Taurine accumulation by the heart of embryonic skates, Raja eglanteria

The hearts of elasmobranchs, like those of other vertebrates, contain high concentrations of the β-amino acid taurine, which is thought to play an osmoregulatory role in cardiac tissue. Although mammals and certain invertebrates can synthesize taurine from sulfur-containing precursors, e.g. cysteine, fish are unable to do so since they lack at least one enzyme involved in taurine biosynthesis: cysteine sulfinate decarboxylase (Jacobsen and Smith, 1968). Therefore, taurine must be acquired by fish via their diet (King et al. 1986). This poses a problem for the embryos of those fish that lay eggs and, for the early period of development at least, have no direct contact with amino acids in the external environment. Although solutes in the egg-case fluids can exchange with the sea water (Evans, 1981), the taurine concentration gradient between the egg case and the environment actually favors outward movement of taurine rather than uptake (L. Golstein, unpublished results). Therefore, we chose to study the mechanism by which the embryonic skate heart accumulates and maintains elevated trations of taurine in its heart. Concentrations of about 50 mmol l−1 taurine are typically found in cardiac muscle of adult specimens of the little skate Raja erinacea (Boyd et al. 1977).