Fish cardiorespiratory physiology in an era of climate change

This review examines selected areas of cardiovascular physiology where there have been impressive gains of knowledge and indicates fertile areas for future research. Because arterial blood is usually fully saturated with oxygen, in- creasing cardiac output is the only means for transferring substantially more oxygen to tissues. Consequently, any behav- ioural or environmental change that alters oxygen uptake typically involves a change in cardiac output, which in fishes can amount to a threefold change. During exercise, not all fishes necessarily have the same ability as salmonids to increase cardiac output by increasing stroke volume; they rely more on increases in heart rate instead. The benefits associated with increasing cardiac output via stroke volume or heart rate are unclear. Regardless, all fishes examined so far show an exqui- site cardiac sensitivity to filling pressure and the cellular basis for this heightened cardiac stretch sensitivity in fish is being unraveled. Even so, a fully integrated picture of cardiovascular functioning in fishes is hampered by a dearth of studies on venous circulatory control. Potent positive cardiac inotropy involves stimulation of sarcolemmal b-adrenoceptors, which in- creases the peak trans-sarcolemmal current for calcium and the intracellular calcium transient available for binding to tro- ponin C. However, adrenergic sensitivity is temperature-dependent in part through effects on membrane currents and receptor density. The membrane currents contributing to the pacemaker action potential are also being studied but remain a prime area for further study. Why maximum heart rate is limited to a low rate in most fishes compared with similar- sized mammals, even when Q10 effects are considered, remains a mystery. Fish hearts have up to three oxygen supply routes. The degree of coronary capillarization circulation is of primary importance to the compact myocardium, unlike the spongy myocardium, where venous oxygen partial pressure appears to be the critical factor in terms of oxygen delivery. Air-breathing fishes can boost the venous oxygen content and oxygen partial pressure by taking an air breath, thereby pro- viding a third myocardial oxygen supply route that perhaps compensates for the potentially precarious supply to the spongy myocardium during hypoxia and exercise. In addition to venous hypoxemia, acidemia and hyperkalemia can ac- company exhaustive exercise and acute warming, perhaps impairing the heart were it not for a cardiac protection mecha- nism afforded by b-adrenergic stimulation. With warming, however, a mismatch between an animal's demand for oxygen (a Q10 effect) and the capacity of the circulatory and ventilatory systems to delivery this oxygen develops beyond an opti- mum temperature. At temperature extremes in salmon, it is proposed that detrimental changes in venous blood composi- tion, coupled with a breakdown of the cardiac protective mechanism, is a potential mechanism to explain the decline in maximum and cardiac arrhythmias that are observed. Furthermore, the fall off in scope for heart rate and cardiac output is used to explain the decrease in aerobic scope above the optimum temperature, which may then explain the field observa- tion that adult sockeye salmon (Oncorhynchus nerka (Walbaum in Artedi, 1792)) have difficulty migrating to their spawn- ing area at temperatures above their optimum. Such mechanistic linkages to lifetime fitness, whether they are cardiovascular or not, should assist with predictions in this era of global climate change. Resume´ : Notre retrospective examine certains domaines choisis de la physiologie cardiovasculaire qui ont connu un avancement impressionnant des connaissances et elle signale des avenues prometteuses pour la recherche future. Parce que le sang arteriel est en general completement satureen oxygene, le seul moyen de transferer plus d'oxygene aux tissus est d'augmenter le debit cardiaque. En consequence, tout changement comportemental ou environnemental qui modifie l'apport d'oxygene implique typiquement une variation du debit cardiaque, ce qui chez les poissons peut representer un changement de l'ordre de trois fois. Durant l'exercice, tous les poissons ne possedent pas necessairement la capacitequ'ont les salmonides d'accrooˆtre le debit cardiaque en augmentant le volume systolique; ils utilisent plutot une acceleration du rythme cardiaque. Les benefices associesal'augmentation du debit cardiaque par le debit systolique ou par le rythme car-