Changes in muscle sympathetic nerve activity and calf blood flow during combined leg and forearm exercise

In order to examine efferent sympathetic nerve control of the peripheral circulation during exercise, muscle sympathetic nerve activity (MSNA), calf blood flow (CBF), heart rate (HR), blood pressure (BP) and oxygen uptake were measured during combined foot and forearm exercise. An initial period of rhythmic foot exercise (RFE) (60 min^-1 at 10% of maximal voluntary contraction (MVC) was followed by the addition of rhythmic handgrip exercise (RFE+OCCL) (60 min at 30% of MVC) and by forearm ischaemia after handgrip exercise while continuing RFE (RFE + OCCL). During RFE, CBF in the working leg, HR and oxygen increased respectively by 560%, 121% and 144% when compared with the control rest period, but MSNA (burst rate) was reduced by 13% (P > 0.05) and BP was unchanged. During RFE+RHG, HR, BP and oxygen uptake were greater than during RFE alone. There was no change in CBF, but a significant increase occurred in calf vascular resistance (CVR) and MSNA increased to 121% of the control level. During RFE + OCCL, MSNA, CVR and BP were all higher than during RFE alone, whereas HR and oxygen uptake decreased slightly, although they remained higher than the control values. The increase in CVR in the working leg and the rise in BP during RFE+RHG or RFE+OCCL might be linked to enhancement of MSNA, which may have been reflexly evoked by input from muscle metabolic receptors in the working forearm.     

Changes in muscle sympathetic nerve activity and calf blood flow during combined leg and forearm exercise.

In order to examine efferent sympathetic nerve control of the peripheral circulation during exercise, muscle sympathetic nerve activity (MSNA), calf blood flow (CBF), heart rate (HR), blood pressure (BP) and oxygen uptake were measured during combined foot and forearm exercise. An initial period of rhythmic foot exercise (RFE) (60 min-1 at 10% of maximal voluntary contraction (MVC) was followed by the addition of rhythmic handgrip exercise (RFE + OCCL) (60 min at 30% of MVC) and by forearm ischaemia after handgrip exercise while continuing RFE (RFE + OCCL). During RFE, CBF in the working leg, HR and oxygen increased respectively by 560%, 121% and 144% when compared with the control rest period, but MSNA (burst rate) was reduced by 13% (P > 0.05) and BP was unchanged. During RFE + RHG, HR, BP and oxygen uptake were greater than during RFE alone. There was no change in CBF, but a significant increase occurred in calf vascular resistance (CVR) and MSNA increased to 121% of the control level. During RFE + OCCL, MSNA, CVR and BP were all higher than during RFE alone, whereas HR and oxygen uptake decreased slightly, although they remained higher than the control values. The increase in CVR in the working leg and the rise in BP during RFE + RHG or RFE + OCCL might be linked to enhancement of MSNA, which may have been reflexly evoked by input from muscle metabolic receptors in the working forearm.     

Initiation of increase in muscle sympathetic nerve activity delay during maximal voluntary contraction

To investigate the effects of maximal voluntary exercise on sympathetic nerve activity, contraction force and muscle sympathetic nerve activity (MSNA) were recorded during maximal (MVG) and submaximal voluntary isometric handgrip (SVG) for 2 min in eight healthy subjects. MSNA was determined by a microneurographic technique, and handgrip force, heart rate (HR) and arterial blood pressure (ABP) were measured by a non-invasive method during exercise. Grip force decayed rapidly to 58% of maximal grip force (MGF) at 10 s after commencement of exercise and was almost constant (≈ 30% of MGF) 40 s after exercise. MSNA increase was delayed by 20 s during MVG, followed by a gradual increase. HR was elevated immediately after onset of exercise, while mean ABP rise showed a 20 s lag from initiation of MVG exercise. During SVG increases in MSNA, HR and mean ABP were delayed by 50, 40 and 20 s, respectively, relative to commencement of exercise. Thereafter, these parameters increased time-dependently. These results suggested that the MSNA increase during MVG may be predominantly because of the metaboreflex.     

