Translocation mechanism of Na-Ca exchange in single cardiac cells of guinea pig.
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 [( Na]i) 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 KmCao or K'mCao) and the apparent maximum current magnitude (I'max) were estimated at each [Na]i. As [Na]i increased, the K'mCao increased progressively and the value of K'mCao/I'max tended to decrease. These results are consistent with the simultaneous mechanism. The K'mCao/I'max values, however, were small and close to each other, so it was not possible to completely preclude a consecutive mechanism.
Li
J M
JM
National Institute for Physiological Sciences, Okazaki, Japan.
Kimura
J
J
D000818:Animals; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007474:Ion Exchange; D007700:Kinetics; D008564:Membrane Potentials; D008954:Models, Biological; D009206:Myocardium / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*
D012964:Sodium; D002118:Calcium