AMINO ACID TRANSPORT IN THE CORNEA AND CRYSTALLINE LENS OF THE TOAD, BUFO MARINUS.
Item
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Title
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AMINO ACID TRANSPORT IN THE CORNEA AND CRYSTALLINE LENS OF THE TOAD, BUFO MARINUS.
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Identifier
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AAI8023722
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identifier
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8023722
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Creator
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MCGAHAN, MARY CHRISTINE.
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Contributor
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Dr. Peter J . B a r l e y | Dr. Marian Orlowski
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Date
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1980
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Language
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English
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Publisher
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City University of New York.
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Subject
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Biology, Animal Physiology
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Abstract
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The uptake and efflux of amino acids in the cornea and the lens of the toad, Bufo marinus, was studied. Amino acids are accumulated across the aqueous side of the cornea, mainly by the epithelium. It appears that there are several distinct systems that mediate (alpha)-aminoisobutyric acid (AIB) transport into and out of the cornea. AIB uptake is saturable but is not directly dependent on the the presence of Na in the external bathing media. However, the exit site is very sensitive to changes in intracellular Na concentrations. Any factor, such as ouabain and metabolic inhibitors, which increases internal Na markedly stimulates AIB efflux. Under normal conditions efflux is very slow, about 15 percent of the rate of uptake. AIB uptake is inhibitable by alanine and leucine though not completely by either of these amino acids. The inhibitory effects of these compounds are additive, suggesting the presence of 2 distinct uptake sites. There are also sites for the exchange of internal for external amino acids, and this process is Na-dependent. However, Na does not move into or out of the cells with the AIB, as indicated in transmural ('22)Na flux studies. Exchange efflux was more specific than uptake, since both alanine and leucine inhibit AIB uptake, but only alanine stimulates AIB efflux. The effect of various membrane 'probes' and hormones on these transport processes was studied. Although the largest amount of AIB accumulated by the cornea was present in the epithelium, evidence is presented that the endothelium and stromal keratocytes may also concentrate and retain amino acids.;In the mammalian lens it is considered that amino acids are only accumulated by the anterior epithelium and they then 'leak' into the rest of the organ. However, in the present experiments on the amphibian lens, such a process was not found. Removal of the capsule and the anterior epithelium did not abolish accumulation of AIB against a concentration gradient. The probable reasons for this difference from the observations on the mammalian lens are discussed. AIB uptake in the lens is saturable and occurs by a system that is inhibited by alanine and is dependent upon the presence of Na in the external bathing media. Uptake of AIB occurs across all surfaces of the lens but is greater across the anterior epithelium. A gradient in concentration of AIB is built up, highest concentrations are in the periphery and the lowest are in the nucleus. Efflux of AIB is very slow; only 35 percent is lost in 16 hours. The rate of exodus declines exponentially over time and is not related to a decrease in the total concentration of AIB that occurs during this period. It is changed little by removal of the capsule and anterior epithelium or an excess of 'cold' AIB. Efflux can, however, be promoted by a Na-free external bathing solution. The retention of this amino acid by the lens may reflect its unique morphology. The layering of cells appears to protect the lens amino acid pool from the effects of changes in the bathing solution and the presence of toxic substances and drugs.
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
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Program
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Biomedical Sciences
