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

We indirectly assessed if altered transarcolemmal Ca 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 (f) and maximal developed force of spontaneously contracting right atria (F), and maximal developed force of isometrically-contracting ventricular strips (F), to Ni (0.1-10 mM), which respectively blocks T-type Ca channels, L-type Ca channels and the Na-Ca-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). Ni decreased intrinsic f, F and F of 21N tissues in a concentration-dependent manner, but the responses were blunted in 21A tissues in Sim Norm. Similarly, dose-response curves for F and F 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 Ca flux in the cardiac pacemaker is a potential mechanism underlying the slowed intrinsic f of anoxic turtles at 21 °C, but not 5 °C, (2) a downregulation of transarcolemmal Ca 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 Ca flux.