Indirect evidence that anoxia exposure and cold acclimation alter transarcolemmal Ca2+ flux in the cardiac pacemaker, right atrium and ventricle of the red-eared slider turtle (Trachemys scripta)

• Published in: Comparative biochemistry and physiology. Part A, Molecular & integrative physiology Molecular & integrative physiology, 2021-11, Vol.261, p.111043-111043, Article 111043
• Main Authors: Stecyk, Jonathan A.W., Barber, Riley G., Cussins, Jace, Hall, Diarmid
• Format: Article
• Language: English
• Subjects: Anoxia; Cardiac pacemaker; Cardiac regulation; Ion channels; Myocardial contractile properties; Red-eared slider; Temperature
• ISSN: 1095-6433; 1531-4332
• DOI: 10.1016/j.cbpa.2021.111043

We indirectly assessed if altered transarcolemmal Ca2+ flux accompanies the decreased cardiac activity displayed by Trachemys scripta with anoxia exposure and cold acclimation. Turtles were first acclimated to 21 °C or 5 °C and held under normoxic (21N; 5N) or anoxic conditions (21A; 5A). We then compared the response of intrinsic heart rate (fH) and maximal developed force of spontaneously contracting right atria (Fmax,RA), and maximal developed force of isometrically-contracting ventricular strips (Fmax,V), to Ni2+ (0.1–10 mM), which respectively blocks T-type Ca2+ channels, L-type Ca2+ channels and the Na+-Ca2+-exchanger at the low, intermediate and high concentrations employed. Dose-response curves were established in simulated in vivo normoxic (Sim Norm) or simulated in vivo anoxic extracellular conditions (Sim Anx; 21A and 5A preparations). Ni2+ decreased intrinsic fH, Fmax,RA and Fmax,V of 21N tissues in a concentration-dependent manner, but the responses were blunted in 21A tissues in Sim Norm. Similarly, dose-response curves for Fmax,RA and Fmax,V of 5N tissues were right-shifted, whereas anoxia exposure at 5 °C did not further alter the responses. The influence of Sim Anx was acclimation temperature-, cardiac chamber- and contractile parameter-dependent. Combined, the findings suggest that: (1) reduced transarcolemmal Ca2+ flux in the cardiac pacemaker is a potential mechanism underlying the slowed intrinsic fH of anoxic turtles at 21 °C, but not 5 °C, (2) a downregulation of transarcolemmal Ca2+ flux may aid cardiac anoxia survival at 21 °C and prime the turtle myocardium for winter anoxia and (3) confirm that altered extracellular conditions with anoxia exposure can modify turtle cardiac transarcolemmal Ca2+ flux. [Display omitted] •The slowed intrinsic heart rate of anoxic turtles at 21°C, but not 5°C, may be due to reduced transarcolemmal Ca2+ flux.•Decreased transarcolemmal Ca2+ flux may aid cardiac anoxia survival at 21 °C.•Decreased transarcolemmal Ca2+ flux may prepare the turtle heart for prolonged anoxia exposure in winter.•Changes to the extracellular milieu have the potential to modify turtle cardiac transarcolemmal Ca2+ flux.