The metabolic origin of hypertonically induced intracellular free amino acid (FAA) accumulation in the freshwater pulmonate snail, Helisoma trivolvis was examined. The changes in FAA patterns in response to hypertonic stress were common in haemolymph and tissues; but FAA concentrations were much lower in the former. Transport experiments with 3H-glycine and detection by high-performance liquid chromatography (HPLC) showed that isolated kidneys in vitro took up FAA against a concentration gradient. Fluorography demonstrated that injected '*C-amino acids were incorporated into haemolymph protein significantly less in hypertonically stressed snails than in control snails. These results suggest that, in H. triuolvis, reduction in synthesis of haemolymph protein and FAA transport from haemolymph into cells contribute to intracellular FAA increase during the initial stage of hypertonic stress.In marine and brackish-water bivalve molluscs, intracellular free amino acids (FAA) have been considered to be involved in cell volume regulation as a solute during osmotic stress (Pierce, '71 creased in response to external salinity increase and that the brain extracts showed no effect on hypertonic FAA accumulation (Matsushima et al., '89). However, the mechanisms whereby intracellular FAA increased have not been investigated.The present study was designed to explore the source of hypertonically induced FAA accumulation in H . triuolvis. Changes in FAA levels in haemolymph and tissues during hypertonic stress and FAA transport by isolated kidneys in vitro were examined. Fluorography was employed to study the effect of hypertonic stress on the balance between FAA and haemolymph protein. The results showed a possible involvement of transport mechanisms and a reduced rate of haemolymph protein synthetic activity in the intracellular FAA accumulation during hypertonic stress.
MATERIALS AND METHODSHelisoma trivolvis (shell diameter = about 10 mm) were reared as described previously (Khan and Saleuddin, 79a). For the development of hy-