-2 Adrenergic Transmission and Human Baroreflex Regulation

We observed earlier that central -2 adrenoceptor stimulation in mice greatly augments parasympathetic tone. To test the effects in humans, we assessed autonomic vasomotor tone and baroreflex regulation in 9 normal young adults on 2 occasions, once with and once without clonidine. We determined heart rate (HR), beat-by-beat blood pressure (BP), and muscle sympathetic nerve activity. HR variability was analyzed in the time and frequency domain. Pharmacological baroreflex slopes were determined using incremental phenylephrine and nitroprusside infusions. Clonidine lowered resting BP (122 4/73 3 versus 100 7/55 3 mm Hg, P 0.01), muscle sympathetic nerve activity (18 3 versus 4 2 bursts/min, P 0.01), and HR (62 3 versus 56 3 bpm, P 0.05). The baroreflex heart rate curve was reset to much lower HR values and showed no saturation at low HR. HR variability profoundly increased during clonidine plus phenylephrine (total power: 3224 843 versus 8943 2329 ms, P 0.05). High-frequency power was 1451 520 at baseline and 672