Characterisation of intestinal amino acid and oleic acid absorption and their interaction in the Pacific spiny dogfish (Squalus suckleyi)

Elasmobranchs are commonly carnivores and are important in energy transfer across marine ecosystems. Despite this, relatively few studies have examined the physiological underpinnings of nutrient acquisition in these animals. Here, we investigated the mechanisms of uptake at the spiral valve intestine for two representative amino acids (L-alanine, L-leucine) and one representative fatty acid (oleic acid), each common to the diet of a carnivore, the Pacific spiny dogfish (Squalus suckleyi). Transport was saturable for all three nutrients, depending upon transport calculation metric (i.e., mucosal disappearance, serosal appearance, or tissue accumulation). Over 0-10 mM range of amino acids the concentration at which 1/2 maximal transport occurred (Km; a measure of transporter affinity) was 11.9 and 11.2 mM for tissue accumulation of alanine and leucine, respectively. Oleic acid transport was measured at lower concentrations (0-200 mu M) and tissue accumulation did not reach saturation. Putative amino acid transport systems were delineated upon confirmation of sodium dependence and competitive inhibition with threonine, glycine, and lysine. The interplay of nutrient combinations on the modulation of nutrient acquisition rates, which better mimics the complex composition of both a meal and the internal osmolytes, was next investigated. Here, the application of serosal oleic acid led to diminished mucosal disappearance of leucine. Feeding did not significantly alter transport rates, perhaps indicative of maximal transport of these energy sources whenever the substrate is available given their importance both as metabolic fuels and precursors to the osmolyte urea.