Salinity dependence, uptake kinetics, and specificity of amino-acid absorption across the body surface of the oligochaete annelidEnchytraeus albidus

Abstract

Enchytraeus albidus is able to absorb dissolved14C-labeled neutral amino acids (glycine, L-alanine, L-valine,α-aminoisobutyric acid) and an amino-acid mixture from ambient water across the body surface against considerable concentration gradients. Saturation kinetics and susceptibility of glycine uptake to competitive inhibition by alanine suggest mediated transport. Absorption of neutral amino acids is an active process. Exchange diffusion of preloadedα-aminoisobutyric acid against external glycine orα-aminoisobutyric acid could not be detected. Results on inhibition of glycine uptake by a variety of low-molecular-weight substances indicate that glycine absorption is highly specific for neutral amino acids and somewhat less for basic amino acids; it is unspecific for non-α-amino acids, acidic amino acids, carbohydrates, and organic acids. Rates of transintegumentary net influx of glycine are nearly identical to14C-glycine influx, suggesting that only small amounts of amino acids are released, as compared with the capacity for uptake. Thus,14C-amino-acid influx data are used for characterization of the uptake system. Glycine uptake is positively correlated to external salinity. In fresh water, absorption is nearly zero; between 10 and 20 ‰ S, uptake increases markedly reaching maximum values at 30 ‰ S; these remain almost constant at 40 ‰ S. Transport constants and maximum uptake rates increase with rising salinities. Since absorption of glycine and L-valine is susceptible to sodium depletion, similar mechanisms presumably underly salinity-dependent uptake of amino acids and sodium-dependent solute transport. Oxygen consumption is not significantly modified by different external salinities. Estimates of nutritional profit gained from absorption of amino acids vary between 4 and 15 % of metabolic rate for glycine absorption and between 10 and 39 % for uptake of an amino-acid mixture, according to external concentrations (10 and 50 µM) and salinities (20 and 30 ‰ S).