Elevated [Cl-]i, and [Na+]i inhibit Na+, K+, Cl- cotransport by different mechanisms in squid giant axons.
Bumetanide-sensitive (BS) unidirectional fluxes of (36)Cl- or (22)Na+ were measured in internally dialyzed squid giant axons while varying the intra- or extracellular concentrations of Na+ and/or Cl-. Raising either [Cl-]i or [Na+]i resulted in a concentration-dependent reduction of the BS influx of both (36)Cl- and (22)Na+. Raising [Cl-]i above 200 mM completely blocked BS influxes. However, raising [Na+]i to 290 mM resulted in saturable but incomplete inhibition of both BS Na+ influx and BS Cl- influx. The consequences of varying intracellular Cl- on cotransporter effluxes were complex. At lower [Cl-]i values (below 100 mM) intracellular Cl- activated cotransporter effluxes. Surprisingly, however, raising [Cl-]i levels > 125 mM resulted in a [Cl-]i-dependent inhibition of BS effluxes of both Na+ and Cl-. On the other hand, raising [Na+]i resulted only in the activation of the BS Na+ efflux; intracellular Na+ did not inhibit BS efflux even at 290 mM. The inhibitory effects of intracellular Na+ on cotransporter-mediated influxes, and lack of inhibitory effects on BS effluxes, are consistent with the trans-side inhibition expected for an ordered binding/release model of cotransporter operation. However, the inhibitory effects of intracellular Cl- on both influxes and effluxes are not explained by such a model. These data suggest that Cl may interact with an intracellular site (or sites), which does not mediate Cl transport, but does modulate the transport activity of the Na+, K+, Cl- cotransporter.
Breitwieser
G E
GE
Department of Physiology, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA.
Altamirano
A A
AA
Russell
J M
JM
D000818:Animals; D001369:Axons / Q000737:chemistry / Q000502:physiology*; D002034:Bumetanide / Q000494:pharmacology; D002352:Carrier Proteins / Q000187:drug effects / Q000378:metabolism*; D002712:Chlorides / Q000378:metabolism* / Q000494:pharmacology; D049832:Decapodiformes; D004232:Diuretics / Q000494:pharmacology; D008565:Membrane Proteins / Q000378:metabolism; D011188:Potassium / Q000378:metabolism; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology; D028021:Sodium-Potassium-Chloride Symporters
D002352:Carrier Proteins; D002712:Chlorides; D004232:Diuretics; D008565:Membrane Proteins; D028021:Sodium-Potassium-Chloride Symporters; D002034:Bumetanide; D012964:Sodium; D011188:Potassium