Renal secretion of amino acids in ophidian reptiles

Net renal tubular secretion of endogenous beta-amino acids (taurine, beta-alanine, beta-aminoisobutyric acid) and their endogenous analogue (L-cysteic acid) was revealed in the olive sea snake, Aipysurus laevis, and in the garter snake, Thamnophis sirtalis, by renal clearance methods. The net secretory rates ranged from 16 to 795 nmol X kg-1 X h-1, with taurine being secreted at the highest rates. These rates of secretion are comparable to those observed in marine fish, the only other group of vertebrates exhibiting renal tubular secretion of L-amino acids under physiological conditions. However, only some snakes (8-58%) demonstrated net tubular secretion; the others (0-58%) showed net tubular reabsorption of these amino acids. Net tubular secretion was consistently observed whenever the filtered load of the amino acid was low, and net reabsorption was apparent whenever the filtered load was high; the two variables being significantly correlated. An analysis of the net amino acid transport rates, both secretory and reabsorptive, as a function of the filtered load suggests that in snakes the beta-amino acids are reabsorbed and secreted at discrete tubular sites, with the secretory sites located beyond the major reabsorptive sites. Taurine, beta-alanine, and L-cysteic acid appear to share a common transport system in the snake renal tubule cells.