Search

This paper presents an impressionistic summary of the formation, activities, accomplishments, and impact of the Copenhagen Muscle Research Centre (CMRC) from 1994 to 2004. The history of the CMRC is viewed in the context of the goals of the original program, the tradition of excellence in exercise physiology in Copenhagen since the time of August Krogh, and the structure of the center. The key role of Professor Bengt Saltin as a visionary, flexible, and inclusive leader is highlighted., (© 2015 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Whoever breaks 2 h will likely have outstanding running economy and small body size along with exposure to high altitude and significant physical activity early in life. However, neither of these factors nor any specific suite of genotypes appear to be obligatory for a time this fast. Current trends suggest that an East African will be the first to break 2 h. However periods of regional dominance in distance running are not unique to the East Africans: athletes from Finland, Eastern Europe, Australia, and New Zealand have all had extended periods of success at a range of distances. From a physiological perspective, more information is clearly needed on the relationship between VO(2max) and running economy and the influence of running economy and body size on thermoregulation and fuel use.

In this review we integrate ideas about regional and systemic circulatory capacities and the balance between skeletal muscle blood flow and cardiac output during heavy exercise in humans. In the first part of the review we discuss issues related to the pumping capacity of the heart and the vasodilator capacity of skeletal muscle. The issue is that skeletal muscle has a vast capacity to vasodilate during exercise [approximately 300 mL (100 g)(-1) min(-1)], but the pumping capacity of the human heart is limited to 20-25 L min(-1) in untrained subjects and approximately 35 L min(-1) in elite endurance athletes. This means that when more than 7-10 kg of muscle is active during heavy exercise, perfusion of the contracting muscles must be limited or mean arterial pressure will fall. In the second part of the review we emphasize that there is an interplay between sympathetic vasoconstriction and metabolic vasodilation that limits blood flow to contracting muscles to maintain mean arterial pressure. Vasoconstriction in larger vessels continues while constriction in smaller vessels is blunted permitting total muscle blood flow to be limited but distributed more optimally. This interplay between sympathetic constriction and metabolic dilation during heavy whole-body exercise is likely responsible for the very high levels of oxygen extraction seen in contracting skeletal muscle. It also explains why infusing vasodilators in the contracting muscles does not increase oxygen uptake in the muscle. Finally, when approximately 80% of cardiac output is directed towards contracting skeletal muscle modest vasoconstriction in the active muscles can evoke marked changes in arterial pressure.

Large interindividual differences exist in resting sympathetic nerve activity (SNA) among normotensive humans with similar arterial pressure (AP). We recently showed inverse relationships of resting SNA with cardiac output (CO) and vascular adrenergic responsiveness that appear to balance the influence of differences in SNA on blood pressure. In the present study, we tested whether nitric oxide (NO)-mediated vasodilation has a role in this balance by evaluating hemodynamic responses to systemic NO synthase (NOS) inhibition in individuals with low and high resting muscle SNA (MSNA). We measured MSNA via peroneal microneurography, CO via acetylene uptake and AP directly, at baseline and during increasing systemic doses of the NOS inhibitor NG-monomethyl-L-arginine (L-NMMA). Baseline MSNA ranged from 9 to 38 bursts/min (13 to 68 bursts/100 heartbeats). L-NMMA caused dose-dependent increases in AP and total peripheral resistance and reflex decreases in CO and MSNA. Increases in AP with L-NMMA were greater in individuals with high baseline MSNA (PANOVA<0.05). For example, after 8.5 mg/kg of L-NMMA, in the low MSNA subgroup (n=6, 28+/-4 bursts/100 heartbeats), AP increased 9+/-1 mmHg, whereas in the high-MSNA subgroup (n=6, 58+/-3 bursts/100 heartbeats), AP increased 15+/-2 mmHg (P<0.01). The high-MSNA subgroup had lower baseline CO and smaller decreases in CO with L-NMMA, but changes in total peripheral resistance were not different between groups. We conclude that differences in CO among individuals with varying sympathetic traffic have important hemodynamic implications during disruption of NO-mediated vasodilation.

