Is the teleost heart oxygen limited? - Insights using "hyperoxic" incubations of contracting cardiac tissue from rainbow trout

Mechanisms of cardiac collapse at high temperature in a marine teleost (Girella nigrians)

Heat-induced mortality in ectotherms may be attributed to impaired cardiac performance, specifically a collapse in maximum heart rate (fHmax), although the physiological mechanisms driving this phenomenon are still unknown. Here, we tested two proposed factors which may restrict cardiac upper thermal limits: noxious venous blood conditions and oxygen limitation. We hypothesized elevated blood [K+] (hyperkalemia) and low oxygen (hypoxia) would reduce cardiac upper thermal limits in a marine teleost (Girella nigricans), while high oxygen (hyperoxia) would increase thermal limits. We also hypothesized higher acclimation temperatures would exacerbate the harmful effects of an oxygen limitation. Using the Arrhenius breakpoint temperature test, we measured fHmax in acutely warmed fish under control (saline injected) and hyperkalemic conditions (elevated plasma [K+]) while exposed to hyperoxia (200% air saturation), normoxia (100% air saturation), or hypoxia (20% air saturation). We also measured ventricle lactate content and venous blood oxygen partial pressure (PO2) to determine if there were universal thresholds in either metric driving cardiac collapse. Elevated [K+] was not significantly correlated with any cardiac thermal tolerance metric. Hypoxia significantly reduced cardiac upper thermal limits (Arrhenius breakpoint temperature [TAB], peak fHmax, temperature of peak heart rate [TPeak], and temperature at arrhythmia [TARR]). Hyperoxia did not alter cardiac thermal limits compared to normoxia. There was no evidence of a species-wide threshold in ventricular [lactate] or venous PO2. Here, we demonstrate that oxygen limits cardiac thermal tolerance only in instances of hypoxia, but that other physiological processes are responsible for causing temperature-induced heart failure when oxygen is not limited.

Getting to the heart of anatomical diversity and phenotypic plasticity: fish hearts are an optimal organ model in need of greater mechanistic study

Natural selection has produced many vertebrate 'solutions' for the cardiac life-support system, especially among the approximately 30,000 species of fishes. For example, across species, fish have the greatest range for central arterial blood pressure and relative ventricular mass of any vertebrate group. This enormous cardiac diversity is excellent ground material for mechanistic explorations. Added to this species diversity is the emerging field of population-specific diversity, which is revealing that cardiac design and function can be tailored to a fish population's local environmental conditions. Such information is important to conservation biologists and ecologists, as well as physiologists. Furthermore, the cardiac structure and function of an individual adult fish are extremely pliable (through phenotypic plasticity), which is typically beneficial to the heart's function when environmental conditions are variable. Consequently, exploring factors that trigger cardiac remodelling with acclimation to new environments represents a marvellous opportunity for performing mechanistic studies that minimize the genetic differences that accompany cross-species comparisons. What makes the heart an especially good system for the investigation of phenotypic plasticity and species diversity is that its function can be readily evaluated at the organ level using established methodologies, unlike most other organ systems. Although the fish heart has many merits as an organ-level model to provide a mechanistic understanding of phenotypic plasticity and species diversity, bringing this potential to fruition will require productive research collaborations among physiologists, geneticists, developmental biologists and ecologists.

No evidence for pericardial restraint in the snapping turtle (Chelydra serpentina) following pharmacologically-induced bradycardia at rest or during exercise

Most animals elevate cardiac output during exercise through a rise in heart rate (f_H), whilst stroke volume (V_S) remains relatively unchanged. Cardiac pacing reveals that elevating f_H alone does not alter cardiac output, which is instead largely regulated by the peripheral vasculature. In terms of myocardial oxygen demand, an increase in f_H is more costly than that which would incur if V_S instead were to increase. We hypothesized that f_H must increase because any substantial rise in V_S would be constrained by the pericardium. To investigate this hypothesis, we explored the effects of pharmacologically-induced bradycardia, with ivabradine treatment, on V_S at rest and during exercise in the common snapping turtle (Chelydra serpentina) with intact or opened pericardium. We first showed that, in isolated myocardial preparations, ivabradine exerted a pronounced positive inotropic effect on atrial tissue, but only minor effects on ventricle. Ivabradine reduced f_H in vivo, such that exercise tachycardia was attenuated. Pulmonary and systemic V_S rose in response to ivabradine. The rise in pulmonary V_S largely compensated for the bradycardia at rest, leaving total pulmonary flow unchanged by ivabradine, although ivabradine reduced pulmonary blood flow during swimming (exercise x ivabradine interaction, P<0.05). Although systemic V_S increased, systemic blood flow was reduced by ivabradine both at rest and during exercise, in spite of ivabradine's potential to increase cardiac contractility. Opening the pericardium had no effect on f_H, V_S or blood flows before or after ivabradine, indicating that the pericardium does not constrain V_S in turtles, even during pharmacologically-induced bradycardia.

