MORPHOMETRIC AND BIOCHEMICAL CHARACTERISTICS OF VENTRICULAR HYPERTROPHY IN MALE RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

We examined the morphometric and biochemical effects of ventricular hy pertrophy in male rainbow trout (Oncorhynchus mykiss) during sexual ma turation. Our investigation focused on characterizing the growth of ve ntricular layers, on cardiomyocyte dimensions (length, cross-sectional area and cell volume) and on the activities of enzymes involved in in termediary metabolism. Relative ventricle mass (100xventricle mass/bod y mass) increased by as much as 2.4-fold during sexual maturation [as defined by an increasing gonadosomatic index (100xgonad mass/body mass )], and this resulted in an increased proportion of epicardium relativ e to endocardium, Ventricular enlargement was associated with increase d length (+31%) and transverse cross-sectional area (+83%) of cardiomy ocytes, which resulted in an expansion of up to 2.2-fold in mean myocy te volume (from 1233 to 2751 mu m(3)) These results indicate that sexu al maturation induces ventricular enlargement through myocyte hypertro phy. Cell length and cross-sectional area were similar in both myocard ial layers, and myocytes were elliptical rather than circular in trans verse cross section. Ventricular hypertrophy did not alter transverse cell shape, perhaps reflecting the maintenance of short diffusion dist ances for small molecules as cells hypertrophy, Myocyte hypertrophy co uld not account entirely for the sevenfold range of ventricle masses f rom different-sized fish, indicating that myocyte hyperplasia contribu tes substantially to ventricular growth as trout grow. Measurements of the maximal activities of metabolic enzymes demonstrated that ventric ular hypertrophy was associated with (1) higher epicardial but not end ocardial activities of citrate synthase (by 23%) and beta-hydroxyacyl- CoA dehydrogenase (by 20%); (2) lower activities of hexokinase (by 50% ) in both layers, and (3) no change in lactate dehydrogenase or pyruva te kinase activities, which were also similar between layers. These re sults suggest that the energetic needs of the hypertrophied trout vent ricle may be met through increased reliance on fatty acid oxidation, p articularly by the endocardium, but decreased reliance on glucose as a metabolic fuel in both layers.