**6. Environmental influences**

cling of calcium during contraction must account for about 45% of the total calcium transi‐ ent [86]. On the other hand, measurements of the NCX activity showed a forward NCX-rate under physiological conditions of 36 amol pF–1 s–1, which is sufficient to remove the total cal‐

In agreement with the notion that calcium cycling by the SR takes place on a beat-to-beat basis and could contribute to the activation of contraction, measurements of SR calcium load with rapid caffeine application and specific protocols used to measure steady-state and max‐ imal SR calcium loading, showed that the SR calcium content in trout ventricular and atrial myocytes can be up to 10 times higher than in mammalian myocytes [58,61,77,84], and that the rate of SR calcium uptake under physiological conditions is also sufficient to remove the total calcium required for the activation of contraction between beats at physiological beat‐ ing rates and temperatures in trout atrial and ventricular myocytes [75,95]. In the presence of β-adrenergic stimulation, the relationship between SR calcium uptake and the cytosolic calcium level was shifted towards lower calcium concentrations, but when normalized to the total calcium transport SR calcium uptake was only slightly increased from 35 to 41% by

It is interesting to note that the SR calcium load in fish cardiomyocytes is more than 10-fold higher than the amount of calcium released on each beat [84, 95, 103]. The reason for such an excessive SR calcium load compared to the calcium requirements on a beat-to-beat basis re‐ mains elusive, but it definitely provides fish species with an intracellular calcium reservoir that allows the heart to function under adverse conditions where SR calcium refilling is im‐ paired for extended periods of time. This also explains why the inhibition of SR calcium up‐ take (with cyclopazonic acid) or increasing SR calcium leak (with ryanodine) may have a

Atrial cardiomyocytes have both greater SR Ca2+ loads [105-106] and make greater use of SR Ca2+ than ventricular cardiomyocytes [60, 105]. The degree of atrio-ventricular difference in SR Ca2+ load and usage is also species specific, being larger in rainbow trout than crucian

The reliance on intracellular Ca2+ leads to more rapid contraction kinetics of the fish at‐ rium due to faster calcium release and reuptake rates via the SR relative to ventricular tis‐ sue in a variety of teleost species (e.g. rainbow trout - [60]; burbot - [105]). Lowering of cytosolic Ca2+ concentration in this manner is via sarco-endoplasmic reticulum Ca2+-AT‐ Pase (SERCA), and thus the higher SERCA protein expression found in atrial contributes to this shorter duration of isometric contraction relative to ventricular myocardium [107]. In addition, the strength of contraction in teleost atrial muscle has greater ryanodine sensi‐ tivity than that in ventricles [60, 62] and is similar in magnitude to what is typically seen

Interestingly, the maximal capacity and steady-state SR Ca2+ load does not necessarily cor‐ relate with SERCA activity or ryanodine sensitivity in all ectotherms. Inverse relationships between ryanodine sensitivity and SR Ca2+ load have been found in rainbow trout, burbot and crucian carp [105]. While atrial contraction is more strongly inhibited by ryanodine

cium required for the activation of contraction in a few hundred ms [76].

much slower effect on contraction in fish than in mammalian hearts.

β-adrenergic stimulation [102].

232 New Advances and Contributions to Fish Biology

carp or burbot [105].

in many mammals [82].

The interspecific variation of fish cardiac anatomy and physiology is guided by evolutionary adaptation to different habitats, modes of life and activity levels. Because ectothermic fish are exposed to different temperatures either seasonally or via the vertical thermocline, tem‐ perature-dependent regulation of cardiac contractility is crucial. Many fish face unpredicta‐ ble temperature changes, meaning that fish hearts must have intrinsic mechanisms or rely on post-translational modifications to protect against acute temperature changes when there is not enough time for transcriptional regulation to take effect [108, 34].

