Effects of forearm bier block with bretylium on the hemodynamic and metabolic responses to handgrip

Sections of ¹ Cardiology, ² Biostatistics, and ³ Anesthesiology and ⁴ Department of Radiology, Center for Nuclear Magnetic Resonance Research, The Pennsylvania State University College of Medicine, The Milton S. Hershey Medical Center, Hershey 17033; ⁵ Lebanon Veterans Affairs Medical Center, Lebanon, Pennsylvania 17042; and ⁶ School of Kinesiology, University of Western Ontario, London, Ontario, Canada N2L 3G1

	ABSTRACT

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

We tested the hypothesis that a reduction in sympathetic tone to exercising forearm muscle would increase blood flow, reduce muscle acidosis, and attenuate reflex responses. Subjects performed a progressive, four-stage rhythmic handgrip protocol before and after forearm bier block with bretylium as forearm blood flow (Doppler) and metabolic (venous effluent metabolite concentration and ³¹P-NMR indexes) and autonomic reflex responses (heart rate, blood pressure, and sympathetic nerve traffic) were measured. Bretylium inhibits the release of norepinephrine at the neurovascular junction. Bier block increased blood flow as well as oxygen consumption in the exercising forearm (P < 0.03 and P < 0.02, respectively). However, despite this increase in flow, venous K⁺ release and H⁺ release were both increased during exercise (P < 0.002 for both indexes). Additionally, minimal muscle pH measured during the first minute of recovery with NMR was lower after bier block (6.41 ± 0.08 vs. 6.20 ± 0.06; P < 0.036, simple effects). Meanwhile, reflex effects were unaffected by the bretylium bier block. The results support the conclusion that sympathetic stimulation to muscle during exercise not only limits muscle blood flow but also appears to limit anaerobiosis and H⁺ release, presumably through a preferential recruitment of oxidative fibers.

blood flow; autonomic nervous system

	INTRODUCTION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

DURING EXERCISE THE SYMPATHETIC nervous system is activated. The neural systems responsible for sympathetic activation are unclear, although it is widely believed that engagement of the muscle reflex plays an important role (18, 22). Once the sympathetic nervous system is engaged, there is an increase in blood pressure and vasoconstrictor tone to inactive skeletal muscle beds (31, 32, 37). The effect of sympathetic activation on flow in exercising muscle remains an area of considerable study. Work from a number of laboratories has focused on the concept that sympathetic tone to exercising muscle is "lysed" by some substance released or present during muscle exercise (15, 29). Other groups, including our own, have emphasized the role the sympathetic nervous system plays in restraining flow to exercising muscle (7, 26, 35).

In this study, we were interested in determining whether the sympathetic nervous system restrains limb blood flow and oxygen consumption in human subjects during exercise. To examine this issue, we utilized a recently described bretylium bier block technique (15). The bier block method allows investigators to isolate the pharmacological effects of infused substances to a single upper extremity of human subjects. Bretylium inhibits norepinephrine release at the neurovascular junction and in the process causes a "pharmacologically induced sympatholysis."

We also examined the impact of the bretylium bier block on muscle metabolism and the reflex responses to forearm exercise. To examine muscle metabolism, we measured venous effluent metabolites and obtained ³¹P-NMR spectra during forearm exercise. To examine the effects of sympathetic stimulation on muscle reflex activation, we measured the blood pressure, heart rate, and peroneal nerve muscle sympathetic nerve activity (MSNA; microneurography). Our hypotheses were that after bier block with bretylium 1) forearm exercise flow would increase, 2) forearm oxygen consumption would remain constant, 3) muscle acidosis would be reduced, and 4) reflex responses to handgrip would be attenuated. Our results suggest that the bier block increased both flow and forearm oxygen consumption. Despite this, we observed an increase in muscle acidosis and no change in muscle reflex engagement.

	METHODS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Subjects

Fifteen healthy subjects (male) volunteered for this study. The subjects were 27 ± 2 (SE) yr of age (range 20-40 yr) and weighed 78 ± 2 kg. All subjects provided informed written consent to the experimental protocol, which was approved by the Institutional Review Board of The Milton S. Hershey Medical Center. All subjects were in good health.

