Reflex responses in plasma catecholamines caused by static contraction of skeletal muscle

Cardiovascular and autonomic responses to passive arm or leg movement in men and women

PURPOSE

Women display an attenuated mechanoreflex during leg movement; however, sex differences in the response to arm movement are unknown.

METHODS

Men (n = 12) and women (n = 10) performed passive arm or leg movement where either the right elbow or right knee was passively flexed/extended for 3 min at 30 times/min. Mean arterial pressure (MAP), cardiac output index (Qi), and heart rate (HR) were continuously measured and 1-min averages along with peak values were obtained. Heart rate variability was measured at baseline and throughout 3 min of passive movement.

RESULTS

Men had a greater average HR (P = 0.006) and Qi (P = 0.05) responses to passive limb movement compared to women. Men also had a greater (P = 0.02) and faster (P = 0.04) peak Qi response compared to women. During arm movement, men exhibited a greater change of average MAP compared to both women (P = 0.002) and leg movement (P = 0.05). Movement of either limb in both sexes decreased low-frequency power (LF; P = 0.04), decreased low-frequency to high-frequency ratio (LF/HF; P = 0.03), and increased high-frequency power (HF; P = 0.01) of heart rate variability. Women had lower pulse wave velocity (P = 0.02), higher root mean square of the successive differences (RMSSD; P = 0.04), lower LF power (P = 0.04), higher HF power (P = 0.03), and higher cardiovagal baroreceptor sensitivity (P = 0.003) compared to men at all time points.

CONCLUSIONS

We have found sex- and limb-dependent responses where men exhibit higher blood pressure in response to passive arm movement compared to women and compared to leg movement.

Reflex arc of the teeth clenching-induced pressor response in rats

Although "teeth clenching" induces pressor response, the reflex tracts of the response are unknown. In this study, dantrolene administration inhibited teeth clenching generated by electrical stimulation of the masseter muscles and completely abolished the pressor response. In addition, trigeminal ganglion block or hexamethonium administration completely abolished the pressor response. Local anesthesia of molar regions significantly reduced the pressor response to 27 ± 10%. Gadolinium (mechanoreceptor blocker of group III muscle afferents) entrapment in masticatory muscles also significantly reduced the pressor response to 62 ± 7%. Although atropine methyl nitrate administration did not change the pressor response, a significant dose-dependent augmentation of heart rate was observed. These results indicate that both periodontal membrane and mechanoreceptors in masticatory muscles are the receptors for the pressor response, and that the afferent and efferent pathways of the pressor response pass through the trigeminal afferent nerves and sympathetic nerves, respectively.

Differential contribution of ACh-muscarinic and β-adrenergic receptors to vasodilatation in noncontracting muscle during voluntary one-legged exercise

We have demonstrated the centrally induced cholinergic vasodilatation in skeletal muscle at the early period of voluntary one-legged exercise and during motor imagery in humans. The purpose of this study was to examine whether central command may also cause β-adrenergic vasodilatation during the exercise and motor imagery. Relative changes in oxygenated hemoglobin concentration (Oxy-Hb) of bilateral vastus lateralis (VL) muscles, as index of tissue blood flow, and femoral blood flow to nonexercising limb were measured during one-legged cycling and mental imagery of the exercise for 1 min before and after propranolol (0.1 mg/kg iv). The Oxy-Hb of noncontracting muscle increased (P < 0.05) at the early period of exercise and the increase was sustained throughout exercise, whereas the Oxy-Hb of contracting muscle increased at the early period but thereafter decreased. We subtracted the Oxy-Hb response with propranolol from the control response in individual subjects to identify the propranolol-sensitive component of the Oxy-Hb response during exercise. In both noncontracting and contracting VL muscles, the increase in Oxy-Hb at the early period of one-legged exercise did not involve a significant propranolol-sensitive component. However, as the exercise proceeded, the propranolol-sensitive component of the Oxy-Hb response was developed during the later period of exercise. Propranolol also failed to affect the initial increases in femoral blood flow and vascular conductance of nonexercising leg but significantly attenuated (P < 0.05) their later increases during exercise. Subsequent atropine (10-15 μg/kg iv) abolished the initial increases in Oxy-Hb of both VL muscles. Mental imagery of the one-legged exercise caused the bilateral increases in Oxy-Hb, which were not altered by propranolol but abolished by subsequent atropine. It is likely that the rapid cholinergic and delayed β-adrenergic vasodilator mechanisms cooperate to increase muscle blood flow during exercise.

