Influência do regime alimentar sobre o metabolismo, excreção e osmorregulação de jovens de Tubarão Bambu (Chiloscyllium punctatum) em diferentes salinidades.
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Data
2018-01-30
Autores
Santos, Rafael Silva dos
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Universidade Estadual Paulista (Unesp)
Resumo
Elasmobrânquios marinhos são reconhecidamente ureotélicos, ou seja, sintetizam, retém e excretam preferencialmente ureia como produto do metabolismo do nitrogênio, mantendo os seus fluídos corpóreos iso-osmóticos, ou ligeiramente hiper-osmóticos, em relação ao meio externo. Assim, o alto requerimento por nitrogênio nesses animais, tanto para o crescimento quanto para a manutenção de sua estratégia ureosmótica, pode tornar os elasmobrânquios limitados pela disponibilidade de alimento. O presente estudo buscou avaliar o efeito do regime de alimentação e da aclimatação a salinidade reduzida sobre respostas fisiológicas e bioquímicas relacionadas ao metabolismo e excreção de nitrogênio em jovens de tubarão bambu (Chiloscyllium punctatum). Para tanto, foram determinados as formas preferenciais de excreção de nitrogênio (amônia_N e ureia_N), consumo de oxigênio, coeficiente de nitrogênio, níveis plasmáticos de compostos nitrogenados (amônia e ureia), principais osmólitos (proteínas totais, [Na+], [Cl-]; osmolaridade total) e glicose em jovens C. punctatum aclimatados em salinidade 32 e 20ppm, e mantidos sob regime regular de alimentação (2-3% da biomassa) e privação alimentar por 7 dias. Nossos resultados evidenciaram que a ureia é a principal excreta nitrogenada nos jovens de tubarão bambu mantidos em ambos os regimes de alimentação e em ambas as salinidades testadas. Após a alimentação os animais em 32ppm aumentam significativamente a excreção de nitrogênio na forma de amônia, efeito não observado nos animais mantidos à 20ppm. No entanto, nos animais aclimatados a salinidade reduzida houve um aumento quantitativo na excreção total de nitrogênio dos animais, particularmente nos jovens C. punctatum mantidos em jejum, como visto pelas taxas aumentadas de excreção de amônia e ureia. Tanto o regime de alimentação quanto a aclimatação a baixa salinidade não influenciaram a taxa de consumo de oxigênio dos animais. O coeficiente de nitrogênio evidenciou que o metabolismo aeróbico dos jovens de tubarão bambu, em ambas as salinidades testadas, é altamente dependente da oxidação de proteínas e aminoácidos, tal como observado para outras espécies de elasmobrânquios. Nos animais em 32ppm foi observado um aumento do catabolismo de proteínas relacionados a estratégia de conservação de nitrogênio pelos animais, particularmente para síntese de amônia, enquanto em 20ppm foi observado um aumento do anabolismo associado a síntese proteica e de ureia pelos animais. A aclimatação a salinidade reduzida promoveu a redução da osmolaridade e das concentrações de ureia e cloreto plasmático nos jovens de C. punctatum, sugerindo que esses animais apresentam a capacidade de regular a concentração desses osmólitos nos fluidos internos a fim de manterem seu equilíbrio osmótico com relação ao meio externo. Em conjunto nossos resultados indicam que em menores salinidades os jovens de C. punctatum são fortemente limitados pelo aporte de nitrogênio via alimentação para a manutenção de sua estratégia ureosmótica, e que longos períodos sob privação alimentar em salinidades baixas podem influenciar negativamente a capacidade osmorregulatória e a retenção de nitrogênio pelos animais.
