Taurine transport characteristics of the embryonic skate (Raja eglanteria) heart

The hearts of elasmobranchs, like those of other vertebrates, accumulate high concentrations of the amino acid taurine (Boyd et al. 1977), which is used to maintain osmotic equilibrium with the extracellular fluid. We recently demonstrated that embryonic hearts of the clearnose skate (Raja eglanteria Bosc) were able to accumulate taurine equally as well as hearts of adult skates and, as in adults, the accumulation was the result of transport from extracellular taurine across the cell membrane against a steep concentration gradient (Goldstein et al. 1990). In adult skate hearts, the uptake of taurine is Na+-dependent and competitively inhibited by β-alanine (Forster and Hannafin, 1980a), indicating that transport occurs by the β-amino acid system found in many vertebrate hearts (Huxtable, 1992). During hypotonic stress, adult skate hearts release taurine and other amino acids. The Na+/taurine cotransport system permits the skate heart to maintain high concentrations of intracellular taurine, while the hypotonicity-activated taurine release aids in cell volume regulation following dilution of the extracellular fluid. The aims of the present study were to determine whether taurine uptake in the embryonic skate heart is Na+-dependent as in the adult and whether the skate heart has the capability of cell volume regulation (during hypotonic stress) before hatching.