Cardiac influence of the β3-adrenoceptor in the goldfish (Carassius auratus): a protective role under hypoxia?

Abstract

The goldfish (Carassius auratus) exhibits a remarkable capacity to survive and remain active under prolonged and severe hypoxia, representing a well-suited model to study cardiac function when oxygen availability represents a limiting factor. Under hypoxia, the goldfish heart increases its performance, this representing a putative component of hypoxia tolerance; however, underlying mechanisms have not been yet elucidated. We aimed to investigate the role of β3-ARs in the mechanisms which modulate the goldfish heart performance, also in relation to oxygen levels. By western blotting analysis, we found that the goldfish heart expresses β3-ARs, and this expression increases under hypoxia. Effects of β3-ARs stimulation were analysed by using an ex vivo working heart preparation. Under normoxia, the β3-ARs selective agonist BRL37344 (10−12-10−7M) elicited a concentration-dependent increase of contractility abolished by the specific β3-AR antagonist (SR59230A; 10−8M), but not by α/β1/β2-ARs inhibitors (phentolamine, nadolol, and ICI118,551; 10−7M). Under acute hypoxia, BRL37344 did not affect the goldfish heart performance. However, SR59230A, but not phentolamine, nadolol, and ICI118,551, abolished the time-dependent enhancement of contractility which characterizes the hypoxic goldfish heart. Under both normoxia and hypoxia, adenylate cyclase and cAMP were found to be involved in the β3-ARs-dependent downstream transduction pathway. Our findings suggest the presence of functional β3-ARs in the goldfish heart, whose activation modulates the basal performance and contributes to the hypoxia-dependent increase of contractility.

The Hypoxia Tolerance of the Goldfish (Carassius auratus) Heart: The NOS/NO System and Beyond

“…The ex vivo isolated and perfused goldfish heart showed basal values of preload, afterload, HR, CO, SV and SW (Table 1) comparable to those previously reported [normoxia: [8,28,36,38]; hypoxia: [8,12,38]].…”

“…Hearts were isolated, cannulated and connected to a perfusion apparatus as described [12,36]. They were perfused with a solution containing (in mmol L −1 ) NaCl 124.9, KCl 2.49, MgSO 4 0.94, NaH 2 PO 4 1.0, Glucose 5.0, NaHCO 3 15.0, and CaCl 2 1.2, equilibrated with a mixture of either 99.5% O 2 and 0.5% CO 2 (normoxia), or 10% O 2 , 0.5% CO 2 and 89.5% N 2 (hypoxia) [8]. The pH was adjusted to 7.7-7.9.…”

“…Oxygen concentration in the input reservoir was continuously monitored by an oxygen analyzer (Milwaukee, SM600, Szeged, Hungary). Concentrations of 8.4 ± 0.2 mg O 2 L −1 (normoxia), and 2.5 ± 0.3 mg O 2 L −1 (hypoxia), means ± s.e.m., were chosen on the basis of previous data [8,37].…”

“…The remarkable ability of the goldfish heart to enhance its basal performance when exposed to a hypoxic milieu has been largely documented by studies from our laboratory [7,8,12]. These studies reported that in C. auratus, exposure to hypoxia is accompanied by an increased expression of cardiac HIF-1α (hypoxia-inducible factor 1α) and NOS.…”

“…Thus, they represent valuable experimental models for studying the physiological strategies that allow animals to survive with reduced oxygen. In several fish species, the resistance to protracted hypoxia/anoxia is supported by a preserved cardiac activity and autonomic cardiovascular control [ 7 , 8 , 9 ]. This allows them not only to mobilize glucose from hepatic glycogen stores to all tissues, but also to transport lactate to the muscle, where it is converted into the less harmful ethanol, rapidly removed via the branchial epithelium [ 10 , 11 ].…”