Effects of Partial Neuromuscuiar Blockade on Sympathetic Nerve Responses to Static Exercise in Humans

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1 468 Effects of Partial Neuromuscuiar Blockade on Sympathetic Nerve Responses to Static Exercise in Humans Ronald G. Victor, Susan L. Pryor, Niels H. Secher, and Jere H. Mitchell Downloaded from by guest on June 14, 218 We used intraneural recordings of sympathetic nerve activity in conscious humans to determine if central command increases sympathetic discharge to resting skeletal muscle during static exercise. In nine healthy subjects, we measured arterial pressure, heart rate, and muscle sympathetic nerve activity with microelectrodes in the peroneal nerve of the resting leg during 1) static handgrip at 15% and 3% maximal voluntary contraction and 2) attempted handgrip during partial neuromuscuiar blockade produced by systemic administration of tubocurarine chloride (.75 mg/kg i.v.). During curare, subjects reported that they used near-maximal motor effort to attempt a sustained handgrip contraction, but they generated almost no force. Without sustained contraction, the intent to exercise alone, that is, central command, caused statistically significant (/><.5) increases in muscle sympathetic nerve activity as well as in arterial pressure and heart rate. However, the increases in muscle sympathetic nerve activity (+56±16% over control) and in mean arterial pressure (+12±2 mm Hg) during attempted handgrip were much smaller (p<.5) than the sympathetic nerve response (+217±37%> over control) and pressor response (+25±3 mm Hg) during an actual static handgrip at 3% maximal voluntary contraction. In contrast, heart rate increased as much during the attempted contraction (+18±2 beats/min) as during the actuaj contraction at 3% maximal voluntary contraction (+16±4 beats/min). In 11 additional subjects, the heart rate responses during curare were greatly attenuated (/><.5) by atropine but were not significantly affected by propranolol. From these observations, we conclude that during static handgrip in humans central command plays a major role in the regulation of parasympathetic outflow to the sinus node and a minor role in the activation of sympathetic outflow to nonexercising skeletal muscle. The new concept suggested by these data is that central command governs vagally mediated increases in heart rate at all levels of static exercise but contributes to activation of skeletal muscle sympathetic outflow only at near-maximal levels of static handgrip. {Circulation Research 1989;65: ) Static exercise causes increases in arterial pressure and heart rate that are mediated by increases in sympathetic and decreases in parasympathetic efferent activity. 1-2 These autonomic adjustments have been attributed to the central neural drive associated with voluntary motor effort, termed central command, 3-7 and to an exercise pressor reflex arising in the contracting muscles During voluntary exercise, the motor From the Department of Anesthesiology, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark. Supported in part by the Lawson and Rogers Lacy Research Fund in Cardiovascular Diseases, by the Frank M. Ryburn, Jr. Chair in Heart Research, by the Harry S. Moss Heart Center, and by the Danish Medical Research Council. Address for correspondence: Ronald G. Victor, MD, Internal Medicine/Cardiology, U.T. Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX Received September 7, 1988; accepted February 7, command signal from the rostral brain is thought to project to vasomotor circuits in the brainstem and directly activate sympathetic outflow. 12 At the same time, mechanical and chemical changes in the contracting muscles activate thin fiber afferents that reflexly increase sympathetic outflow The traditional thinking has been that central command sets the initial pattern of autonomic activation to the heart and blood vessels at the onset of exercise and that chemosensitive muscle afferents, once engaged, then modulate the basic pattern of sympathetic response. 3 ' More recently, however, microneurographic studies have suggested that during static exercise central command does not increase but rather may decrease skeletal muscle sympathetic outflow. 18 That interpretation, however, was based on indirect evidence from experiments that were designed primarily to isolate the

