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 Ca2+-Ca2+ exchanges with a goal to elucidate the rate-limiting me chanisms. The initial rates (t = 1 s) of Na-i- and Ca-i-dependent Ca-4 5 uptakes were measured in the sarcolemma vesicles. At pH 7.4 the Ca2-Ca2+ exchange shows a bell-shaped temperature curve with a maximum at 27-29 degrees C. This effect is not caused by irreversible inactivati on of the exchanger. The increase of pH from pH 6.0 to 7.4 in the K+-f ree medium decelerates the Ca2+-Ca2+ exchange 1.5-2.0-fold, while the addition of K+ accelerates the Ca2+-Ca2+ exchange 2.0-3.0-fold. Theref ore, the accelerating effect of K+ opposes the decelerating effect of deprotonation. Temperatures increase (6-45 degrees C) in the K+-free m edium (pH 7.4) elevates the Na+-Ca2+/Ca2+-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 C a2+-Ca2+ exchange 2-3-fold. At 6-45 degrees C and fixed pH 7.4, the in side-positive potential (Delta psi greater than or equal to +200 mV) a ccelerates the Na+-Ca2+ exchange 1.7-2.0-fold, suggesting that the sam e rate-limiting reaction controls the Na+-Ca2+ exchange at various tem peratures. 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+ exchan ge, while the Ca2+ efflux limits the Ca2+-Ca2+ exchange. (b) At pH < 6 .1 (6-45 degrees C and 0-100 mM K+) the voltage-insensitive Ca2+ influ x limits both Na+-Ca2+ and Ca2+-Ca2+ exchanges (this may represent a r educed voltage sensitivity of Na+-Ca2+ exchange and the similar rates of Na+-Ca2+ and Ca2+-Ca2+ exchanges). (c) The bell-shaped temperature curve of Ca2+-Ca2+ exchange cannot be described by a simple reversible reaction involving two species (the exchange has to involve at least three species), (d) K+ interacts with a deprotonated species (pH > 6.1 ) accelerating the rate-limiting Ca2+ efflux of Ca2+-Ca2+ exchange.