Vascular distensibilities have minor effects on intracardiac shunt patterns in reptiles.
TL;DR: No general linear relation between total blood flow and systemic or pulmonary blood flows is found and cardiac output alterations cannot be used as a reliable reference for prediction of shunt patterns in both T. scripta and C. durissus.
About: This article is published in Zoology.The article was published on 2017-06-01. It has received 11 citations till now. The article focuses on the topics: Cardiac shunt & Cardiac output.
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TL;DR: It is argued that when blood flow changes by a greater relative magnitude than blood pressure, conductance yields a more faithful representation of cardiovascular status.
Abstract: Vascular resistance and conductance are reciprocal indices of vascular tone that are often assumed to be interchangeable. However, in most animals in vivo , blood flow (i.e. cardiac output) typically varies much more than arterial blood pressure. When blood flow changes at a constant pressure, the relationship between conductance and blood flow is linear, whereas the relationship between resistance and blood flow is non-linear. Thus, for a given change in blood flow, the change in resistance depends on the starting point, whereas the attendant change in conductance is proportional to the change in blood flow regardless of the starting conditions. By comparing the effects of physical activity at different temperatures or between species – concepts at the heart of comparative cardiovascular physiology – we demonstrate that the difference between choosing resistance or conductance can be marked. We also explain here how the ratio of conductance in the pulmonary and systemic circulations provides a more intuitive description of cardiac shunt patterns in the reptilian cardiovascular system than the more commonly used ratio of resistance. Finally, we posit that, although the decision to use conductance or resistance should be made on a case-by-case basis, in most circumstances, conductance is a more faithful portrayal of cardiovascular regulation in vertebrates.
17 citations
Cites background from "Vascular distensibilities have mino..."
...The pulmonary and systemic vascular tone are regulated by the autonomic nervous system and a suite of other endocrine and paracrine factors (Filogonio et al., 2017; Hicks and Comeau, 1994; Joyce and Wang, 2014; Overgaard et al., 2002; Skovgaard et al., 2005) that alter vascular tone and hence divert blood flow between the two circulations....
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TL;DR: It is argued that deposing the ‘monolithic’ view of shunting leads to a more nuanced view of vertebrate cardiovascular systems, and suggests new paradigms for testing the function of shunts.
Abstract: This review explores the long-standing question: 'Why do cardiovascular shunts occur?' An historical perspective is provided on previous research into cardiac shunts in vertebrates that continues to shape current views. Cardiac shunts and when they occur is then described for vertebrates. Nearly 20 different functional reasons have been proposed as specific causes of shunts, ranging from energy conservation to improved gas exchange, and including a plethora of functions related to thermoregulation, digestion and haemodynamics. It has even been suggested that shunts are merely an evolutionary or developmental relic. Having considered the various hypotheses involving cardiovascular shunting in vertebrates, this review then takes a non-traditional approach. Rather than attempting to identify the single 'correct' reason for the occurrence of shunts, we advance a more holistic, integrative approach that embraces multiple, non-exclusive suites of proposed causes for shunts, and indicates how these varied functions might at least co-exist, if not actually support each other as shunts serve multiple, concurrent physiological functions. It is argued that deposing the 'monolithic' view of shunting leads to a more nuanced view of vertebrate cardiovascular systems. This review concludes by suggesting new paradigms for testing the function(s) of shunts, including experimentally placing organ systems into conflict in terms of their perfusion needs, reducing sources of variation in physiological experiments, measuring possible compensatory responses to shunt ablation, moving experiments from the laboratory to the field, and using cladistics-related approaches in the choice of experimental animals.
14 citations
TL;DR: Alteration in adrenergic control and relative cardiac size (which was increased in hypoxic-programmed alligators) may account for the differences between the experimental groups.
