Introduction
With over 20 000 species of teleost fish, considerable interspecific diversity in cardiac anatomy and physiology is expected. This is the outcome of evolutionary adaptation to different habits, modes of life and activity levels. For example, athletic species have a more powerful heart than sedentary species, and fish such as hagfish, carp and eel normally show a much higher degree of myocardial hypoxia tolerance than species such as salmonids (Farrell, 1991
; Farrell and Jones, 1992). Plasticity in cardiac form and function has also been demonstrated during ontogeny, and the cardiovascular flexibility exhibited during embryonic and larval development, is nicely reviewed by Pelster (2003). What is less well appreciated, however, is the high degree of intraspecific cardiac plasticity displayed by post-larval fishes. Accordingly, this review explores what is known about intraspecific cardiac plasticity among juvenile and adult fishes. This intraspecific plasticity, like that exhibited during development, may well reflect individual variability on which natural selection could act.
In this review, we focus primarily on temperature effects, which are relatively well studied, and on the effects of other environmental and biological factors that modify cardiac anatomy and physiology, including food deprivation, sexual maturation, exercise training and rearing under aquaculture conditions. Further, we summarize recent work on cardiac preconditioning and myocardial hypoxia tolerance in fishes, and discuss the potential implications of this work. Preconditioning is a short-term form of cardiac plasticity that has the potential to protect the heart from insults that might normally lead to cardiac damage, dysfunction or death. Preconditioning has been the focus of several thousand mammalian studies (e.g. see review by Yellon and Downey, 2003), and so the handful of recent studies in fish, which already point to important intraspecific differences, may find application outside the piscine world. Similarly, researchers who wish to stimulate cardiac growth to replace damaged myocardial tissue in mammals, may be heartened to discover that fish cardiac tissue, unlike the mammalian heart, does not lose its ability for hyperplastic growth with age. In fact, we suspect that the high degree of intraspecific plasticity that we describe below is partly related to the fact that fish hearts grow through hyperplasia as well as hypertrophy.
The heart powers an internal convection system for the whole animal, and in this context, global comparisons of cardiac function (e.g. cardiac output, stroke volume) are best represented in units of ml min–1 kg–1 body mass. However, relative ventricular mass (RVM) can vary considerably (e.g. by 50% intraspecifically, see below), and thus of units of ml min–1 g–1 ventricular mass or cardiac power output (mW g–1 ventricular mass) allow us to interpret whether differences in cardiac function are due to changes in heart size, and/or plasticity in cellular physiology. We utilize both measurements of cardiac function in this review, because as a more mechanistic understanding of cellular plasticity emerges, elucidating the roles of these cellular changes will require increasingly refined comparators of cardiac performance.
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