The Ontogeny Of Digesitive System And Kinetics Of Carnitine Palmitoyltransferase â…  Of Chinese Sucker(Myxocyprinus Asiaticus)

Chinese sucker(Myxocyprinus asiaticus) was the only species of the family Catostomidae in Asia, which had been an endangered species listed in the Category II of the endangered aquatic and land animal species in China. It had an important scientific value in geographic classification. At present, although several studies were involved in the nutrition of juvenile Chinese sucker and the production of the fish species, little information was available about the ontogeny of digesitive system and kinetics of carnitine palmitoyltransferase I in Chinese sucker (M. asiaticus). The aim of this study was to describe the ontogeny of digestive system, and analyse the development of digestive enzymes and the kinetics of carnitine palmitoyltransferase I (CPT I) in Chinese sucker, which would contribute to the information about morphology, nutriology and biology of development of this fish species. The main results are shown as follows.1. The development of digestive system in Chinese sucker (M. asiaticus)The present study was conducted to determine the ontogenetic development of the digestive tract and its accessory structures (liver, pancreas and gall bladder) in agastric larval Chinese sucker M. asiaticus with the histological and ultrastructural approaches from hatching to56days after hatching (DAH). Based on its feeding mode, and analysing the main histological features of the digestive system, larval development in Chinese sucker was divided into three stages from hatching:stage1(endotrophic period):1-6DAH; stage2(endoexotrophic period):7-14DAH; stage3(exclusively exotrophic period):from15DAH onwards. At hatching, the digestive tract of the larvae consisted of an undifferentiated straight tube. At4DAH, the mouth opened, and the digestive tract was differentiated into buccopharyngeal cavity, esophagus and intestine. At7DAH, fish started to feed exogenously. Yolk sac was completely exhausted at15DAH. Until56DAH, the digestive tract of the larvae displayed regularly arranged microvilli, abundant vacuoles and protein inclusion bodies. The pancreas, liver and gall bladder were functional from6DAH, which enabled larvae to ingest, digest, and assimilate the first exogenous food. In comparison with teleosts that have a stomach, the development of the digestive tract of the agastric Chinese sucker seemed relatively slow. 2. Developmental changes of selected digestive enzymes in Chinese sucker (M. asiaticus) during larval ontogenyThe ontogenesis and specific activities of pancreatic (trypsin and amylase) and intestinal enzymes (alkaline phosphatase and aminopeptidase N) were investigated in Chinese sucker M. asiaticus larvae from hatching to40day after hatching (DAH). All of assayed digestive enzymes were detected in newly hatched Chinese sucker larvae. However, the maximum activities varied among different digestive enzymes. From3to14DAH specific activity of amylase in Chinese sucker larvae increased, indicating the synthesis of amylase in early ontogenesis and the carbohydrates were actively catalyzed during the stage. The decrease of amylase from14to17DAH followed by an increase at20DAH in Chinese sucker larvae may be possibly due to progressive changes in the digestive tract and the subsequent response to composition and amount of the available food. The secretion level of amylase remained at the relatively low level between30and40DAH, suggesting the carnivorous feeding habitat for juvenile Chinese sucker. Significant changes of trypsin specific activities were observed from14to25DAH. The specific activity of alkaline phosphatase sharply increased between7and14DAH, indicating the formation of brush border enterocytes. Its activity decreased after peaking at14DAH, and remained constant between20and40DAH. The aminopeptidase N specific activity remained at a constant low level before12DAH. The activity of the enzyme reached the first peak at14DAH and a second peak on20DAH. Based on these observations above, the maturation of the digestive tract seemed fully achieved at20DAH in Chinese sucker. From this date, this species has an adult mode of digestion, so weaning at20DAH could be considered reasonable for Chinese sucker larvae.3. Changes of kinetics of CPT I in Chinese sucker (M. asiaticus)The total carnitine in liver tissues of Chinese sucker gradually increased from hatching to1year. The increase of hepatic total carnitine may be indicative of the enhanced biosynthesis capacity for carnitine in liver tissues. From68-day-old to1-year-old Chinese sucker, both free and total carnitine concentrations in muscle gradually increased. This increase would be a need to generate energy through β-oxidation of fatty acids for the rapid growth of the muscle during this developmental period. Both acyl and total concentrations in intestine at different stages changed violently, which may be explained by the fact that digestive tract went through a series of dramatic changes during the course of maturation. The ratio of AC/FC in liver progressively increased from hatching (1.03) to4months (1.61), but at the age of1year it significantly decreased to0.71, and then reached1.73by2years. In muscle tissues the ratio of AC/FC was highest at the age of68-days-old and lowest at4months. The whole Chinese sucker larvae at hatching had a high value of apparent Km (4.51±0.16mM) for carnitine; the kinetic parameter Vmax measured in the whole Chinese sucker larvae at hatching was4.53±0.09nmol/min/mg protein. I the kinetic parameter Vmax measured in intestine tissue showed an increasing trend with age. In our study, the free carnitine concentrations in the examined tissues (liver, muscle and intestine) at all stages of development were all less than the respective Km. The data imply that Chinese sucker may require supplemental carnitine to ensure the activity of CPT I.