Abstract
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 Nao,
Nai, Cao, Cai, 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 Cao and of Ca influx on Cai. 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.
- 591918
- active,
- adenosine
- animals,
- axons,
- binding
- biological
- biological,
- cell
- energy
- equilibrium,
- exchange,
- gov't,
- in
- ion
- kinetics,
- mathematics,
- membrane,
- metabolism,
- models,
- musculoskeletal
- p.h.s.,
- potassium,
- research
- sites,
- sodium,
- squid,
- support,
- transport,
- triphosphate,
- u.s.
- vitro,
Users
Please
log in to take part in the discussion (add own reviews or comments).