Free amino acids in tissues of the skate Raja erinacea and the stingray Dasyatis sabina: effects of environmental dilution.

Concentrations of individual free amino acids were determined in various tissues of the skate (Raja erinacea) and the stingray (Dasyatis sabina), and the relationship of cellular free amino acid concentrations to intracellular osmoregulation was investigated by adapting these elasmobranchs to half-strength seawater. Each tissue characteristically had high levels of certain specific amino acids. Skate sing muscle contained high concentrations of sarcosine and beta-alanine, skate heart had high concentrations of taurine, and skate erythrocytes had high levels of taurine and beta-alanine. Amino acid levels in skate plasma were very low. High concentrations of aturine and glutamate were found in stingray brain. Adaptation of skates and stingrays to half-strength seawater was accompanied by significant decreases in concentrations of the major free amino acids in skate wing muscle and erythrocytes and in stingray brain, but not in skate heart. The data suggest that in these elasmobranchs certain specific free amino acids are selectively involved in intracellular osmoregulatory mechanisms.