Marine elasmobranchs are ureothelic organisms which synthesize, retain, and excretes preferentially urea as the end-product of nitrogen metabolism, maintaining their internal body fluids iso-osmotic or slightly hyperosmotic relative to the external marine environment. The high requirement for nitrogen in these animals for both growth and maintenance of their ureosmotic strategy, might make elasmobranchs limited by the availability of food. Thus, the present study aimed to evaluate the effect of feeding regime and acclimation to lowered salinity on physiological and biochemical responses associated to nitrogen metabolism and excretion in juvenile of brownbanded bamboo shark (Chiloscyllium punctatum). For this purpose, we determine the rates of waste excretion of the main nitrogen end-products (ammonia_N and urea_N), oxygen consumption, nitrogen quotient, and levels of nitrogen compounds (ammonia and urea), osmolality, major ions (Na+ and Cl-), total protein and glucose in plasma of juvenile C. punctatum acclimated to both 32 and 20ppm, and kept on regular feeding regime (2-3% of body weight) or fasting for 7 days. Our results showed that urea is the main nitrogen product excreted by brownbanded bamboo shark kept at both fasting and feeding regime, at both salinities tested. After feeding, sharks at 32ppm significantly increased ammonia excretion, an effect not seen in animals acclimated to 20ppm. However, at lower salinity, animals quantitatively increased the total nitrogen excretion, particularly fasted sharks, as seen by the increased ammonia and urea excretion rates in comparison with animals maintained at 32ppm. Oxygen consumption was not affect by either fasting or feeding regime at both salinities tested. The nitrogen coefficient showed that the aerobic metabolism of brownbanded bamboo shark acclimated to both 32 and 20ppm is highly dependent on the oxidation of proteins and amino acids, as already reported to other elasmobranchs species. At 32ppm, sharks displayed an increase in catabolism of proteins related to the strategy of nitrogen conservation, particularly related to ammonia synthesis, while an increase in anabolism associated with synthesis of both urea and protein was seen in animals acclimated to 20ppm. The acclimation to lowered salinity resulted in decreases in plasmatic osmolarity, as well as in both urea and chloride levels of juveniles of C. punctatum, suggesting that these animals have the ability to regulate the concentration of these osmolytes in their internal fluids, in order to maintain their osmotic balance with the external marine environment. Overall, our results indicate that in lowered salinities the juveniles of brownbanded bamboo shark are strongly limited by the nitrogen acquired through feeding in order to maintain their ureosmotic strategy, and that fasting for prolonged periods at low salinities can negatively disturb their osmoregulatory homeostasis and affect the retention of nitrogen by the animals
Marine elasmobranchs are ureothelic organisms which synthesize, retain, and excretes preferentially urea as the end-product of nitrogen metabolism, maintaining their internal body fluids iso-osmotic or slightly hyperosmotic relative to the external marine environment. The high requirement for nitrogen in these animals for both growth and maintenance of their ureosmotic strategy, might make elasmobranchs limited by the availability of food. Thus, the present study aimed to evaluate the effect of feeding regime and acclimation to lowered salinity on physiological and biochemical responses associated to nitrogen metabolism and excretion in juvenile of brownbanded bamboo shark (Chiloscyllium punctatum). For this purpose, we determine the rates of waste excretion of the main nitrogen end-products (ammonia_N and urea_N), oxygen consumption, nitrogen quotient, and levels of nitrogen compounds (ammonia and urea), osmolality, major ions (Na+ and Cl-), total protein and glucose in plasma of juvenile C. punctatum acclimated to both 32 and 20ppm, and kept on regular feeding regime (2-3% of body weight) or fasting for 7 days. Our results showed that urea is the main nitrogen product excreted by brownbanded bamboo shark kept at both fasting and feeding regime, at both salinities tested. After feeding, sharks at 32ppm significantly increased ammonia excretion, an effect not seen in animals acclimated to 20ppm. However, at lower salinity, animals quantitatively increased the total nitrogen excretion, particularly fasted sharks, as seen by the increased ammonia and urea excretion rates in comparison with animals maintained at 32ppm. Oxygen consumption was not affect by either fasting or feeding regime at both salinities tested. The nitrogen coefficient showed that the aerobic metabolism of brownbanded bamboo shark acclimated to both 32 and 20ppm is highly dependent on the oxidation of proteins and amino acids, as already reported to other elasmobranchs species. At 32ppm, sharks displayed an increase in catabolism of proteins related to the strategy of nitrogen conservation, particularly related to ammonia synthesis, while an increase in anabolism associated with synthesis of both urea and protein was seen in animals acclimated to 20ppm. The acclimation to lowered salinity resulted in decreases in plasmatic osmolarity, as well as in both urea and chloride levels of juveniles of C. punctatum, suggesting that these animals have the ability to regulate the concentration of these osmolytes in their internal fluids, in order to maintain their osmotic balance with the external marine environment. Overall, our results indicate that in lowered salinities the juveniles of brownbanded bamboo shark are strongly limited by the nitrogen acquired through feeding in order to maintain their ureosmotic strategy, and that fasting for prolonged periods at low salinities can negatively disturb their osmoregulatory homeostasis and affect the retention of nitrogen by the animals
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Palavras-chave
Ureia, Amônia, Jejum, Consumo de oxigênio, Coeficiente de nitrogênio, Urea, Ammonia, Fasting, Oxygen consumption, Nitrogen quotient