When the rock crab Cancer irroratus is exposed to hypoosmotic artificial seawater (ASW) its muscle cell volume is regulated, in part, by a reduction of the intracellular glycine concentration. Therefore, ihe effect of reduced ASW osmolarity on the unidirectional glycine influx and efflux of isolated rock crab muscle cells was assessed to determine whether changes in these fluxes might contribute to the decreased intracellular glycine concentration. The glycine influx (extracellular glycine concentration = 1mM) into muscle cells exposed to 60% ASW decreased from 810 nmol/g dry wt x 8 min (100% ASW) to 470 nmole/g dry wt x 8 min, whereas the glycine efflux increased from 3,410 nmole/g dry wt x 8 min (100% ASW) to 5,930 nmole/g dry wt x 8 min. The decrease in influx was due to the reduced osmolarity rather than the reduced Na+ concentration of the 60% ASW. Further, even the total replacement of Na+ (Li' and Tris substituted) reduced the glycine influx by only 34%. Thus Na+-coupled glycine influx is not an important factor in adjusting the intracellular glycine concentration of muscle cells exposed to reduced ASW osmolarity. Since under isosmotic conditions glycine efflux exceeds influx by a factor of four, the hypoosmotically induced increase in glycine efflux is largely responsible for the decrease of the intracellular glycine concentration during the initial stages of hypoosmotic stress.The cells of marine and estuarine inverte-pled to Na+ transport by a common membrates contain high concentrations of free brane-associated carrier protein (Wyban et amino acids (FAA) (Pierce, '82; Gilles, '78; al., '80). Since Na+ makes up approximately Evans, '73a). During salinity stress this FAA 45% of the blood osmotic concentration in pool is used as osmotic solute to control cell marine animals (Prosser, '731, the reduction volume. For example, in a n osmoconformer, in blood Na+ concentration, such as occurs a reduction in external osmolarity causes an in a hypoosmotically stressed osmoconinflux of water into the cell and, thus, cell former, might reduce the transmembrane swelling. In response, the cellular amino acid Na + electrochemical potential gradient and content declines, osmotically obligated water thereby the driving force for cellular uptake is lost, and cell volume is, at least partially, of amino acids. This should contribute to regrestored (Pierce, '82; Gilles, '78; Lange, '72). ulation of cell volume.Since the blood FAA concentrations are This report describes the results of experiusually much lower than intracellular con-ments designed to test this notion using isocentrations, large FAA chemical gradients lated muscle cells of the rock crab Cancer exist across the plasma membranes of these irroratus. The rock crab was a suitable choice cells. Typically, cel1:plasma FAA ratios of for this study. Preliminary experiments in-200:l to 9OO:l are found (Evans, '73a; Gerard dicated that this crab was a moderately euand Gilles, '72). These gradients are main-ryhaline osmoconformer and that muscle cell tained eit...
