The Journal of General Physiology, Vol 70, 681-695, Copyright © 1977 by The Rockefeller University Press
A mechanism for Na/Ca transport
LJ Mullins
A model is developed which requires the binding of 4 Na+ to a carrier
before a Ca binding site is induced on the opposite side of the membrane.
Upon binding Ca, this carrier translocates Na and Ca. The existence of
partially Na-loaded but nonmobile forms for the carrier (NaX, Na2X, Na3X)
suffices to explain both the activating and the inhibitory effects of Na on
the Ca transport reaction. Analytical expressions for Ca efflux and influx
in terms of [Na]o, [Na]i, [Ca]o, [Ca]i, and Em are developed for the Na/Ca
exchange system at equilibrium; these provide for a quantitative
description of Ca fluxes. Under nonequilibrium conditions, appropriate
modifications of the flux equations can be developed. These show a
dependence of Ca efflux on [Ca]o and of Ca influx on [Ca]i. The large
effect of internal ATP on Ca efflux and influx in squid axons, with no
change in net Ca flux, can be understood on the single assumption that ATP
changes the affinity of the carrier for Na at both faces of the membrane
without providing an energy input to the transport reaction.
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