ABSTRACT
Starfish, unlike other echinoderms such as sea cucumbers and sea urchins, lack an effective intestine for nutrient absorption. Instead, they possess digestive diverticula called pyloric caeca which extend from the stomach wall into each arm and branch laterally into numerous blind-ended, epithelial-lined tubules. These tubules exhibit a number of physiological activities including the secretion of extracellular proteolytic enzymes (Lawrence, 1982), the storage of carbohydrates and lipids as energy reservoirs (Lawrence and Lane, 1982) and the absorption of low molecular weight organic solutes resulting from digestive activities in the diverticulum lumen (Ferguson, 1979, 1982; Jangoux, 1982; Lawrence, 1982). While it is clear from these studies that starfish pyloric caeca participate in nutrient absorption, there has been no direct physiological characterization of a transapical or transcellular transport process for amino acids or sugars by the epithelial cells of this organ. The present investigation is the first to show, using purified brushborder membrane vesicles from asteroid pyloric caeca, that amino acids can be transported across this membrane by at least one carrier-mediated process that displays properties in common with gastrointestinal absorptive organs of animals from a variety of phyla.