2 Victor et al Sympathetic Responses to Exercise During Neuromuscular Blockade 469 Downloaded from by guest on June 14, 218 reflex effects of muscle afferents while eliminating central command. Accordingly, we now have designed experiments to isolate the autonomic effects of central command while controlling or minimizing the input from the muscle afferents. We performed microelectrode recordings of sympathetic nerve discharge to resting leg muscles during attempted handgrip contraction after the systemic administration of curare. The rationale was that attempted contraction during partial neuromuscular blockade would augment central command because an exaggerated degree of voluntary motor effort would be needed to generate tension in the weakened muscles. The goal of this study was to determine if central command increases or decreases muscle sympathetic nerve activity (MSNA) in humans. Subjects and Methods Subjects A total of 16 men and seven women participated in this study. Data from three subjects were excluded from analysis because stable recordings of sympathetic nerve activity could not be maintained throughout the experiment. All subjects were normotensive (supine blood pressures < 14/9 mm Hg), were taking no medications, and had no evidence of cardiopulmonary disease by history and physical examination at the time of the study. The studies were approved by the Municipal Ethical Committee of Copenhagen, and each subject gave informed consent to participate. Measurements Subjects were studied in the supine position. Arterial pressure, heart rate, and sympathetic nerve discharge to nonexercising skeletal muscles in the leg were studied during static handgrip. Intraarterial pressure (radial artery catheter), heart rate (electrocardiogram), respiratory excursions (pneumograph), force of muscle contraction (force transducer), and MSNA were recorded continuously on a physiological recorder (Mingograph 8, Siemens- Elema Ltd.) at a paper speed of 5 mm/sec. Respiratory excursions were measured to detect inadvertent performance of a Valsalva maneuver or of a held exhalation because these respiratory maneuvers have been shown to markedly stimulate MSNA At the end of each exercise period, subjects were asked to rate their perceived effort on a scale of 6 (minimal effort) to 2 (maximal effort) as a subjective index of central command. 21 Microneurography Multiunit recordings of postganglionic sympathetic nerve activity were obtained from muscle nerve fascicles in the right peroneal nerve posterior to the fibular head by microneurography. The details of this technique have been described previously. 2 ' 22 Briefly, unipolar recordings of MSNA were obtained with tungsten intraneural microelectrodes. The neural signals were amplified by a factor of 2-5 thousand and filtered with a bandwidth of 7-2, Hz. The filtered neurogram was rectified and integrated with a resistance-capacitance circuit (time constant.1 second) to obtain a mean voltage display of the MSNA. A recording of MSNA was considered acceptable when 1) electrical stimulation (1-3 V,.2 msec, 1 Hz) through the intraneural electrode produced muscle twitches but not paresthesias, 2) the receptive field of the impaled mechanoreceptor afferents could be plotted by tapping or stretching muscles or tendons but not by lightly stroking the skin that is innervated by the peroneal nerve, and 3) the neurogram revealed spontaneous, pulse-synchronous bursts that increased during prolonged exhalation and phases II and III of a Valsalva maneuver but not during arousal stimuli (loud noise, skin pinch). Neurograms that revealed spontaneous skin sympathetic activity were not accepted. Inadvertent contraction of the leg muscles adjacent to the recording electrode produces electromyographic artifacts that are easily distinguished from sympathetic bursts. Before beginning the protocol, subjects rested quietly for 1 minutes to ensure a stable baseline. Partial Neuromuscular Blockade Tubocurarine chloride (curare) (Nordisk Droge) was infused into a forearm vein in an initial dose of.75 nig/kg body wt. Small supplemental doses were administered until the subject's maximal voluntary handgrip contraction was decreased to a value that was equivalent to approximately 5% of the initial maximal contraction before curare. Because the duration of the neuromuscular blocking action of intravenous curare was approximately 1-15 minutes, subjects received several infusions of curare in the course of an experimental session. Arterial blood gases were monitored repeatedly during the experiments. Experimental Protocols Protocol 1: Static handgrip before curare infusion. In nine subjects, we