A Comprehensive Analysis of Voluntary Wheel Running Effects on Neural Control of the Heart Rate in Rats

Aerobic exercise training is aimed to prevent and correct various cardiovascular disorders. To study its effects, various rodent models are currently in use, among which the rat model of voluntary wheel running is of particular interest, as its pattern of motor activity is close to natural rat locomotion, while being non-stressful. This work was aimed at a comprehensive investigation of wheel running effects on the neural control of heart rate (HR) in rats. The animals, aged 6 weeks, were divided into two groups: training (TR, free access to wheels, n = 11) and sedentary control (CON, n = 12). After a 6-week training, ECG was recorded in freely moving rats using skin electrodes in three modes: at rest, after autonomic blockade of cardiotropic influences, and during 4-min air-jet stress. Autonomic neural influences were analyzed by administering the β1-adrenoceptor blocker atenolol (2 mg/kg) and the peripheral M-cholinoceptor antagonist methylatropine (1 mg/kg), as well as by analyzing HR variability via spectral and wavelet analyses. At rest, the TR group showed a decrease in the baseline HR level vs. CON group. Аtenolol decreased HR equally in two groups, but methylatropine elicited a more significant increase in HR in the TR vs. CON group. After atenolol and methylatropine co-administration, HR levels in two groups were similar. TR rats showed an increased contribution of high-frequency (0.75–3 Hz) oscillations to the total RR interval power spectrum. During emotional (air-jet) stress, the TR group demonstrated a more pronounced increase in HR vs. CON group. In addition, stressed TR rats exhibited a decrease in the amplitude of HR high-frequency oscillations, which was absent in the CON group. Thus, voluntary wheel running in rats is accompanied by an increase in parasympathetic influences on the heart, as manifested in an increase both in respiratory sinus arrhythmia and in vagal effects on the resting HR level (increased tachycardia). Moderate resting bradycardia promotes a more pronounced increase in HR under emotional stress conditions due to suppression of parasympathetic cardiac influences.