Protocols

The exercise protocols to be described are similar to those previously reported by our group (35). Two separate sets of experiments were performed. The first set was performed in the General Clinical Research Center of the Hershey Medical Center (study A). The second group was performed in the NMR facilities of the medical center (study B). In study A (n = 8), we measured forearm blood flow (Doppler method; model 500 M, Multigon, Yonkers, NY), skin blood flow (SBF; laser-Doppler technique; Laserflo BPM², Vasomedics, St. Paul, MN), MSNA, heart rate (electrocardiogram), blood pressure (Finapres, Ohmeda, Madison, WI; and Dinamap, Critikon, Tampa, FL), venous concentrations of lactate (StatPlus 2300, Yellow Springs Instrument, Yellow Springs OH), lactate release (index of flow × venous lactate concentration), venous K⁺ concentration (ion-selective electrode, Roche Diagnostics, Indianapolis, IN), and venous K⁺ release. In addition, venous blood gases were analyzed (model ABL 510, Radiometer, Copenhagen, Denmark) to calculate venous H⁺ concentration and release and venous oxygen saturation and forearm oxygen consumption (index of flow × oxygen extraction). The saturation values obtained were corrected for the effects of pH an oxygen-hemoglobin dissociation. From the venous oxygen saturation and the measured hemoglobin concentration, venous oxygen content was determined.

Study A. On arrival at the General Clinical Research Center, each subject read and signed the informed consent and underwent a routine history and physical examination. A Doppler probe was placed over the brachial artery. The laser-Doppler diode was placed on the forearm skin, and an intravenous catheter was placed in a retrograde fashion into an antecubital vein in the exercising forearm. A second intravenous catheter was placed in a forearm vein of the opposite arm for the purpose of providing systemic intravenous access. Microneurography electrodes were placed in the peroneal nerve and in the adjacent subcutaneous tissue.

After at least 5 min of rest, each subject performed 1 min of rhythmic isometric handgrip exercise at each of four increasing workloads. Each subject performed rhythmic handgrip for 1 min at workloads of 11, 21, 31, and 41 lbs., respectively. The contractions at each workload were performed with the nondominant forearm in a work-rest schedule of 1 s: 1 s. There were no rest periods between the workloads. Subjects exercised using a handgrip dynamometer, which was electronically linked to an analog meter. This meter provided visual feedback to the subjects, enabling them to maintain the prescribed tension. Exercise was followed by a 5-min recovery period. On the basis of prior experiments, we believed that this paradigm would lead to a high level of perceived effort by the end of the fourth workload (35).

Study B. In this study, ³¹P-NMR spectra were obtained from the flexor digitorum superficialis muscle of the exercising forearm. We examined muscle metabolism during forearm exercise in one group before and after bretylium (n = 9) and in a second group in whom forearm handgrip was performed after receiving an infusion of saline (no bretylium) before the second trial (n = 7). Four subjects performed both study A and B, and two subjects performed study A and were a control in study B. Three subjects in study B received bretylium and also served as controls (received saline). The protocols for study A and B are shown in Fig. 1.

View larger version (21K): [in this window] [in a new window]   Fig. 1.   Schematic representation of the experimental protocol. CPT, cold pressor test; G1, G2, G3, minutes 1-3 of incremental handgrip, respectively; EG, end grip.

Bretylium Bier Block (Performed in Both Study A and B)

Inhibition of norepinephrine release was accomplished by infusing bretylium tosylate (1 mg/kg; bier block) into the antecubital vein of the exercising forearm after sequential exsanguination of forearm blood volume [compression bandage placed on the forearm and circulatory arrest (15, 33)]. Circulatory arrest was applied by using a compression cuff around the upper arm inflated to 250-300 mmHg. The circulatory arrest was maintained for 20 min. After a 60-min rest period, testing was resumed. The efficacy of sympathetic blockade was determined by measuring the forearm flow during a 90-s cold pressor test. For this test, the foot without the peroneal nerve electrode was placed in iced saline.

NMR Studies (Performed in Study B)

Muscle metabolism during forearm skeletal muscle exercise was assessed by using ³¹P-NMR spectroscopy. This method allows for the noninvasive repetitive assessment of muscle phosphocreatine (PCr), P_i, and pH. The high-energy phosphate measurements were obtained with a 1.9-T 26 cm-bore superconducting magnet (Oxford Instruments, Abbington, UK) interfaced with a radio frequency transmitter and receiver (Nicolet Instrument, Madison, WI). A coil 2.5 cm in diameter was placed over the flexor digitorum superficialis muscle. The obtained spectra were collected at 32.5 MHz and represented the Fourier transformation of 32 transients averaged over 60 s during rest, during each exercise workload, and during each minute of recovery. The relative concentrations of PCr were calculated from the area under the resonance curve and are expressed in arbitrary units. Intracellular pH was calculated from the shift of the P_i resonance relative to PCr. The NMR technique has been described previously (3).