Cardiac performance may change through independent changes in stroke volume and heart rate, with relative contributions of each varying substantially among fish. During ex‐ ercise, more athletic fish modulate cardiac output preferentially by increasing SV, while less active species preferentially modulate HR in different species [54-55]. With environ‐ mental hypoxia, reflex bradycardia occurs and increased stroke volume is necessary to maintain cardiac output [109-110]. Higher temperatures have been shown to increase heart rate while colder temperatures decrease stroke volume in crucian carp [10]. For this review, the impact of temperature on cardiac contractility will be focused on. Mechanisms of control of both stroke volume and heart rate must be considered when looking at tem‐ perature sensitivity of the fish heart.

There appears to be seasonal variation with the activity of pacemaker cells (marine plaice - [111]). Heart rate is maintained despite the lower temperatures without the need for extrin‐ sic modification. Changes in heart rate seen with temperature are not solely dependent on pacemaker cells. Therefore, changes that are seen may be due to ion channel temperature sensitivity. Chronic temperature stress can also modify K+ channel conductances and reduce action potential duration to enable higher heart rates (trout - [112]). Thermal acclimation modifies functional properties and subunit composition of KIR2 channels [113]. Cold temper‐ atures induce decreased IK1, possibly increasing the excitability of cardiomyocytes but in‐ creased IKr leading to limited AP duration and decreased refractoriness of the heart [114]. These changes in IK appear when it is the predominant current, namely in ventricular cardi‐ omyocytes [66] while the changes in IKr appear in both atrial and ventricular cardiomyocytes [114]. The density in particular of ventricular IK1 increases in warm-acclimated vs. cold accli‐ mated fish (trout - [113]).

ventricular cardiomyocytes, with ventricular maximum SR Ca2+ load varying only in the

Functional and Structural Differences in Atria Versus Ventricles in Teleost Hearts

http://dx.doi.org/10.5772/53506

235

As previously mentioned, isoform compositional changes can lead can lead to variation in contractility. Modulation of MHC isoform expression patterns with temperature also leads to changes in myofibrillar ATPase activity (crucian carp - [10]). This may also be a speciesrelated difference, since not all species display the same changes. The degree to which each chamber varies is still unclear. Differing paralog usage between the chambers may be linked to temperature-induced Ca2+ sensitivity differences. Examples of this may exist in important of the contractile element. Troponin I, the inhibitory subunit of troponin, has multiple paral‐ ogs expressed in cardiac tissue. The relative expression of these paralogs in the ventricle shifts with temperature, possibly leading to changes in the calcium affinity of the entire tro‐ ponin complex [117]. However, while paralog profiles vary between tissues (heart, slow skeletal) no information is known about the atrium. TnC isoform differences between cham‐ bers [107] may also lend insight into Ca2+ sensitivity changes with temperature as the affinity of cTnC in trout is temperature dependent [118]. As previously mentioned, the very pres‐ ence of multiple fish-specific paralogs of important components of the contractile element may be indicative of how fish have evolved mechanisms to thrive in variable environments. Incorporating the chamber-specific variation with temperature response reveals the com‐

The unique structure of the fish heart accommodates the variability in function that have al‐ lowed species to exploit many different environments. The increased importance of the at‐ rial contraction in guiding changes in cardiac output indicates that some of this physiological versatility may be due to atrio-ventricular differences.These two main cham‐ bers of the fish heart differ in terms of contractile properties from the molecular level of E-C coupling and AP morphology, and the whole heart morphology and function reflect this. The fish heart is representative of early embryological stages of higher vertebrates, these studies on both the development and functioning of chamber-specific properties lends in‐ sight into proper functioning of the heart not only in fish. However, the degree of atrio-ven‐ tricular differences vary between species making it very difficult to generalize speciesspecific results across phylogeny. The species-specific differences in these two chambers lend insight into how evolutionary history guides responses to environmental factors such as ambient temperature. Fish-specific genome duplications may have allowed for multiple chamber-specific genes with variable functions that allow for flexibility in function. Further work is required to determine how genome duplication has shaped the structure and func‐ tion of the fish heart and to clarify how this wide-range of species-specific phenotypes has

ventricle of most scombrids [112].

plexity of these mechanisms in achieving survival.