studied responses during static handgrip at 15% and at 3% of maximal voluntary contraction (MVC) to examine responses to two levels of effort before curare infusion. With the subject in the supine position, MVC was determined before each exercise protocol with an isometric handgrip dynamometer that was connected to a Peekel measuring bridge amplifier. During handgrip, subjects were given visual feedback of force output on a voltmeter. Subjects were instructed to avoid performance of a Valsatva maneuver or a held exhalation and to avoid contraction of nonexercising muscles during handgrip. A pneumatic cuff was inflated to suprasystolic pressure (25 mm Hg) on the exercising arm in the last 5 seconds before the cessation of handgrip, and the arrested forearm circulation was maintained for

3 Downloaded from by guest on June 14, Circulation Research Vol 65, No 2, August 1989 an additional 2 minutes in the postexercise period. This maneuver was used to maintain the chemical stimulation of muscle afferents while eliminating central command. Each exercise sequence consisted of 2 minutes each of control, handgrip, posthandgrip forearm muscle ischemia, and recovery (i.e., restoration of blood flow to the relaxed forearm muscles). The order of the two interventions (handgrip at 15% MVC, handgrip at 3% MVC) was randomized, and there were 1-minute rest periods between interventions. Protocol 2: Attempted handgrip during curare infusion. As soon as enough curare had been infused to decrease the subjects' MVC by about 5%, they attempted to sustain a handgrip contraction for 2 minutes at the tension equivalent to 15% of the precurare maximum. Because curare caused dosedependent decreases in the amount of time for which subjects could sustain a submaximal level of handgrip, two types of experimental protocols were produced (protocols 2a and 2b). In 21 experiments conducted on eight subjects (protocol 2a), the subject was able to generate muscle tension at a force equivalent to 15% of the precurare maximum for only a few seconds until force production decreased markedly despite continued maximal effort. This protocol, therefore, was designed to maximize the input from central command while minimizing the input from muscle afferents. In 11 experiments conducted on eight subjects (protocol 2b), the subject was given a somewhat smaller dose of curare than that administered in protocol 2a. With this smaller dose, the subject was able to maintain force production for a full 2 minutes at a level equivalent to 15% of the precurare MVC. The rational for this protocol was that the handgrip contraction during curare infusion would produce the same muscle afferent stimulation (same force) as the handgrip contraction at 15% MVC before curare infusion but would require more central command (more effort) to generate the muscle tension. Each experimental sequence consisted of 2 minutes each of control, attempted handgrip, posthandgrip forearm ischemia, and recovery with 1-minute rest periods between interventions. Protocol 3: Cold pressor test. The aim of this protocol was to determine if curare, which is a nicotinic antagonist, decreased ganglionic transmission. In seven subjects, we compared effects of the higher dose of curare on responses to handgrip with effects of the same dose of curare on responses to the cold pressor test (hand in ice water for 2 minutes) used as an internal control, that is, as a nonexercise stimulus to sympathetic outflow. 23 Protocol 4: Effects of autonomic blockade on heart rate responses during attempted handgrip. To examine effects of central command on parasympathetic and sympathetic regulation of heart rate, in 11 additional subjects we studied heart rate responses to attempted handgrip during the higher dose of curare before and after intravenous infusion of 1) propranolol hydrochloride,.15 mg/kg (six experiments), 2) atropine sulfate,.4 mg/kg (five experiments), and 3) atropine followed by propranolol (five experiments). Data Analysis Sympathetic bursts were identified by inspection from the mean voltage neurogram and expressed in 1) bursts per minute and 2) bursts per minute times mean burst amplitude, a measure of integrated (total) nerve activity. The intraobserver variability in identifying bursts is less than 5%, and the interobserver variability is less than 1%. 