Osmotic stimulation of Na(+)-K(+)-Cl- cotransport in squid giant axon is [Cl-]i dependent.
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.
Breitwieser
G E
GE
Department of Physiology, Johns Hopkins School of Medicine, Baltimore, Maryland 21205.
Altamirano
A A
AA
Russell
J M
JM
D000818:Animals; D001369:Axons / Q000187:drug effects / Q000502:physiology*; D002034:Bumetanide / Q000494:pharmacology; D002352:Carrier Proteins / Q000378:metabolism*; D002712:Chlorides / Q000378:metabolism / Q000494:pharmacology*; D049832:Decapodiformes; D066298:In Vitro Techniques; D007700:Kinetics; D009994:Osmolar Concentration; D011188:Potassium / Q000378:metabolism; D012623:Seawater; D012964:Sodium / Q000378:metabolism; D028021:Sodium-Potassium-Chloride Symporters
D002352:Carrier Proteins; D002712:Chlorides; D028021:Sodium-Potassium-Chloride Symporters; D002034:Bumetanide; D012964:Sodium; D011188:Potassium