<prompt> Na: Ca binding ratio on the sodium-calcium exchanger NCX in the myocardial fibers is:-1:011:033:014:01  <response>: 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 Ca2 . Now perfusion is established nutrients are available because of ICF Ca2 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. 