Twenty-four-hour non-invasive monitoring of systemic haemodynamics and cerebral blood flow velocity in healthy humans

Acute short-term changes in blood pressure (BP) and cardiac output (CO) affect cerebral blood flow (CBF) in healthy subjects. As yet, however, we do not know how spontaneous fluctuations in BP and CO influence cerebral circulation throughout 24 h. We performed simultaneous monitoring of BP, systemic haemodynamic parameters and blood flow velocity in the middle cerebral artery (MCAV) in seven healthy subjects during a 24-h period. Finger BP was recorded continuously during 24 h by Portapres and bilateral MCAV was measured by transcranial Doppler (TCD) during the first 15 min of every hour. The subjects remained supine during TCD recordings and during the night, otherwise they were seated upright in bed. Stroke volume (SV), CO and total peripheral resistance (TPR) were determined by Modelflow analysis. The 15 min mean value of each parameter was assumed to represent the mean of the corresponding hour. There were no significant differences between right vs. left, nor between mean daytime vs. night time MCAV. Intrasubject comparison of the twenty-four 15-min MCAV recordings showed marked variations (P < 0.001). Within each single 15-min recording period, however, MCAV was stable whereas BP showed significant short-term variations (P < 0.01). A day-night difference in BP was only observed when daytime BP was evaluated from recordings in the seated position (P < 0.02), not in supine recordings. Throughout 24 h, MCAV was associated with SV and CO (P < 0.001), to a lesser extent with mean arterial pressure (MAP; P < 0.005), not with heart rate (HR) or TPR. These results indicate that in healthy subjects MCAV remains stable when measured under constant supine conditions but shows significant variations throughout 24 h because of activity. Moreover, changes in SV and CO, and to a lesser extent BP variations, affect MCAV throughout 24 h.     

Twenty‐four‐hour non‐invasive monitoring of systemic haemodynamics and cerebral blood flow velocity in healthy humans

Acute short‐term changes in blood pressure (BP) and cardiac output (CO) affect cerebral blood flow (CBF) in healthy subjects. As yet, however, we do not know how spontaneous fluctuations in BP and CO influence cerebral circulation throughout 24 h. We performed simultaneous monitoring of BP, systemic haemodynamic parameters and blood flow velocity in the middle cerebral artery (MCAV) in seven healthy subjects during a 24‐h period. Finger BP was recorded continuously during 24 h by Portapres and bilateral MCAV was measured by transcranial Doppler (TCD) during the first 15 min of every hour. The subjects remained supine during TCD recordings and during the night, otherwise they were seated upright in bed. Stroke volume (SV), CO and total peripheral resistance (TPR) were determined by Modelflow analysis. The 15 min mean value of each parameter was assumed to represent the mean of the corresponding hour. There were no significant differences between right vs. left, nor between mean daytime vs. night time MCAV. Intrasubject comparison of the twenty‐four 15‐min MCAV recordings showed marked variations (P < 0.001). Within each single 15‐min recording period, however, MCAV was stable whereas BP showed significant short‐term variations (P < 0.01). A day–night difference in BP was only observed when daytime BP was evaluated from recordings in the seated position (P < 0.02), not in supine recordings. Throughout 24 h, MCAV was associated with SV and CO (P < 0.001), to a lesser extent with mean arterial pressure (MAP; P < 0.005), not with heart rate (HR) or TPR. These results indicate that in healthy subjects MCAV remains stable when measured under constant supine conditions but shows significant variations throughout 24 h because of activity. Moreover, changes in SV and CO, and to a lesser extent BP variations, affect MCAV throughout 24 h.     

