Cardiorespiratory reflexes in white sturgeon (Acipenser transmontanus): Lack of cardiac baroreflex response to blood pressure manipulation?

Arterial pressure (Pa) regulation is essential to adequately distribute nutrients to metabolizing tissues, remove wastes and avoid lesions associated with hypertension. In vertebrates, short-term Pa regulation is achieved through the baroreflex, which elicits inversely proportional changes in heart rate (fH) and vascular resistance to restore Pa. The cardiac limb of this reflex has been reported in all vertebrate groups studied to date: teleosts, amphibians, snakes, lizards, crocodiles, birds and mammals - which led to the suggestion that the baroreflex is an ancient trait present in all vertebrate species. However, it is not clear whether more basal groups of vertebrates, such as cyclostomes, elasmobranchs and chondrosteans, manifest baroreflex regulation of fH. Thus, the aim of this study was to determine whether the white sturgeon (Acipenser transmontanus; Chondrostei: Acipenseridae) exhibits a cardiac baroreflex. To do so, we induced Pa perturbations through injections of phenylephrine, sodium nitroprusside (SNP) and saline solution (hypervolemia), and examined possible fH baroreflex responses. We also investigated whether fH responses triggered by fright and chemoreflex were present in this species, in order to confirm the potential of sturgeon to perform reflexive cardiac adjustments. The findings indicate that A. transmontanus exhibits reflex bradycardia in response to fright and chemoreceptor stimulation, illustrating its capacity for short-term cardiac regulation. However, this species does not display baroreflex control of fH across its physiological range. This dissociation suggests that while the nervous and cardiovascular systems of A. transmontanus are primed for rapid reflex responses, a cardiac baroreflex mechanism remains absent.

The baroreflex in aquatic and amphibious teleosts: Does terrestriality represent a significant driving force for the evolution of a more effective baroreflex in vertebrates?

All vertebrates have baroreflexes that provide fast regulation of arterial blood pressure (PA) to maintain adequate tissue perfusion and avoid vascular lesions from excessive pressures. The baroreflex is a negative feedback loop, where altered PA results in reciprocal changes in heart rate (fH) and systemic vascular conductance to restore pressure. In terrestrial environments, gravity usually leads to blood pooling in the lower body reducing venous return, cardiac filling, cardiac output and PA. Conversely, in aquatic environments, the hydrostatic pressure of surrounding water mitigates blood pooling and prevents vascular distensions. In this context, we aimed to test the hypothesis that vertebrate species that were exposed to gravity-induced hemodynamic disturbances throughout their evolutionary histories have a more effective barostatic reflex than those that were not. We examined the cardiac baroreflex of fish that perform (Clarias gariepinus and Hoplerythrinus unitaeniatus) and do not perform (Hoplias malabaricus and Oreochromis niloticus) voluntary terrestrial sojourns, using pharmacological manipulations of PA to characterize reflex changes in fH using a four-variable sigmoidal logistic function (i.e. the "Oxford technique"). Our results revealed that amphibious fish exhibit higher baroreflex gain and responsiveness to hypotension than strictly aquatic fish, suggesting that terrestriality and the gravitational circulatory stresses constitute a relevant driving force for the evolution of a more effective baroreflex in vertebrates. We also demonstrate that strictly aquatic teleosts have considerable baroreflex gain, supporting the view that the baroreflex is an ancient cardiovascular trait that appeared before vertebrates colonized the gravity-dominated realm of land.

The autonomic control of upright posture tachycardia in the arboreal lizard Iguana iguana.

In terrestrial environments, upright spatial orientation can dramatically influence animals' hemodynamics. Generally, large and elongated species are particularly sensitive to such influence due to the greater extent of their vascular beds being verticalized, favoring the establishment of blood columns in their bodies along with caudal blood pooling, and thus jeopardizing blood circulation through a cascade effect of reductions in venous return, cardiac filling, stroke volume, cardiac output, and arterial blood pressure. This hypotension triggers an orthostatic-(baroreflex)-tachycardia to normalize arterial pressure, and despite the extensive observation of this heart rate (fH ) adjustment in experiments on orthostasis, little is known about its mediation and importance in ectothermic vertebrates. In addition, most of the knowledge on this subject comes from studies on snakes. Thus, our objective was to expand the knowledge on this issue by investigating it in an arboreal lizard (Iguana iguana). To do so, we analyzed fH , cardiac autonomic tones, and fH variability in horizontalized and tilted iguanas (0°, 30°. and 60°) before and after muscarinic blockade with atropine and double autonomic blockade with atropine and propranolol. The results revealed that I. Iguana exhibits significant orthostatic-tachycardia only at 60o inclinations-a condition that is primarily elicited by a withdrawal of vagal drive. Also, as in humans, increases in low-frequency fH oscillations and decreases in high-frequency fH oscillations were observed along with orthostatic-tachycardia, suggesting that the mediation of this fH adjustment may be evolutionarily conserved in vertebrates.

