Low-Intensity Exercise Training Attenuates Cardiac β-Adrenergic Tone During Exercise in Spontaneously Hypertensive Rats

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Abstract
Acute and chronic exercise decrease peripheral sympathetic nerve activity, but the effect of exercise training of varying intensity on the sympathetic control of heart rate of spontaneously hypertensive rats has not yet been described. The effect of low and high intensities of exercise training on the vagal and sympathetic activities that control heart rate at rest and during dynamic exercise at 0.5, 0.8, and 1.0 mph for 4 minutes per stage was investigated in sedentary (SED, n=11), high-intensity (HT, n=12), and low-intensity exercise–trained (LT, n=13) spontaneously hypertensive rats. Exercise training was performed on a treadmill for 60 minutes, 5 days per week for 18 weeks, at 55% maximum oxygen consumption for the LT group and 85% for the HT group. Vagal and sympathetic activities were studied after administration of methylatropine (3 mg/kg) and propranolol (4 mg/kg), respectively. The LT group had a significantly lower heart rate (at 0.5, 0.8, 1.0 mph versus rest: 410±7, 426±7, 464±9, and 295±6 beats per minute [bpm], respectively) than the HT (440±6, 453±7, 474±5, and 315±4 bpm) and the SED (474±11, 500±11, 523±10, and 327±3 bpm) groups. Sympathetic effect (LT: 84±10, 88±12, 105±12, and 9±4; HT: 123±8, 125±7, 133±7, and 34±7; SED: 130±13, 143±12, 150±10, and 38±7 bpm) and sympathetic tonus (LT: 125±6, 121±5, 112±6, and 91±6; HT: 145±9, 136±6, 142±8, and 118±7; SED: 136±6, 129±6, 132±7, and 118±8 bpm) were significantly decreased by low-intensity exercise training. In conclusion, low- but not high-intensity exercise training causes resting bradycardia and attenuation of tachycardiac response during progressive dynamic exercise in spontaneously hypertensive rats. This effect can be attributed to a significantly decreased β-adrenergic tone that controls heart rate.