Exercise‐induced sympathetic activation is correlated with cerebral hemisphere laterality,but not handedness

To investigate whether sympathetic responses are correlated with central laterality or handedness, muscle sympathetic nerve activity (MSNA), heart rate (HR) and blood pressure (BP) were compared between right (RA) and left arm (LA) grip exercise with volitional maximum effort (MVHG) for 2 min and post‐exercise arterial occlusion (PEAO) in right‐ and left‐handed volunteers. MVHG and PEAO led to a greater increase in MSNA in RA than in LA exercise (180 vs. 150%, P=0.004; 140 vs. 85%, P=0.005). MVHG elevated HR to a significantly lesser extent in RA than in LA (35 vs. 46%, P=0.030), and the difference was maintained during PEAO. The BP rise during MVHG and PEAO was the same in RA and in LA. Muscle sympathetic nerve activity, HR and BP responses during MVHG and PEAO showed no difference between the dominant and non‐dominant arm. These results suggested that the effects of central motor command and metaboreflex on sympathetic outflow to the vasculature and the heart may be selectively modulated partly by hemispherical laterality.     

Exercise-induced sympathetic activation is correlated with cerebral hemisphere laterality, but not handedness

To investigate whether sympathetic responses are correlated with central laterality or handedness, muscle sympathetic nerve activity (MSNA), heart rate (HR) and blood pressure (BP) were compared between right (RA) and left arm (LA) grip exercise with volitional maximum effort (MVHG) for 2 min and post-exercise arterial occlusion (PEAO) in right- and left-handed volunteers. MVHG and PEAO led to a greater increase in MSNA in RA than in LA exercise (180 vs. 150%, P=0.004; 140 vs. 85%, P=0.005). MVHG elevated HR to a significantly lesser extent in RA than in LA (35 vs. 46%, P=0.030), and the difference was maintained during PEAO. The BP rise during MVHG and PEAO was the same in RA and in LA. Muscle sympathetic nerve activity, HR and BP responses during MVHG and PEAO showed no difference between the dominant and non-dominant arm. These results suggested that the effects of central motor command and metaboreflex on sympathetic outflow to the vasculature and the heart may be selectively modulated partly by hemispherical laterality.     

Middle cerebral artery blood velocity,arterial diameter and muscle sympathetic nerve activity during post-exercise muscle ischaemia

During exercise the transcranial Doppler determined mean blood velocity (V_mean) increases in the middle cerebral artery (MCA) and reflects cerebral blood flow when the diameter at the site of investigation remains constant. Sympathetic activation could induce MCA vasoconstriction and in turn elevate V_mean at an unchanged cerebral blood flow. In 12 volunteers we evaluated whether V_mean relates to muscle sympathetic nerve activity (MSNA) in the peroneal nerve during rhythmic handgrip and post-exercise muscle ischaemia (PEMI). The luminal diameter of the dorsalis pedis artery (AD) was taken to reflect the MSNA influence on a peripheral artery. Rhythmic handgrip increased heart rate (HR) from 74 ± 20 to 92 ± 21 beats min^?1 and mean arterial pressure (MAP) from 87 ± 7 to 105 ± 9 mmHg (mean ± SD; P < 0.05). During PEMI, HR returned to pre-exercise levels while MAP remained elevated (101 ± 9 mmHg). During handgrip contralateral MCA V_mean increased from 65 ± 10 to 75 ± 13 cm s^?1 and this was more than on the ipsilateral side (from 63 ± 10 to 68 ± 10 cm s^?1; P < 0.05). On both sides of the brain V_mean returned to baseline during PEMI. MSNA did not increase significantly during handgrip (from 56 ± 24 to 116 ± 39 units) but the elevation became statistically significant during PEMI (135 ± 86 units, P < 0.05), while AD did not change. Taken together, during exercise and PEMI, V_mean changed independent of an elevation of MSNA by more than 140% and the dorsalis pedis artery diameter was stable. The results provide no evidence for a vasoconstrictive influence of sympathetic nerve activity on medium size arteries of the limbs and the brain during rhythmic handgrip and post-exercise muscle ischaemia.     

