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

Abstract

Throughout the life cycle of the rainbow trout (Oncorhynchus mykiss), the heart exhibits periods of enhanced growth. Two such instances are cardiac enlargement associated with sexual maturity in males and heart growth at seasonally low environmental temperatures. Heart growth includes a parallel increase in the number of mitochondria. These natural models of heart growth have 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 tracking the incorporation of l-[2,6-(3)H]phenylalanine into trichloracetic-acid-precipitable protein. Amino acid incorporation into mitochondrial protein was linear with respect to time and was inhibited by chloramphenicol. Radiolabel was selectively enhanced in molecular mass fractions over the same size range as polypeptides known to be encoded by the mitochondrial genome. Protein synthesis was measured in mitochondria isolated from sexually mature animals and from animals subjected to different thermal regimes. The relative ventricular mass of sexually mature male rainbow trout was significantly greater than that of sexually mature females (0. 104+/-0.004 versus 0.087+/-0.002; mean +/- s.e.m.). Mitochondria isolated from the heart 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 phenylalanine mg(-)(1 )protein min(-)(1)). That is, 'male' mitochondria are inherently predisposed to synthesize protein at faster rates. We speculate that the difference may result from higher levels of mitochondrial RNA in males than in females. Mitochondria isolated from the heart of sexually immature rainbow trout acclimated to 13 degrees C synthesized protein at the same rate at 25 degrees C (0.456+/-0.075 pmolphenylalanine mg(-)(1 )protein min(-)(1)) and 15 degrees C (0.455+/-0.027 pmol phenylalanine mg(-)(1 )protein min(-)(1)). However, the 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 the temperature range 25 to 5 degrees C, the rate of mitochondrial protein synthesis is not directly coupled to the rate of respiration. Thermal acclimation to 5 degrees C did not result in positive thermal compensation in either the rate of protein synthesis or the rate of oxygen consumption by isolated mitochondria. In a further series of experiments, total protein synthesis and oxygen consumption 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 between 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 approximately 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 and 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 isolated from the heart of females such that the former are able to synthesize protein at a faster rate. The rate of mitochondrial protein synthesis does not correlate with the greater than twofold changes in rates of oxygen consumption induced by acute changes in assay temperature, suggesting that protein synthesis is not directly coupled to rates of ATP or GTP synthesis.