Measurements of MSNA (Performed in Study A)

Multiunit recordings of postganglionic MSNA were obtained from the peroneal nerve with an insulated 200-µm-diameter tungsten electrode with a tapered uninsulated 1- to 5-µm diameter tip. A microelectrode was inserted transcutaneously into the peroneal nerve (just posterior to the fibular head), and a reference electrode was positioned subcutaneously 1-3 cm from the recording site. Neuronal activity was amplified (×50,000-100,000), the signal was filtered (bandwidth of 700-2,000 Hz), rectified, and integrated, yielding a mean voltage neurogram. MSNA data were analyzed according to the burst frequency (11). Resting burst frequency has been shown to be reproducible (8, 39) when readings are performed on different days. Complete sets of the MSNA data were collected in six of the eight subjects studied.

Forearm Flow Measurements (Performed in Study A)

Brachial artery limb blood flow was determined by combined pulsed and echo Doppler ultrasound. A 4- or 5-MHz flat probe was used to measure blood flow velocity. The probe was positioned over the brachial artery in the antecubital fossa. In each case, the probe was fixed to the skin with hypoallergenic tape. During measurements, small adjustments of the probe were necessary to maintain the optimal signal. Signal strength was optimized by using both visual and auditory feedback of the Doppler shift frequencies. The depth and gate size of the ultrasound beam were adjusted to insonate the entire vessel lumen.

The Doppler shift signal was demodulated to provide instantaneous mean blood velocity (MBV) data in real time. The signal was calibrated by an electronically generated phase shift of 1 mV, allowing conversion of the demodulated data to MBV values calculated and stored on a computer-based system at 100 Hz.

Blood flow was determined from both the blood velocity and the vessel cross-sectional area (blood flow = MBV × r², where r is the vessel radius). Vessel dimensions were measured at each workload by using echo Doppler sonography as two-dimensional B-mode images were obtained with a 7.5-MHz transducer. Vessel dimensions were determined by three investigators, and the coefficient of variation was calculated to be <5%. Vascular resistance and conductance were calculated from the mean arterial pressure and forearm blood flow. Relative changes in SBF were determined by using a laser-Doppler flow probe that was placed on the forearm, and the position was marked. During the bier block procedure, it was necessary to remove the probe. It was replaced in the marked location on the forearm.

The "release" of K⁺, H⁺, and lactate was determined by measuring the flow and multiplying this value by the concentration of the various substances in the venous effluent. In these calculations, we assumed that the increases in flow and changes in metabolite concentrations with exercise were due predominantly to increases in muscle flow. Prior literature suggests that this approach is valid (4, 34). Forearm oxygen consumption was determined by the following equation: forearm flow × [arterial oxygen saturation  venous oxygen saturation × hemoglobin × 1.39]. In these studies we assumed an arterial oxygen saturation of 100%.

Statistics

The longitudinal design of this study consisted of two factors repeated for each subject: bretylium effect (pre- and postbier block) and handgrip. To assess the effects of bretylium and handgrip, as well as their possible interaction, on the blood flow, metabolism, and reflexes, doubly repeated-measures analysis of variance models were fit to the data (21). The fit of these models were assessed by using residual diagnostics. When the interaction effect was significant, simple effects were tested and adjusted for multiple comparisons via Bonferroni's procedure. Descriptive data for continuous variables (e.g., blood flow) are presented as means ± SE. A P value  0.05 was considered statistically significant. The data were analyzed by using the MIXED procedure from the SAS statistical software (SAS Institute, Cary, NC).

For six of the eight subjects in our study, the vessel diameters were known for each phase of the exercise paradigm. For two of the eight subjects, however, only baseline vessel diameter was known. Because vessel diameter was necessary to calculate blood flow, we imputed values for the missing vessel diameters for these two subjects. On the basis of the empirical distribution of vessel diameters from the six subjects with complete data, we selected two of these subjects, whose baseline diameters closely matched the baseline diameters of the two subjects with missing data, and imputed their values for the missing vessel diameters. This method of imputation is preferable to other methods, such as imputing the mean values of nonmissing diameters, because it does not dampen the variability of the vessel diameter distribution.

