Please use this identifier to cite or link to this item: http://scholarbank.nus.edu.sg/handle/10635/15651
Title: Excretory nitrogen metabolism in the Chinese soft-shelled turtle, pelodiscus sinensis
Authors: LEE MIN LIN, SERENE
Keywords: Ammonia, urea, turtle, nitrogen metabolism, soft-shelled, excretion
Issue Date: 7-Jan-2007
Source: LEE MIN LIN, SERENE (2007-01-07). Excretory nitrogen metabolism in the Chinese soft-shelled turtle, pelodiscus sinensis. ScholarBank@NUS Repository.
Abstract: This study aimed to determine effects of four experimental conditions, namely feeding, salinity stress, emersion and acute ammonia toxicity, on nitrogen metabolism and excretion in the Chinese soft-shelled turtle Pelodiscus sinensis. Pelodiscus sinensis is ureogenic and primarily ureotelic in freshwater. Our results reveal that a major portion of the urea was excreted through the buccopharyngeal epithelium. After feeding, postprandial surges in ammonia contents in the various tissues were effectively prevented by an increased urea synthesis rate (apparently increased 7-fold), with a major portion of assimilated nitrogen converted to urea before being excreted. In addition, postprandial ammonia toxicity was ameliorated by increased transamination and synthesis of certain amino acids in the liver and muscle. Turtles exposed to a progressive increase in salinity through a 6-day period had significant increases in plasma osmolality, [Na+] and [Cl-] by day 6. Free amino acids and urea were accumulated in various tissues for cell volume regulation with accumulations facilitated by increases in proteolysis and catabolism of certain amino acids respectively. Consequently, the rate of urea synthesis increased 1.4-fold. During 6 days of emersion, turtles maintain levels of haematocrit, plasma osmolality, [Na+] and [Cl-] by reducing water loss through urine output. A consequent significant decrease in excretion of nitrogenous waste, with urea excretion decreased more drastically (buccopharyngeal excretion precluded by lack of water) than ammonia excretion, resulted in a switch to ammonoteley. In emersion, turtles apparently decreased urea synthesis following suppression of amino acid catabolism with a proportional reduction in proteolysis. The brain of P. sinensis had high tolerance of ammonia at cellular and sub-cellular levels, tolerating transient increases in ammonia contents of the brain up to 16 A?mol g-1 brain after injection with a sub-lethal dose of NH4Cl. Through the use of MK801 (an NMDA receptor antagonist) and MSO (a glutamine synthetase inhibitor) prior to a lethal dose of NH4Cl, it appears that ammonia toxicity in P. sinensis involves the activation of NMDA receptors and that glutamine accumulation could not be the major cause of death.
URI: http://scholarbank.nus.edu.sg/handle/10635/15651
Appears in Collections:Ph.D Theses (Open)

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