The effects of elevated potassium, acidosis, reduced oxygen levels, and temperature on the functional properties of isolated myocardium from three elasmobranch fishes: clearnose skate (Rostroraja eglanteria), smooth dogfish (Mustelus canis), and sandbar shark (Carcharhinus plumbeus)

Elevated plasma potassium levels (hyperkalemia), reduced plasma pH (acidosis), reduced blood oxygen content, and elevated temperatures are associated with species-specific rates of at-vessel and post-release mortality in elasmobranch fishes. The mechanism linking these physiological disturbances to mortality remains undetermined however, and we hypothesize that the proximate cause is reduced myocardial function. We measured changes in the functional properties of isolated ventricular myocardial strips from clearnose skate (Rostroraja eglanteria), smooth dogfish (Mustelus canis), and sandbar shark (Carcharhinus plumbeus) when subjected to the following stressors (both in isolation and in combination): hyperkalemia (7.4 mM K⁺), acidosis (from 7.9 to 7.1), and reduced oxygen (to 31% O₂ saturation) applied at temperatures 5 °C above and below holding temperatures. We selected these species based on phylogenetic distance, diverse routine activity levels, and their tolerance to capture and transport. Stressors had a few significant species-specific detrimental impacts on myocardial function (e.g., a 33-45% decrease in net force under acidosis + low O₂). Net force production of myocardial strips from clearnose skate and smooth dogfish approximately doubled following exposure to isoproterenol, demonstrating that these species possess beta-adrenergic receptors and that their stimulation could provide a mechanism for preservation of cardiac function during stress. Our results suggest that disruption of physiological homeostasis associated with capture may fatally impair cardiac function in some elasmobranch species, although research with more severe stressors is needed.

Effects of hypoxic acclimation on contractile properties of the spongy and compact ventricular myocardium of steelhead trout (Oncorhynchus mykiss)

The trout ventricle has an outer compact layer supplied with well-oxygenated arterial blood from the coronary circulation, and an inner spongy myocardium supplied with oxygen poor venous blood. It was hypothesized that: (1) the spongy myocardium of steelhead trout (Oncorhynchus mykiss), given its routine exposure to low partial pressures of oxygen (PO₂), would be better able to maintain contractile performance (work) when exposed to acute hypoxia (100 to 10% air saturation) relative to the compact myocardium, and would show little benefit from hypoxic acclimation; and (2) the compact myocardium from hypoxia-acclimated (40% air saturation) fish would be better able to maintain work during acute exposure to hypoxia relative to normoxia-acclimated individuals. Consistent with our expectations, when PO₂ was acutely lowered, net work from the compact myocardium of normoxia-acclimated fish declined more (by ~ 73%) than the spongy myocardium (~ 50%), and more than the compact myocardium of hypoxia-acclimated fish (~ 55%), and hypoxic acclimation did not benefit the spongy myocardium in the face of reduced PO₂. Further, while hypoxic acclimation resulted in a 25% (but not significant) decrease in net work of the spongy myocardium, the performance of the compact myocardium almost doubled. This research suggests that, in contrast to the spongy myocardium, performance of the compact myocardium is improved by hypoxic acclimation; and supports previous research suggesting that the decreased contractile performance of the myocardium upon exposure to lowered PO₂ may be adaptive and mediated by mechanisms within the muscle itself.

Hypoxic acclimation negatively impacts the contractility of steelhead trout (Oncorhynchus mykiss) spongy myocardium

Cardiac stroke volume (SV) is compromised in Atlantic cod and rainbow trout following acclimation to hypoxia (i.e., 40% air saturation; ~8 kPa O2) at 10-12°C, and this is not due to changes in heart morphometrics or maximum achievable in vitro end-diastolic volume. To examine if this diminished SV may be related to compromised myocardial contractility, we used the work-loop method to measure work and power in spongy myocardial strips from normoxic- and hypoxic-acclimated steelhead trout when exposed to decreasing Po2 levels (21 to 1.5 kPa) at several frequencies (30-90 contractions/min) at 14°C (their acclimation temperature). Work required to lengthen the muscle, as during filling of the heart, was strongly frequency dependent (i.e., increased with contraction rate) but was not affected by hypoxic acclimation or test Po2. In contrast, although shortening work was less frequency dependent, this parameter and network (and power) 1) were consistently lower (by ~30-50 and ~15%, respectively) in strips from hypoxic-acclimated fish and 2) fell by ~40-50% in both groups from 20 to 1.5 kPa Po2, despite the already-reduced myocardial performance in the hypoxic-acclimated group. In addition, strips from hypoxic-acclimated trout showed a poorer recovery of net power (by ~15%) when returned to normoxia. These results strongly suggest that hypoxic acclimation reduces myocardial contractility, and in turn, may limit SV (possibly by increasing end-systolic volume), but that this diminished performance does not improve the capacity to maintain myocardial performance under oxygen limiting conditions.