AJP - Cell Physiology, Vol 258, Issue 4 C749-C753, Copyright © 1990 by American Physiological Society
Osmotic stimulation of Na(+)-K(+)-Cl- cotransport in squid giant axon is [Cl-]i dependent
G. E. Breitwieser, A. A. Altamirano and J. M. Russell
Department of Physiology, Johns Hopkins School of Medicine, Baltimore, Maryland 21205.
The effects of increasing extracellular osmolality on unidirectional Cl-
fluxes through the Na(+)-K(+)-Cl- cotransporter were studied in internally
dialyzed squid giant axons. Hyperosmotic seawater stimulated
bumetanide-sensitive Cl-influx at 150 mM intracellular Cl- concentration
([Cl-]i), whereas Cl- efflux was unaffected under comparable ionic
conditions. Stimulation of bumetanide-sensitive Cl- influx was proportional
to the increase in extracellular osmolality. Bumetanide-sensitive Cl-
influx began to increase after a latency of approximately 20 min after a
stepwise increase of extracellular osmolality and continued to increase for
at least 70 min. The increased bumetanide-sensitive Cl- influx measured
after 65 min of exposure to hyperosmotic external fluid was a function of
the intracellular Cl- concentration; stimulation by hyperosmotic external
fluids was observed at physiological [Cl-]i levels (greater than 100 mM)
but not at lower [Cl-]i levels. Under both normo- and hyperosmotic
conditions, intracellular Cl- inhibited Na(+)-K(+)-Cl- cotransport influx
in a concentration-dependent manner. However, in hyperosmotic seawater, the
dose dependence of inhibition by intracellular Cl- was shifted to higher
[Cl-]i values. Therefore, we conclude that hyperosmotic extracellular
fluids stimulate influx via the Na(+)-K(+)-Cl- cotransport by resetting the
relation between [Cl-]i and transport activity.
