Most mammals are not capable of surviving at low body temperature for extended periods. Although some degree of hypothermia provides an advantage in medical treatment, such as organ plantation and open heart surgery, through depression of the energy consumption and prevention of accumulation of harmful metabolites, the lowering of body temperature causes serious cold-damage to cells, probably by overloading Ca²⁺, resulting from a perturbation of membrane ion regulation. Mammalian hibernators undergo extreme hypothermia (near 0°C) during hibernation without any damage to their bodies, indicating the existence of an effective strategy for the protection of cells and tissues against cold. Recent studies on excitation-contraction coupling of cardiac muscles of hibernators found dramatic changes in Ca²⁺ regulatory systems, Ca channels and sarcoplasmic reticulum (SR), during hibernation. The contraction usually depends on Ca²⁺ influx through Ca channels, whereas during hibernation, Ca²⁺ for contraction is derived mainly from intracellular Ca stores, SR. Further electrophysiological and biochemical studies in hibernating animals revealed that these are attributed to both the marked enhancement of the Ca²⁺ uptake ability of SR and much less activation of Ca channels. These changes greatly contribute to the reduction of cytoplasmic Ca²⁺ after contraction. Thus, during hibernation, hearts are kept functioning normally at low body temperature through avoiding Ca²⁺ overload. The present mechanism of adaptation to cold provides a useful model for studying a strategy for overcoming cold injury.

View full abstract

Thirty-five clinically healthy dogs aged from 4 months to 2 years were investigated under anesthesia to estimate cardiac function, on the basis of the patterns of ultrasonic pulsed Doppler waveforms as well as various other measurements. The Doppler waveforms were similar to those of human subjects. The mean value and coefficient of variation in each measurement suggested that changes in peak rates, A/E of waveforms of left ventricular inflow tract flow, ET of waveforms of rihgt and left ventricular outflow tract flow, TVI associated with determination of the flow volume, and AT/ET of waveforms of right ventricular outflow tract flow, were clinically useful in evaluating cardiac functions.

View full abstract

Electrocardiograms (ECG) in limb leads were recorded from 100 beagle dogs (6-40 months old, weighing 7.0-12.0 kg) in the right lateral recumbency without anesthesia, and the T wave, variation of the QT intervals, and a correlation between the QT and RR intervals were investigated.
1. The patterns of T waves were variable, which included positive, negative, notched, biphasic and flat. In one dog, the pattern of T wave changed from positive to biphasic, and then turned back to positive during short time.
2. Among 96 dogs, the high and positive T waves were found mainly in leads II, III and aVF, and the negative ones in leads I, aVR and aVL.
3. Time-dependent variation of the RR and QT intervals were examined in the ECG taken from 4 dogs over 2-4 min on 2 successive days. In 2 of 4 dogs, the RR intervals gradually prolonged with time ; however, in the remaining 2 dogs RR intervals did not change throughout the recording period. The QT intervals gradually prolonged with time in all 4 dogs.
4. It was found that the QT and RR intervals were not always well correlated in each dog; when 1000 pairs of the QT and RR intervals were compared in ECG taken from 4 dogs over 2-4 min on successive 5-7 days. The coefficient of correlation between the mean QT and RR intervals of 4 dogs over 4-8 sec was greater than that from the individual values in each ECG.
As mentioned above, the patterns of T waves were considerably variable in dogs, and the QT intervals can also change within a short time. These findings suggest that more detailed examinations on the T wave and QT-interval are necessary to take up the QT interval as a diagnostic parameter in dogs.

View full abstract

In our previous study, we found that Rhode Island Red chickens (RIR) showed the highest incidences of spontaneously occurring arrhythmias among seven breeds of chickens. The present study was carried out in the hope of defining the pathology of the heart and extrinsic cardiac nerves in 18 RIR showing ventricular premature depolarization, ventricular tachycardia, ventricular preexcitation, atrial premature depolarization or atrial fibrillation, 4 Japanese Game chickens (JG) with ventricular preexcitation or atrial premature depolarization and 161 chickens of four different breeds (RIR, White Leghorn (WL), JG and the first filial generation obtained by mating RIR with JG (RIRxJG)) with normal sinus rhythm as controls. Microscopically predominant changes in the heart were vascular lesions of the intramural coronary artery. The vascular lesions consisted of intimal thickening with muscular or fibromuscular proliferation. In addition, the lesions were frequently accompanied by narrowing of the vascular lumen and interstitial and perivascular fibrosis. Severe vascular lesions were observed in 16 out of 22 chickens with arrhythmia (72.7% ; RIR : 83.3% and JG : 25.0%) and 27 out of 161 chickens with normal sinus rhythm (16.8%; RIR : 48.5%; JG : 8.2%, WL : 13.6%; RIRxJG : 2.9%). Moreover, foci of myocardial fibrosis were present in greater or lesser numbers depending on the severity of the lesions in both arrhythmia-affected chickens and ones with normal sinus rhythm. The former included17 of the 18 RIR and 1 of the 4 JG, and the latter 7 of 33 RIR, 2 of 39 JG and 1 of 44 WL. The rate of occurrence of the severe vascular lesions was significantly higher in RIR with arrhythmia than in JG with arrhythmia (p<0.05) and in RIR with arrhythmia than in RIR with normal sinus rhythm (p<0.05). Among chickens with normal sinus rhythm, RIR showed a significantly higher incidence of severe vascular lesions than all other breeds (p<0.001) as well. On the other hand, the extrinsic cardiac nerves showed foci of mild lesions consisting of edematous loosening in part of the nerve bundles, and partial deposits of calcium salts were observed in extrinsic cardiac nerves in a few animals. The presence of these nerve lesions appeared to be unrelated to the occurrence of arrhythmia. Our results indicate that the high incidence of arteriosclerosis of the intramural coronary artery may be related to the high incidence of arrhythmia in RIR.

View full abstract