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.