Characterization of a Na+-dependent betaine transporter with Cl- channel properties in squid motor neurons.
Most marine invertebrates, including squids, use transporters to accumulate organic osmolytes such as betaine, to prevent water loss when exposed to elevated salinity. Although a limited number of flux studies have shown the Na+ dependence of betaine transport, nothing is known about the electrogenic properties of osmolyte transporters. We used whole cell and perforated-patch voltage-clamp techniques to characterize the electrical properties of the betaine transporter in giant fiber lobe motor neurons of the squid Lolliguncula brevis. Betaine activated a large, Cl--selective current that was reversibly blocked by 100 microM niflumic acid (97 +/- 2% block after 40 s, SD; n = 7) and partially inhibited by 500 microM SITS (29 +/- 11%; n = 5). The Cl- current was Na+ dependent and was virtually eliminated by isotonic replacement of Na+ with Li+, NMDG+, or Tris+. Concentration-response data revealed an EC50 in a physiologically relevant range for these animals of 5.1 +/- 0.9 mM (n = 11). In vertebrates, the betaine transporter is structurally related to the GABA transporter, and although GABA did not directly activate the betaine-induced current, it reversibly reduced betaine responses by 34 +/- 14% (n = 8). Short-term changes in osmolality alone did not activate the Cl- current, but when combined with betaine, Cl- currents increased in hypertonic solutions and decreased in hypotonic solutions. Activation of the betaine transporter and Cl- current in hypertonic conditions may affect both volume regulation and excitability in L. brevis motor neurons. This study is the first report of a novel betaine transporter in neurons that can act as a Cl- channel.
Petty
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Department of Physiology, University of Utah School of Medicine, Salt Lake City, Utah 84108, USA.
Lucero
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D000818:Animals; D000894:Anti-Inflammatory Agents, Non-Steroidal / Q000494:pharmacology; D001622:Betaine / Q000494:pharmacology; D002352:Carrier Proteins / Q000032:analysis / Q000502:physiology*; D018118:Chloride Channels / Q000032:analysis / Q000502:physiology*; D002712:Chlorides / Q000378:metabolism; D049832:Decapodiformes; D004305:Dose-Response Relationship, Drug; D050485:GABA Plasma Membrane Transport Proteins; D005765:Gastrointestinal Agents / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009046:Motor Neurons / Q000737:chemistry* / Q000378:metabolism; D009544:Niflumic Acid / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D012623:Seawater; D012964:Sodium / Q000502:physiology*; D014882:Water-Electrolyte Balance / Q000502:physiology; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
D000894:Anti-Inflammatory Agents, Non-Steroidal; D002352:Carrier Proteins; D018118:Chloride Channels; D002712:Chlorides; D050485:GABA Plasma Membrane Transport Proteins; D005765:Gastrointestinal Agents; C072207:betaine plasma membrane transport proteins; D001622:Betaine; D009544:Niflumic Acid; D005680:gamma-Aminobutyric Acid; D012964:Sodium