title: Intracellular osmoregulatory role of amino acids and urea in marine elasmobranchs.
abstract: Little skates, Raja erinacea, and stingrays, Dasyatis americana, were gradually transferred over a period of 4-5 days from full strength to approximatley 50% seawater. Plasma and muscle osmolarity fell. Hematocrits were essentially unchanged. Extracellular fluid volume (ECF) of muscle, estimated as the chloride space, increased 70% during this period. Regulation of muscle cell volume was associated with sharp declines in cellular concentrations of total amino acids (ninhydrin-positive materials) and urea. The osmoregulatory importance of the free amino acid pool in erythrocytes and muscle was a particularly prominent feature in both species. Intracellular amino acid concentration in R. erinacea muscle fell from 214 to 144 mmol/liter during transfer to 50% seawater, urea from 398 to 264, and trimethylamine oxide (TMAO) dropped from 63.9 to 35.8 mmol/liter. TMAO plasma levels were similar in stingray and skate, but muscle TMAO concentrations were much higher in the former. Urea content in stingray plasma greatly exceeded that in R. erinacea-630 and 574 mmol/liter in two specimens-perhaps the highest recorded.
mesh_terms: D000596:Amino Acids / Q000097:blood / Q000378:metabolism*; D000818:Animals; D001835:Body Weight; D002712:Chlorides / Q000378:metabolism; D004912:Erythrocytes; D005260:Female; D005399:Fishes / Q000502:physiology*; D006400:Hematocrit; D008297:Male; D009132:Muscles / Q000378:metabolism; D011188:Potassium / Q000378:metabolism; D012623:Seawater; D012964:Sodium / Q000378:metabolism; D014508:Urea / Q000378:metabolism*; D014882:Water-Electrolyte Balance
chemical_list: D000596:Amino Acids; D002712:Chlorides; D014508:Urea; D012964:Sodium; D011188:Potassium