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The influence of daily spontaneous running on the sympathetic and parasympathetic components of the spontaneous arterial baroreflex control of heart rate was examined in 22 female spontaneously hypertensive rats 12 sedentary and 10 daily spontaneous running. Following 8 weeks of sedentary control or daily spontaneous running, animals were chronically instrumented with an arterial catheter. Daily spontaneous running resulted in an increased heart weight/body weight ratio (5.2 +/- 0.27 vs 4.3 +/- 0.01 g/kg) and a resting bradycardia (321+/- 8 bpm vs 360 +/- 6). The spontaneous changes in arterial pressure and the reflex responses of heart rate were examined under three experimental conditions: 1) pre-blockade, 2) following beta1-adrenergic receptor blockade, and 3) following muscarinic-cholinergic receptor blockade. Daily spontaneous running attenuated the spontaneous gain of the arterial baroreflex control of heart rate (56%). After muscarinic-cholinergic receptor blockade, the spontaneous gain remained reduced in daily spontaneous running rats (57%). In contrast, after beta1-adrenergic receptor blockade the spontaneous gain was not different between sedentary control and daily spontaneous running animals. Results demonstrate that daily spontaneous running decreased the sympathetic component resulting in an apparently greater influence of the parasympathetic component on the spontaneous arterial baroreflex control of heart rate.
Collins et al. (2000) studied this question.