VOLTAGE-DEPENDENCE OF NA-CA EXCHANGER CONFORMATIONAL CURRENTS

Properties of a transient current (I-cort) believed to reflect a confo rmational change of the Na-Ca exchanger molecules after Ca2+ binding w ere investigated. Intracellular Ca2+ concentration jumps in isolated c ardiac myocytes were generated with flash photolysis of caged Ca2+ dim ethoxynitrophenamine, and membrane currents were simultaneously measur ed using the whole-cell variant of the patch-clamp technique. A previo usly unresolved shallow voltage dependence of I-cort was revealed afte r developing an experimental protocol designed to compensate for the p hotoconsumption of the caged compound. This voltage dependence can be interpreted to reflect the distribution of Na-Ca exchanger conformatio nal states with the Ca2+ binding site exposed to the inside of the cel l immediately before the flash. Analysis performed by fitting a Boltzm ann distribution to the observed data suggests that under control cond itions most exchanger molecules reside in states with the Ca2+ binding site facing the outside of the cell. Dialysis of the cytosol with 3', 4'-dichlorobenzamil, an organic inhibitor of the Na-Ca exchange, incre ased the magnitude of I-cort and changed the voltage dependence, consi stent with a parallel shift of the charge/voltage curve. This shift ma y result from intracellular DCB interfering with an Na+-binding or Na-translocating step. These observations are consistent with I-cort ari sing from a charge movement mediated by the Na-Ca exchanger molecules after binding of Ca2+.