Comprehensive analysis of voluntary wheel training effects on neural control of the heart rate in rats

Aerobic physical training is used to prevent and correct many cardiovascular disorders. To study the effects of such physical exercise, various methods of training are used in rodents, among which voluntary wheel running is of particular interest, because it is close to the natural rat locomotion in terms of the pattern of motor activity and is devoid of stressful influence. The aim of this work was a comprehensive study of the effects of wheel running on the nervous control of heart rate (HR) in rats. At the age of 6 weeks, the animals were divided into two groups: training (TR, free access to wheels, n = 11) and sedentary control (CON, n = 12). After 6 weeks of training, ECG was recorded in freely moving rats using skin electrodes in baseline, after blockade of cardiac nervous influences and under air-jet stress (4 min). The effects of the autonomic nervous system were analyzed by administering a β1-adrenergic blocker and a peripherally acting M-cholinoceptor blocker, and by analyzing cardiac rhythm variability using spectral and wavelet analyses. TR group showed a decrease in the baseline HR level compared to the CON group. The decrease in HR upon administration of atenolol (2 mg/kg) did not differ between the groups, but methylatropine (1 mg/kg) caused a more significant increase in HR in the TR group than in the CON group. With the combined action of atenolol and methylatropine, HR levels did not differ between the groups. The rats of the TR group showed an increase in the contribution of high-frequency (0.75–3 Hz) oscillations to the total spectrum power of the RR interval. During air-jet stress, TR group showed a more pronounced increase in HR compared to the CON group. In addition, in the TR group, a decrease in the amplitude of HR high-frequency oscillations was observed during stress, while in the CON group, such a decrease was absent. Thus, the voluntary wheel running is accompanied in the rat by an increase in parasympathetic influences on the heart, which is manifested in an increase in respiratory sinus arrhythmia and in an increase in vagal influence on the resting HR level. Moderate bradycardia at rest provides the possibility of a more pronounced increase in HR during emotional stress because of the suppression of parasympathetic cardiac influences.