Control of osmoregulation and excretion
Like many homeostatic functions, osmoregulation is controlled mainly by hormones. Some of these hormones act quickly to help fishes cope with rapid changes in the osmotic concentration of their environment by controlling the activity of existing cell membrane transporters or channels. Others act slowly and for longer time periods by regulating he synthesis of proteins that create the channels and transporters, and may also play a role in restructuring osmoregulatorytissues (see Takei & Loretz 2006). Prolactin appears to play a large role in adaptation to fresh water by decreasing the permeability of gill, kidney, bladder, and intestinal membranes to water and stimulating the uptake of sodium and chloride by the mitochondria-rich cells of the gills. Cortisol, a stress hormone, apparently assists with sodium and chloride uptake by freshwater fishes, and C-type natriuretic peptides seem to help with sodium uptake and retention inhypo osmotic environments.
Cortisol also plays an important role in saltwater adaptation, apparently by increasing the size and number of mitochondria-rich cells responsible for reducing levels of sodium and chloride in the blood and modifying the lining of theintestine to increase water absorption (Takei & Loretz2006). Blood levels of cortisol increase when euryhalinefishes are transferred to salt water. Growth hormone also increases the size and number of chloride-transporting mitochondria-rich cells and enhances the activity of the enzymes associated with sodium–potassium exchange. It also enhances the expression of genes responsible for the protein involved in ion transport across epithelial cell membranes. Vasopressin (from the posterior pituitary) and urotensins(from the urophysis) may also play a role in osmoregulation, but the evidence is not conclusive. Atrial natriuretic peptide and related hormones appear to help with short-term adaptation to high salinity environmentsby inhibiting swallowing of salt water and reducing the uptake of sodium by the intestine. And although the specific function of the guanylins is unknown, the genes responsible for their production become activated when eels are transferred to salt water, suggesting a role in the transition ofanguillid eels from a freshwater or estuarine juvenile to an ocean-dwelling, spawning adult (Takei & Loretz 2006).