Attenuated adrenergic responses to exercise in women with fibromyalgia - A controlled study

The pathogenesis of widespread pain and fibromyalgia (FM) is unknown. Altered responses from the hypothalamus-pituitary-adrenal axis, sympathetic nervous system and muscular system have been suggested as being of importance. The present study was undertaken to determine: (i) whether the sympathoadrenal response to repetitive isometric contractions until exhaustion is altered in patients with FM, and (ii) whether sympathoadrenal responses are associated with muscle fatigue and pain during exercise. Nineteen women with FM, and 19 healthy women matched for age, smoking and self-reported physical activity, participated. Maximal voluntary contraction (MVC), repetitive isometric contractions (6s contraction and 4s resting phases) were performed with both quadriceps muscles at 30% of MVC until exhaustion. Muscle activity was recorded from the quadriceps muscles by surface electromyography (EMG). Plasma adrenalin (Adr), noradrenalin (NAdr) and cortisol were measured and perceived exertion and pain reported during exercise. Attenuated Adr responses (p<0.001) with normal plasma NAdr and cortisol (p>0.19) responses were found during exercise in the FM group compared with the control group. Significantly higher EMG amplitude (%EMG(max)) during the contraction phases (p=0.001) was found in the FM than in the control group. Perceived exertion and pain responses to exercise were higher in the FM group than in the controls (p<0.001), without relationship to the sympathoadrenal responses. In conclusion; the exercise was perceived as being more painful and strenuous in the FM group. Muscle performance was altered with increased muscle activity during the exercise. Women with FM showed an attenuated Adr response to repetitive isometric exercise.

Progressive arteriolar vasoconstriction and fatigue during tetanic contractions of rat skeletal muscle are inhibited by α-receptor blockade

Voluntary muscle contractions activate sympathetic efferent pathways. Using a fatiguing electrical stimulation protocol designed specifically to enhance sympathetically-mediated vasoconstrictor tone, we explored the temporal profile and mechanistic bases of the evoked vasoconstrictor response and its role in muscle fatigue. Spinotrapezius muscles of Wistar rats were exteriorized and stimulated tetanically (100 Hz, 6-8 V, stimulus duration 700 ms) every 3 s for 2.5 min. The extent and time course of diameter changes in arterioles (1A and 2A) and venules (1V and 2V) were determined after each of 10 discrete sets of muscle stimulation at 5-min intervals. At first, to compare the effect of stimulation parameters in this preparation, stimulations were performed with rectangular pulses of either 0.2- or 4-ms pulse duration. As expected the 0.2-ms pulse stimulation did not affect arteriolar diameter or muscle fatigability. In contrast, during and following 4-ms pulse stimulations, there was a surprising arteriolar vasoconstriction rather than the expected vasodilation. Arteriolar (but not venular) vasoconstriction (reduced arteriolar diameter by 38.6 ± 2.6% in the 10th set) increased progressively with muscle fatigue (to 29 ± 12% of initial tension in the 10th set) for the 4-ms pulse condition. Superfusion with the selective α1-adrenergic receptor antagonist prazosin (1 μM) and/or α2-adrenergic receptor antagonist rauwolscine (10 μM) abolished both the arteriolar vasoconstriction and significantly reduced fatigue (i.e., % initial tension, α1: 46.8 ± 10.3%; α2: 39.0 ± 5.8%; α1 + α2: 48.7 ± 16.3% in the 10th set; all P < 0.05 vs. control). We conclude that sequential bouts of contractions induce a progressively greater degree of α-adrenergic receptor-induced arteriolar (but not venular) vasoconstriction which contributes significantly to fatigue in this model.

Centrally evoked increase in adrenal sympathetic outflow elicits immediate secretion of adrenaline in anaesthetized rats