	RESULTS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Study A

The bier block caused a marked reduction in forearm resistance during the cold pressor maneuver (pre-bretylium 1.98 ± 0.19 resistance units, post-bretylium 0.82 ± 0.12 resistance units). The bier block procedure also attenuated the blood pressure seen during the cold pressor intervention (change pre-bretylium 19.0 ± 2.2 mmHg; change post-bretylium 10.3 ± 1.6 mmHg; P < 0.03). This suggested that this intervention had systemic as well as local effects.

Forearm exercise led to a large increase in forearm flow, a rise in forearm conductance, a reduction in venous oxygen saturation, and an increase in oxygen consumption (Fig. 2).

View larger version (30K): [in this window] [in a new window]   Fig. 2.   Effects of bretylium bier block and handgrip exercise on flow, conductance, venous oxygen saturation (venous SAT), and oxygen consumption (O2). B, baseline. Values are means ± SE for 8 subjects. There was a bretylium bier block main effect for flow, conductance, and O2. * P0.05 for values of B pre- and post-bretylium.

After bier block, resting flow and forearm conductance were higher (Fig. 2) than before bretylium block (flow: pre-bretylium 59.4 ± 9.0 ml/min; post-bretylium 163.5 ± 33.6 ml/min; conductance: pre-bretylium 0.623 ± 0.076 ml · min¹ · mmHg¹; post-bretylium 1.682 ± 0.336 ml · min¹ · mmHg¹). The bier block increased forearm blood flow (bretylium effect, P < 0.03), the flow velocity (P < 0.0001), and the forearm vascular conductance (P < 0.02) in the exercising forearm. This was associated with a greater forearm oxygen consumption (bretylium effect, P < 0.023; Fig. 2).

In Fig. 3, the change in oxygen consumption as a function of the change in flow is presented. This figure suggests that changes in flow above are not sufficient to explain the greater oxygen consumption seen after bier block. SBF during exercise was also greater after bier block (bretylium main effect P < 0.04; end grip values: pre-bretylium 5.6 ± 0.5 arbitrary units; post-bretylium 7.5 ± 1.3 arbitrary units).

View larger version (10K): [in this window] [in a new window]   Fig. 3.   Change in O2 (O2) vs. change in flow (flow) demonstrates that the rise in O2 was not due solely to an increase in flow, because the slope of the line was steeper post-bretylium than pre-bretylium. Units for x- and y-axis presented in the text.

Exercise led to an increase in the release of K⁺ and H⁺ but not of lactate (Fig. 4).

View larger version (17K): [in this window] [in a new window]   Fig. 4.   Effects of bretylium bier block and handgrip exercise on lactate, K+, and H+ release. Note that bretylium bier block resulted in an increase in K+ and H+ release but not in lactate release. Values are means ± SE for 8 subjects. R1, first minute of recovery.

After bier blockade, resting MSNA was much higher, whereas heart rate and blood pressure were unchanged by the intervention (Fig. 5). Bretylium did not alter heart rate and blood pressure during exercise. Although we observed a main effect for MSNA (Fig. 5) during exercise, bretylium did not alter the change in MSNA.

View larger version (15K): [in this window] [in a new window]   Fig. 5.   Effects of bretylium bier block and handgrip exercise on mean arterial pressure (MAP), heart rate (HR), and muscle sympathetic nerve activity bursts. Values are means ± SE for 8 subjects. bpm, Beats/min. Muscle sympathetic nerve activity was higher post-bretylium; however, MAP and HR were not significantly changed by the intervention.

Study B (Fig. 6)

In the bretylium-treated subjects we observed a lower minimum intracellular pH (simple effects during minute 1 of recovery, P < 0.036). No such sequence effect was seen in the saline-treated (control) subjects. The bier block had no effect on the P_i ratio (P_i/PCr + P_i) in either group.

View larger version (36K): [in this window] [in a new window]   Fig. 6.   NMR data for intracellular pH and Pi ratio (Pi/PCr + Pi) in bretylium-treated and saline-treated subjects. Values are means ± SE for 9 for bretylium-treated subjects and 7 saline-treated subjects. In the bretylium-treated subjects, pH at R1 was lower after blockade than before. * P < 0.05 for pre- and post-bretylium at R1.

	DISCUSSION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Study Findings

There are several findings in the present paper worthy of mention: 1) bier block with bretylium increased the forearm blood flow during exercise; 2) this was associated with an increase in forearm oxygen consumption during exercise; 3) despite greater levels of forearm flow after bier block, there was a greater release of H⁺ and a lower minimal pH (measured during the first minute of recovery with NMR); and 4) the increase in forearm flow did not reduce the heart rate, blood pressure, or sympathetic nerve responses to forearm exercise. In the DISCUSSION, we will examine the possible mechanisms for these observations as well as their implications.