**7. Summary**

been maintained.

The plasticity of E-C coupling in fish has been suggested to be based, at least in part, on these temperature-induced changes in IK [114]. These modifications lead to changes in Ca2+ influx and hence variation in E-C coupling. Ca2+ pumps and ion channels them‐ selves are also known to be sensitive to acute temperature change [103]. The large SR calcium stores and low calcium sensitivity of ryanodine receptors have been proposed to play a critical role in maintaining contraction at lower temperatures than mammals [81]. Higher SR Ca2+ content may be necessary to initiate and maintain SR Ca2+ release events at lower temperatures [80,115].

Different species may have variations in response to temperature depending on their evolu‐ tionary history. Eurythermal fish such as trout must be able to cope with seasonal tempera‐ ture fluctuations as well as more acute temperatures through the thermal gradient. Other stenothermal species are only able to cope with a more limited temperature range, though this range may be considered on the extreme end of the range for a eurythermal fish (e.g. Antarctic fish). Interspecies variation in excitation-contraction coupling seen with tempera‐ ture acclimation may be indicative of the capacity for thermal niche expansion. This sub‐ stantial variation in SR Ca2+ release relative to SL Ca2+ influx ratio exists between species as well as being dependent on acute and chronic temperature changes. Cold-tolerant active fish have a larger capacity to store Ca2+ in the SR than mammals (e.g. active teleosts - [115]; scombroids - [116]). The atrio-ventricular differences in Ca2+ loading are also more pro‐ nounced in cold-acclimated than warm acclimated fish (trout - [112].

However, the contribution of the SR to cytosolic Ca2+ management varies with tempera‐ ture in fish hearts. With cold acclimation, SR function is also enhanced more strongly in atrial than ventricular myocytes (trout - [60]). More cold tolerant species may also possess greater SR Ca2+ content to begin with (bluefin tuna and mackerel - [112]). The degree of modification of ICa to offset the negative effects of colder temperatures appears to be spe‐ cies specific, with many teleosts such as trout demonstrating relatively temperature-insen‐ sitive Ca2+ flux [103] whereas certain scombroids show significant reductions in ICa with decreasing temperatures [112]. This temperature sensitivity may differ between atrial and ventricular cardiomyocytes, with ventricular maximum SR Ca2+ load varying only in the ventricle of most scombrids [112].

As previously mentioned, isoform compositional changes can lead can lead to variation in contractility. Modulation of MHC isoform expression patterns with temperature also leads to changes in myofibrillar ATPase activity (crucian carp - [10]). This may also be a speciesrelated difference, since not all species display the same changes. The degree to which each chamber varies is still unclear. Differing paralog usage between the chambers may be linked to temperature-induced Ca2+ sensitivity differences. Examples of this may exist in important of the contractile element. Troponin I, the inhibitory subunit of troponin, has multiple paral‐ ogs expressed in cardiac tissue. The relative expression of these paralogs in the ventricle shifts with temperature, possibly leading to changes in the calcium affinity of the entire tro‐ ponin complex [117]. However, while paralog profiles vary between tissues (heart, slow skeletal) no information is known about the atrium. TnC isoform differences between cham‐ bers [107] may also lend insight into Ca2+ sensitivity changes with temperature as the affinity of cTnC in trout is temperature dependent [118]. As previously mentioned, the very pres‐ ence of multiple fish-specific paralogs of important components of the contractile element may be indicative of how fish have evolved mechanisms to thrive in variable environments. Incorporating the chamber-specific variation with temperature response reveals the com‐ plexity of these mechanisms in achieving survival.