Renal sympathetic and circulatory responses to activation of the exercise pressor reflex in rats

We investigated the role played by the exercise pressor reflex in sympathetic regulation of the renal circulation in rats. In mid-collicular decerebrate rats, mean arterial pressure (MAP), heart rate (HR), left renal cortical blood flow (RCBF) and left renal sympathetic nerve activity (RSNA) were recorded before and during 30 s of static contraction of the left triceps surae muscles evoked by electrical stimulation of the tibial nerve, which activates both metabo- and mechanosensitive muscle afferents, and during 30 s of passive stretch of the left Achilles tendon, which selectively activates mechanosensitive muscle afferents. Static contraction (n = 17, +344 +/- 34 g developed tension) significantly (P < 0.05) increased MAP (+14 +/- 3 mmHg), HR (+6 +/- 1 beats min(-1)) and RSNA (n = 11, +19 +/- 5%) and significantly decreased renal cortical vascular conductance (RCVC, n = 11, -11 +/- 2%). Passive stretch (n = 20, +378 +/- 11 g) also significantly increased MAP (+11 +/- 2 mmHg), HR (+7 +/- 2 beats min(-1)) and RSNA (n = 15, +14 +/- 4%) and significantly decreased RCVC (n = 11, -12 +/- 3%). RCBF showed no significant changes during static contraction or passive stretch. Renal denervation abolished the decrease in RCVC during contraction (n = 12) or stretch (n = 13). These data indicate that both the exercise pressor reflex and its mechanically sensitive component, the muscle mechanoreflex, induced renal cortical vasoconstriction through sympathetic activation in rats.     

The influence of straining maneuvers on the pressor response during isometric exercise

Summary Experiments were performed to determine to what extent increments in esophageal and abdominal pressure would have on arterial blood pressure during fatiguing isometric exercise. Arterial blood pressure was measured during handgrip and leg isometric exercise performed with both a free and occluded circulation to active muscles. Handgrip contractions were exerted at 33 and 70% MVC (maximum voluntary contraction) by 4 volunteers in a sitting position and calf muscle contractions at 50 and 70% MVC with the subjects in a kneeling position. Esophageal pressure measured at the peak of inspirations did not change during either handgrip or leg contractions but peak expiratory pressures increased progressively during both handgrip and leg contractions as fatigue occurred. These increments were independent of the tensions of the isometric contractions exerted. Intra-abdominal pressures measured at the peak of either inspiration or expiration did not change during inspiration with handgrip contractions but increased during expiration. During leg exercise, intraabdominal pressures increased during both inspiration and expiration, reaching peak levels at fatigue. The arterial blood pressure also reached peak levels at fatigue, independent of circulatory occlusion and tension exerted, averaging 18.5–20 kPa (140–150 mm Hg) for both handgrip and leg contrations. While blood pressure returned to resting levels following exercise with a free circulation, it declined by only 2.7–3.8 kPa after leg and handgrip exercise, respectively, during circulatory occlusion. These results indicate that straining maneuvers contribute 3.5 to 7.8 kPa to the change in blood pressure depending on body position.     

