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

Abstract

The extraordinary capacity of the goldfish (Carassius auratus) to increase its cardiac performance under acute hypoxia is crucial in ensuring adequate oxygen supply to tissues and organs. However, the underlying physiological mechanisms are not yet completely elucidated. By employing an ex vivo working heart preparation, we observed that the time-dependent enhancement of contractility, distinctive of the hypoxic goldfish heart, is abolished by the Nitric Oxide Synthase (NOS) antagonist L-NMMA, the Nitric Oxide (NO) scavenger PTIO, as well as by the PI3-kinase (PI3-K) and sarco/endoplasmic reticulum Ca2+-ATPase 2a (SERCA2a) pumps’ inhibition by Wortmannin and Thapsigargin, respectively. In goldfish hearts exposed to hypoxia, an ELISA test revealed no changes in cGMP levels, while Western Blotting analysis showed an enhanced expression of the phosphorylated protein kinase B (pAkt) and of the NADPH oxidase catalytic subunit Nox2 (gp91phox). A significant decrease of protein S-nitrosylation was observed by Biotin Switch assay in hypoxic hearts. Results suggest a role for a PI3-K/Akt-mediated activation of the NOS-dependent NO production, and SERCA2a pumps in the mechanisms conferring benefits to the goldfish heart under hypoxia. They also propose protein denitrosylation, and the possibility of nitration, as parallel intracellular events.

The goldfish Carassius auratus: an emerging animal model for comparative cardiac research

“…This increases myocardial efficiency (i.e., the force generated per O 2 consumed), thus contributing to maintain myocardial function (Stecyk et al 2004). In line with these observations, studies from our lab revealed in the isolated goldfish heart that acute hypoxia (~ 1.5 mg O 2 L −1 ) is accompanied by an increased myocardial NOS expression (Imbrogno et al 2014), and that the NOS-derived NO is crucial for the hypoxia-dependent increase of myocardial contractility, typical of this teleost (Filice et al 2020b). This is supported by the hypoxiainduced activation of the phosphatidylinositol-3 kinase (PI3-K)/protein kinase B (Akt) pathway, a well-known player in the NOS-dependent NO production (Carrillo-Sepulveda et al 2010), and SERCA2a pumps modulation, whose inhibition under hypoxia significantly reduced the timecourse increase of the goldfish heart performance (Filice et al 2020b).…”

Hypoxic and Thermal Stress: Many Ways Leading to the NOS/NO System in the Fish Heart

“…The same occurs in hypoxia-tolerant fish in which, as observed in the perfused goldfish heart, exposure to low O 2 is accompanied by the activation of these kinases [75]. Interestingly, in the hypoxic goldfish heart, the molecular events of downstream NO generation exclude the involvement of the cGMP-dependent signalling [75]. Non-cGMP-dependent pathways represent an important route for NO to control its molecular targets.…”

“…In the mammalian heart, under basal conditions and in the presence of low O 2 , the PI3-K/Akt pathway controls eNOS activity, and thus NO generation and the consequent NO-dependent signalling, being protective for the heart [76,77]. The same occurs in hypoxia-tolerant fish in which, as observed in the perfused goldfish heart, exposure to low O 2 is accompanied by the activation of these kinases [75]. Interestingly, in the hypoxic goldfish heart, the molecular events of downstream NO generation exclude the involvement of the cGMP-dependent signalling [75].…”

“…In contrast, a significant reduction of the myocardial sensitivity to stretch is observed, if NO is removed from the tissue by PTIO and if sGC is inhibited by ODQ. This is of relevance, since it indicates that the effects elicited by NO involve the cGMP cascade but are NOS-independent, thus requiring other routes for NO generation [ 75 ].…”

“…Proteins encountering denitrosylation in the hypoxic goldfish heart and the related functional significance have not yet been identified. However, it is reasonable to hypothesize that this event, by activating still undefined protective programs, contributes to preserving myocardial function when challenged by hypoxia [ 75 ].…”