18 All neurograms were analyzed with the investigator blinded to the experimental condition. Arterial pressure and heart rate were measured from the arterial pressure tracing. Mean arterial pressure was calculated as one third of the pulse pressure plus the diastolic pressure. Values for arterial pressure and heart rate reflect the average for the last 3 seconds of each measurement period. Statistical analysis was performed by repeatedmeasures analysis of variance with the Bonferroni adjustment for multiple comparisons. Values of p<.5 were considered significant. Results are expressed as mean±sem. Results Protocol 1 Responses to static handgrip and posthandgrip forearm ischemia before curare infusion. Static handgrip at 15% and 3% MVC before curare infusion produced graded increases in arterial pressure and heart rate. Handgrip at 15% MVC had no effect on MSNA, whereas handgrip at 3% MVC caused a marked increase in the frequency and the amplitude of the sympathetic bursts; total MSNA increased by 216 ±62% over control values (mean± SEM,p<.5) by the end of the second minute of handgrip (Table 1; Figures 1 and 2). When handgrip at 3% MVC was followed by posthandgrip forearm ischemia, heart rate returned to the control values whereas the increases in arterial pressure and in MSNA were maintained above control (p<.5). Protocol 2 Effects of curare on baseline variables. The doses of curare used in these experiments had no effect on resting blood pressure, heart rate, and MSNA (Table 1 and Table 2, Figure 1). Resting values of arterial ph (7.39±.1 vs. 7.4±.1), PcOj (37.±1.3 vs. 36.2±.9 mm Hg), and POj (88±8 vs. 91±6 mm Hg) were comparable before and during curare infusion. None of the subjects showed any evidence of respiratory difficulty during curare infusion. Responses to attempted handgrip during curare infusion. During protocol 2a (higher dose of curare),

4 Victor et al Sympathetic Responses to Exercise During Nenromnscular Blockade 471 Downloaded from by guest on June 14, 218 TABLE 1. Responses to Handgrip Before Curare Static handgrip at 15% MVC Control period Handgrip Posthafldgrip forearm ischemia Recovery period Static handgrip at 3% MVC Control period Handgrip Posthandgrip forearm ischemia Recovery period Mean arterial pressure (mm Hg) 86±4 9±2 95±2* 89±3 9±2 86±1 86±1 99±2* 111+3* 17±2* 17±3* 86±2 Heart rate (beats/min) 66±3 69±4 71 ±4* ±3 67±4 67±4 79±5* 84±5* 71 ±4 7±4 66±3 Muscle sympathetic nerve activity (bursts/min x mean (bursts/min) burst amplitude) 21 ±2 2±3 25±4 25±2 24±3 19±2 2±3 22±2 38±5* 32 ±3* 33 ±3* 25±3 Values are mean±sem for nine subjects. MVC, maximal voluntary contraction. *Significantry different from control (/><.5). subjects could maintain force production for only the first 15-3 seconds of attempted handgrip, and then force development declined rapidly and progressively despite continued effort; in the second minute of this attempted handgrip, subjects reported that they used near-maximal motor effort (RPE= 19±1), but they generated almost no force (Table 2, Figures 1 and 2). Under these conditions, total MSNA increased by +56±16% over resting values MSNA FORCE (kg) MSNA FORCE (kg) MSNA 16 r o' 16 r o< ^JLJJL LL_^ BEFORE CURARE - Handgrip 15% MVC- -Handgrip 3% MVC- DU RING CURARE -^*A-.o» -^V. JJULJWAJU^, FORCE 16 r- (kg) ot- Altonpttd Hondgrip~ 227±22 29±22 244±3 282± ± ±41 553±16* * 576±126* * Handgrip force (kg) 6.8±.5 6.8± ± ±1. (/><.5), and mean arterial pressure increased by + 12 ±2 mm Hg (p<.5). Although calculated total MSNA (burst frequency times mean burst amplitude) increased during attempted handgrip, neither burst frequency alone (2±2 vs. 25±2 bursts/min, />>.1) nor burst amplitude alone (11.3±.6 vs. 14.±1.5 mm,/7=o.o8) increased significantly. The pressor response was one half as large (/?<.5) as that to the normal handgrip at 3% MVC. The small FIGURE 1. Segments of an illustrative record from one subject showing muscle sympathetic nerve activity (MSNA) and handgrip force during static handgrip at 15% and at 3% maximal voluntary contraction (MVC) before curare infusion and attempted handgrip during curare infusion (high dose). Before curare, static handgrip at 15% MVC had no effect on MSNA, whereas handgrip at 3% MVC caused a marked increase in the frequency and amplitude of the sympathetic bursts. During curare infusion, the subject could maintain a force equivalent to that of a normal handgrip at 15% MVC for less than 3 seconds after which force output fell almost to zero despite continued maximal effort. Note the jagged force tracing that is characteristic of the partial neuromuscular blockade caused by curare. Without sustained contraction, the intent to exercise alone caused a small increase in MSNA.

