The effect of formaldehyde exposure on the transmembrane distribution of free amino acids in muscles of Mytilus edulis

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.