Studies On The Relationship Between The Liver, Kidney Mitochondrial Metabolism And The Resting Metabolism Of The Southern Catfish

The effects of water temperature on the resting metabolic rate and on the metabolic rates of liver and kidney mitochondria in the southern catfish(Silurus meridionalis Chen)were tested,The relationship between the mitochondrial metabolism and the resting metabolism of the catfish was also examined and analyzed in this study.In February,2007,40 one-year-old juveniles,from the same batch of this fish by artificial fertilization,were divided into 5 groups for different temperature-treatments.The resting metabolic rate of each fish was measured using a continuous-flow respirometer.The tested fish was then killed and the oxygen consumption rates of mitochondria isolated from its liver and kidney were determined using an oxygen electrode,respectively,at the same temperature as its resting metabolism measurement.The main results were as follows: (1)In order to test metabolic substrate preference of the mitochondria isolated from the liver of the fish,the oxygen consumption rates of the liver mitochondria were determined with succinate and pyruvate + malate as metabolic substrate, respectively.The results showed that the rate with succinate oxidation was 21.56±1.70 n mol O₂/min/mg and that with pyruvate + malate oxidation was 13.96±0.86 n mol O₂/min/mg,and the former was significantly higher than the latter(p＜0.05).(2)The oxygen consumption rate of liver and kidney mitochondria with succinate as metabolic substrate was determined at 2.5,17.5,22.5,27.5,and 32.5℃.The oxygen consumption rates per mg of mitochondrial protein(Mp)of the liver mitochondria were 12.24,11.76,16.28,21.56,and 31.43 n mol O₂/min/mg,and the Mp of kidney were 11.68,17.84,23.33,39.16,and 39.61 nmolO₂/min/mg, respectively.The Mp of kidney was not significantly different from that of the liver at 12.5℃,but the former was significantly higher than the latter at other temperatures(p＜0.05).(3)At the temperatures of 12.5,17.5,22.5,27.5,and 32.5℃,the resting metabolic rates(Ms:mg O₂/h/kg)of the fish were 26.64,49.03,73.64,102.40,and 156.72 mg O₂/h/kg,respectively.The ratios of gross oxygen consumption of the liver mitochondrial metabolism to that of the resting metabolism were 7.14%,3.19%, 2.93%,1.72%,and 2.00%,while the ratios of the kidney were 4.51,2.75,2.19, 1.91,and 1.16%,respectively.The ratio in either liver or kidney at 12.5℃was significantly higher(p＜0.05)than those at other temperatures.(4)Ms,Mp of liver and kidney increased respectively with increasing temperature. There were significant double-logarithmic linear correlations in each of the three kinds of metabolic rates with temperature.The slopes(temperature exponents b) in the equation for Ms,Mp of liver and kidney were 1.880,1.163,and 1.369.The covariance analysis showed that differences among the three values of b were significant,i.e.the pattern of individual resting metabolism changing with temperature was different from that of the mitochondrial metabolism,in either liver or kidney.The main conclusions were as follows:(1)It showed by the discussion that the mitochondria isolated from the liver prefer succinate as the metabolic substrate,in common with the fishes reported,which should be a functional reference for formulating the experimental methods in mitochondrial bioenergetics of fish.(2)Oxygen consumption rate of the kidney mitochondria was higher than that of liver mitochondria at each of the tested temperature,and temperature sensitivity in the former showed a higher degree compared with the latter.The patterns of Mp changing with temperature were different between the two organs.The optimal range of temperature for the kidney mitochondria was 27.5～32.5℃, whereas for the liver mitochondria it was higher than 32.5℃.It indicated from the phenomena above that there should be considerable differences in mitochondrial metabolism between different tissues in this fish.(3)The ratio of gross oxygen consumption of the mitochondrial metabolism to that of the resting metabolism decreased with increasing temperature.Mitochondrial metabolism in the either liver or kidney could maintain a relatively high oxygen consumption rate at the low temperature,which should not be induced by increase in mitochondrial volume density,but rather by that in metabolic capacity of mitochondria.The trial in this study for comparison between the two levels of metabolism at the whole animal and their mitochondria provided the experimental evidence to evaluate the adaptive mechanism in mitochondria for ectothermic vertebrate to resist ambient cold impact.