5 Downloaded from by guest on June 14, Circulation Research Vol 65, No 2, August 1989 ATOTALMSNAdrits) soo 1X.3 2 3r 1% 3% MVC MVC Before Cunrs A MAP (mm Hg) During" Curare 3 ATOTALMSNAOU 1% 3% Altwipfd MVC MVC FIGURE 2. Graphs of peak increases in total muscle sympathetic nerve activity (MSNA), mean arterial pressure (MAP), and heart rate (HR) caused by static handgrip at 15% MVC (open bars) and at 3% MVC (hatched bars) before curare infusion and by attempted handgrip during a high dose of curare (solid bars). During curare infusion, subjects used near-maximal effort to attempt sustained handgrip but generated almost no force. Without sustained contraction, the intent to exercise alone (L e., central command) caused much smaller increases in MSNA and arterial pressure than normally caused by an actual static handgrip at 3% MVC even though the effort was greater with the attempted than with the actual contraction. In contrast, heart rate increased as much with the attempted handgrip as with the actual handgrip at 3% MVC. Entries are mean±semfor eight subjects. increases in MSNA and arterial pressure during attempted handgrip were not maintained during posthandgrip forearm ischemia. In contrast to MSNA and arterial pressure, heart rate increased as much with attempted handgrip during curare infusion (+18 ±2 beats/min) as with static handgrip at 3% MVC before curare (+16±4 beats/min). During protocol 2b (lower dose of curare), subjects maintained for 2 minutes the same force (i.e., kg) as that during static handgrip at 15% MVC before curare but rated the effort to be higher (RPE=11±1 before curare vs. 15 ±1 during curare; p<.5) (Table 2). Despite the increased effort, sustained handgrip still had no effect on MSNA. Curare augmented the heart rate response to static handgrip at this level of force (+5±2 beats/min before vs. +1±l during curare, p<.5) but did not augment the pressor response to this maneuver (+8±1 mm Hg before vs. +1±2 mm Hg during curare infusion,p>.1). Protocol 3 Effects of curare on responses to the cold pressor test. Before curare, cold pressor stimulation produced significant increases in arterial pressure, heart rate, and MSNA. The highest doses of curare used in these experiments had no effect on the increases in blood pressure, heart rate, and MSNA evoked by the cold pressor test (Figure 3). Protocol 4 Effects of autonomic blockade on heart rate responses during attempted handgrip. The increases in heart rate produced by attempted handgrip during curare (high dose) were not attenuated by propranolol but were greatly attenuated (/?<.5) by atropine alone and by combined administration of atropine plus propranolol (Figure 4). Discussion The traditional thinking has been that central command plays a major role in the initiation of sympathetic neural activation during static exercise ' 24 This study provides direct measurements of sympathetic nerve activity during attempted static exercise after partial neuromuscular blockade in humans. The principal new conclusions are twofold. First, during mild and moderate levels of static handgrip, central command plays a primary role in the withdrawal of parasympathetic outflow to the sinus node but has little if any effect on sympathetic outflow to skeletal muscle. Moderate levels of muscle weakness augmented the heart rate response to static handgrip at 15% MVC but did not augment the MSNA and arterial pressure responses evoked by this maneuver. Second, during near-maximal levels of handgrip effort, central command plays a small but statistically significant role in the stimulation of muscle sympathetic outflow. During curare, attempted handgrip increased MSNA only when subjects were so weak that even maximal motor effort failed to maintain force output. Without sustained contraction, the intent to exercise alone, that is, central command, caused large increases in heart rate due to vagal withdrawal but only small increases in muscle sympathetic outflow and arterial pressure. These observations suggest that, during static handgrip, central command has a much larger influence on parasympathetic outflow to the heart than on sympathetic outflow to skeletal muscle. The simplest explanation for the increases in MSNA, heart rate, and arterial pressure evoked by attempted handgrip during curare is that partial neuromuscular blockade augmented central command. During handgrip, the rating of perceived exertion, a subjective index of central command, was directly related to the degree of curare-induced