Abstract: Reptilian embryos naturally experience fluctuating oxygen levels in ovo, and developmental hypoxia has been established to have long-term impacts on cardiovascular function in vertebrates In the present study, we investigated the impact of developmental 21% (normoxia) and 10% O2 (hypoxia) on juvenile (4-year-old) American alligator cardiovascular function in animals at rest and during swimming In both experimental groups, combined right aortic and right subclavian blood flow approximately doubled during swimming Carotid blood flow increased during swimming in the hypoxia-programmed animals only, and both carotid and left aortic blood flow reached higher values in swimming hypoxic-programmed animals compared to the normoxic group However, pulmonary blood flow, which increased two to threefold during swimming (in both groups), was higher in normoxic-programmed animals at both rest and swimming The differences between programming groups were preserved after cholinergic blockade (atropine), but reduced by adrenergic receptor antagonists (propranolol and phentolamine) Propranolol and phentolamine also blunted the incremental increases in blood flows during swimming, which was especially clear in the hypoxia-programmed animals Alteration in adrenergic control and relative cardiac size (which was increased in hypoxic-programmed alligators) may account for the differences between the experimental groups
12 citations
Cites background from "Vascular distensibilities have mino..."
...In non-crocodilian reptiles, atropine has the opposite effect and decreases the right-to-left shunt by decreasing pulmonary vascular resistance (Galli et al. 2004; Filogonio et al. 2017)....
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TL;DR: The hypothesis that exposure to hypercarbia during embryonic development has long term effects on organ morphology and cardiovascular responses of C. serpentina is supported.
Abstract: Reptilian embryos often face challenging environmental gas compositions during incubation, which may inflict long-lasting effects in the individuals' physiological responses. These conditions can have a lasting effect on the animal into juvenile life as chronic prenatal exposure to hypercarbia results in enlarged hatchling organ size, higher growth rate and resting metabolic rate, although relatively smaller increment in metabolic scope during digestion. Therefore, we wanted to verify whether prenatal hypercarbia exposure would cause persistent effects on morphology and physiological responses in C. serpentina. We measured organ masses and cardiovascular parameters in five years old turtles incubated either under 3.5% hypercarbia (H3.5) or normoxia (N21). We expected that: i) organ masses of H3.5 would be bigger than N21; ii) acute exposure to hypoxia should decrease blood flows in H3.5, since metabolic scope is presumably reduced in this group. As hypoxia exposure elicits catecholamine release, we also tested cardiovascular responses to adrenaline injection. Lungs and stomach exhibited higher growth rates in H3.5. Divergent cardiovascular responses between groups to adrenaline injection were observed for heart rate, pulmonary blood flow, pulmonary mean arterial pressure, blood shunt, systemic stroke volume, and stomach perfusion. Hypoxia caused decreased systemic blood flow and cardiac output, systemic and total stroke volume, and systemic vascular conductance in H3.5. These variables were unaffected in N21, but pulmonary flow and stroke volume, and stomach blood perfusion were reduced. These data support the hypothesis that exposure to hypercarbia during embryonic development has long term effects on organ morphology and cardiovascular responses of C. serpentina.
9 citations
TL;DR: The mechanical properties of the dorsal aorta and the pulmonary artery during fasting and after feeding reveal increased pulmonary artery distensibility and elasticity during digestion, which possibly improves the Windkessel effect.
