STUDIES ON MEMBRANE TRANSPORT. I. A COMMON TRANSPORT SYSTEM FOR SUGARS AND AMINO ACIDS? 1 CLARK P. READ Marine Biological Laboratory, ll-'oods Hole, Massachusetts 02543 and Department of Biology, Rice University, Houston, Texas 77001 It has been reported that sugars, particularly galactose, interfere with the absorption of amino acids by intestinal tissues (Newey and Smyth, 1964; Saunders and Isselbacher, 1965; Alvarado, 1966; Annegers, 1966; Chez ct al, 1966), by microorganisms (Ames, 1964; Kepes, 1964), and by tapeworms (Kilejian, 1966; Read et al., unpublished data). Various suggestions have been made concerning the mode of action of sugars, including competition for energy sources (Newey and Smyth, 1964), formation of a toxic metabolite (Saunders and Isselbacher, 1965), and direct allosteric effects in a polyfunctional carrier system. Kilejian (1966), working in the present author's laboratory, showed clearly that previously absorbed glucose inhibits the subsequent absorption of proline by the rat tapeworm, Hymenolepis diniinuta. None of the suggested mechanisms for sugar inhibition of amino acid transport seem to be consistent with Kilejian's findings. The present paper is concerned with interactions of sugars and amino acids in their absorption by intestinal mucosa of the smooth dogfish, Mustelus canis. Preparations of the spiral intestine from elasmobranchs have advantageous qualities for experiments on tissue uptake of metabolites ; both sides of the flat excised valve are constituted of mucosal cells and the reproducibility of multiple samples is satisfactory (Read et al., 1960). MATERIALS AND METHODS Smooth dogfish were used within 1 to 5 days after capture in the waters off Woods Hole, Massachusetts. Animals were killed by blows on the head and the spiral intestine rapidly removed. The third or fourth valve was excised and spread flat on a chilled plate. Replicate samples about 1 cm. 2 (60 to 80 mg. of wet tissue) were cut with sharp scissors and removed to chilled elasmobranch saline containing 300 mM urea (Read et al., 1960). As many as 36 samples were readily obtained from a single valve and were used immediately in experiments. Incubations of single samples were carried out at 20 C. in 8-or 10-ml. volumes of media. Preliminary experiments showed that gassing with 95% O 2 5% CO 2 or 95% air 5% CO 2 , with the addition of bicarbonate buffer, did not increase the rate of amino acid transport. Hence, the incubations were performed in a shaker bath 1 This work was supported by a grant from the National Institutes of Health, U. S. Public Health Service (AI-01384). Some technical assistance was made available through an Office of Naval Research contract, Nonr 4785(00). Appreciation is expressed to the Supply Depart-ment of the Marine Biological Laboratory for the collection of many dogfish in the summer of 1966. 630
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