To examine whether feedforward control by central command activates preganglionic adrenal sympathetic nerve activity (AdSNA) and releases catecholamines from the adrenal medulla, we investigated the effects of electrical stimulation of the hypothalamic locomotor region on preganglionic AdSNA and secretion rate of adrenal catecholamines in anaesthetized rats. Pre- or postganglionic AdSNA was verified by temporary sympathetic ganglionic blockade with trimethaphan. Adrenal venous blood was collected every 30 s to determine adrenal catecholamine output and blood flow. Hypothalamic stimulation for 30 s (50 Hz, 100-200 microA) induced rapid activation of preganglionic AdSNA by 83-181% depending on current intensity, which was followed by an immediate increase of 123-233% in adrenal adrenaline output. Hypothalamic stimulation also increased postganglionic AdSNA by 42-113% and renal sympathetic nerve activity by 94-171%. Hypothalamic stimulation induced preferential secretion of adrenal adrenaline compared with noradrenaline, because the ratio of adrenaline to noradrenaline increased greatly during hypothalamic stimulation. As soon as the hypothalamic stimulation was terminated, preganglionic AdSNA returned to the prestimulation level in a few seconds, and the elevated catecholamine output decayed within 30-60 s. Adrenal blood flow and vascular resistance were not affected or slightly decreased by hypothalamic stimulation. Thus, it is likely that feedforward control of catecholamine secretion from the adrenal medulla plays a role in conducting rapid hormonal control of the cardiovascular system at the beginning of exercise.

Muscle mechanosensitive reflex is suppressed in the conscious condition: effect of anesthesia

To test the hypothesis that a muscle mechanosensitive reflex is suppressed in the conscious condition, we examined the effect of anesthesia on the cardiovascular responses to passive mechanical stretch of the hindlimb triceps surae muscle in six conscious cats. The triceps surae muscle was manually stretched for 30 s by extending the hip and knee joints and subsequently by dorsiflexing the ankle joint; the lateral gastrocnemius muscle was lengthened by 19 +/- 2.6 mm. Heart rate (HR) and mean arterial blood pressure (MAP) did not change significantly during passive stretch of the muscle in the conscious condition. At 10-40 min after intravenously administering pentobarbital sodium (20-25 mg/kg), the identical passive stretch of the triceps surae muscle was able to induce the cardiovascular responses; HR and MAP were increased by 14 +/- 1.3 beats/min and 14 +/- 1.4 mmHg, respectively, and the cardiovascular responses were sustained throughout the passive stretch. In contrast, stretching skin on the triceps surae muscle evoked no significant changes in HR and MAP in the anesthetized condition. When anesthesia became light 40-90 min after injection of pentobarbital and the animals started to show spontaneous body movement, the cardiovascular response to passive muscle stretch tended to be blunted again. It is therefore concluded that passive mechanical stretch of skeletal muscle is capable of evoking the reflex cardiovascular response, which is suppressed in the conscious condition but exaggerated by anesthesia.

Deciphering the metabolic and mechanical contributions to the exercise-induced circulatory response: insights from eccentric cycling

Metabolic demand and muscle mechanical tension are closely coupled during exercise, making their respective drives to the circulatory response difficult to establish. This coupling being altered in eccentric cycling, we implemented an experimental design featuring eccentric vs. concentric constant-load cycling bouts to gain insights into the control of the exercise-induced circulatory response in humans. Heart rate (HR), stroke volume (SV), cardiac output (Q̇), oxygen uptake (V̇o2), and electromyographic (EMG) activity of quadriceps muscles were measured in 11 subjects during heavy concentric (heavy CON: 270 ± 13 W; V̇o2 = 3.59 ± 0.20 l/min), heavy eccentric (heavy ECC: 270 ± 13 W, V̇o2 = 1.17 ± 0.15 l/min), and light concentric (light CON: 70 ± 9 W, V̇o2 = 1.14 ± 0.12 l/min) cycle bouts. Using a reductionist approach, the circulatory responses observed between heavy CON vs. light CON (difference in V̇o2 and power output) was ascribed either to metabolic demand, as estimated from heavy CON vs. heavy ECC (similar power output, different V̇o2), or to muscle mechanical tension, as estimated from heavy ECC vs. light CON (similar V̇o2, different power output). 74% of the Q̇ response was determined by the metabolic demand, also accounting for 65% and 84% of HR and SV responses, respectively. Consequently, muscle mechanical tension determined 26%, 35%, and 16% of the Q̇, HR, and SV responses, respectively. Q̇ was significantly related to V̇o2 ( r2 = 0.83) and EMG activity ( r2 = 0.82; both P < 0.001). These results suggest that the exercise-induced circulatory response is mainly under metabolic control and support the idea that the level of muscle activation plays a role in the cardiovascular regulation during cycle exercise in humans.