Abstract: In animals with functional division of blood systemic and pulmonary pressures, such as mammals, birds, crocodilians and a few exceptions among non-crocodilian reptiles, the vessel walls of systemic and pulmonary arteries are exquisitely adapted to endure different pressures during the cardiac cycle, systemic arteries being stronger and stiffer than pulmonary arteries. However, the typical non-crocodilian reptile heart possesses an undivided ventricle that provides similar systolic blood pressures to both circuits. This raises the question whether in these species the systemic and pulmonary mechanical vascular properties are similar. Snakes also display large organ plasticity and increased cardiac output in response to digestion, and we speculate how the vascular circuit would respond to this further stress. We addressed these questions by testing the mechanical vascular properties of the dorsal aorta and the right pulmonary artery of fasted and fed yellow anacondas, Eunectes notaeus , a snake without functional ventricular separation that also exhibits large metabolic and cardiovascular responses to digestion. Similar to previous studies, the dorsal aorta was thicker, stronger, stiffer and more elastic than the pulmonary artery. However, unlike any other species studied so far, the vascular distensibility ( i.e. the relative volume change given a pressure change) was similar for both circuits. Most striking, the pulmonary artery elasticity ( i.e. its capacity to resume its original form after being stretched) and distensibility increased during digestion, which suggests that this circuit is remodeled to accommodate the larger stroke volume and enhance the Windkessel effect, thus providing a more constant blood perfusion during digestion.
8 citations
References
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TL;DR: Monosynaptic excitatory post‐synaptic potentials produced in triceps surae motoneurones of the cat by stimulation of single afferent fibres in the muscle nerve were recorded with intracellular electrodes before and after the administration of thiopentone or pentobarbitone.
Abstract: 1. Monosynaptic excitatory post-synaptic potentials (EPSPs) produced in triceps surae motoneurones of the cat by stimulation of single afferent fibres in the muscle nerve were recorded with intracellular electrodes before and after the administration of thiopentone or pentobarbitone.
2. The average amplitude of the ‘unit’ EPSP was 0·11–0·21 mV and remained unchanged after the administration of the barbiturates (10 mg/kg, I.V.).
3. Mean quantum content (m) ranged from 1·9 to more than 5 before drug administration. The m was reduced by thiopentone (10 mg/kg, I.V.) and pentobarbitone (10 mg/kg, I.V.) by 23·1 and 24·7% respectively.
4. The barbiturates, in the doses employed, produced no alterations in the input resistance of the motoneurone membrane or in the strength—duration relation obtained by passing depolarizing current pulses through the micro-electrode.
5. It is concluded that the action of thiopentone and pentobarbitone, in the doses used, is confined to the presynaptic nerve terminals and results in a reduction in the amount of transmitter released by afferent impulses.
430 citations
"Vascular distensibilities have mino..." refers background in this paper
...This is probably because this mechanism is only discernible at very special occasions when animals present very high cardiac outputs (Hillman et al., 2014), or perhaps during anesthesia when autonomic regulation is dampened down (Weakly, 1969; Tsyrlin and Gerasimenko, 1973)....
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TL;DR: Blood flow and pressures in pulmonary and systemic circuits were studied in intact, unanesthetized turtles in both nondiving and diving states, suggesting that hypercarbia is involved in their maintenance.
Abstract: WHITE, FRED N., AND GORDON Ross. Circulatory changes during experimental diving in the turtle. Am. J. Physiol. 2 I I ( I) : I 518. I g66.-Blood flow (electromagnetic flowmeters) and pressures in pulmonary and systemic circuits were studied in intact, unanesthetized turtles (Pseudemys scripta) in both nondiving and diving states. Cardiac output (combined pulmonary, left and right arch flows) in the predive state was in the order of 50-60 ml/kg per min with 60% distributed to the pulmonary circuit (left-to right shunt). During diving, bradycardia and a reduction and redistribution of cardiac output (right-to-left shunt) occurred after an interval varying from 8 to 180 min. The development of the shunt was accompanied by a decrease in the systemic and increase in the pulmonary resistances. These changes were rapidly reversed on surfacing into air or nitrogen, but persisted when the turtles surfaced into carbon dioxide, suggesting that hypercarbia is involved in their maintenance.
113 citations
TL;DR: Three‐dimensional models of the anatomy and physiology of squamate hearts are generated and it is concluded that the squamate heart has a highly consistent design including a disproportionately large right side probably due to prevailing pulmonary bypass (intraventricular shunting).