6 Victor et al Sympathetic Responses to Exercise During Neuromuscular Blockade 473 Downloaded from by guest on June 14, 218 TABLE 2. Responses to Handgrip During Curare Higher dose of curare Control period Attempted handgrip Posthandgrip forearm ischemia Recovery period Lower dose of curare Control period Attempted handgrip Posthandgrip forearm ischemia Recovery period Mean arterial pressure (mm Hg) 86±2 95±2' 99±2* 89±2 88±2 86±2 84±2 91±1* 94±2* 88±2 87±2 83±2 Values are mean±sem for eight subjects in each group. *Significantly different from control (p<.5). Heart rate (beats/min) 69±3 81±4* 87±3* 7±4 72±4 71 ±4 68±4 73±4* 77±5* 71±4 71±4 68±4 muscle weakness. However, the slow onset of the MSNA responses to attempted handgrip could be explained either by a progressive increase in central command or by chemical activation of muscle afferents. We therefore considered the possibility that curare itself might increase or facilitate the chemical stimulus to the exercise pressor reflex. Administration of curare can stimulate the release of histamine from mast cells, 25 and infusion of histamine into the arterial supply of the cat hind limb stimulates the discharge of group IV muscle afferents and elicits a reflex pressor response In addition, Petrofsky and Lind 28 have suggested that contraction of fast-twitch muscle fibers may evoke a larger reflex pressor response than does contraction of slow-twitch fibers. Because curare preferentially blocks slow-twitch rather than fast-twitch fibers, 29 muscle contractions performed during curare might activate a greater than normal percentage of the muscle fibers that have a high potential to stimulate muscle metaboreceptors and to reflexly increase sympathetic outflow. It is unlikely that chemical sensitization of muscle afferents explains the increases in sympathetic activity and arterial pressure during attempted handgrip because these increases were not maintained when attempted handgrip was followed by forearm vascular occlusion. Rather, we suggest that the responses during the attempted exercise were caused by central command because MSNA and arterial pressure returned rapidly to control values with the cessation of motor effort. Muscle sympathetic nerve activity (bursts/min x mean (bursts/min) burst amplitude) 2±2 24±3 25±2 23±3 21±3 18±2 18±3 2±3 2±3 23±3 22±3 2O±3 225±21 262±44 343±4* 247±39 228±37 188±25 19O±32 199±25 237±45 252± ±35 229±34 Handgrip force (kg) 2.1±.6 1.3±.4 6.5±.5 6.5±.5 RESPONSES to the COLD PRESSOR TEST BEFORE and DURING CURARE A TOTAL MSNA 8 bnttv) Before During Cunr* Curar* AMAP () B*fore During Curar* Curar* ATOTAL MSNA B*for* Owing Curar* Curara AHR (beatt/mln) Bafor* Durhg Curar* Curar* FIGURE 3. Effects of curare on responses to cold pressor stimulation. Curare had no effect on the increases in muscle sympathetic nerve activity (MSNA), mean arterial pressure (MAP), and heart rate (HR) evoked by cold pressor stimulation. Entries are mean±sem for seven subjects.

7 474 Circulation Research Vol 65, No 2, August 1989 Heal Rate PROPRANOLOL ATRGPWE ATRGPHE PLUS PROPRANCLOL 55 Control Attempted Control Attempted Control Artwnptto FIGURE 4. Increases in heart rate evoked by attempted handgrip during the higher dose of curare before (solid lines) and after (dashed lines) autonomic blockade with propranolol, atropine, and atropine plus propranolol. *Significantty different (p<.5)from responses to attempted handgrip during curare alone. The heart rate response to attempted handgrip was not attenuated by propranolol alone but was greatly attenuated by atropine alone and by atropine plus propranolol. Entries are mean ±SEM for six subjects with propranolol and for five subjects with atropine and with atropine plus propranolol. Downloaded from by guest on June 14, 218 Lind 3 has emphasized the importance of inadvertent straining maneuvers in the interpretation of the autonomic responses to static exercise in humans. While we cannot totally eliminate any effect of straining maneuvers, several lines of evidence strongly suggest that straining maneuvers were not important in causing the increases in MSNA in our experiments. First, we detected only four Valsalva maneuvers and three prolonged exhalations in 55 exercise periods; these seven periods were excluded from analysis. These are the two straining maneuvers that would have been most likely to have surreptitiously increased MSNA. Second, the normal handgrip-induced increases in MSNA indeed are caused by exercise of the muscles in the forearm because, in the postexercise period, these increases are maintained completely by arresting the circulation to those specific muscles. Third, because curare was administered systemically rather than locally, subjects were too weak to produce sustained contraction of any muscle groups during attempted handgrip. Several observations suggest that the sympathoexcitatory