Left ventricular mechanoreceptors: a haemodynamic study

1. To study the function of the left ventricular mechanoreceptors, a working left ventricle preparation was devised in dogs which permitted control of pressure and flow of the isolated perfused coronary circulation and of the flow of the isolated, separately perfused systemic circulation. The systemic circulation was perfused at a constant rate so that changes in systemic pressure reflected changes in systemic resistance.2. Increases in myocardial contractility produced by injection of catecholamines into the isolated, perfused coronary circulation produced a fall in the pressure (resistance) of the isolated, separately perfused (at a constant rate) systemic circulation.3. Completeness of isolation of the coronary and systemic circulations was shown by the marked difference in appearance times between the reflex hypotensive responses from catecholamine injections into the isolated coronary circulation and the direct hypertensive response from a similar injection when the circulations were connected as well as by the marked difference between the pressure pulses recorded simultaneously on both sides of the aortic balloon separating the two circulations.4. Myocardial beta receptor blockade produced by injection of propranolol into the isolated coronary circulation abolished or attenuated the changes in left ventricular myocardial contractility as well as the subsequent hypotensive responses following the similar injection of catecholamines.5. Electrical stimulation of a sympathetic nerve innervating the heart resulted in increases in left ventricular myocardial contractility and subsequent systemic hypotensive responses indistinguishable from those following injection of catecholamines.6. That distortion of the mechano- or stretch receptors in the left ventricular myocardium was the cause of the hypotensive responses was demonstrated by increasing left ventricular myocardial contractility by mechanically obstructing the left ventricular outflow which produced hypotensive responses similar to those following the injection of catecholamines or nerve stimulation.7. Bilateral high cervical vagotomy abolished the hypotensive responses following injection of catecholamines into the isolated coronary circulation or following left ventricular outflow obstruction in all but one instance, indicating the importance of vagal fibres to the afferent arm of the reflex.8. It is suggested that the left ventricular mechanoreceptors function normally to reduce the peripheral resistance in order to prepare the systemic circulation to receive the left ventricular output and, especially during exercise, to prepare the systemic circulation to receive the augmented cardiac output with a minimum alteration in the systemic blood pressure and to distribute this augmented output preferentially to the skeletal muscles.     

Differences in muscle sympathetic nerve response to isometric exercise in different muscle groups

The aim of this study was to examine the effects of muscle fibre composition on muscle sympathetic nerve activity (MSNA) in response to isometric exercise. The MSNA, recorded from the tibial nerve by a microneurographic technique during contraction and following arterial occlusion, was compared in three different muscle groups: the forearm (handgrip), anterior tibialis (foot dorsal contraction), and soleus muscles (foot plantar contraction) contracted separately at intensities of 20%, 33% and 50% of the maximal voluntary force. The increases in MSNA relative to control levels during contraction and occlusion were significant at all contracting forces for handgrip and at 33% and 50% of maximal for dorsal contraction, but there were no significant changes, except during exercise at 50%, for plantar contraction. The size of the MSNA response correlated with the contraction force in all muscle groups. Pooling data for all contraction forces, there were different MSNA responses among muscle groups in contraction forces (P = 0.0001, two-way analysis of variance), and occlusion periods (P = 0.0001). The MSNA increases were in the following order of magnitude: handgrip, dorsal, and plantar contractions. The order of the fibre type composition in these three muscles is from equal numbers of types I and II fibres in the forearm to increasing number of type I fibres in the leg muscles. The different MSNA responses to the contraction of different muscle groups observed may have been due in part to muscle metaboreflex intensity influenced by their metabolic capacity which is related to by their metabolic capacity which is related to the fibre type.     

Blood pressure and heart rate responses during a fatiguing isometric exercise in paraplegic men with hypertension

A group of 45 male subjects were examined in a cross-sectional study to compare the blood pressure response that occurs during isometric exercise maintained to fatigue among control subjects and paraplegic patients with (PH) and without essential hypertension (PN). Two muscle groups were examined: the handgrip muscles (voluntary effort) and the quadriceps muscles. The tension chosen for the contraction was 40% of the muscles maximum strength for both muscle groups. While the paraplegic groups had more strength in their handgrip muscles than that found for the controls, the control subjects had more strength in their quadriceps muscles than either of the paraplegic groups. During the fatiguing isometric contractions, the rate of rise and absolute systolic blood pressure was higher in the PH than the other groups of subjects. The diastolic pressure of the PH group, while elevated during exercise, was only elevated to the same degree as the increase in resting diastolic pressure above normal. Heart rate changes during exercise was the same in all groups of subjects for handgrip contractions. The controls had the same heart rate response to handgrip as to leg exercise. The paraplegic groups showed no heart rate change during fatiguing contractions of their quadriceps muscles. The PH group actually showed a reduction in heart rate during the leg exercise. Accepted: 10 July 2000     

