ABSTRACT The Alaska blackfish ( Dallia pectoralis ) remains active at cold temperatures when experiencing aquatic hypoxia without air access. To discern the cardiophysiological adjustments that permit this behaviour, we quantified the effect of acclimation from 15°C to 5°C in normoxia (15N and 5N fish), as well as chronic hypoxic submergence (6–8 weeks; ∼6.3–8.4 kPa; no air access) at 5°C (5H fish), on in vivo and spontaneous heart rate ( f H ), electrocardiogram, ventricular action potential (AP) shape and duration (APD), the background inward rectifier ( I K1 ) and rapid delayed rectifier ( I Kr ) K + currents and ventricular gene expression of proteins involved in excitation–contraction coupling. In vivo f H was ∼50% slower in 5N than in 15N fish, but 5H fish did not display hypoxic bradycardia. Atypically, cold acclimation in normoxia did not induce shortening of APD or alter resting membrane potential. Rather, QT interval and APD were ∼2.6-fold longer in 5N than in 15N fish because outward I K1 and I Kr were not upregulated in 5N fish. By contrast, chronic hypoxic submergence elicited a shortening of QT interval and APD, driven by an upregulation of I Kr . The altered electrophysiology of 5H fish was accompanied by increased gene expression of kcnh6 (3.5-fold; K v 11.2 of I Kr ), kcnj12 (7.4-fold; K ir 2.2 of I K1 ) and kcnj14 (2.9-fold; K ir 2.4 of I K1 ). 5H fish also exhibited a unique gene expression pattern that suggests modification of ventricular Ca 2+ cycling. Overall, the findings reveal that Alaska blackfish exposed to chronic hypoxic submergence prioritize the continuation of cardiac performance to support an active lifestyle over reducing cardiac ATP demand.

ABSTRACT At critically high temperature, cardiac output in fish collapses as a result of depression of heart rate (bradycardia). However, the cause of bradycardia remains unresolved. To investigate this, rainbow trout ( Oncorhynchus mykiss ; acclimated at 12°C) were exposed to acute warming while electrocardiograms were recorded. From 12°C to 25.3°C, electrical excitation between different parts of the heart was coordinated, but above 25.3°C, atrial and ventricular beating rates became partly dissociated because of 2:1 atrioventricular (AV) block. With further warming, atrial rate increased to a peak value of 188±22 beats min −1 at 27°C, whereas the ventricle rate peaked at 124±10 beats min −1 at 25.3°C and thereafter dropped to 111±15 beats min −1 at 27°C. In single ventricular myocytes, warming from 12°C to 25°C attenuated electrical excitability as evidenced by increases in rheobase current and the size of critical depolarization required to trigger action potential. Depression of excitability was caused by temperature-induced decrease in input resistance (sarcolemmal K + leak via the outward I K1 current) of resting myocytes and decrease in inward charge transfer by the Na + current ( I Na ) of active myocytes. Collectively, these findings show that at critically high temperatures AV block causes ventricular bradycardia owing to the increased excitation threshold of the ventricle, which is due to changes in the passive (resting ion leak) and active (inward charge movement) electrical properties of ventricular myocytes. The sequence of events from the level of ion channels to cardiac function in vivo provides a mechanistic explanation for the depression of cardiac output in fish at critically high temperature.

ABSTRACT We used electrocardiogram (ECG) telemeters to measure the heart rate of hibernating Ictidomys tridecemlineatus (thirteen-lined ground squirrel). An increase in heart rate from 2.2 to 5 beats min –1 accurately identified arousal from torpor before any change in body temperature was detected. Variability in raw heart rate data was significantly reduced by a forward–backward Butterworth low-pass filter, allowing for discrete differential analysis. A decrease in filtered heart rate to 70% of maximum values in interbout euthermia (from approximately 312 to 235 beats min –1 ) accurately detected entrance into torpor bouts. At this point, body temperature had fallen from 36.1°C to only 34.7°C, much higher than the 30°C typically used to identify entrance. Using these heart rate criteria allowed advanced detection of entrance and arousal (detected 51.9 and 76 min earlier, respectively), compared with traditional body temperature criteria. This method will improve our ability to detect biochemical and molecular markers underlying these transition periods, during which many physiological changes occur.

