Cardiac plasticity in fishes: environmental influences and intraspecific differences

The heart as a working model to explore themes and strategies for anoxic survival in ectothermic vertebrates

Most vertebrates die within minutes when deprived of molecular oxygen (anoxia), in part because of cardiac failure, which can be traced to an inadequate matching of cardiac ATP supply to ATP demand. Cardiac power output (PO; estimated from the product of cardiac output and central arterial pressure and an indirect measure of cardiac ATP demand) is directly related to cardiac ATP supply up to some maximal level during both normoxia (ATP supply estimated from myocardial O(2) consumption) and anoxia (ATP supply estimated from lactate production rates). Thus, steady state PO provides an excellent means to examine anoxia tolerance strategies among ectothermic vertebrates by indicating a matching of cardiac glycolytic ATP supply and demand. Here, we summarize in vitro measurements of PO data from rainbow trout, freshwater turtles and hagfishes to provide a reasonable benchmark PO of 0.7 mW g(-1) for maximum glycolytic potential of ectothermic hearts at 15 degrees C, which corresponds to a glycolytic ATP turnover rate of about 70 nmol ATP g(-1) s(-1). Using this benchmark to evaluate in vivo PO data for hagfishes, carps and turtles, we identify two cardiac survival strategies, which in conjunction with creative waste management techniques to reduce waste accumulation, allow for long-term cardiac survival during anoxia in these anoxia-tolerant species. Hagfish and crucian carp exemplify a strategy of evolving such a low routine PO that routine cardiac ATP demand lies within the range of the maximum cardiac glycolytic potential. Common carp and freshwater turtles exemplify an active strategy of temporarily and substantially decreasing cardiac and whole body metabolism so that PO is below maximum cardiac glycolytic potential during chronic anoxia despite being quite close to this potential under normoxia.

Endurance exercise differentially stimulates heart and axial muscle development in zebrafish (Danio rerio)

Mechanical load is an important factor in the differentiation of cells and tissues. To investigate the effects of increased mechanical load on development of muscle and bone, zebrafish were subjected to endurance swim training for 6 h/day for 10 wk starting at 14 days after fertilization. During the first 3 wk of training, trained fish showed transiently increased growth compared with untrained (control) fish. Increased expression of proliferating cell nuclear antigen suggests that this growth is realized in part through increased cell proliferation. Red and white axial muscle fiber diameter was not affected. Total cross-sectional area of red fibers, however, was increased. An improvement in aerobic muscle performance was supported by an increase in myoglobin expression. At the end of 10 wk of training, heart and axial muscle showed increased expression of the muscle growth factor myogenin and proliferating cell nuclear antigen, but there were major differences between cardiac and axial muscle. In axial muscle, expression of the "slow" types of myosin and troponin C was increased, together with expression of erythropoietin and myoglobin, which enhance oxygen transport, indicating a shift toward a slow aerobic phenotype. In contrast, the heart muscle shifts to a faster phenotype but does not become more aerobic. This suggests that endurance training differentially affects heart and axial muscle.

Hybridization among Divergent Stocks of Largemouth Bass (Micropterus salmoides) Results in Altered Cardiovascular Performance: The Influence of Genetic and Geographic Distance

Animal populations exhibit wide ranges of divergence associated with both geographic and genetic distances. Here, we examined the role of crossing distance on the cardiovascular response to exhaustive exercise among differentiated stocks of largemouth bass Micropterus salmoides at 10 degrees C and 20 degrees C. Stocks of 2+ fish were produced using adults from three regions in the midwestern United States (southeastern Wisconsin, northwestern Wisconsin, and west central Minnesota) and were crossed with fish from central Illinois. Doppler flow probes were used to quantify cardiac output, heart rate, and stroke volume. Cardiac variables (both resting and maximal) were consistently lowest in pure Illinois fish relative to the F(1) interstock hybrids. Additionally, when exposed to exercise, cardiac variables for F(1) interstock hybrids required approximately 40% longer to return to resting levels compared with the pure Illinois stock. However, the time required to exhaust fish was similar across stocks. Interestingly, all of the stocks (including the interstock hybrids and pure Illinois) maintained cardiac scope. In general, the patterns observed in cardiovascular performance were consistent for both water temperatures. Multiple regression analysis was used to determine which of the divergence metrics contributed to variation in cardiovascular performance in interstock hybrids. Mitochondrial DNA data (genetic distance) were infrequently identified as a significant source of variation in cardiovascular performance. However, genetic distance data for the neutral allozyme markers revealed that these stocks have experienced significant divergence. Latitude (geographic distance) accounted for between 31% and 45% of variation observed in the recovery parameters. This study suggests that the magnitude of stock divergence is an important determinant in the degree to which cardiovascular performance of bass is altered from interstock hybridization and associated breakdown of coadapted gene complexes. Furthermore, these data suggest that translocating bass among even geographically discrete regions or among stocks with reasonable genetic similarity can result in substantial performance alterations at the F(1) generation.

