ca2+ load complementing ATP-dependent Ca2+ ATPases PMCA and SERCA. Structural studies show eight transmembrane segments with alpha-repeat forming ion binding cluster where Na+ and Ca2+ compete. Pharmacological inhibitors KB-R7943 preferentially block reverse mode, used experimentally to study arrhythmogenesis. Such detailed mechanistic insight is frequently examined in competitive tests including NEET, CUET, CSIR-NET and GATE where transporter classification, energetics and disease linkage are integrated into problem-solving questions.
Ref: Hilgemann et al., Annu Rev Physiol 2013, NCX mechanism; Khananshvili, Biochem Soc Trans 2013.