Baroreflex control of renal sympathetic nerve activity and spontaneous rhythms at Mayer waveʹs frequency in rats

The effects of sedation with pentobarbital sodium (15 mg/kg followed by 15.9±1.2 mg/kg/h, i.v.) on arterial pressure (AP) Mayer waves and accompanying oscillations of renal sympathetic nerve activity (RSNA) were examined in rats (n=8). As compared with values observed in the conscious state, pentobarbital significantly (P<0.05) decreased AP (from 119±2 to 93±3 mm Hg), heart rate (HR; from 427±11 to 355±11 beats/min) and RSNA (from 1.20±0.27 to 0.62±0.13 μV). The baroreflex control of RSNA was analyzed by fitting a sigmoid logistic function to changes in RSNA and AP observed during nitroprusside and phenylephrine administrations. During pentobarbital infusion, the RSNA-AP relationship was reset towards lower AP values, but neither its maximum gain nor its gain at resting AP were significantly altered (from 6.3±1.0 to 5.8±1.4 and from 3.2±0.5 to 3.8±1.3 normalized units (n.u.)/mm Hg, respectively). Spectral power in the frequency band containing Mayer waves (0.29–0.73 Hz) was significantly decreased by pentobarbital for both AP (from 4.65±0.90 to 0.95±0.25 mm Hg2) and RSNA (from 1437±245 to 488±79 n.u.2). This effect was mainly secondary to the attenuation of strongly coherent oscillations of both variables at ~ 0.4 Hz. Although previous experimental evidence points to a major involvement of the sympathetic limb of the arterial baroreceptor reflex in the genesis of Mayer waves, the present study indicates that the amplitude of these oscillations cannot be used as a quantitative index of sympathetic baroreflex sensitivity.