Cl- transport in the lobster stretch receptor neurone

Experiments were performed to identify mechanisms underlying non-leakage an d non-H+/HCO3--linked transmembrane Cl- transports in the slowly adapting s tretch receptor neurone of the European lobster, using intracellular microe lectrode and pharmacological techniques. In methodological tests, it was es tablished that direct estimates of intracellular Cl- with ion-sensitive mic roelectrodes are statistically identical with indirect estimates by means o f a GABA method, where 1-2 mM GABA is transforming the cell's membrane volt age into its Cl- equilibrium voltage from which the Cl- concentration is in ferred by the Nernst equation. From experiments using sodium orthovanadate and ethacrynic acid, supposed to block primary Cl- pumps, and bumetanide, s upposed to block Na-K-Cl co-transporters, it appeared that neither of the t wo Cl- transport systems exists in the stretch receptor neurone. It could b e shown, however, that the cell is equipped with an electroneutral K-Cl co- transporter that (a) is blockable by furosemide in high (K-m approximate to 350 mu M), by 4-acetamido-4'-isothiocyanato-stilbene-2,2-disulphonic acid (SITS) in medium-high (K-m approximate to 35 mu M), and by 4,4'-diisothiocy anostilbene-2,2'-disulphonic acid (DIDS) in low (K-m 15 mu M) doses, (b) is (transiently) activatable by (1 mM) n-ethylmaleimide, (c) is not suppresse d by extracellular Rb+ or NH4+, and (d) is not directly coupled to any tran smembrane transports of Na+, H+ or HCO3-. From functional tests, with varyi ng transmembrane K+ and Cl- gradients, evidence obtained that the K-Cl co-t ransporter is able to reverse its transport direction and to adjust its tra nsport rate in a considerable range. As a whole, the results speak in favou r of the K-Cl co-transporter being responsible (a) for normally keeping the intracellular Cl- concentration at low levels, for an optimization of the cell's inhibitory system, and (b) for achieving fast transmembrane shifts o f K+ (and Cl-), as a means of stabilizing the cell's membrane excitability in conditions of varying extracellular K+ concentrations.