Onset of exercise increases lumbar sympathetic nerve activity in rats

We hypothesized that lumbar sympathetic nerve activity (LSNA) increases at the onset of whole-body dynamic exercise in the rat. To test this hypothesis, we recorded LSNA, heart rate (HR), and arterial pressure (AP) at rest and during a graded exercise test in six adult rats. Rats were instrumented with arterial and venous catheters and recording electrodes around the lumbar sympathetic trunk. Following recovery, each rat ran continuously on a hand- driven or motorized treadmill at 6 m·min^-1, 12 m·min^-1, and 18 m-min^{- 1} on a 10% grade for approximately 3 min at each workload. Before exercise, mean arterial pressure (MAP), HR, and LSNA averaged 108 ± 4 mm Hg, 385 ± 20 bpm, and 100%, respectively. As hypothesized, all variables increased abruptly and dramatically at the onset of treadmill exercise. For example, MAP (117 ± 5 mm Hg), HR (450 ± 15 bpm), and LSNA (225 ± 19%) all increased significantly within the first 25 s of treadmill running at 6 m·min^-1. As the exercise continued, there was a progressive increase in HR; however, MAP plateaued at 6 m·min^-1 and LSNA plateaued at 12 m·min^-1. Since LSNA increased at the onset of whole-body dynamic exercise in the rat, we suggest that the increase in LSNA at the onset of exercise is mediated by a central (feed forward) mechanism.