effect of central command was small in our experiments and that central command alone cannot explain the normal activation of muscle sympathetic outflow evoked by static handgrip. Whereas static handgrip at 3% MVC before curare infusion caused a 2% increase in total MSNA due to large increases in both the frequency and the amplitude of the sympathetic bursts, attempted handgrip during curare infusion caused only a 5% increase in total MSNA and did not cause statistically significant increases either in burst frequency alone or in burst amplitude alone. Thus, the stimulation of MSNA during attempted handgrip was much smaller than the normal response to static handgrip at 3% MVC even though the motor command signal presumably was much larger with the attempted than with the actual contraction. By way of comparison, the increases in MSNA during attempted handgrip were about one tenth as large as those during a strenuous Valsalva maneuver 31 and also were much smaller than those produced by cold pressor stimulation, 23 by mild lower-body negative pressure, 32 and by small reductions in arterial pressure during vasodilator infusion. 2 It is important to emphasize that our conclusions are based on measurements of sympathetic discharge targeted specifically to inactive leg muscles. We, therefore, cannot exclude the possibility that central command may have a larger effect on sympathetic outflow to the skin and viscera than on sympathetic outflow to skeletal muscle. However, it is unlikely that central command produced widespread sympathetic activation and vasoconstriction in our experiments because attempted exercise during curare administration produced only small increases in arterial pressure. Furthermore, we provided evidence that central command increases heart rate during static handgrip mainly through parasympathetic withdrawal rather than through sympathetic activation because the increases in heart rate during attempted handgrip were greatly attenuated by muscarinic blockade but were unaffected by /3-adrenergic blockade. Because we measured sympathetic activity that is postganglionic, we considered the possibility that a ganglionic-blocking action of curare 33 caused us to underestimate the sympathoexcitatory effect of central command. This possibility is unlikely because curare did not decrease baseline sympathetic activity and had no effect on the increases in sympathetic activity, arterial pressure, and heart rate evoked by cold pressor stimulation. Previously, Mark et al 18 suggested that during static exercise, central command tends to decrease rather than increase MSNA. This interpretation was based on three lines of evidence, each of which was indirect and subject to alternative explanations. 18 First, MSNA did not increase but rather tended to decrease with the engagement of central command at the onset of static handgrip. While this observation now has been replicated repeatedly, the precise explanation remains unknown and may be related to arterial baroreceptor buffering. Second, MSNA was higher when central command was eliminated during posthandgrip forearm ischemia than when central command was engaged during handgrip exercise. Our data replicate this finding

8 Victor et al Sympathetic Responses to Exercise During Neuromuscular Blockade 475 Downloaded from by guest on June 14, 218 but only when MSNA is expressed per 1 heart beats and not when MSNA is expressed as activity per unit time, the latter presumably reflecting the physiological stimulus to neurotransmitter release. Third, MSNA decreased during voluntary biceps contraction but increased when central command was eliminated during electrically evoked involuntary biceps contraction. Although recorded force output was comparable with the voluntary and involuntary contractions, it is unlikely that muscle afferent stimulation was equivalent because these two types of contraction cause very different patterns of motor unit recruitment. 34 In contrast to postexercise forearm ischemia and involuntary muscle contractions that were designed to eliminate central command but maintain muscle afferent stimulation, the curare paradigm allowed us to study effects of increasing central command while controlling or minimizing muscle afferent stimulation. Ourfindingssupport the concept of Mark et al 18 that during static handgrip, central command is more important than muscle afferents in increasing heart rate whereas muscle afferents are more important than central command in stimulating muscle sympathetic outflow. However, our present findings refute the hypothesis that central command decreases sympathetic outflow to skeletal muscle during this form of exercise. During static