Rapid charge translocation by the cardiac Na(+)-Ca2+ exchanger after a Ca2+ concentration jump.
The kinetics of Na(+)-Ca2+ exchange current after a cytoplasmic Ca2+ concentration jump (achieved by photolysis of DM-nitrophen) was measured in excised giant membrane patches from guinea pig or rat heart. Increasing the cytoplasmic Ca2+ concentration from 0.5 microM in the presence of 100 mM extracellular Na+ elicits an inward current that rises with a time constant tau 1 < 50 microseconds and decays to a plateau with a time constant tau 2 = 0.65 +/- 0.18 ms (n = 101) at 21 degrees C. These current signals are suppressed by Ni2+ and dichlorobenzamil. No stationary current, but a transient inward current that rises with tau 1 < 50 microseconds and decays with tau 2 = 0.28 +/- 0.06 ms (n = 53, T = 21 degrees C) is observed if the Ca2+ concentration jump is performed under conditions that promote Ca(2+)-Ca2+ exchange (i.e., no extracellular Na+, 5 mM extracellular Ca2+). The transient and stationary inward current is not observed in the absence of extracellular Ca2+ and Na+. The application of alpha-chymotrypsin reveals the influence of the cytoplasmic regulatory Ca2+ binding site on Ca(2+)-Ca2+ and forward Na(+)-Ca2+ exchange and shows that this site regulates both the transient and stationary current. The temperature dependence of the stationary current exhibits an activation energy of 70 kj/mol for temperatures between 21 degrees C and 38 degrees C, and 138 kj/mol between 10 degrees C and 21 degrees C. For the decay time constant an activation energy of 70 kj/mol is observed in the Na(+)-Ca2+ and the Ca(2+)-Ca2+ exchange mode between 13 degrees C and 35 degrees C. The data indicate that partial reactions of the Na(+)-Ca2+ exchanger associated with Ca2+ binding and translocation are very fast at 35 degrees C, with relaxation time constants of about 6700 s-1 in the forward Na(+)-Ca2+ exchange and about 12,500 s-1 in the Ca(2+)-Ca2+ exchange mode and that net negative charge is moved during Ca2+ translocation. According to model calculations, the turnover number, however, has to be at least 2-4 times smaller than the decay rate of the transient current, and Na+ inward translocation appears to be slower than Ca2+ outward movement.
Max-Planck-Institut für Biophysik, Frankfurt/Main, Germany. kappl@kennedy.mpibp.uni-frankfurt.de
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D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism*; D002474:Cell-Free System; D003593:Cytoplasm / Q000502:physiology; D004553:Electric Conductivity; D006168:Guinea Pigs; D006321:Heart / Q000502:physiology; D007700:Kinetics; D009206:Myocardium / Q000648:ultrastructure; D009532:Nickel / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D051381:Rats; D012964:Sodium / Q000502:physiology; D019831:Sodium-Calcium Exchanger; D013696:Temperature
D002352:Carrier Proteins; D019831:Sodium-Calcium Exchanger; D000584:Amiloride; D009532:Nickel; C064515:2',4'-dichlorobenzamil amiloride; D012964:Sodium; D002118:Calcium