Na+: Ca++ binding ratio on the sodium-calcium exchanger (NCX) in the myocardial fibers is:-

Sodium-calcium Exchanger (NCX) is an example of counter-transpo /antipo (a type of secondary active transpo) It carries 3 sodium Ions into the cell & one calcium Ion out of the cell It is critically dependent on primary active transpo (Na+-K+ pump). Two mechanisms can be explained on basis of this exchanger:(i) Action of digitalis: Digitalis binds with Na+-K+ pump & inhibits it; thus Na+ cannot be pumped out, its concentration gradient cannot be created therefore NCX will not work. Ca+ accumulates inside the cell which increases myocardial contractility. (ii) Reperfusion Injury: After MI there is failure of Na+-K+ pump results in NCX failure, Thus Intracellular Ca+2 |. Now perfusion is established (nutrients are available) because of |ICF Ca+2 cardiac fibers contract very strongly. Already muscle fibers are fragile due to M.I, now this contraction results in fuher damage. This is reperfusion Injury.