Free amino acids in the posterior adductor muscle of mussels (Mytilus edulis) occur in a high-energy gradient group, with energy gradients of 15­18 kJ mol-1 (aspartate, beta-alanine, glycine, taurine and threonine), and a low-energy gradient group, with energy gradients around 12 kJ mol-1 (the rest of the amino acids). Two of the amino acids, glycine and taurine, are present at intracellular concentrations of 100­150 mmol kg-1, while the other amino acids occur at concentrations below 50 mmol kg-1. Exposure of mussels to formaldehyde causes a marked influx of Na+ into the muscle cells and an increase in cellular water content. The Na+ gradient, which provides the energy for the cellular accumulation of free amino acids, is reduced. The drop in the Na+ gradient is accompanied by a nearly proportional reduction in the energy gradients of all amino acids in the high-energy gradient group and a 150 mmol kg-1 reduction in the total intracellular concentration of free amino acids. Most of this reduction is made up by the Na+-dependent amino acids aspartate, glycine and threonine, the concentrations of which are reduced by about 120 mmol kg-1. The transmembrane distribution of the low-energy gradient amino acids seems to be independent of the Na+ gradient, and these amino acids display only moderate reductions in their intracellular concentrations when the Na+ gradient is reduced. The reduction in the concentrations of the free amino acids appears to be a volume-regulatory response, serving to bring the cell volume back to its optimal level after the formaldehyde-induced Na+ influx has caused a cellular swelling. The basis of these differences in Na+-dependence is discussed. Taurine, which is the quantitatively dominating organic solute and an important volume-regulatory osmolyte in mussels, does not take part in the volume-regulatory response. This may be due the role of taurine in the protection against potentially toxic Ca2+, which enters the cells in large quantities when mussels are exposed to formaldehyde.

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