Mitochondrial protein synthesis in rainbow trout (Oncorhynchus mykiss) heart is enhanced in sexually mature males but impaired by low temperature

Throughout the life cycle of the rainbow trout (Oncorhynchus mykiss), the h eart exhibits periods of enhanced growth. Two such instances are cardiac en largement associated with sexual maturity in males and heart growth at seas onally low environmental temperatures. Heart growth includes a parallel inc rease in the number of mitochondria. These natural models of heart growth h ave been exploited to study protein synthesis directed by the mitochondrial genome. Methods were developed to assess protein synthesis in mitochondria isolated from the heart of rainbow trout. Protein synthesis was assessed by trackin g the incorporation of L-[2,6-H-3]phenylalanine into trichloracetic-acid-pr ecipitable protein. Amino acid incorporation into mitochondrial protein was linear with respect to time and was inhibited by chloramphenicol. Radiolab el was selectively enhanced in molecular mass fractions over the same size range as polypeptides known to be encoded by the mitochondrial genome. Prot ein synthesis was measured in mitochondria isolated from sexually mature an imals and from animals subjected to different thermal regimes. The relative ventricular mass of sexually mature male rainbow trout was sig nificantly greater than that of sexually mature females (0.104 +/- 0.004 ve rsus 0.087 +/- 0.002; mean +/- S.E.M.). Mitochondria isolated from the hear t of males synthesized protein at a faster rate than mitochondria isolated from the heart of females (0.22 +/- 0.02 versus 0.11 +/- 0.02 pmol phenylal anine mg(-1) protein min(-1)). That is,'male' mitochondria are inherently p redisposed to synthesize protein at faster rates. We speculate that the dif ference may result from higher levels of mitochondrial RNA in males than in females. Mitochondria isolated from the heart of sexually immature rainbow trout acc limated to 13 degrees C synthesized protein at the same rate at 25 degrees C (0.456 +/- 0.075 pmol phenylalanine mg(-1) protein min(-1)) and 15 degree s C (0.455 +/- 0.027 pmol phenylalanine mg(-1) protein min(-1)). However, t he rate of protein synthesis was severely impaired at 5 degrees C (0.125 +/ - 0.02 pmol phenylalanine mg(-1) protein min(-1)). Since the rate of state 3 respiration by isolated mitochondria decreased in a linear fashion over t he temperature range 25 to 5 degrees C, the rate of mitochondrial protein s ynthesis is not directly coupled to the rate of respiration. Thermal acclim ation to 5 degrees C did not result in positive thermal compensation in eit her the rate of protein synthesis or the rate of oxygen consumption by isol ated mitochondria. In a further series of experiments, total protein synthesis and oxygen cons umption were measured in isolated myocytes. The rate of oxygen consumption by myocytes remained constant over the temperature range 25 to 5 degrees C. There was no difference in the rate of total cell protein synthesis betwee n 25 degrees C (1.73 +/- 0.29 pmol phenylalanine 10(6) cells(-1) h(-1)) and 15 degrees C (2.12 +/- 0.19 pmol phenylalanine 10(6) cells(-1) h(-1)), but at 5 degrees C protein synthesis was substantially impaired to approximate ly one-sixth of the level observed at 15 degrees C. As such, rates of total cell protein synthesis were not directly coupled to rates of respiration a nd were curtailed at low temperature. In vitro studies show that mitochondria isolated from the heart of sexually mature male rainbow trout are inherently different from mitochondria isola ted from the heart of females such that the former are able to synthesize p rotein at a faster rate. The rate of mitochondrial protein synthesis does n ot correlate with the greater than twofold changes in rates of oxygen consu mption induced by acute changes in assay temperature, suggesting that prote in synthesis is not directly coupled to rates of ATP or GTP synthesis.