Blood pressure and heart rate responses during a fatiguing isometric exercise in paraplegic men with hypertension

A group of 45 male subjects were examined in a cross-sectional study to compare the blood pressure response that occurs during isometric exercise maintained to fatigue among control subjects and paraplegic patients with (PH) and without essential hypertension (PN). Two muscle groups were examined: the handgrip muscles (voluntary effort) and the quadriceps muscles. The tension chosen for the contraction was 40% of the muscles maximum strength for both muscle groups. While the paraplegic groups had more strength in their handgrip muscles than that found for the controls, the control subjects had more strength in their quadriceps muscles than either of the paraplegic groups. During the fatiguing isometric contractions, the rate of rise and absolute systolic blood pressure was higher in the PH than the other groups of subjects. The diastolic pressure of the PH group, while elevated during exercise, was only elevated to the same degree as the increase in resting diastolic pressure above normal. Heart rate changes during exercise was the same in all groups of subjects for handgrip contractions. The controls had the same heart rate response to handgrip as to leg exercise. The paraplegic groups showed no heart rate change during fatiguing contractions of their quadriceps muscles. The PH group actually showed a reduction in heart rate during the leg exercise.     

Exteroceptive reflexes in jaw-closing muscle EMG during rhythmic jaw closing and clenching in man

Exteroceptive jaw reflexes might play a role in normal functions of the mouth such as mastication. Until now these reflexes have only been studied under isometric conditions. The aim of this study was to compare exteroceptive reflexes in jaw muscle EMG during the closing phase of rhythmic open-close movements and clenching, at the same jaw gape and with similar muscle EMG. Reflexes consisting of successive waves of decreased and increased muscle activity (the Q, R, S and T waves of the post-stimulus electromyographic complex (PSEC)), evoked by light noxious electrical stimulation of the vermillion border of the lower lip, were recorded from the jaw closing muscles of 17 subjects. Differences between the two tasks occurred in two phases of the PSEC: (1) in an early phase, around the R wave, there was significantly less EMG during jaw closing (mean EMG ratio between jaw-closing and clenching 0.71), and (2) in a late phase, around the transition between the S to the T wave, there was significantly more EMG during jaw closing (mean EMG ratio: 1.40). The decrease in EMG activity around the R wave during jaw closing may be due to a change in reflex sensitivity at an interneuron level. The increase in EMG activity around the transition between the S and T waves during jaw closing might, at least in part, be due to a proprioceptive stretch reflex. This reflex is mediated by muscles spindles that are activated by the deceleration of the jaw evoked by the lip stimulus. The finding of inhibitory reflex mechanisms that predominate more during rhythmic jaw movements than during clenching in an early phase of the PSEC might be related to protecting oral tissues from trauma when the jaw is closing with potentially a large muscle force. In contrast, when food is held between the teeth, a possible inhibitory influence of light noxious stimuli is diminished.     

Autonomic nervous control of the heart rate during isometric exercise in normal man

The relative contribution of the efferent components of the autonomic nervous system to the regulation of tachycardia induced by isometric exercise was assessed in 23 normal males. The isometric exercise (handgrip) was performed at the maximum intensity tolerated by the individual over a period of 10 s (maximal voluntary contraction — MVC) and at levels equivalent to 75, 50 and 25% of MVC for 20, 40 and 10 s, respectively. The study was performed both under control conditions and after pharmacological blockade with atropine (12 individuals) or propranolol (11 individuals). Under control conditions, the heart rate (HR) responses to isometric effort were dependent on the intensity and duration of the exercise, showing a tendency towards progressive elevation with the maintenance of muscular contraction at the levels studied. The tachycardia evoked by this effort was of considerable magnitude and of rapid onset, especially at the more intense levels of activity. Parasympathetic blockade markedly decreased tachycardia, which manifested itself during the first 10 s of exercise at all levels of intensity, whereas sympathetic blockade markedly modified the HR response after 10 s of effort at the 75 and 50% MVC levels. A slight depression of the tachycardiac response could be observed already after 10 s of maximum effort after propranolol. The present results suggest that the autonomic regulation of these responses is based on a biphasic mechanism, with the initial phase depending on the rapid withdrawal of the parasympathetic influence, followed by a marked sympathetic contribution to the induction of tachycardia after 10 s of isometric contraction or even a little before at maximum exertion.     

