Rate-limiting mechanisms of exchange reactions in the cardiac sarcolemma Na(+)-Ca2+ exchanger.
The effects of temperature, pH, voltage and K+ were tested on Na(+)-Ca2+ and Ca(2+)-Ca2+ exchanges with a goal to elucidate the rate-limiting mechanisms. The initial rates (t = 1 s) of Nai- and Cai-dependent 45Ca uptakes were measured in the sarcolemma vesicles. At pH 7.4 the Ca(2+)-Ca2+ exchange shows a bell-shaped temperature curve with a maximum at 27-29 degrees C. This effect is not caused by irreversible inactivation of the exchanger. The increase of pH from pH 6.0 to 7.4 in the K(+)-free medium decelerates the Ca(2+)-Ca2+ exchange 1.5-2.0-fold, while the addition of K+ accelerates the Ca(2+)-Ca2+ exchange 2.0-3.0-fold. Therefore, the accelerating effect of K+ opposes the decelerating effect of deprotonation. Temperatures increase (6-45 degrees C) in the K(+)-free medium (pH 7.4) elevates the Na(+)-Ca2+/Ca(2+)-Ca2+ exchange ratio from 0.8 to 5.0. With varying temperatures (6-37 degrees C) and pH 5.0-9.7, K+ has no considerable effect on Na(+)-Ca2+ exchange but accelerates the Ca(2+)-Ca2+ exchange 2-3-fold. At 6-45 degrees C and fixed pH 7.4, the inside-positive potential (delta psi > or = +200 mV) accelerates the Na(+)-Ca2+ exchange 1.7-2.0-fold, suggesting that the same rate-limiting reaction controls the Na(+)-Ca2+ exchange at various temperatures. It is concluded that (a) At pH > 6.5 (6-45 degrees C and 0-100 mM K+) the voltage-sensitive Na+ efflux limits the Na(+)-Ca2+ exchange, while the Ca2+ efflux limits the Ca(2+)-Ca2+ exchange.(ABSTRACT TRUNCATED AT 250 WORDS)
Khananshvili
D
D
Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv, Israel.
Shaulov
G
G
Weil-Maslansky
E
E
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism*; D002417:Cattle; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007700:Kinetics; D008564:Membrane Potentials; D009206:Myocardium / Q000378:metabolism*; D011188:Potassium / Q000737:chemistry; D012964:Sodium / Q000378:metabolism*; D019831:Sodium-Calcium Exchanger; D013696:Temperature
D002352:Carrier Proteins; D019831:Sodium-Calcium Exchanger; D012964:Sodium; D011188:Potassium; D002118:Calcium