Magnetic resonance imaging (MRI) reveals high cardiac ejection fractions in red-footed tortoises (Chelonoidis carbonarius)

How the trabecular layer impacts on left ventricular function

The ventricular trabecular layer is crucial in embryonic life. In adults, the proportion of trabecular-to-compact myocardium varies substantially between individuals, within individuals over time, and yet exhibits almost no correlation to pump function since most individuals with excessive trabeculation are asymptomatic. The question of how functional is the myocardium of the trabecular layer, relative to the myocardium of the compact layer, has been difficult to answer but it is often assumed to be inferior. An answer is now emerging from recent advances and it can improve our understanding of how the trabecular layer impacts on pathogenicity. This narrative review concerns natural variation in trabeculation, tissue organization, transcriptomics, immunohistochemistry, vascularization, electrical propagation, diastolic function and compliance, systolic function, and ejection fraction. There are no overt transcriptional differences in the adult stage, and the myocardium is equally equipped with sarcomeric proteins, mitochondria, and vascular supply. The similar structural features are consistent with myocardium with a similar stroke work per gram tissue, along with a high ejection fraction of the trabecular layer. In conclusion, the myocardium of the trabecular and compact layers is highly similar and this offers a logical explanation for the reproducible observations that most individuals with excessive trabeculation are asymptomatic.

The Elusive Hypertrophy of the Python Heart

The Burmese python, one of the world's largest snakes, has reached celebrity status for its dramatic physiological responses associated with digestion of enormous meals. The meals elicit a rapid gain of mass and function of most visceral organs, particularly the small intestine. There is also a manyfold elevation of oxygen consumption that demands the heart to deliver more oxygen. It therefore made intuitive sense when it was reported that the postprandial response entailed a 40% growth of heart mass that could accommodate a rise in stroke volume. Many studies, however, have not been able to reproduce the 40% growth of the heart. We collated published values on postprandial heart mass in pythons, which include several instances of no change in heart mass. On average, the heart mass is only 15% greater. The changes in heart mass did not correlate to the mass gain of the small intestine or peak oxygen consumption. Hemodynamic studies show that the rise in cardiac output does not require increased heart mass but can be fully explained by augmented cardiac filling and postprandial tachycardia. Under the assumption that hypertrophy is a contingent phenomenon, more recent experiments have employed two interventions such as feeding with a concomitant reduction in hematocrit. The results suggest that the postprandial response of the heart can be enhanced, but the 40% hypertrophy of the python heart remains elusive.

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.

A Physio-Logging Journey: Heart Rates of the Emperor Penguin and Blue Whale

Physio-logging has the potential to explore the processes that underlie the dive behavior and ecology of marine mammals and seabirds, as well as evaluate their adaptability to environmental change and other stressors. Regulation of heart rate lies at the core of the physiological processes that determine dive capacity and performance. The bio-logging of heart rate in unrestrained animals diving at sea was infeasible, even unimaginable in the mid-1970s. To provide a historical perspective, I review my 40-year experience in the development of heart rate physio-loggers and the evolution of a digital electrocardiogram (ECG) recorder that is still in use today. I highlight documentation of the ECG and the interpretation of heart rate profiles in the largest of avian and mammalian divers, the emperor penguin and blue whale.

What determines systemic blood flow in vertebrates?

In the 1950s, Arthur C. Guyton removed the heart from its pedestal in cardiovascular physiology by arguing that cardiac output is primarily regulated by the peripheral vasculature. This is counterintuitive, as modulating heart rate would appear to be the most obvious means of regulating cardiac output. In this Review, we visit recent and classic advances in comparative physiology in light of this concept. Although most vertebrates increase heart rate when oxygen demands rise (e.g. during activity or warming), experimental evidence suggests that this tachycardia is neither necessary nor sufficient to drive a change in cardiac output (i.e. systemic blood flow, Q̇ _sys) under most circumstances. Instead, Q̇ _sys is determined by the interplay between vascular conductance (resistance) and capacitance (which is mainly determined by the venous circulation), with a limited and variable contribution from heart function (myocardial inotropy). This pattern prevails across vertebrates; however, we also highlight the unique adaptations that have evolved in certain vertebrate groups to regulate venous return during diving bradycardia (i.e. inferior caval sphincters in diving mammals and atrial smooth muscle in turtles). Going forward, future investigation of cardiovascular responses to altered metabolic rate should pay equal consideration to the factors influencing venous return and cardiac filling as to the factors dictating cardiac function and heart rate.