The influence of midazolam on heart rate arises from cardiac autonomic tones alterations in Burmese pythons, Python molurus.

The GABAA receptor agonist midazolam is a compound widely used as a tranquilizer and sedative in mammals and reptiles. It is already known that this benzodiazepine produces small to intermediate heart rate (HR) alterations in mammals, however, its influence on reptiles' HR remains unexplored. Thus, the present study sought to verify the effects of midazolam on HR and cardiac modulation in the snake Python molurus. To do so, the snakes' HR, cardiac autonomic tones, and HR variability were evaluated during four different experimental stages. The first stage consisted on the data acquisition of animals under untreated conditions, in which were then administered atropine (2.5mgkg-1; intraperitoneal), followed later by propranolol (3.5mgkg-1; intraperitoneal) (cardiac double autonomic blockade). The second stage focused on the data acquisition of animals under midazolam effect (1.0mgkg-1; intramuscular), which passed through the same autonomic blockade protocol of the first stage. The third and fourth stages consisted of the same protocol of stages one and two, respectively, with the exception that atropine and propranolol injections were reversed. By comparing the HR of animals that received midazolam (second and fourth stages) with those that did not (first and third stages), it could be observed that this benzodiazepine reduced the snakes' HR by ~60%. The calculated autonomic tones showed that such cardiac depression was elicited by an ~80% decrease in cardiac adrenergic tone and an ~620% increase in cardiac cholinergic tone - a finding that was further supported by the results of HR variability analysis.

The Effects of Feeding on Cardiac Control of the Broad-Nosed Caiman (Caiman latirostris): The Role of the Autonomic Nervous System and NANC Factors.

The metabolic increment that occurs after feeding demands cardiovascular adjustments to be maintained, as increased heart rate (fH ) and cardiac output. In mammals, postprandial tachycardia seems to be triggered by an increase in adrenergic activity and by nonadrenergic noncholinergic (NANC) factors, while in ectothermic vertebrates, this adjustment seems to be linked to a withdrawal of vagal drive as well as to NANC factors. Because the factors behind postprandial tachycardia have not yet been investigated in crocodilians, the present study sought to evaluate the postprandial tachycardia mediators in the broad-nosed caiman. To this end, fasting and digesting animals were instrumented with intraperitoneal cannula and subcutaneous electrocardiogram electrodes (for the measurement of fH , cardiac autonomic tones, and total fH variability, as well as for a power spectral analysis of fH ). Data were then collected with the animals in an untreated state, as well as after muscarinic cholinergic blockade with atropine (2.5 mg kg-1 ) and after double autonomic blockade with atropine and propranolol (5.0 mg kg-1 ). Fasting animals' fH was ∼18 bpm, a value which increased to ∼30 bpm during digestion. After the double autonomic blockade, fasting animals exhibited an fH of ∼15 bpm, while digesting animals' fH was ∼23 bpm. This result is evidence of the presence of NANC factors with positive chronotropic effects acting during digestion. The calculated autonomic tones showed that, after feeding, the adrenergic tone increased while the cholinergic tone remained unchanged. Finally, fH variability analyses revealed that this adrenergic increase is primarily derived from circulating catecholamines.

Gill denervation eliminates the barostatic reflex in a neotropical teleost, the tambaqui (Colossoma macropomum).

The baroreflex is one of the most important regulators of cardiovascular homeostasis in vertebrates. It begins with the monitoring of arterial pressure by baroreceptors, which constantly provide the central nervous system with afferent information about the status of this variable. Any change in arterial pressure relative to its normal state triggers autonomic responses, which are characterized by an inversely proportional change in heart rate and systemic vascular resistance and which tend to restore pressure normality. Although the baroreceptors have been located in mammals and other terrestrial vertebrates, their location in fish is still not completely clear and remains quite controversial. Thus, the objective of this study was to locate the baroreceptors in a teleost, the Colossoma macropomum. To do so, the occurrence and efficiency of the baroreflex were both analyzed when this mechanism was induced by pressure imbalancements in intact fish (IN), first-gill-denervated fish (G1), and total-gill-denervated fish (G4). The pressure imbalances were initiated through the administration of the α1-adrenergic agonist phenylephrine (100 µg kg(-1)) and the α1-adrenergic antagonist prazosin (1 mg kg(-1)). The baroreflex responses were then analyzed using an electrocardiogram that allowed for the measurement of the heart rate, the relationship between pre- and post-pharmacological manipulation heart rates, the time required for maximum chronotropic baroreflex response, and total heart rate variability. The results revealed that the barostatic reflex was attenuated in the G1 group and nonexistent in G4 group, findings which indicate that baroreceptors are exclusively located in the gill arches of C. macropomum.