Cardiac contractility of the African sharptooth catfish, Clarias gariepinus : role of extracellular Ca 2+ , sarcoplasmic reticulum, and β-adrenergic stimulation

This study investigated the dependence of contraction from extracellular Ca 2+ , the presence of a functional sarcoplasmic reticulum (SR), and the effects of β-adrenergic stimulation using isometric cardiac muscle preparations. Moreover, the expression of Ca 2+ -handling proteins such as SR-Ca 2+ -ATPase (SERCA), phospholamban (PLN), and Na + /Ca 2+ exchanger (NCX) were also evaluated in the ventricular tissue of adult African sharptooth catfish, Clarias gariepinus , a facultative air-breathing fish. In summary, we observed that (1) contractility was strongly regulated by extracellular Ca 2+ ; (2) inhibition of SR Ca 2+ -release by application of ryanodine reduced steady-state force production; (3) ventricular myocardium exhibited clear post-rest decay, even in the presence of ryanodine, indicating a decrease in SR Ca 2+ content and NCX as the main pathway for Ca 2+ extrusion; (4) a positive force-frequency relationship was observed above 60 bpm (1.0 Hz); (5) ventricular tissue was responsive to β-adrenergic stimulation, which caused significant increases in twitch force, kept a linear force-frequency relationship from 12 to 96 bpm (0.2 to Hz), and improved the cardiac pumping capacity (CPC); and (6) African catfish myocardium exhibited similar expression patterns of NCX, SERCA, and PLN, corroborating our findings that both mechanisms for Ca 2+ transport across the SR and sarcolemma contribute to Ca 2+ activator. In conclusion, this fish species displays great physiological plasticity of E-C coupling, able to improve the ability to maintain cardiac performance under physiological conditions to ecological and/or adverse environmental conditions, such as hypoxic air-breathing activity.