ELEVATED [CL-](I) AND [NA- COTRANSPORT BY DIFFERENT MECHANISMS IN SQUID GIANT-AXONS(](I) INHIBIT NA+,K+,CL)

Bumetanide-sensitive (BS) unidirectional fluxes of Cl-36(-) or Na-22() 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 red uction of the BS influx of both Cl-36(-) and Na-22(+). 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 B S Na+ influx and BS Cl- influx. The consequences of varying intracellu lar Cl- on cotransporter effluxes were complex. At lower [Cl-](i) valu es (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 t he other hand, raising [Na+]i resulted only in the activation of the B S Na+ efflux; intracellular Na+ did not inhibit BS efflux even at 290 mM. The inhibitory effects of intracellular Na+ on cotransporter-media ted influxes, and lack of inhibitory effects on BS effluxes, are consi stent with the trans-side inhibition expected for an ordered binding/r elease model of cotransporter operation. However, the inhibitory effec ts of intracellular Cl- on both influxes and effluxes are not explaine d by such a model. These data suggest that Cl- may interact with an in tracellular site (or sites), which does nor mediate Cl- transport, but does modulate the transport activity of the Na+, K+, Cl- cotransporte r.