Characterization of a Na+-dependent betaine transporter with Cl- channel properties in squid motor neurons.

Abstract

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.