handgrip with various levels of curare-induced muscle weakness, MSNA either did not change or increased slightly but never decreased below the resting level of activity. In summary, we have applied microelectrode recordings of sympathetic nerve activity to the model of central command produced by partial neuromuscular blockade. The findings provide the first direct evidence in humans that during static handgrip central command plays a major role in the regulation of parasympathctic outflow to the sinus node and a minor role in the activation of sympathetic outflow to nonexercising skeletal muscle. In contrast to the traditional thinking, the new concept suggested by these data is that central command governs vagally mediated increases in heart rate at all levels of static exercise but contributes to activation of skeletal muscle sympathetic outflow only at near-maximal, not at mild and moderate, levels of static handgrip. Acknowledgments We would like to thank Ms. Lilia Unas for research assistance and Ms. Janet Wright and Mrs. Pamela Maass for expert secretarial assistance. References 1. Lind AR, Taylor SH, Humphreys PW, Kennelly BM, Donald KW: The circulatory effects of sustained voluntary muscle contraction. Clin Sci 1964;27: Martin CE, Shaver JA, Leon DF, Thompson ME, Reddy PS, Leonard JJ: Autonomic mechanisms in hemodynamic responses to isometric exercise. JClinlnvest 1974^54: Krogh A, Lindhard J: The regulation of respiration and circulation during the initial stages of muscular work. J Physiol (Lond) 1913:47: Freyschuss U: Cardiovascular adjustments to somatomotor activation. The elicitation of increments in heart rate, aortic pressure and venomotor tone with the initiation of muscle contraction. Ada Physiol Scand 197;342(suppl):l Goodwin GM, McCloskey DI, Mitchell JH: Cardiovascular and respiratory responses to changes in central command during isometric exercise at constant muscle tension. / Physiol (Lond) 1972;226: Leonard B, Mitchell JH, Mizuno M, Rube N, Saltin B, Secher NH: Partial neuromuscular blockade and cardiovascular responses to static exercise in man. J Physiol (Lond) 1985;359: Hobbs SF, Gandevia SC: Cardiovascular responses and the sense of effort during attempts to contract paralyzed muscles: Role of the spinal cord. Neurosci Lett 1985;57: Volkman AW: Die Bewegungen des Athmens und Schhickens, mit bosonderer Beruckskhtigung neurologischer Streitfagen, in Archiv fur Anatomie, Physiologie und Wissenschlaftliche Medicine. Berlin, G Eichler, 1841, pp Coote JH, Hilton SM, Perez-Gonzalez JF: The reflex nature of the pressor response to muscular exercise. / Physiol (Lond) 1971;215: McCloskey DI, Mitchell JH: Reflex cardiovascular and respiratory responses originating in exercising muscle. J Physiol (Lond) 1972;224: Mitchell JH, Kaufman MP, Iwamoto GA: The exercise pressor reflex: Its cardiovascular effects, afferent mechanisms, and central pathways. Anna Rev Physiol 1983;45: Eldridge FL, Millhorn DE, Kiley JP, Waldrop TG: Stimulation by central command of locomotion, respiration, and circulation during exercise. Respir Physiol 1985;59: Kaufman MP, Longhurst JC, Rybicki KJ, Wallach JH, Mitchell JH: Effects of static muscular contraction on impulse activity of groups 111 and IV afferents in cats. JAppl Physiol 1983;55: Mitchell JH, Schmidt RF: Cardiovascular reflex control by afferent fibers from skeletal muscle receptors, in Shepherd JT, Abboud FM (eds): Handbook of Physiology, Volume III, Part 2. 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10 Effects of partial neuromuscular blockade on sympathetic nerve responses to static exercise in humans. R G Victor, S L Pryor, N H Secher and J H Mitchell Downloaded from by guest on June 14, 218 Circ Res. 1989;65: doi: /1.RES Circulation Research is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX Copyright 1989 American Heart Association, Inc. All rights reserved. Print ISSN: Online ISSN: The online version of this article, along with updated information and services, is located on the World Wide Web at: Permissions: Requests for permissions to reproduce figures, tables, or portions of articles originally published in Circulation Research can be obtained via RightsLink, a service of the Copyright Clearance Center, not the Editorial Office. Once the online version of the published article for which permission is being requested is located, click Request Permissions in the middle column of the Web page under Services. Further information about this process is available in the Permissions and Rights Question and Answer document. Reprints: Information about reprints can be found online at: Subscriptions: Information about subscribing to Circulation Research is online at: