NUTRIENT TRANSPORT IN STARFISH. II. UPTAKE OF NUTRIENTS BY ISOLATED ORGANS

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Abstract
Iso-lated digestive glands, gonads, rectal caeca, and cardiac stomach from the starfish, Asterias forbesi, were able to absorb labeled amino acids and glucose from dilute solutions in sea water and coelomic fluid. This absorption could be largely inhibited with 2 x 10-4 M sodium iodo-acetate. Analyses of the rates of absorption of labeled nutrients by isolated digestive glands have led to the conclusion that these substances are very rapidly turned over through all the body fluids of the starfish. Calculations indicate that the maximum possible rate of movement of stored nutrients (amino acids) from the digestive glands to the other tissues is equivalent to nearly 0.5 g (dry weight) of protein per day. The maximum rate for glucose is about 12 mg per day. These quantities probably exceed the amount of nutrients normally ingested by the animals. Similar studies indicate that related compounds may differ markedly in their importance in nutrient transport. The absorption of glycine is much slower from coelomic fluid than from sea water. It is suggested that the concentrations of this amino acid build up in the body fluids because its reabsorption may be inhibited by other amino acids. Experiments measuring the passive diffusion of nitrogenous-materials from isolated digestive glands have further verified that a rapid flux of organic nutrients occurs between the internal fluids and tissues of the starfish.