Osmotic effects on arginine kinase function in living muscle of the blue crab Callinectes sapidus

Flux was examined through the reaction catalyzed by arginine kinase in intact blue crab (Callinectes sapidus) muscle during simulated changes in salinity. Isolated dark levator muscles from the swimming leg were superfused with a saline solution that had an osmolarity equivalent to that of the hemolymph under different salinity regimes. Animals were acclimated for 7 days to a salinity of 5, 17 or 35 per thousand, which corresponds to a hemolymph osmolarity of 640, 720 or 960 mosmoll(-1), respectively. Experiments were conducted under control conditions, in which the osmolarity of the superfusion medium matched that of the acclimated hemolymph, as well as under hypo- and hyperosmotic conditions. These latter treatments were meant to simulate a rapid change in environmental salinity. Pseudo-first-order unidirectional rate constants and flux rates were measured for arginine kinase in the forward and reverse directions using a (31)P-nuclear magnetic resonance saturation transfer method. There were no differences in the rate constants or flux rates among the controls, indicating that arginine kinase function is not modulated by salinity if the animal has had sufficient acclimation time. However, the rate constants and flux rates of arginine kinase varied over a modest 1.7-fold range across the three types of osmotic treatments, although the range for the flux data was reduced when cell volume changes were taken into account. The hyperosmotic treatments led to a reduction in arginine kinase flux, while the hypo-osmotic treatments led to an enhanced arginine kinase flux. We propose that this effect is mediated by an increase in the concentration of perturbing inorganic ions under hyperosmotic conditions and a decrease in the concentration of such ions during the hypo-osmotic treatments.