## **7. Summary**

There appears to be seasonal variation with the activity of pacemaker cells (marine plaice - [111]). Heart rate is maintained despite the lower temperatures without the need for extrin‐ sic modification. Changes in heart rate seen with temperature are not solely dependent on pacemaker cells. Therefore, changes that are seen may be due to ion channel temperature sensitivity. Chronic temperature stress can also modify K+ channel conductances and reduce action potential duration to enable higher heart rates (trout - [112]). Thermal acclimation modifies functional properties and subunit composition of KIR2 channels [113]. Cold temper‐ atures induce decreased IK1, possibly increasing the excitability of cardiomyocytes but in‐ creased IKr leading to limited AP duration and decreased refractoriness of the heart [114]. These changes in IK appear when it is the predominant current, namely in ventricular cardi‐ omyocytes [66] while the changes in IKr appear in both atrial and ventricular cardiomyocytes [114]. The density in particular of ventricular IK1 increases in warm-acclimated vs. cold accli‐

The plasticity of E-C coupling in fish has been suggested to be based, at least in part, on these temperature-induced changes in IK [114]. These modifications lead to changes in Ca2+ influx and hence variation in E-C coupling. Ca2+ pumps and ion channels them‐ selves are also known to be sensitive to acute temperature change [103]. The large SR calcium stores and low calcium sensitivity of ryanodine receptors have been proposed to play a critical role in maintaining contraction at lower temperatures than mammals [81]. Higher SR Ca2+ content may be necessary to initiate and maintain SR Ca2+ release events

Different species may have variations in response to temperature depending on their evolu‐ tionary history. Eurythermal fish such as trout must be able to cope with seasonal tempera‐ ture fluctuations as well as more acute temperatures through the thermal gradient. Other stenothermal species are only able to cope with a more limited temperature range, though this range may be considered on the extreme end of the range for a eurythermal fish (e.g. Antarctic fish). Interspecies variation in excitation-contraction coupling seen with tempera‐ ture acclimation may be indicative of the capacity for thermal niche expansion. This sub‐ stantial variation in SR Ca2+ release relative to SL Ca2+ influx ratio exists between species as well as being dependent on acute and chronic temperature changes. Cold-tolerant active fish have a larger capacity to store Ca2+ in the SR than mammals (e.g. active teleosts - [115]; scombroids - [116]). The atrio-ventricular differences in Ca2+ loading are also more pro‐

However, the contribution of the SR to cytosolic Ca2+ management varies with tempera‐ ture in fish hearts. With cold acclimation, SR function is also enhanced more strongly in atrial than ventricular myocytes (trout - [60]). More cold tolerant species may also possess greater SR Ca2+ content to begin with (bluefin tuna and mackerel - [112]). The degree of modification of ICa to offset the negative effects of colder temperatures appears to be spe‐ cies specific, with many teleosts such as trout demonstrating relatively temperature-insen‐ sitive Ca2+ flux [103] whereas certain scombroids show significant reductions in ICa with decreasing temperatures [112]. This temperature sensitivity may differ between atrial and

nounced in cold-acclimated than warm acclimated fish (trout - [112].

mated fish (trout - [113]).

234 New Advances and Contributions to Fish Biology

at lower temperatures [80,115].

The unique structure of the fish heart accommodates the variability in function that have al‐ lowed species to exploit many different environments. The increased importance of the at‐ rial contraction in guiding changes in cardiac output indicates that some of this physiological versatility may be due to atrio-ventricular differences.These two main cham‐ bers of the fish heart differ in terms of contractile properties from the molecular level of E-C coupling and AP morphology, and the whole heart morphology and function reflect this. The fish heart is representative of early embryological stages of higher vertebrates, these studies on both the development and functioning of chamber-specific properties lends in‐ sight into proper functioning of the heart not only in fish. However, the degree of atrio-ven‐ tricular differences vary between species making it very difficult to generalize speciesspecific results across phylogeny. The species-specific differences in these two chambers lend insight into how evolutionary history guides responses to environmental factors such as ambient temperature. Fish-specific genome duplications may have allowed for multiple chamber-specific genes with variable functions that allow for flexibility in function. Further work is required to determine how genome duplication has shaped the structure and func‐ tion of the fish heart and to clarify how this wide-range of species-specific phenotypes has been maintained.
