Water household of the common carp, Cyprinus carpio, when submitted to an osmotic challenge, as determined by diffusion-weighted magnetic resonance imaging at 7 T.

In vivo diffusion-weighted magnetic resonance imaging (MRI) was used to determine the effects of an osmotic challenge (1% NaCl) to a freshwater fish, the common carp (Cyprinus carpio). The imaged region covered organs such as the swimbladder, the liver, the kidney, the intestine, the spinal cord, and muscle tissue. A striking difference between salt-treated and control fish was found in the liver. The apparent diffusion coefficient value of livers from control fish was (0.39 +/- 0.16) 10(-9) m2/s and of salt-treated fish was (1.23 +/- 0.14) 10(-9) m2/s, which points to an increase in extracellular water content. These results were partially confirmed by a decrease in dry/wet weight ratio of the liver tissue. We also found increased levels of stress proteins in liver tissue. The Q factor of the applied radiofrequency coil dropped dramatically when we performed experiments with salt-exposed fish, indicating an increased conductivity resulting from the increased ion concentration and osmolarity of the fish. The data on plasma osmolarity of salt-exposed fish confirm a significant osmolarity increase upon salt exposure (from 334 to 430 mOsm/kg) and exceeded the osmolarity of the salt water (324 mOsm/kg), indicating that carp tend to cope with an increased salinity by increasing the internal osmolarity (hyperosmotic regulation). These data demonstrate that diffusion-weighted MRI might be a useful and noninvasive tool in the study of osmotic challenges of aquatic organisms.