ABSTRACT Temperature sensitivity of electrical excitability is a potential limiting factor for performance level and thermal tolerance of excitable tissues in ectothermic animals. To test whether the rate and rhythm of the heart acclimatize to seasonal temperature changes, thermal sensitivity of cardiac excitation in a eurythermal teleost, the roach ( Rutilus rutilus ), was examined. Excitability of the heart was determined from in vivo electrocardiograms and in vitro microelectrode recordings of action potentials (APs) from winter and summer roach acclimatized to 4 and 18°C, respectively. Under heat ramps (3°C h −1 ), starting from the acclimatization temperatures of the fish, heart rate increased to maximum values of 78±5 beats min −1 (at 19.8±0.5°C) and 150±7 beats min −1 (at 28.1±0.5°C) for winter and summer roach, respectively, and then declined in both groups. Below 20°C, heart rate was significantly higher in winter than in summer roach ( P <0.05), indicating positive thermal compensation. Cardiac arrhythmias appeared with rising temperature as missing QRS complexes, increase in variability of heart rate, episodes of atrial tachycardia, ventricular bradycardia and complete cessation of the heartbeat (asystole) in both winter and summer roach. Unlike winter roach, atrial APs of summer roach had a distinct early repolarization phase, which appeared as shorter durations of atrial AP at 10% and 20% repolarization levels in comparison to winter roach ( P <0.05). In contrast, seasonal acclimatization had only subtle effects on ventricular AP characteristics. Plasticity of cardiac excitation appears to be necessary for seasonal improvements in performance level and thermal resilience of the roach heart.

Heart rate and peripheral blood flow distribution are the primary determinants of the rate and pattern of oxygen store utilisation and ultimately breath-hold duration in marine endotherms. Despite this, little is known about how otariids (sea lions and fur seals) regulate heart rate ( f H ) while diving. We investigated dive f H in five adult female California sea lions ( Zalophus californianus ) during foraging trips by instrumenting them with digital electrocardiogram (ECG) loggers and time depth recorders. In all dives, dive f H (number of beats/duration; 50±9 beats min −1 ) decreased compared with surface rates (113±5 beats min −1 ), with all dives exhibiting an instantaneous f H below resting (<54 beats min −1 ) at some point during the dive. Both dive f H and minimum instantaneous f H significantly decreased with increasing dive duration. Typical instantaneous f H profiles of deep dives (>100 m) consisted of: (1) an initial rapid decline in f H resulting in the lowest instantaneous f H of the dive at the end of descent, often below 10 beats min −1 in dives longer than 6 min in duration; (2) a slight increase in f H to ~10–40 beats min −1 during the bottom portion of the dive; and (3) a gradual increase in f H during ascent with a rapid increase prior to surfacing. Thus, f H regulation in deep-diving sea lions is not simply a progressive bradycardia. Extreme bradycardia and the presumed associated reductions in pulmonary and peripheral blood flow during late descent of deep dives should (a) contribute to preservation of the lung oxygen store, (b) increase dependence of muscle on the myoglobin-bound oxygen store, (c) conserve the blood oxygen store and (d) help limit the absorption of nitrogen at depth. This f H profile during deep dives of sea lions may be characteristic of deep-diving marine endotherms that dive on inspiration as similar f H profiles have been recently documented in the emperor penguin, another deep diver that dives on inspiration.

The upper thermal tolerance and mechanisms of heat-induced cardiac failure in the brown trout ( Salmo trutta fario ) was examined. The point above which ion channel function and sinoatrial contractility in vitro , and electrocardiogram (ECG) in vivo , started to fail (break point temperature, BPT) was determined by acute temperature increases. In general, electrical excitation of the heart was most sensitive to heat in the intact animal (electrocardiogram, ECG) and least sensitive in isolated cardiac myocytes (ion currents). BPTs of Ca 2+ and K + currents of cardiac myocytes were much higher (>28°C) than BPT of in vivo heart rate (23.5±0.6°C) ( P <0.05). A striking exception among sarcolemmal ion conductances was the Na + current ( I Na ), which was the most heat-sensitive molecular function, with a BPT of 20.9±0.5°C. The low heat tolerance of I Na was reflected as a low BPT for the rate of action potential upstroke in vitro (21.7±1.2°C) and the velocity of impulse transmission in vivo (21.9±2.2°C). These findings from different levels of biological organization strongly suggest that heat-dependent deterioration of Na + channel function disturbs normal spread of electrical excitation over the heart, leading to progressive variability of cardiac rhythmicity (missed beats, bursts of fast beating), reduction of heart rate and finally cessation of the normal heartbeat. Among the cardiac ion currents I Na is ‘the weakest link’ and possibly a limiting factor for upper thermal tolerance of electrical excitation in the brown trout heart. Heat sensitivity of I Na may result from functional requirements for very high flux rates and fast gating kinetics of the Na + channels, i.e. a trade-off between high catalytic activity and thermal stability.