Effects of Sympathetic Tone of Forearm Blood Flow

The interrelationship between metabolic vasodilation and neurally mediated vasoconstriction has been an area of considerable investigation. It is interesting to note that a number of substances that may contribute to metabolic vasodilation alter norepinephrine release and reuptake (41) and in the process antagonize the effects of norepinephrine, leading to the classic concept of "functional sympatholysis" (29). Microvascular studies (20, 24) and recent near-infrared spectroscopy data in humans (15) provide further support for this concept. However, it is known that muscle blood flow capacity can greatly exceed maximal cardiac output (1, 2, 25). Given the fact that blood pressure does not fall during exercise, it must be concluded that these peak levels of muscle flow are not achieved. Rowell (30) has hypothesized that myogenic and sympathetic vasoconstrictor influences prevent muscle flow from outstripping cardiac output during maximal exercise. It must be emphasized that, in addition to this teleological approach to the problem, a number of studies using both animal and human models have shown that the sympathetic nervous system is reflexly engaged, evoking a vasoconstrictor response that actively opposes the profound local vasodilation seen within the exercising muscle (7, 12, 17, 26, 28, 35, 36, 38, 42).

We believe the findings of the present study provide evidence that sympathetic tone does restrain muscle blood flow during exercise. After the bier block, total forearm blood flow (Doppler) as well as SBF increased. Our data do not support the notion that the increase in SBF was responsible for the majority of the increase in total forearm flow, because the skin contributes only ~11% of the vascular forearm tissue capable of vasodilation (10). Because blood pressure was similar before and after the bier block, the data suggest that the higher exercise flows postblock were not due to differences in myogenic tone but rather to "lysis" of sympathetic tone directed predominantly to skeletal muscle.

Effects of Sympathetic Withdrawal on Oxygen Consumption

In this study, we observed that after bier block, oxygen consumption during forearm exercise was increased. This confirms prior work by Joyner et al. (17), who demonstrated that, after stellate ganglion block, exercise limb flow and oxygen consumption were greater than before block. The present report differs from the one by Joyner et al. in that we directly measured limb flow (Doppler) and the metabolic consequence of flow. Additionally, Joyner et al. used a stellate ganglion blockade, whereas we used a bretylium bier block approach.

Additionally, we believe this finding is consistent with the concept that, under certain circumstances, oxygen delivery can play an important regulatory role in cellular respiration during muscle contraction (9); i.e., increases in oxygen delivery can lead to an increase in the amount of oxygen consumed (6, 17, 43, 44).

The increase in oxygen consumption found in our study was not solely due to the increase in oxygen delivery because the slope of line relating oxygen consumption to limb flow was greater after the bretylium block. The mechanism for this finding is not entirely clear, although we did observe a greater efflux of H⁺ after bier block. This would tend to shift the partial pressure of oxygen at which 50% hemoglobin is saturated (P₅₀) for hemoglobin to the right and in the process lead to a greater release of oxygen to the exercising muscle (14).

Effects of Sympathetic Withdrawal on Cellular Metabolism

In this study, we observed that, despite higher levels of forearm blood flow, H⁺ and K⁺ efflux and the maximal cellular H⁺ concentration (NMR) were increased after the bretylium bier block. The mechanism for these findings is not entirely clear, although we propose the following scenario. It is well known that increased sympathetic drive can increase the amount of muscle tension developed during contraction. This effect of catecholamines on muscle contractile function has been described in great detail for the heart (16) and diaphragm (13, 19). For example, it is known that increased catecholamines evoke Ca²⁺ release in the sarcoplasmic reticulum (16), a process thought to be mediated by a protein termed the dihydropyridine (DHP) receptor (5). This leads to increased contractile function.

Recently it has been shown that the cardiac isoform of the ₁-subunit of the DHP protein is expressed in slow- but not in fast-twitch skeletal muscle (27). Accordingly, on the basis of our results, we propose the following scenario: as muscle workload increased, the muscle reflex was eventually engaged, leading to increased sympathetic tone directed to exercising muscle. This led to the release of norepinephrine, which stimulated the DHP receptors, thereby preserving slow-twitch muscle fiber contractility. The bier block reduced norepinephrine levels during handgrip and in the process reduced contractile activity by the muscle oxidative fibers. This necessitated the recruitment of more glycolytic muscle fibers to maintain muscle tension. It is known that glycolytic fibers as opposed to oxidative ones produce more lactic acid and utilize oxygen less efficiently (23, 45). We further speculate that this led to the greater oxygen consumption and the increased production of metabolic byproducts of contraction that was seen after bier block. This line of reasoning is consistent with prior rat model work demonstrating that systemic bretylium infusions lead to the preferential utilization of glycogen from fast-twitch skeletal muscle (40).

