Natural variation in enzyme activity of the African cichlid Pseudocrenilabrus multicolor victoriae

Contrasting strategies of hypoxic cardiac performance and metabolism in cichlids and armoured catfish

The heart of tropical fishes is a particularly useful model system in which to investigate mechanisms of hypoxic tolerance. Here we focus on insights gained from two groups of fishes, cichlids and armoured catfishes. Cichlids respond to hypoxia by entering a sustained hypometabolism with decreased heart performance to match whole animal circulatory needs. Heart rate is decreased along with protein turnover to reduce adenosine triphosphate demand. This occurs despite the inherent capacity for high levels of cardiac power development. Although highly hypoxic tolerant at the whole animal level, the heart of cichlids does not have high constitutive activities of glycolytic enzymes compared to other species. Information is conflicting with respect to changes in glycolytic gene expression and enzyme activity following hypoxic exposure with some studies showing increases and others decreases. In contrast to cichlids, species of armoured catfish, that are routinely exposed to water of low oxygen content, do not display hypoxic bradycardia. Under hypoxia there are early changes in glucose trafficking suggestive of activation of glycolysis before lactate accumulation. Thereafter, heart glycogen is mobilized and lactate accumulates in both heart and blood, in some species to very high levels. Heart performance under hypoxia is enhanced by defense of intracellular pH. A functional sarcoplasmic reticulum and binding of hexokinase to the outer mitochondrial membrane may also play a role in cardioprotection. Maintenance of heart performance under hypoxia may relate to a tradeoff between air breathing via a modified stomach and circulatory demands for digestion.

Hypoxia tolerance is unrelated to swimming metabolism of wild, juvenile striped bass (Morone saxatilis)

Juvenile striped bass residing in Chesapeake Bay are likely to encounter hypoxia that could affect their metabolism and performance. The ecological success of this economically valuable species may depend on their ability to tolerate hypoxia and perform fitness-dependent activities in hypoxic waters. We tested whether there is a link between hypoxia tolerance (HT) and oxygen consumption rate (ṀO2 ) of juvenile striped bass measured while swimming in normoxic and hypoxic water, and to identify the interindividual variation and repeatability of these measurements. HT (loss of equilibrium) of fish (N=18) was measured twice collectively, 11 weeks apart, between which ṀO2  was measured individually for each fish while swimming in low flow (10.2 cm s-1) and high flow (∼67% of critical swimming speed, Ucrit) under normoxia and hypoxia. Both HT and ṀO2  varied substantially among individuals. HT increased across 11 weeks while the rank order of individual HT was significantly repeatable. Similarly, ṀO2  increased in fish swimming at high flow in a repeatable fashion, but only within a given level of oxygenation. ṀO2  was significantly lower when fish were swimming against high flow under hypoxia. There were no clear relationships between HT and ṀO2  while fish were swimming under any conditions. Only the magnitude of increase in HT over 11 weeks and an individual's ṀO2  under low flow were correlated. The results suggest that responses to the interacting stressors of hypoxia and exercise vary among individuals, and that HT and change in HT are not simple functions of aerobic metabolic rate.

Hypoxia-induced plasticity in the metabolic response of a widespread cichlid

The African cichlid, Pseudocrenilabrus multicolor victoriae is a eurytopic fish that exhibits high levels of developmental plasticity in response to dissolved oxygen availability. In this study, F1 offspring from three sites in the Mpanga River drainage of Western Uganda characterized by different dissolved oxygen (D.O.) regimes were reared under normoxic or hypoxic conditions. After 1 year, enzymes were measured to determine the tissue metabolic capacity of four different tissues: muscle, heart, brain and liver. The enzymes measured were pyruvate kinase [PK], lactate dehydrogenase [LDH], citrate synthase [CS], and cytochrome C oxidase [CCO], and an additional two, malate dehydrogenase (MDH) and fructose 1,6-bisphosphatase (FBPase), were examined in the liver only. Individuals reared under hypoxia exhibited elevated levels of LDH and CCO in the heart; and depressed activity levels of brain CS and liver CCO and MDH relative to normoxia-reared sibs. Results from this study demonstrate that long-term exposure to hypoxia during development can induce changes in the metabolic capacities of P. multicolor. This flexibility may be important in facilitating persistence in variable and/or novel environments, and in the face of increasing global hypoxia.