Integumentary Amino Acid Transport and Metabolism in the Apodous Sea Cucumber, Chiridota Rigida

Abstract

ABSTRACT The apodous sea cucumber, Chiridota rigida, was found to transport exogenous amino acids to intracellular free amino acid pools in the integument, the gastrointestinal epithelium making only a minimal contribution to total animal uptake. Amino acids entering the integumentary free amino acid pool were either completely catabolized to CO2, incorporated into large molecular weight compounds, such as proteins, or left unchanged. The latter fraction was available for efflux through both apical and basal cellular membranes through saturable or non-saturable transport processes. Influx of natural (glycine) and analog (AIB) amino acids into the integumentary epithelium was achieved by a combination of saturable and non-saturable transport mechanisms: the former displaying Michaelis-Menten kinetics, being Na+-dependent, and transporting both glycine and AIB; the latter involving a simple diffusion of amino acid across the apical cell membranes and/or through the intercellular septate desmosomes. Preloaded AIB exhibited both homo-and hetero-exchange diffusion with exogenous amino acids when a relatively large internal analog pool had been established. With a small internal pool of AIB, homo-exchange diffusion persisted, but high external concentrations of glycine inhibited the washout of the analog. Transintegumentary influx of AIB to the coelomic fluid was a linear function of the exogenous amino acid concentration and was unchanged when choline chloride was used as a substitute saline. At steady-state the coelomic fluid analog concentration was maintained at a much lower level than the incubation medium or the epithelial free amino acid pool, due to apparent absorptive processes of the dermis and muscle layers of the integument, as well as by possible carrier-mediated re-uptake mechanisms on the basal membranes of the epithelial cells. A tentative model of integumentary amino acid transport and metabolism in C. rigida is presented.