Effects of Sympathetic Withdrawal on MSNA

In this study, we observed that, after sympathetic blockade, the MSNA response to the progressive handgrip protocol was unchanged. We speculate that a greater flow delivery and the greater removal of interstitial metabolites would tend to reduce reflex responses. However, these effects were likely offset by the greater recruitment of muscle fibers with a more glycolytic profile that produced greater quantities of metabolites capable of stimulating muscle afferents.

Conclusion

The present study provides convincing evidence that flow to skeletal muscle is in fact restrained by sympathetic tone. We believe this ability to limit flow to exercising muscle contributes to the maintenance of blood pressure seen with vigorous exercise. The intriguing part of this study, however, is the fact that, after pharmacological sympathetic blockade, muscle acidosis and metabolite release were greater than before blockade. This suggests that the capacity of the sympathetic nervous system to influence exercise performance is multifactorial. Moreover, under the conditions of the present paradigm, we believe that the sympathetic nervous system's ability to directly alter metabolism may be more important than its ability to modulate flow.

	ACKNOWLEDGEMENTS

The authors greatly appreciate the nursing support of Cindy Hogeman from the General Clinical Research Center, the technical support of Michael Herr, and the secretarial support of Jennie Stoner in the preparation of the manuscript.

	FOOTNOTES

This work was supported by National Aeronautics and Space Administration Grant NAG 9-1044 (to L. I. Sinoway); a Veterans Administration Merit Review Award (to L. I. Sinoway); a National Heart, Lung, and Blood Institute K24 Midcareer Investigator Award in Patient-Oriented Research K24 HL-04011 (to L. I. Sinoway); and a National Institutes of Health General Clinical Research Center with Division of Research Resources Grant M01 RR-10732.

Address for reprint requests and other correspondence: L. I. Sinoway, Sect. of Cardiology, MC H047, The Pennsylvania State Univ., The Milton S. Hershey Medical Center, PO Box 850, Hershey, PA 17033 (E-mail:lsinoway{at}psu.edu).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Received 24 September 1999; accepted in final form 11 February 2000.

	REFERENCES

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

1.   Andersen, P, and Saltin B. Maximal perfusion of skeletal muscle in man. J Physiol (Lond) 366: 233-249, 1985[Abstract/Free Full Text].

2.   Armstrong, RB, and Laughlin MH. Exercise blood flow patterns within and among rat muscles after training. Am J Physiol Heart Circ Physiol 246: H59-H68, 1984[Abstract/Free Full Text].

3.   Batman, BA, Hardy JC, Leuenberger UA, Smith MB, Yang QX, and Sinoway LI. Sympathetic nerve activity during prolonged rhythmic forearm exercise. J Appl Physiol 76: 1077-1081, 1994[Abstract/Free Full Text].

4.   Blaak, EE, van Baak MA, Kemerink GJ, Pakbiers MT, Heidendal GA, and Saris WH. Total forearm blood flow as an indicator of skeletal muscle blood flow: effect of subcutaneous adipose tissue blood flow. Clin Sci (Colch) 87: 559-566, 1994[Medline].

5.   Booth, FW, and Baldwin KM. Muscle plasticity: energy demand and supply processes. In: Handbook of Physiology. Exercise: Regulation and Integration of Multiple Systems. Bethesda, MD: Am. Physiol. Soc, 1996, sect. 12, pt. III, chapt. 24, p. 1075-1123.

6.   Brechue, WF, Barclay JK, O'Drobinak DM, and Stainsby WN. Differences between O₂ maxima of twitch and tetanic contractions are related to blood flow. J Appl Physiol 71: 131-135, 1991[Abstract/Free Full Text].

7.   Buckwalter, JB, Mueller PJ, and Clifford PS. Sympathetic vasoconstriction in active skeletal muscles during dynamic exercise. J Appl Physiol 83: 1575-1580, 1997[Abstract/Free Full Text].

8.   Chen, X, Rahman MA, and Floras JS. Effects of forearm venous occlusion on peroneal muscle sympathetic nerve activity in healthy subjects. Am J Cardiol 76: 212-214, 1995[ISI][Medline].