Muscle mechanosensitive receptors close to the myotendinous junction of the Achilles tendon elicit a pressor reflex

Feedback regulation by activation of mechanosensitive afferents in the exercising muscle causes the cardiovascular and sympathetic nerve responses, which follow tension development and are almost identical between static contraction and passive stretch. The precise location of the mechanoreceptors contributing to the exercise pressor reflex, however, remained unknown. To test the hypothesis that the mechanoreceptors will be located around the myotendinous junction to monitor a change in muscle tension than a change in muscle length, we examined the reflex cardiovascular responses to passive stretch of the triceps surae muscle in anesthetized rats with three interventions; systemic injection of gadolinium, cutting the Achilles tendon, and local injection of lidocaine into the myotendinous junction. Gadolinium (42 micromol/kg iv) blunted the increases in heart rate and mean arterial blood pressure during passive stretch by 36 and 22-26%, respectively, suggesting that the reflex cardiovascular responses were evoked by stimulation of muscle mechanosensitive receptors. The cardiovascular responses to passive stretch were not different between the cut Achilles tendon and the intact tendon in the same rats, suggesting that any mechanoreceptors, terminated in the more distal part of the tendon, did not contribute to the reflex cardiovascular responses. Lidocaine (volume, 0.04-0.1 ml) injected into the myotendinous junction blunted the stretch-induced increases in heart rate and mean arterial blood pressure by 37-49 and 27-34%, respectively. We conclude that the muscle mechanosensitive receptors evoking the reflex cardiovascular responses at least partly locate at or close to the myotendinous junction of the Achilles tendon.

Responses of hepatic glucose output to electro-acupuncture stimulation of the hindlimb in anaesthetized rats

Responses of hepatic glucose output (HGO) to electro-acupuncture (EA) stimulation of the hindlimb were investigated in anaesthetized rats, focusing on involvement of the somatic afferent and autonomic efferent nerves. HGO was measured with a microdialysis probe implanted into the left lateral lobe of the liver. Stainless steel needles with a diameter of 0.25 mm were inserted into the right tibialis anterior muscle and connected to an electrical stimulator. The EA stimulation was delivered for 10 min at 10 mA, 20 Hz. Atropine was injected in order to block the action of the parasympathetic nerves, whereas phentolamine and propranolol were injected in order to block the action of the sympathetic nerves. Furthermore, adrenal sympathetic nerves were crushed bilaterally to block the reflex secretion of adrenal medullary hormones. The EA stimulation significantly increased HGO for 20 min after the onset of stimulation. The increases of HGO were abolished by severing the femoral and sciatic nerves, demonstrating that the responses are elicited via activation of somatic afferent nerves. Furthermore, the increases were diminished after severance of the adrenal sympathetic nerves, which regulate catecholamine secretion from the adrenal medulla. The increases were totally abolished after pretreatment with phentolamine, an alpha-adrenergic blocker, and propranolol, a beta-adrenergic blocker. On the other hand, the increases of HGO in response to the EA stimulation were augmented after pretreatment with atropine, a muscarinic cholinergic blocker. The present results demonstrate that EA stimulation to a hindlimb can reflexly increase HGO via activation of somatic afferents and, thereby, sympathetic efferents, including sympathetic efferents to the adrenal medulla. The present results further show that the increases of HGO in responses to EA stimulation are simultaneously reflexly inhibited via the parasympathetic nerves.

Peripheral norepinephrine exacerbates neuritis-induced hyperalgesia

Inflammation of a peripheral nerve (neuritis) causes mechanical and thermal hyperalgesia in the region in which the inflamed nerve innervates. We investigated whether peripherally applied norepinephrine (NE) would exacerbate mechanical hyperalgesia in rats with neuritis. After inflammation of the left L5 spinal nerve with complete Freund's adjuvant, the foot withdrawal thresholds to mechanical stimuli applied to the affected hind paw (mechanical thresholds) were decreased significantly, indicating the development of mechanical hyperalgesia. An intradermal injection of NE to the affected paw further aggravated mechanical hyperalgesia transiently (1-3 days) and then recovered to the pre-NE injection levels afterwards. This responsiveness to NE (adrenergic sensitivity) was observed not only while rats were showing inflammatory hyperalgesia but also after recovering from it. The effect of NE on mechanical hyperalgesia was mediated by both peripheral alpha(1)- and alpha(2)-adrenoceptors. Immunohistochemical study of the previously inflamed nerve showed that proinflammatory cytokine tumor necrosis factor immunoreactivity was significantly higher in the rats showing adrenergic sensitivity compared to rats without adrenergic sensitivity. The data thus suggest that peripheral NE, when released in an excessive amount from the sympathetic nervous system, might play an important role in the aggravation of pain in neuritis.