Osmoregulation of the common carp ( Cyprinus carpio) when exposed to an osmotic challenge assessed in-vivo and non-invasively by diffusion- and T 2-weighted magnetic resonance imaging

Abstract

The regulation of the total, extracellular and intracellular water content in the muscle and liver of the stenohaline common carp in response to an exposure of 1 wt.% NaCl was studied in an in-vivo and non-invasive way in conscious fish. The method of choice was in-vivo magnetic resonance imaging (MRI), which allowed study of dynamic changes in the free water content (T 2) and the intra- versus extracellular water content of tissues (apparent diffusion coefficient, ADC). Also plasma osmolarity data were obtained which confirmed that the common carp is a hyperregulator and which were correlated in time with water content data. The demonstrated drop in T 2-weighted signal intensity (T 2w SI) values revealed water loss in both liver and muscle during the first days of salt exposure. As soon as the plasma osmolarity increased, recovery from the water loss was discerned in both tissues, as demonstrated by the increased T 2w SI values, reaching after 21 days of exposure control values in muscle and twice the control value in the liver, reflecting the massive water gain of this organ. The observed ADC values increased linearly, revealing cell shrinkage the first day of exposure with subsequent continuous increase of the extracellular volume. No cell volume repair was observed during the investigated period, not even in the muscle tissue whose water content was restored after 21 days.