Influence of local adaptation and interstock hybridization on the cardiovascular performance of largemouth bassMicropterus salmoides

In this study, the cardiovascular response to exhaustive exercise among differentiated stocks of largemouth bass Micropterus salmoides was compared at 10°C and 20°C to assess the level of their local adaptation. In addition, the impact that interstock hybridization had on adaptive differences was assessed using F1 hybrids. To accomplish these assessments, four genetically distinct stocks of fish were produced using adults from two regions in the midwestern United States identified as distinct conservation management units (central Illinois, IL and southeastern Wisconsin, WI): both P1 stocks and both reciprocal F1interstock hybrids. Cardiac variables (both resting and maximal) were consistently lowest for the IL×IL stock relative to the WI×WI stock and both F1 interstock hybrids. Interestingly, however, all groups of fish were able to maintain similar levels of cardiac scope. All fish responded to exercise by increasing heart rate and decreasing stroke volume,consistent with the notion that largemouth bass modulate cardiac output via frequency. After exercise, cardiac variables returned to resting levels 25-35% more rapidly for IL×IL fish relative to all other groups at 20°C. At 10°C, recovery rates for both P1 stocks were similar but more rapid than the interstock hybrids. Collectively, these results indicate that the locally adapted stock (IL×IL) exhibited cardiovascular adaptations that enabled rapid cardiovascular recovery and maintenance of low resting cardiac output and heart rate. Conversely, the translocated stock (WI×WI) and the interstock hybrids required longer for cardiovascular variables to recover after exercise and exhibited higher resting levels of cardiac output and heart rate. This study provides some of the first direct evidence of a physiological mechanism by which mixing stocks could potentially decrease fitness and illustrates the magnitude of the intraspecific diversity of cardiovascular performance.

Linking swimming performance, cardiac pumping ability and cardiac anatomy in rainbow trout

We exploited the inherent individual diversity in swimming performance of rainbow trout Oncorhynchus mykiss to investigate the hypothesis that maximum cardiac performance is linked to active metabolic rate (AMR) and critical swimming speed (Ucrit). Six hundred juveniles(body mass ∼150 g) were screened using a swimming challenge of 1.2 m s-1 to identify `poor swimmers' and `good swimmers', i.e. the first and last 60 fish to fatigue, respectively. These 120 fish were individually tagged and then reared in common tanks for 9 months, where they grew at similar rates and achieved a similar body mass of approximately 1100 g. Critical swimming speed (Ucrit) was then measured individually in tunnel respirometers, with simultaneous recordings of cardiac output via a ventral aortic flow probe. The group of individuals that were screened as poor swimmers remained so, with a significantly (27%) lower Ucrit than good swimmers [89±10 cm s-1vs 123±5 cm s-1 (mean ± s.e.m.), respectively, N=6], a 19%lower AMR (147±12 μmol min-1 kg-1vs181±11 μmol min-1 kg-1, respectively), and a 30% lower maximum in vivo cardiac output (47.3±4.7 ml min-1 kg-1vs 68.0±5.2 ml min-1 kg-1, respectively). When cardiac performance was compared with an in situ heart preparation, hearts from poor swimmers had a significantly (26%) lower maximum cardiac output (45.9±1.9 ml min-1 kg-1vs 56.4±2.3 ml min-1 kg-1, respectively) and a 32% lower maximum cardiac power output at a high afterload (3.96±0.58 mW g-1vs 5.79±1.97 mW g-1, respectively). Cardiac morphology was visualised in vivo by Doppler echography on anaesthetised individual fish and revealed that poor swimmers had a significantly more rounded ventricle (reduced ventricle length to height ratio) compared with good swimmers, which in turn was correlated with fish condition factor. These results provide clear evidence that maximum cardiac performance is linked to AMR and Ucrit and indicate that a simple screening test can distinguish between rainbow trout with lower active metabolic rate, Ucrit, maximal cardiac pumping capacity and a more rounded ventricular morphology. These distinguishing traits may have been retained for 9 months despite a common growing environment and growth.