9.   Connett, RJ, and Sahlin K. Control of glycolysis and glycogen metabolism. In: Handbook of Physiology. Exercise: Regulation and Integration of Multiple Systems. Bethesda, MD: Am. Physiol. Soc., 1996, sect. 12, pt. III, chapt. 19, p. 870-911.

10.   Cooper, KE, Edholm OG, and Mottram RF. The blood flow in skin and muscle of the human forearm. J Physiol (Lond) 128: 258-267, 1955.

11.   Delius, W, Hagbarth K-E, Hongell A, and Wallin BG. Manoeuvres affecting sympathetic outflow in human muscle nerves. Acta Physiol Scand 84: 82-94, 1972[ISI][Medline].

12.   Donald, DE, Rowlands DJ, and Ferguson DA. Similarity of blood flow in the normal and the sympathectomized dog hind limb during graded exercise. Circ Res 26: 185-199, 1970[Abstract/Free Full Text].

13.   Evans, RH, and Smith JW. The effect of catecholamines on the influx of calcium and the development of tension in denervated mouse diaphragm muscle. Br J Pharmacol 58: 109-116, 1976[ISI][Medline].

14.   Ganong, WF. Gas transport between the lungs & the tissues. In: Review of Medical Physiology (17th ed.), edited by Dolan J, and Langan C.. Norwalk, CN: Appleton-Lange, 1995, chapt. 35, p. 608-614.

15.   Hansen, J, Thomas GD, Harris SA, Parsons WJ, and Victor RG. Differential sympathetic neural control of oxygenation in resting and exercising human skeletal muscle. J Clin Invest 98: 584-596, 1996[ISI][Medline].

16.   Hussain, M, and Orchard CH. Sarcoplasmic reticulum Ca²⁺ content, L-type Ca²⁺ current and the Ca²⁺ transient in rat myocytes during beta-adrenergic stimulation. J Physiol (Lond) 505: 385-402, 1997[ISI][Medline].

17.   Joyner, MJ, Nauss LA, Warner MA, and Warner DO. Sympathetic modulation of blood flow and O₂ uptake in rhythmically contracting human forearm muscles. Am J Physiol Heart Circ Physiol 263: H1078-H1083, 1992[Abstract/Free Full Text].

18.   Kaufman, MP, and Forster HV. Reflexes controlling circulatory, ventilatory and airway responses to exercise. In: Handbook of Physiology. Exercise: Regulation and Integration of Multiple Systems. Bethesda, MD: Am. Physiol. Soc, 1996, sect. 12, pt. II, chapt. 10, p. 381-447.

19.   Kimura, I, Kimura M, and Kimura M. External Ca²⁺-dependent Ca²⁺ release in directly stimulated diaphragm muscles of mice. Jpn J Pharmacol 44: 510-514, 1987[Medline].

20.   Kurjiaka, DT, and Segal SS. Interaction between conducted vasodilation and sympathetic nerve activation in arterioles of hamster striated muscle. Circ Res 76: 885-891, 1995[Abstract/Free Full Text].

21.   Littell, RC, Milliken GA, Stroup WW, and Wolfinger RD. SAS System for Mixed Models. Cary, NC: SAS Institute, 1996.

22.   Mark, AL, Victor RG, Nerhed C, and Wallin BG. Microneurographic studies of the mechanisms of sympathetic nerve responses to static exercise in humans. Circ Res 57: 461-469, 1985[Abstract/Free Full Text].

23.   McAllister, RM, Ogilvie RW, and Terjung RL. Functional and metabolic consequences of skeletal muscle remodeling in hypothyroidism. Am J Physiol Endocrinol Metab 260: E272-E279, 1991[Abstract/Free Full Text].

24.   McGillivray-Anderson, KM, and Faber JE. Effect of acidosis on contraction of microvascular smooth muscle by alpha1- and alpha2-adrenoceptors. Implications for neural and metabolic regulation. Circ Res 66: 1643-1657, 1990[Abstract/Free Full Text].

25.   Musch, TI, Haidet GC, Ordway GA, Longhurst JC, and Mitchell JH. Training effects on regional blood flow response to maximal exercise in foxhounds. J Appl Physiol 62: 1724-1732, 1987[Abstract/Free Full Text].

26.   O'Leary, DS, Robinson ED, and Butler JL. Is active skeletal muscle functionally vasoconstricted during dynamic exercise in conscious dogs? Am J Physiol Regulatory Integrative Comp Physiol 272: R386-R391, 1997[Abstract/Free Full Text].

