The Efflux of Amino Acids from the Olfactory Organ of the Spiny Lobster: Biochemical Measurements and Physiological Effects.

The amino acids taurine and glycine are odorants that activate specific chemosensory cells in the olfactory sensilla (aesthetascs) of the spiny lobster, Panulirus argus. We show that the aesthetascs themselves contain large intracellular concentrations of taurine (≈2 mM) and glycine (≈ 85 mM); these concentrations are more than 10,000-fold greater than the response thresholds of the chemosensory cells. A net efflux of at least five amino acids occurs when the olfactory organ is immersed in amino acid-free seawater. With taurine and glycine, efflux continues until an apparent equilibrium is reached between the sensilla and the external medium; for taurine the equilibrium with seawater occurs at ≈12 to 28 nM, and for glycine at ≈100 to 500 nM. Aesthetascs may achieve these equilibria within 300 ms. Hence, even during the brief interval between consecutive flicks of the antennule, olfactory receptors are exposed to a background of odorants escaping from intracellular stores. Electrophysiological studies show that both the spontaneous and evoked activities of taurine-sensitive chemosensory cells are markedly affected by a taurine background simulating that measured in the efflux studies. Uptake systems may participate in establishing the equilibria between sensilla and seawater since (1) the net efflux of amino acids increases in sodium-free seawater; and (2) guanidinoethane sulfonate, a competitor for taurine uptake, selectively increases net taurine efflux. Effluxes from an olfactory organ may contribute noise to the chemosensory process; alternatively, background substances could contribute functionally by affecting membrane proteins.