SUMMARY Anaesthesia and minor surgery to place electrocardiogram recording electrodes in the short-horned sculpin caused a decrease in mean normal beat(R–R) interval and heart rate variability (HRV), measured as the standard deviation in the R–R interval (SDRR). Mean R–R interval increased to a steady state value (1.9±2.9 s) 72 h post-surgery, but SDRR took 120 h to stabilise (0.56±0.09 s). Power spectral analysis applied to recordings of instantaneous heart rate showed no spectral peaks immediately after surgery, with the development of twin peaks (at 0.02 and 0.05 Hz) that also became stable 120 h post surgery. Bilateral cardiac vagotomy abolished the variability in beat-to-beat interval, and both the high and low frequency peaks, suggesting that much of the regulation of heart rate and HRV in sculpin was under parasympathetic, cholinergic control that was withdrawn as a result of surgical and handling stress. Rate of oxygen consumption \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(({\dot{M}}_{\mathrm{O}_{2}})\) \end{document} and heart rate ( f H ) were monitored simultaneously and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \({\dot{M}}_{\mathrm{O}_{2}}\) \end{document} showed a good correlation with both mean R–R interval( r 2 =–0.89) and SDRR ( r 2 =0.93),although a more significant (ANCOVA, P =0.02) covariance existed between the post-surgical decrease in \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \({\dot{M}}_{\mathrm{O}_{2}}\) \end{document} and increase in SDRR. These data suggest that sculpin use f H as a way of moderating oxygen consumption, fine-tuned on a beat-to-beat basis by cholinergic control. We conclude that power spectral analysis is a useful method of determining HRV in fish, and that HRV is a more sensitive measure of recovery from disturbance than f H alone.

SUMMARY Alligators and other crocodilians have a cog-wheel valve located within the subpulmonary conus, and active closure of this valve during each heart beat can markedly and phasically increase resistance in the pulmonary outflow tract. If this increased resistance causes right ventricular pressure to rise above that in the systemic circuit, right ventricular blood can flow into the left aorta and systemic circulation, an event known as pulmonary-to-systemic shunting. To understand better how this valve is controlled, anaesthetized American alligators ( Alligator mississippiensis ) were used to examine the relationships between depolarization of the right ventricle,depolarization/contraction of the cog-wheel valve muscle and the resultant right ventricular, pulmonary artery and systemic pressures. Depolarization swept across the right ventricle from the apex towards the base (near where the cog-wheel valve muscle is located) at a velocity of 91±23 cm s -1 (mean ± S.E.M., N =3). The cog-wheel valve electrocardiogram (ECG) (and thus contraction of the valve) trailed the right ventricular ECG by 248±28 ms ( N =3), which was equivalent to 6-35 % of a cardiac cycle. This long interval between right ventricular and valve depolarization suggests a nodal delay at the junction between the base of the right ventricle and the cog-wheel valve. The delay before valve closure determined when the abrupt secondary rise in right ventricular pressure occurred during systole and is likely to strongly influence the amount of blood entering the pulmonary artery and thus to directly control the degree of shunting. Left vagal stimulation (10-50 Hz) reduced the conduction delay between the right ventricle and cog-wheel valve by approximately 20 % and reduced the integrated cog-wheel ECG by 10-20 %. Direct application of acetylcholine (1-2 mg) also reduced the integrated cog-wheel ECG by 10-100 %;however, its effect on the conduction delay was highly variable (-40 to +60%). When the cog-wheel valve muscle was killed by the application of ethanol,the cog-wheel ECG was absent, right ventricular and pulmonary pressures remained low and tracked one another, the secondary rise in right ventricular pressure was abolished and shunting did not occur. This study provides additional, direct evidence that phasic contraction of the cog-wheel valve muscle controls shunting, that nervous and cholinergic stimulation can alter the delay and strength of valve depolarization and that this can affect the propensity to shunt.