Translocation mechanism of Na-Ca exchange in single cardiac cells of guinea pig.

Abstract

We have studied in single cardiac ventricular cells of guinea pig the ionic translocation mechanism of the electrogenic Na-Ca exchange, i.e., whether Na and Ca ions countercross the membrane simultaneously or consecutively with "ping pong" kinetics. The dose-response relation between the external Ca concentrations ([Ca]o) and the current density of the outward Na-Ca exchange current were measured at three different intracellular Na concentrations ([Nail) in the absence of external Na. Nonlinear regression curves of the dose-response relation obtained by computer revealed Michaelis-Menten type hyperbola from which the [Ca]o giving a half-maximal response (apparent K, Ca o or K'Cao) and the apparent maximum current magnitude (I~x) were estimated at each [Na]i. As [Na]i increased, the K'Cao increased progressively and the value of K'Cao/l'r~ tended to decrease. These results are consistent with the simultaneous mechanism. The K'Cao/l'm~ values, however, were small and close to each other, so it was not possible to completely preclude a consecutive mechanism.

Inactivation of outward Na(+)‐Ca2+ exchange current in guinea‐pig ventricular myocytes.

“…The current amplitude declines at all membrane potentials during Ca2" application, but the decline seems to be more significant at negative membrane potentials. This tendency is consistent with I-V relation changes of the outward exchange current in giant membrane patches in response to an increase of the cytoplasmic Ca2+ concentration or a decrease of cytoplasmic Na+ concentration (Matsuoka & Hilgemann, 1992 (Khananshvili, 1990;Li & Kimura, 1990Hilgemann, Nicoll & Philipson, 1991;Niggli & Lederer, 1991). A consecutive model with one transition to an inactive state (Scheme 1), taking place from the three Na+-loaded exchanger conformation with cytoplasmic orientation of binding sites, described well many experimental results on the inactivation of exchange current in giant patches (Hilgemann et al 1992b).…”

“…The outward exchange current from giant membrane patches revealed a similar shift for the apparent steady-state cytoplasmic Na+ affinity, related to inactivation (Hilgemann et al 1992b). Our Kd for the peak current is substantially higher than the previously reported value by Li & Kimura (1990) (0-46 mm at 150 mM Na+ in the pipette). The reason for the discrepancy is not certain, but it may be due at least in part to the greater speed of the solution switch in our experiments.…”

“…Activation of outward Na+-Ca2" exchange current by chymotrypsin and STDS A consecutive transport model is at present the most reasonable model of the exchange cycle (Khananshvili, 1990;Li & Kimura, 1990Hilgemann, Nicoll & Philipson, 1991;Niggli & Lederer, 1991). A consecutive model with one transition to an inactive state (Scheme 1), taking place from the three Na+-loaded exchanger conformation with cytoplasmic orientation of binding sites, described well many experimental results on the inactivation of exchange current in giant patches (Hilgemann et al 1992b).…”

Cardiac Na+ ‐Ca2+ Exchange System in Giant Membrane Patches

“…A further important reason for assuming a consecutive model is that it appears possible to isolate an electrogenic ion translocation step of the cycle via cytoplasmic ion jumps. To attempt to isolate the individual ion translocation reactions of the exchanger, either a small concentration of calcium (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) 3B). By all these same criteria, the possibility of negative charge movement in the translocation of calcium has not been supported by the equivalent experiments with cytoplasmic calcium pulses.…”

Contribution of sarcolemmal sodium-calcium exchange and intracellular calcium release to force development in isolated canine ventricular muscle.

“…An important characteristic of the cardiac sarcolemmal Na+-Ca 2+ exchange protein (Nicoll, Longoni, and Phillipson, 1990) is its ability to transport Ca 2+ ions across the plasma membrane in either a forward or reverse direction during changes in membrane potential associated with various phases of the cardiac cycle (Mullins, 1981;Eisner and Lederer, 1985;Hilgemann and Noble, 1987). The extent to which Ca 2+ can enter or leave the cell via this mechanism appears to be due to the physiochemical properties of the exchanger, including its stoichiometry (Phillipson and Nishimoto, 1982;Phillipson, 1985;Reeves, 1985), kinetics (Miura and Kimura, 1989;Crespo, Grantham, and Cannell, 1990;Li and Kimura, 1990), binding affinities for intracellular and extracellular Na + and Ca 2+ ions (Miura and Kimura, 1989;Hilgemann, 1990;Li and Kimura, 1990), and the voltage across the sarcolemma at a given time in the cycle of contraction to relaxation. Accordingly, quantitative relationships have been descrilzed between the inotropic state of mammalian cardiac muscle and the concentration of Na + ions in the perfusate, [Na+]o (Reuter and Sietz, 1968;Reuter, 1974;Bers, 1987;Watanabe, Ishide, and Takishima, 1987), intracellular Na + activity, anal (Cohen, Fozzard, and Sheu, 1982;Eisner, Lederer, and Vaughan-Jones, 1984;Boyett, Hart, Levi, and Roberts, 1987;Brill, Fozzard, Makielski, and Wasserstrom, 1987;Wang, Chae, Gong, and Lee, 1988), and various time-, voltage-, and stimulation-dependent ionic currents attributed to an electrogenic Na+-Ca 2+ exchange process (Hume and Uehara, 1986;Kimura, Miyamae, and Noma, 1987;Fedida, Noble, Shimoni, and Spindler, 1987;Beuckelmann and Wier, 1989;Egan, Noble, Powell, Spindler, and Twist, 1989;Terrar and Whit...…”