Lithium in marine elasmobranchs as a natural marker of rectal gland contribution in sodium balance.

Natural concentrations of lithium in blood plasma and urine of several species of elasmobranchs and teleosts from the Black Sea and in rectal gland fluid of the former were determined by mass-spectrometric isotope dilution techniques. Unlike the teleosts, the elasmobranchs showed a prominent shift of Li/Na ratio in blood plasma with respect to the surrounding water, the plasma Li/Na ratio being five times lower than that of sea-water. Li-Na selectivity was found to be high in the kidneys and negligible in the rectal gland. Differences in Li-Na selectivity between kidneys and rectal gland are used as a basis for the method of estimation of relative contributions of these excretory organs in sodium excretion. Permanent contributions of the kidneys and rectal gland in sodium excretion of the ray Dasyatis pastinaca were found to be nearly equal.