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Soluble adenylyl cyclase (sAC) is a HCO3 - -stimulated enzyme that produces the ubiquitous signalling molecule cAMP, and deemed an evolutionarily conserved acid-base sensor. However, its presence is not yet confirmed in bony fishes, the most abundant and diverse of vertebrates. Here, we identified sAC genes in various cartilaginous, ray-finned and lobe-finned fish species. Next, we focused on rainbow trout sAC (rtsAC) and identified 20 potential alternative spliced mRNAs coding for protein isoforms ranging in size from 28 to 186 kDa. Biochemical and kinetic analyses on purified recombinant rtsAC protein determined stimulation by HCO3 - at physiologically relevant levels for fish internal fluids (EC50 ∼ 7 mM). rtsAC activity was sensitive to KH7, LRE1, and DIDS (established inhibitors of sAC from other organisms), and insensitive to forskolin and 2,5-dideoxyadenosine (modulators of transmembrane adenylyl cyclases). Western blot and immunocytochemistry revealed high rtsAC expression in gill ion-transporting cells, hepatocytes, red blood cells, myocytes and cardiomyocytes. Analyses in the cell line RTgill-W1 suggested that some of the longer rtsAC isoforms may be preferentially localized in the nucleus, the Golgi apparatus and podosomes. These results indicate that sAC is poised to mediate multiple acid-base homeostatic responses in bony fishes, and provide cues about potential novel functions in mammals.

Background: The three-ringed polycyclic aromatic hydrocarbon (PAH) phenanthrene (Phe) has been implicated in the cardiotoxicity of petroleum-based pollution in aquatic systems, where it disrupts the contractile and electrical function of the fish heart. Phe is also found adsorbed to particulate matter and in the gas phase of air pollution, but to date, no studies have investigated the impact of Phe on mammalian cardiac function. Objectives: Our objectives were to determine the arrhythmogenic potential of acute Phe exposure on mammalian cardiac function and define the underlying mechanisms to provide insight into the toxicity risk to humans. Methods: Ex vivo Langendorff-perfused mouse hearts were used to test the arrhythmogenic potential of Phe on myocardial function, and voltageand current-clamp recordings were used to define underlying cellular mechanisms in isolated cardiomyocytes. Results: Mouse hearts exposed to ~8 [micro]M Phe for 15-min exhibited a significantly slower heart rate (p = 0.0006, N =10 hearts), a prolonged PR interval (p = 0.036, N = 8 hearts), and a slower conduction velocity (p = 0.0143, N = 7 hearts). Whole-cell recordings from isolated cardiomyocytes revealed action potential (AP) duration prolongation (at 80% repolarization; p = 0.0408, n = 9 cells) and inhibition of key murine repolarizing currents-transient outward potassium current ([I.sub.to]) and ultrarapid potassium current ([I.sub.Kur])--following Phe exposure. A significant reduction in AP upstroke velocity (p = 0.0445, n = 9 cells) and inhibition of the fast sodium current ([I.sub.Na]; p = 0.001, n = 8 cells) and calcium current ([I.sub.Ca]; p = 0.0001) were also observed, explaining the slowed conduction velocity in intact hearts. Finally, acute exposure to ~8 [micro]M Phe significantly increased susceptibility to arrhythmias (p = 0.0455, N = 9 hearts). Discussion: To the best of our knowledge, this is the first evidence of direct inhibitory effects of Phe on mammalian cardiac electrical activity at both the whole-heart and cell levels. This electrical dysfunction manifested as an increase in arrhythmia susceptibility due to impairment of both conduction and repolarization. Similar effects in humans could have serious health consequences, warranting greater regulatory attention and toxicological investigation into this ubiquitous PAH pollutant generated from fossil-fuel combustion. https://doi.org/10.1289/EHP12775, Introduction Air pollution is one of the leading risk factors for global disease burden, (1-7) and accounts for an estimated 6.7 million premature deaths each year worldwide. (8) With 99% [...]

Hypoxia is a pervasive problem in coastal environments and is predicted to have enduring impacts on aquatic ecosystems. Intraspecific variation in hypoxia tolerance is well documented in fish; however, the factors underlying this variation remain unknown. Here, we investigate the role of the heart in individual hypoxia tolerance of the European sea bass (Dicentrarchus labrax). We found individual whole-animal hypoxia tolerance is a stable trait in sea bass for more than 18 months (duration of study). We next examined in vitro cardiac performance and found myocardial muscle from hypoxia-tolerant individuals generated greater force, with higher rates of contraction and relaxation, than hypoxic-sensitive individuals during hypoxic exposure. Thus, whole-animal hypoxia tolerance is associated with cardiac hypoxia tolerance. As the occurrence of aquatic hypoxia is expected to increase in marine ecosystems, our experimental data suggest that cardiac performance may influence fish survival and distribution.