Human investigations into the exercise pressor reflex

During exercise, neural input from skeletal muscles reflexly maintains or elevates blood pressure (BP) despite a maybe fivefold increase in vascular conductance. This exercise pressor reflex is illustrated by similar heart rate (HR) and BP responses to electrically induced and voluntary exercise. The importance of the exercise pressor reflex for tight cardiovascular regulation during dynamic exercise is supported by studies using pharmacological blockade of lower limb muscle afferent nerves. These experiments show attenuation of the increase in BP and cardiac output when exercise is performed with attenuated neural feedback. Additionally, there is no BP response to electrically induced exercise with paralysing epidural anaesthesia or when similar exercise is evoked in paraplegic patients. Furthermore, BP decreases when electrically induced exercise is carried out in tetraplegic patients. The lack of an increase in BP during exercise with paralysed legs manifests, although electrical stimulation of muscles enhances lactate release and reduces muscle glycogen. Thus, the exercise pressor reflex enhances sympathetic activity and maintains perfusion pressure by restraining abdominal blood flow, while brain, skin and muscle blood flow may also become affected because the reflex 'resets' arterial baroreceptor modulation of vascular conductance, making BP the primarily regulated cardiovascular variable during exercise.     

Cardiovascular and catecholamine responses to static exercise in partially curarized humans

Neural control of the circulation was evaluated during static exercise in 19 subjects by the determination of heart rate (HR), mean arterial pressure (MAP), cardiac output (CO) and plasma catecholamines. Influence from central command was evaluated during contractions with weakened muscles following partial curarization and reflex influence from metaboreceptors was assessed by post-exercise muscle ischaemia. Static handgrip increased HR and more so MAP and CO and MAP remained elevated during post-exercise muscle ischaemia. With partial curarization plasma catecholamines were also increased (P<0.05). Two-leg extension increased all variables and during post-exercise muscle ischaemia elevations of HR, MAP and CO were maintained (P<0.05). With partial curarization HR, MAP and plasma noradrenaline were even greater during the contraction. With the involvement of both legs during static exercise, reflex influence from the muscles elevated blood pressure by way of HR and CO and the importance of central command was detectable for HR and MAP as plasma catecholamines became elevated. However, the results indicate a separation between a central command influence on HR and CO related to an increase in plasma catecholamines during a handgrip, while the reflex influence on blood pressure was directed towards total peripheral resistance.     

Autonomic and cardiovascular effects of central neuromedin U in rats

Previous studies have demonstrated that neuromedin U (NMU) affects cardiovascular functions such as blood pressure (BP) or heart rate (HR) in rats. Here, we examined the effects of the lateral cerebral ventricular (ICV) injection of various doses of NMU on renal sympathetic nerve activity (RSNA) and BP in urethane-anesthetized rats. ICV injection of NMU elevated RSNA, BP and HR in a dose-dependent manner.Moreover, neither ICV pretreatment of thioperamide, an antagonist of histaminergic H3-receptor, or of diphenhydramine, an antagonist of histaminergic H1-receptor, abolished increasing effects of NMU on RSNA, BP and HR In addition, ICV injection of NMU suppressed gastric vagal nerve activity (GVNA) and activated brown adipose tissue sympathetic nerve activity (BAT-SNA) of anesthetized rats, and elevated brown adipose tissue temperature (BAT-T) of conscious rats. Thus, these evidence suggest that NMU affects neural activities of autonomic nerves containing RSNA, GVNA or BAT-SNA, and BP by mediating central mechanism.     

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

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.