The temperature challenges on cardiac performance in winter-quiescent and migration-stage eels Anguilla anguilla

The present study was undertaken to examine cardiac responses to some of the temperature challenges that eels encounter in their natural environment. The contractile properties of ventricular muscle was studied on electrically paced tissue strips after long term acclimation at 0 °C, 10 °C, or 20 °C, and following acute ± 10 °C temperature changes. The time-course of contraction, and thus maximal attainable heart rates, was greatly influenced by working temperature, but was independent of acclimation history. The absolute force of contraction and power production (i.e. the product of force and stimulation frequency) was significantly influenced by acute temperature decrease from 20 °C to 10 °C. The role of adrenaline as a modulator of contraction force, power production, rates of contraction and relaxation, and minimum time in contraction was assessed. Increased adrenergic tonus elicited a positive inotropic, temperature-dependent response, but did not influence twitch duration. This suggests that adrenaline acts as an agent in maintaining an adequate contractile force following temperature challenges. A significant increased relative ventricular mass was observed in 0 °C and 10 °C-acclimated eels compared to 20 °C-acclimated, which suggests that at low temperatures, eels secure cardiac output by heart enlargement. Inhibition of specific sarcolemmal Ca(2+) channels by selective drug treatment revealed that, depending on temperature, L-type channels is the major entry site, but also that reverse-mode Na(+)/Ca(2+)-exchange and store operated calcium entry contribute to the pool of activator Ca(2+).

Maintenance of resting tension in the american eel (Anguilla rostrata L.) heart is dependent upon exogenous fuel and the sarcoplasmic reticulum

The relationship between extracellular glucose and management of cell Ca(2+) in the heart of the American eel (Anguilla rostrata) was indirectly assessed by monitoring the performance of isolated ventricular strips at 20 degrees C. Twitch force increased in ventricular strips under specific conditions of 30 bpm pacing and an extracellular Ca(2+) challenge from 1.5 to 9.5 mM. The response was independent of any exogenous metabolic fuel in the medium. Resting tension was maintained when glucose was available, but in the absence of a metabolic fuel, resting tension increased in response to the increase in extracellular Ca(2+) level. When ventricular strips were treated with iodoacetate to inhibit glycolysis, a Ca(2+) challenge resulted in a decrease in twitch force in association with an approximately equivalent increase in resting tension even in the presence of exogenous glucose. However, when pyruvate (5 mM) was substituted as a metabolic fuel, twitch force increased as a function of extracellular Ca(2+), and resting tension was maintained in the presence of iodoacetate. Therefore, there is a need for an extracellular fuel but not a specific metabolic requirement for glucose to maintain the performance characteristics, which are presumably related to the management of intracellular Ca(2+) levels. Ventricular strips were treated with ryanodine to inhibit Ca(2+) release and uptake by the sarcoplasmic reticulum (SR). Ryanodine treatment impaired postrest potentiation at high extracellular Ca(2+) levels. In the presence of ryanodine, the protective effect of glucose on the increase in resting tension in the face of an extracellular Ca(2+) challenge was eliminated. Considered together, the results reveal that the heart of the American eel has a requirement for an extracellular fuel to manage intracellular Ca(2+) at high Ca(2+) loads, and that the SR plays a role in the beat-to-beat regulation of Ca(2+) at a frequency of 30 bpm, high Ca(2+) load, and 20 degrees C.

Activity levels of enzymes of energy metabolism in heart and red muscle are higher in north-temperate-zone than in Amazonian teleosts

Fish living in the Amazon basin typically have body temperatures of about 30°C, whereas freshwater fishes of the north-temperate zone are eurythermal, with typical body temperatures of 10-20°C in summer. Enzyme activity levels in heart and red muscle of Amazonian species, which display various physiological mechanisms for dealing with hypoxic conditions, were compared with those in north-temperate-zone species. Five Amazonian species (acará-açu (Astronotus ocellatus), acari-bodó (Lipossarcus pardalis), tambaqui (Colossoma macropomum), tamoatá (Hoplosternum littorale), and pirarucu (Arapaima gigas)) and four north-temperate-zone species (American eel, bullhead, yellow perch, and rainbow trout) were studied. The Amazonian species included obligate and facultative air breathers. Activities of key indicator enzymes associated with carbohydrate metabolism, fatty acid metabolism, the citric acid cycle, and the electron-transport chain were determined. There was no obvious correlation between cardiac enzyme activity levels and the potential ability of fish to maintain blood oxygen levels in hypoxic water or the capacity of isolated heart preparations to survive anoxia. In heart, activity levels of hexokinase, phosphofructokinase, pyruvate kinase, lactate dehydrogenase, citrate synthase, cytochrome oxidase, and β-hydroxyacyl CoA dehydrogenase were about twice as high in north-temperate-zone species than in Amazonian species. Activities of red-muscle enzymes, especially those associated with aerobic fatty acid metabolism, were significantly higher in comparable north-temperate-zone species relative to Amazonian species. Increased enzyme activity levels in north-temperate-zone species relative to Amazonian species is considered to be an adaptation to generally lower body temperatures. This finding is consistent with earlier comparisions of Antarctic and north-temperate-zone species and with the results of studies of cold acclimation within north-temperate-zone fishes.