Charge movement during Na+ translocation by native and cloned cardiac Na+/Ca2+ exchanger.
Na+/Ca2+ exchange is electrogenic and moves one net positive charge per cycle. Although the cardiac exchanger has a three-to-one Na+/Ca2+ stoichiometry, details of the reaction cycle are not well defined. Here we associate Na+ translocation by the cardiac exchanger with positive charge movement in giant membrane patches from cardiac myocytes and oocytes expressing the cloned cardiac Na+/Ca2+ exchanger. The charge movements are initiated by step increments of the cytoplasmic Na+ concentration in the absence of Ca2+. Giant patches from control oocytes lack both steady-state Na+/Ca2+ exchange current (INaCa) and Na(+)-induced charge movements. Charge movements indicate about 400 exchangers per micron 2 in guinea-pig sarcolemma. Fully activated INaCa densities (20-30 microA cm-2) indicate maximum turnover rates of 5,000 s-1. As has been predicted for consecutive exchange models, the apparent ion affinities of steady state INaCa increase as the counterion concentrations are decreased. Consistent with an electroneutral Ca2+ translocation, we find that voltage dependence of INaCa in both directions is lost as Ca2+ concentration is decreased. The principal electrogenic step seems to be at the extracellular end of the Na+ translocation pathway.
Hilgemann
D W
DW
Department of Physiology, University of Texas Southwestern Medical Center, Dallas 75235.
Nicoll
D A
DA
Philipson
K D
KD
D000818:Animals; D002118:Calcium / Q000502:physiology*; D002352:Carrier Proteins / Q000502:physiology*; D004553:Electric Conductivity; D006168:Guinea Pigs; D066298:In Vitro Techniques; D008564:Membrane Potentials; D009206:Myocardium / Q000378:metabolism; D009865:Oocytes; D011994:Recombinant Proteins; D012964:Sodium / Q000502:physiology*; D019831:Sodium-Calcium Exchanger; D014982:Xenopus laevis
D002352:Carrier Proteins; D011994:Recombinant Proteins; D019831:Sodium-Calcium Exchanger; D012964:Sodium; D002118:Calcium