Abstract: With approximately 7000 species, snakes and lizards, collectively known as squamates, are by far the most species-rich group of reptiles. It was from reptile-like ancestors that mammals and birds evolved and squamates can be viewed as phylogenetically positioned between them and fishes. Hence, their hearts have been studied for more than a century yielding insights into the group itself and into the independent evolution of the fully divided four-chambered hearts of mammals and birds. Structurally the heart is complex and debates persist on rudimentary issues such as identifying structures critical to understanding ventricle function. In seeking to resolve these controversies we have generated three-dimensional (3D) models in portable digital format (pdf) of the anaconda and anole lizard hearts ('typical' squamate hearts) and the uniquely specialized python heart with comprehensive annotations of structures and cavities. We review the anatomy and physiology of squamate hearts in general and emphasize the unique features of pythonid and varanid lizard hearts that endow them with mammal-like blood pressures. Excluding pythons and varanid lizards it is concluded that the squamate heart has a highly consistent design including a disproportionately large right side (systemic venous) probably due to prevailing pulmonary bypass (intraventricular shunting). Unfortunately, investigations on rudimentary features are sparse. We therefore point out gaps in our knowledge, such as the size and functional importance of the coronary vasculature and of the first cardiac chamber, the sinus venosus, and highlight areas with implications for vertebrate cardiac evolution.
93 citations
TL;DR: Alternative views are advanced suggesting that, in reptiles, cardiac shunts represent either an ancestral condition or an embryonic trait.
Abstract: The morphology of the reptilian heart results in the mixing of oxygenated and deoxygenated blood (cardiac shunts). In birds and mammals cardiac shunts are detrimental, but in reptiles this condition is often considered a derived trait, conveying important physiological functions and favored by natural selection. Alternative views are advanced suggesting that, in reptiles, cardiac shunts represent either an ancestral condition or an embryonic trait.
85 citations
TL;DR: Several hypotheses are presented that suggest selective advantages for central vascular shunting in intermittent breathing reptiles with variable body temperature and metabolic rate.
Abstract: Synopsis. Consistent with the great variation in their circulatory morphology, there are distinct variations in the cardiovascular physiology of extant reptiles. The chelonian and squamate reptiles have a complexly structured heart that includes three partially separated ventricular cava. In most species (under most conditions), the ventricle acts as a single pressure pump perfusing both the pulmonary and systemic circuits. However, the varanid lizards provide a striking exception. Subtle evolutionary changes in cardiac morphology allow the ventricle of the varanid lizard to divide functionally during systole into a low pressure, pulmonary pump and a high pressure, systemic pump. The crocodilians represent yet another anatomical and physiological pattern. The ventricle is completely divided into left and right chambers as in homeotherms, but the systemic and pulmonary circuits may still communicate through the left aorta that arises from the right ventricle. A fundamental feature of all reptilian circulations is the ability to regulate the distri? bution of cardiac output between systemic and pulmonary circuits via central vascular blood shunts. Regardless of species, mechanisms for regulating intracardiac shunting involve changes in the balance between peripheral resistance of the pulmonary and sys? temic circulations, and adjustments in cardiac performance per se. Several hypotheses are presented that suggest selective advantages for central vascular shunting in intermittent breathing reptiles with variable body temperature and metabolic rate.
84 citations
"Vascular distensibilities have mino..." refers background in this paper
...…the atrioventricular valves to diminish the degree of admixture between the inflowing blood streams, most reptiles are endowed with the capacity for generating large cardiac shunts (White and Ross, 1967; Burggren, 1987; Hicks and Wang, 1996; Hicks, 1998; Jensen et al., 2014; Joyce et al., 2016)....
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...Although prominent septal divisions within the ventricle act in a concerted manner with the atrioventricular valves to diminish the degree of admixture between the inflowing blood streams, most reptiles are endowed with the capacity for generating large cardiac shunts (White and Ross, 1967; Burggren, 1987; Hicks and Wang, 1996; Hicks, 1998; Jensen et al., 2014; Joyce et al., 2016)....
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