α-Adrenergic effects on low-frequency oscillations in blood pressure and R-R intervals during sympathetic activation

Abstract

The present study was designed to address the contribution of α-adrenergic modulation to the genesis of low-frequency (LF; 0.04-0.15 Hz) oscillations in R-R interval (RRi), blood pressure (BP) and muscle sympathetic nerve activity (MSNA) during different sympathetic stimuli. Blood pressure and RRi were measured continuously in 12 healthy subjects during 5 min periods each of lower body negative pressure (LBNP; −40 mmHg), static handgrip exercise (HG; 20% of maximal force) and postexercise forearm circulatory occlusion (PECO) with and without α-adrenergic blockade by phentolamine. Muscle sympathetic nerve activity was recorded in five subjects during LBNP and in six subjects during HG and PECO. Low-frequency powers and median frequencies of BP, RRi and MSNA were calculated from power spectra. Low-frequency power during LBNP was lower with phentolamine versus without for both BP and RRi oscillations (1.6 ± 0.6 versus 1.2 ± 0.7 ln mmHg 2 , P = 0.049; and 6.9 ± 0.8 versus 5.4 ± 0.9 ln ms 2 , P = 0.001, respectively). In contrast, the LBNP with phentolamine increased the power of high-frequency oscillations (0.15-0.4 Hz) in BP and MSNA (P < 0.01 for both), which was not observed during saline infusion. Phentolamine also blunted the increases in the LBNP-induced increase in frequency of LF oscillations in BP and RRi. Phentolamine decreased the LF power of RRi during HG (P = 0.015) but induced no other changes in LF powers or frequencies during HG. Phentolamine resulted in decreased frequency of LF oscillations in RRi (P = 0.004) during PECO, and a similar tendency was observed in BP and MSNA. The power of LF oscillation in MSNA did not change during any intervention. We conclude that α-adrenergic modulation contributes to LF oscillations in BP and RRi during baroreceptor unloading (LBNP) but not during static exercise. Also, α-adrenergic modulation partly explains the shift to a higher frequency of LF oscillations during baroreceptor unloading and muscle metaboreflex activation.

Physiological Mechanisms Mediating the Coupling between Heart Period and Arterial Pressure in Response to Postural Changes in Humans

“…These data may reflect a type II error due to sample size ( n = 8), which was relatively small, albeit considerable in absolute terms with respect to the demanding experimental protocol of Experiment 3. Nonetheless, our data are in line with previous data that employed lower-body negative pressure to decrease CVP in the supine position (Kiviniemi et al, 2011; Aletti et al, 2012). Activation of the cardiopulmonary baroreflex may enhance the arterial baroreflex control of vascular resistance in human subjects (Victor and Mark, 1985).…”

Individual differences in cardiac and vascular components of the pressor response to isometric handgrip exercise in humans

“…As a result, we found no association between the CO response during exercise and PEMI-induced changes in cardiac parasympathetic tone or BRS. In contrast to our earlier studies (22,65) and the present results, which showed an increase in BRS during PEMI, BRS has also reportedly attenuated during isometric handgrip exercise (32,58,59). This discrepancy might be attributable to differences in cardiac parasympathetic tone, since the reduction in BRS during exercise is linked to vagal withdrawal (45).…”

“…While the increase in arterial pressure during isometric exercise is a well-established response, the responses of the components mediating the pressor response, cardiac output (CO) and total peripheral vascular resistance (TPR), remain controversial. In fact, previous studies have shown that, during isometric handgrip exercise in healthy humans, the pressor response occurs via an increase in CO (16,32,[34][35][36]57), an increase in TPR (6, 17), or both of those (3,31,58,59,61,62). The variability of those findings leads one to speculate that there are considerable individual differences in the components of the pressor response to exercise, which is in stark contrast to the nearly invariant arterial pressure response.…”

Spontaneous fluctuations in the peripheral photoplethysmographic waveform: roles of arterial pressure and muscle sympathetic nerve activity

“…The present results, however, cannot infer the association between the changes in PPG, MAP, and MSNA variabilities within an individual under a physiological stimulus, which will be a separate question that needs to be addressed by further studies. Recently, it has been suggested that the ability of LF spectral power to reflect changes in SNA may depend on the specific mechanism of sympathetic activation (19).…”