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.

Reduction in Pulse Pressure during Standing Can Distinguish Neurogenic Orthostatic Hypotension

Background: We investigated whether changes in the pulse pressure (PP) reduction ratio during the head-up tilt test (HUTT) can aid in distinguishing neurogenic orthostatic hypotension (OH) from non-neurogenic OH. Methods: We enrolled consecutive patients with NOH and non-neurogenic OH between January 2015 and October 2018. We compared the Valsalva ratio, the presence or absence of late phase II and IV overshoot, the pressure recovery time, and the PP reduction ratio during HUTT between the two OH groups. Results: The expiratory–inspiratory (E:I) ratio and Valsalva ratio were significantly decreased in the NOH group (p = 0.026, p < 0.001, respectively). The absence of late phase II and phase IV overshoot was more frequent in the NOH group than in the non-neurogenic OH group (p = 0.001, p < 0.001, respectively). The pressure recovery time was significantly prolonged in the NOH group (p < 0.001), which exhibited increases in the PP reduction ratio (1—minimal PP/baseline PP) during the HUTT (p < 0.001). We calculated the cutoff point for the PP reduction ratio during HUTT, which exhibited an area under the receiver operating characteristic curve of 0.766 (0.659–0.840, 95% confidence interval). The cutoff value for the PP reduction ratio during HUTT (0.571) exhibited sensitivity of 0.879 and specificity of 0.516. Conclusions: Increases in the PP reduction ratio during HUTT may be a meaningful NOH laboratory marker.

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.

Low incidence of atrial septal defects in nonmammalian vertebrates

The atrial septum enables efficient oxygen transport by separating the systemic and pulmonary venous blood returning to the heart. Only in placental mammals will the atrial septum form by the coming-together of the septum primum and the septum secundum. In up to one of four placental mammals, this complex morphogenesis is incomplete and yields patent foramen ovale. The incidence of incomplete atrial septum is unknown for groups with the septum primum only, such as birds and reptiles. We found a low incidence of incomplete atrial septum in 11 species of bird (0% of specimens) and 13 species of reptiles (3% of specimens). In reptiles, there was a trabecular interface between the atrial septum and the atrial epicardium which was without a clear boundary between left and right atrial cavities. In developing reptiles (four squamates and one crocodylian), the septum primum initiated as a sheet that acquired perforations and the trabecular interface developed late. We conclude that atrial septation from the septum primum only results in a low incidence of incompleteness. In reptiles, the atrial septum and atrial wall develop a trabecular interface, but previous studies on atrial hemodynamics suggest this interface has a very limited capacity for shunting.

Distribution and properties of cardiac and pulmonary β-adrenergic receptors in corn snakes (Pantherophis guttatus) and Boa constrictor (Boa constrictor)

The aim of the present study was to characterize β-adrenergic receptors in the snake heart and lung of corn and Boa constrictor snakes. The β-adrenergic receptor binding sites were studied in purified heart and lung membranes using the specific β-adrenergic receptor antagonist [¹²⁵J]-iodocyanopindolol (ICYP) and subtypes using selective β₁-adrenergic receptor antagonist CGP-20712A and selective β₂-adrenergic receptor antagonist ICI-118.551. A saturable and specific β-adrenergic receptor binding site was detected in cardiac membranes with maximal receptor density (B_max) of 43.99 ± 3.86 fmol/mg protein (corn snake) and 58.07 ± 2.88 fmol/mg protein (Boa constrictor) as well as K_D of 24.21 ± 7.38 pM (corn snake) and 21.48 ± 3.85 pM (Boa constrictor) and in lung membranes (B_max fmol/mg protein: 55.95 ± 16.28 (corn snake) and 107.00 ± 14.21 (Boa constrictor); K_D pM: 71.25 ± 21.92 (corn snake) and 55.04 ± 18.68 (Boa constrictor)). Competition-binding studies showed β-adrenergic receptors with low affinities to the β₂-selective adrenergic receptor antagonist and high affinity binding to β₁-selective adrenergic receptor antagonist in both heart and lung tissues of both snake species, suggesting the presence of high population of the post-synaptic β₁-adrenergic receptor subtype. It seems that the presence of the predominant β₁-subtype also in lung tissues may indicate the importance of the vascular system in the snake lung.

Contraction of atrial smooth muscle reduces cardiac output in perfused turtle hearts

Unusual undulations in resting tension (tonus waves) were described in isolated atria from freshwater turtles more than a century ago. These tonus waves were soon after married with the histological demonstration of a rich layer of smooth muscle on the luminal side of the atrial wall. Research thereafter waned and the functional significance of this smooth muscle has remained obscure. Here, we provide evidence that contraction of the smooth muscle in the atria may be able to change cardiac output in turtle hearts. In in situ perfused hearts of the red-eared slider turtle (Trachemys scripta elegans), we demonstrated that activation of smooth muscle contraction with histamine (100 nmol kg^-1 bolus injected into perfusate) reduced cardiac output by decreasing stroke volume (>50% decrease in both parameters). Conversely, inhibition of smooth muscle contraction with wortmannin (10 µmol l^-1 perfusion) approximately doubled baseline stroke volume and cardiac output. We suggest that atrial smooth muscle provides a unique mechanism to control cardiac filling that could be involved in the regulation of stroke volume during diving.

Maximum heart rate does not limit cardiac output at rest or during exercise in the American alligator (Alligator mississippiensis)

In most vertebrates, increases in cardiac output result from increases in heart rate (fH) with little or no change in stroke volume (Vs), and maximum cardiac output (Q̇) is typically attained at or close to maximum fH. We therefore tested the hypothesis that increasing maximum fH may increase maximum Q̇. To this end, we investigated the effects of elevating fH with right atrial pacing on Q̇ in the American alligator ( Alligator mississippiensis) at rest and while swimming. During normal swimming, Q̇ increased entirely by virtue of a tachycardia (29 ± 1 to 40 ± 3 beats/min), whereas Vs remained stable. In both resting and swimming alligators, increasing fH with right atrial pacing resulted in a parallel decline in Vs that resulted in an unchanged cardiac output. In swimming animals, this reciprocal relationship extended to supraphysiological fH (up to ~72 beats/min), which suggests that maximum fH does not limit maximum cardiac output and that fH changes are secondary to the peripheral factors (for example vascular capacitance) that determine venous return at rest and during exercise.

Contribution of active atrial contraction to cardiac output in anesthetized American alligators (Alligator mississippiensis)

Ventricular filling may occur directly from the venous circulation during early diastole or via atrial contraction in late diastole. The contribution of atrial contraction to ventricular filling is typically small in mammals (10-40%), but has been suggested to predominate in reptiles. We investigated the importance of atrial contraction in filling of the ventricle in American alligators (Alligator mississippiensis) by bypassing both atria (with the use of ligatures to prevent atrial filling) and measuring the resultant effects on cardiac output in anesthetized animals. Atrial ligation had no significant effects on total systemic blood flow before or after adrenaline injection. Unexpectedly, pulmonary flow was increased following atrial ligation prior to adrenaline treatment, but was unaffected after it. These findings suggest that the atria are non-essential (i.e. redundant) for ventricular filling in alligators, at least under anesthesia, but may serve as important volume reservoirs.