27.   Péréon, V, Dettbarn C, Lu Y, Westlund KN, Zhang J-T, and Palade P. Dihydropyridine receptor isoform expression in adult rat skeletal muscle. Pflügers Arch 436: 309-314, 1998[ISI][Medline].

28.   Peterson, DF, Armstrong RB, and Laughlin MH. Sympathetic neural influences on muscle blood flow in rats during submaximal exercise. J Appl Physiol 65: 434-440, 1988[Abstract/Free Full Text].

29.   Remensynder, JP, Mitchell JH, and Sarnoff SJ. Functional sympatholysis during muscular activity. Observations of influence of carotid sinus on oxygen uptake. Circ Res 11: 370-380, 1962[Abstract/Free Full Text].

30.   Rowell, LB. Central circulatory adjustments to dynamic exercise. In: Human Cardiovascular Control. New York: Oxford Univ. Press, 1993, chapt. 7, p. 255-301.

31.   Saito, M, Mano T, and Iwase S. Changes in muscle sympathetic nerve activity and calf blood flow during static handgrip exercise. Eur J Appl Physiol 60: 277-281, 1990.

32.   Seals, DR. Sympathetic neural discharge and vascular resistance during exercise in humans. J Appl Physiol 66: 2472-2478, 1989[Abstract/Free Full Text].

33.   Shoemaker, JK, McQuillan PM, and Sinoway LI. Upright posture reduces forearm blood flow early in exercise. Am J Physiol Regulatory Integrative Comp Physiol 276: R1434-R1442, 1999[Abstract/Free Full Text].

34.   Shoemaker, JK, Naylor HL, Hogeman CS, and Sinoway LI. Blood flow dynamics in heart failure. Circulation 99: 3002-3008, 1999[Abstract/Free Full Text].

35.   Shoemaker, JK, Pandey P, Herr MD, Silber DH, Yang QX, Smith MB, Gray K, and Sinoway LI. Augmented sympathetic tone alters muscle metabolism with exercise: lack of evidence for functional sympatholysis. J Appl Physiol 82: 1932-1938, 1997[Abstract/Free Full Text].

36.   Sinoway, L, and Prophet S. Skeletal muscle metaboreceptor stimulation opposes peak metabolic vasodilation in humans. Circ Res 66: 1576-1584, 1990[Abstract/Free Full Text].

37.   Sinoway, L, Prophet S, Gorman I, Mosher T, Shenberger J, Dolecki M, Briggs R, and Zelis R. Muscle acidosis during static exercise is associated with calf vasoconstriction. J Appl Physiol 66: 429-436, 1989[Abstract/Free Full Text].

38.   Strandell, T, and Shepherd JT. The effect in humans of increased sympathetic activity on blood flow to active muscles. Acta Med Scand Suppl 472: 146-167, 1967[Medline].

39.   Sundlöf, G, and Wallin BG. The variability of muscle nerve sympathetic activity in resting recumbent man. J Physiol (Lond) 272: 383-397, 1977[Abstract/Free Full Text].

40.   Trudeau, F, Péronnet F, Béliveau L, and Brisson G. Sympatho-endocrine and metabolic responses to exercise under post-ganglionic blockade in rats. Horm Metab Res 20: 546-550, 1988[ISI][Medline].

41.   Vanhoutte, PM, Verbueren TJ, and Webb RC. Local modulation of adrenergic neuroeffector interaction in the blood vessel wall. Physiol Rev 61: 151-247, 1981[Free Full Text].

42.   Vatner, SF, Franklin D, Van Citters RL, and Braunwald E. Effects of carotid sinus nerve stimulation on blood-flow distribution in conscious dogs at rest and during exercise. Circ Res 27: 495-503, 1970[Abstract/Free Full Text].

43.   Wagner, PD, Erickson BK, Seaman J, Kubo K, Hiraga A, Kai M, and Yamaya Y. Effects of altered FI_O2 on maximum O₂ in the horse. Respir Physiol 105: 123-134, 1996[ISI][Medline].

44.   Welch, HG. Hyperoxia and human performance: a brief review. Med Sci Sports Exerc 14: 253-262, 1982[ISI][Medline].

45.   Yang, HT, Ogilvie RW, and Terjung RL. Peripheral adaptations in trained aged rats with femoral artery stenosis. Circ Res 74: 235-243, 1994[Abstract/Free Full Text].