Effects of prolonged anoxia on electrical activity of the heart in Crucian carp (Carassius carassius)

Abstract

The effects of sustained anoxia on cardiac electrical excitability were examined in the anoxia-tolerant crucian carp (Carassius carassius). The electrocardiogram (ECG) and expression of excitationcontraction coupling genes were studied in fish acclimatised to normoxia in summer (+18°C) or winter (+2°C), and in winter fish after 1, 3 and 6 weeks of anoxia. Anoxia induced a sustained bradycardia from a heart rate of 10.3±0.77 beats min −1 to 4.1±0.29 beats min −1 (P<0.05) after 5 weeks, and heart rate slowly recovered to control levels when oxygen was restored. Heart rate variability greatly increased under anoxia, and completely recovered under reoxygenation. The RT interval increased from 2.8±0.34 s in normoxia to 5.8±0.44 s under anoxia (P<0.05), which reflects a doubling of the ventricular action potential (AP) duration. Acclimatisation to winter induced extensive changes in gene expression relative to summer-acclimatised fish, including depression in those genes coding for the sarcoplasmic reticulum calcium pump (Serca2a_q2) and ATP-sensitive K + channels (Kir6.2) (P<0.05). Genes of delayed rectifier K + (kcnh6) and Ca 2+ channels (cacna1c) were up-regulated in winter fish (P<0.05). In contrast, the additional challenge of anoxia caused only minor changes in gene expression, e.g. depressed expression of Kir2.2b K + channel gene (kcnj12b), whereas expression of Ca 2+ (cacna1a, cacna1c and cacna1g) and Na + channel genes (scn4a and scn5a) was not affected. These data suggest that low temperature pre-conditions the crucian carp heart for winter anoxia, whereas sustained anoxic bradycardia and prolongation of AP duration are directly induced by oxygen shortage without major changes in gene expression.

Cardiophysiological responses of the air-breathing Alaska blackfish to cold acclimation and chronic hypoxic submergence at 5°C

“…The depression of in vivo f H with acclimation to 5°C from 15°C in normoxia is qualitatively and quantitatively similar to the reductions of in vivo f H displayed by the anoxia-tolerant crucian carp (C. carassius) and the severely hypoxia-tolerant common carp (Cyprinus carpio) with cold acclimation (Matikainen and Vornanen, 1992;Stecyk andFarrell, 2002, 2006;Stecyk et al, 2004b;Tikkanen et al, 2017;Vornanen, 1994;Vornanen and Tuomennoro, 1999). However, the bradycardia contrasts with the positive thermal compensation and relatively high in vivo f H displayed by coldactive fishes acclimated to cold temperature, including the burbot (Lota lota) (Tiitu and Vornanen, 2002), navaga cod (Eleginus navaga) (Hassinen et al, 2014) and rainbow trout (Oncorhynchus mykiss) (Aho and Vornanen, 2001).…”

“…Recording and analysis of ECG from live fish ECG recordings were obtained from unrestrained and unanaesthetized fish using established methods (Campbell et al, 2004;Tikkanen et al, 2017). Fish were anaesthetized with buffered tricaine methanesulfonate (MS-222; 0.2 g l −1 +0.2 g l −1 NaHCO 3 ; Sigma Aldrich, St Louis, MO, USA) until opercular movements ceased.…”

The expression of genes involved in excitatory and inhibitory neurotransmission in turtle (Trachemys scripta) brain during anoxic submergence at 21 °C and 5 °C reveals the importance of cold as a preparatory cue for anoxia survival

“…This finding corroborates previous studies on gene expression in turtle heart and brain (Stecyk et al, 2012), the density of ion channels in turtle heart (Stecyk et al, 2007) and the gene expression of heat shock proteins in crucian carp heart and brain (Stensløkken et al, 2010). The finding is also in-line with the extensive changes in cardiac gene expression in winter-acclimatized crucian carp that serve to precondition the heart for winter anoxia (Tikkanen et al, 2017). Thus, our data lends further support to the notion that cold acclimation prepares the turtle for a prolonged exposure to anoxia.…”

“…Statistical analyses were performed using JMP 8 (Aspire Software International, Ashburn, USA) to evaluate whether the different oxygen and temperature regimes affected individual gene expression and genefamily profiles, and PAST 3.22 (Hammer et al, 2001) to determine if the pattern of gene expression differed among exposure conditions. One-way ANOVAs followed by a TukeyHSD post hoc test were used to determine statistically significant effects of anoxia and reoxygenation on the log10 transformed (Hellemans and Vandesompele, 2011) target gene expression at each acclimation temperature (Del Toro et al, 2003;Ellefsen et al, 2008a;Ellefsen et al, 2009;Ellefsen and Stenslokken, 2010;Prentice et al, 2003;Stecyk et al, 2012;Tikkanen et al, 2017;Wilson et al, 2013). Student's t-tests were used to assess the effect of acclimation temperature on gene expression by comparing the log10 transformed (Hellemans and Vandesompele, 2011) target gene expression of normoxic turtles at 21ºC and 5ºC .…”

“…In addition to anoxia-induced responses, acclimation to low temperature is critical for extending anoxia survival time in the ectothermic champions of anoxia tolerance, including the turtles, crucian carp (Carassius carassius) and goldfish (Carassius auratus). Cold acclimation primes their physiological processes for prolonged anoxia survival by inducing whole-body metabolic depression (Hogg et al, 2014;Jackson and Ultsch, 2010;Ultsch, 1985), reduced cardiac function (Hicks and Farrell, 2000;Jackson and Ultsch, 2010;Stecyk, 2017;Stecyk et al, 2012;Stecyk et al, 2007;Stensløkken et al, 2010;Tikkanen et al, 2017;Vornanen et al, 2009) or brain function remodelling (Hogg et al, 2014;Stecyk et al, 2012;Stensløkken et al, 2010). Importantly, during cold acclimation, these organisms actively suppress biological rate processes beyond the direct depression imposed by the reduced kinetic energy of molecules at lowered temperature.…”