Laboratory of Cardiac/Membrane Physiology, The Rockefeller University, New York, New York 10021, USA
Address for correspondence: David C. Gadsby, Laboratory of Cardiac/Membrane Physiology, The Rockefeller University, 1230 York Avenue, New York, NY 10021-6399. Voice: 212-327-8680; fax: 212-327-7589.
gadsby{at}mail.rockefeller.edu
Ann. N.Y. Acad. Sci. 976: 31-40 (2002).
Ion pumps and exchangers are considered to be different from
ion channels for two principal reasons. Ion pumps move ions
against, whereas ion channels allow ions to move
with, the electrochemical
potential gradient, and pumps transport ions relatively slowly,
10
2 s
-1, whereas channels conduct ions rapidly,
10
7 s
-1. However,
the latter high rate refers only to the open pore, and yet all
ion channels contain at least one gate. Not surprisingly, the
conformational changes associated with channel gating occur
with kinetics similar to those of ion pumping. Indeed, ion pumps
may be viewed as ion channels with two gates, one external to,
and the other internal to, the ion binding cavity. The simple
operational rule for such a pump is that the two gates should
never be open simultaneously; otherwise, the pump would become
a channel and conduct dissipative fluxes several orders of magnitude
larger than, and in the opposite direction to, the active transport
fluxes. Analyses of Na
+ ion movements mediated by the Na
+/K
+ pump under various conditions have suggested that in at least
one, short-lived, conformation of the pump, an ion-channel-like
structure, closed at its intracellular end, connects the extracellular
solution with the ion binding sites deep in the protein core.
Here we use the marine toxin, palytoxin, to act on Na
+/K
+ pumps
in outside-out patches excised from cardiac myocytes and so
transform the pumps into nonselective cation channels which
we study using macroscopic, and single-channel, recording. We
find that gating of the palytoxin-induced channels is regulated
by the pump's natural ligands. Thus, external K
+ congeners tend
to close, and external Na
+ tends to open, an extracellular gate,
whereas ATP acts from the cytoplasmic solution to open an intracellular
gate. These gating influences echo the normal ion occlusion
and deocclusion reactions that first entrap two extracellular
K
+ ions within the interior of the pump (between the two gates)
and then release them to the cytoplasmic side in a step accelerated
by ATP. These results offer the promise of being able to examine
ion occlusion and deocclusion steps at the microscopic level
in single Na
+/K
+ pump molecules.
