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

Abstract

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. TheQ 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.