Current models to measure protein turnover across muscle bed are based on many surrogate measures of amino acyl-tRNA. We measured muscle protein turnover based on tracer-to-tracee ratios of the stable isotopes of leucine, phenylalanine, and ketoisocaproate (KIC) in artery and vein and muscle amino acyl-tRNA and muscle tissue fluid (TF) in 26 healthy subjects. A three-compartment model calculation based on arteriovenous and tRNA measurements was first performed and its domain of validity assessed. The results were then compared with those using simpler approaches based on surrogate measures of tRNA such as those of TF and KIC and a one-compartment model based on arteriovenous amino acids. In 96% of cases, the model using tRNA was applicable, but only in a lower percentage of cases were the results using surrogate measures applicable. Protein breakdown, protein synthesis, and shunting of amino acids from artery to vein were consistently underestimated, and fluxes of amino acid from artery to intracellular compartment and from intracellular compartment to vein were overestimated, when surrogate measures were used. The one-compartment model also underestimated protein breakdown and synthesis. Measurements using tissue fluid gave results closer to those based on tRNA. In conclusion, a three-compartment model using arteriovenous samples and amino acyl-tRNA provides measurements of muscle protein turnover of acceptable precision in 96% of cases. The precision was unacceptable in a substantial percentage of cases, and the accuracy of the estimation of protein fluxes was significantly affected when surrogate measures were used.

The idea that there might be sympathetic vasodilator nerves to skeletal muscle is an old concept that fits with the archaic 'fight or flight' model of the sympathetic nervous system. Clear evidence for vasodilator nerves to skeletal muscle began to emerge in animals during the 1930s, when stimulation of selected brainstem areas was shown to evoke hypertension, tachycardia and skeletal muscle vasodilation (i.e. the 'defense reaction'). By the 1940s and 1950s this idea was well established and it was shown in animals that the sympathetic dilator nerves to muscles were cholinergic. During this time, circumstantial evidence began to suggest the existence of sympathetic cholinergic vasodilator fibres in human skeletal muscle. In this context, the well- known forearm vasodilator response to mental stress was shown to be atropine-sensitive, and absent after surgical sympathectomy. However, while there was clear histological evidence for sympathetic cholinergic dilator fibres in animal muscle, such evidence was not seen in humans. Additionally, attempts to record from sympathetic dilator fibres human muscle have never demonstrated clear evidence for dilator nerve traffic, and many 'sympathetic dilator' responses are still present after local anaesthetic nerve block. More recently, the skeletal muscle dilator response to sympathoexcitatory manoeuvres in both humans and animals appears to be nitric oxide (NO)-dependent. While there are clearly atropine-sensitive and NO-dependent dilator nerves to skeletal muscles in animals, our current thinking is that most 'sympathetic dilator' responses in human muscle are due to adrenaline or local cholinergic mechanisms acting to stimulate NO release from the vascular endothelium.

Phenylephrine is generally regarded as a "pure" alpha(1)-agonist. However, after treatment of the forearm with the alpha-adrenergic-blocking drug phentolamine, brachial artery infusion of phenylephrine can cause transient forearm vasodilation. To determine whether this response was beta-receptor mediated, phenylephrine, phentolamine, and propranolol were infused into the brachial arteries of six healthy volunteers. Forearm vascular conductance (FVC) was also calculated and expressed as arbitrary units (units). Infusion of phenylephrine by itself (0.5 microg. dl forearm volume(-1). min(-1)) caused a sustained decrease (P < 0.05) in FVC from 3.5 +/- 0.7 to 0.9 +/- 0.2 units (P < 0.05). Infusion of the alpha-blocker phentolamine increased (P < 0.05) baseline FVC to 5.7 +/- 1.3 units. Subsequent infusion of phenylephrine after alpha-blockade caused FVC to increase (P < 0.05) for ~1 min from 5.7 +/- 1.3 to a peak of 13.1 +/- 1.8 units. Propranolol had no effect on baseline flow, and subsequent phenylephrine infusion after alpha- and beta-blockade caused a small, but significant, sustained decrease in FVC from 5.1 +/- 1.0 to 3.6 +/- 0.8 units. There were no systemic effects from the infusions, and saline infusion at the same rate (1-2 ml/min) had no forearm vasoconstrictor or dilator effects. These data indicate that in humans phenylephrine can exert transient beta(2)-vasodilator activity when its predominant alpha-constrictor effects are blocked.

Evidence for the existence of sympathetic vasodilator nerves in human skeletal muscle is controversial. Manoeuvres such as contralateral ischaemic handgripping to fatigue that cause vasoconstriction in the resting forearm evoke vasodilatation after local alpha-adrenergic receptor blockade, raising the possibility that both constrictor and dilator fibres are present. The purpose of this study was to determine whether this dilatation is neurally mediated. Ten subjects (3 women, 7 men) performed ischaemic handgripping to fatigue before and after acute local anaesthetic block of the sympathetic nerves (stellate ganglion) innervating the contralateral (resting) upper extremity. Forearm blood flow was measured with venous occlusion plethysmography in the resting forearm. In control studies there was forearm vasoconstriction during contralateral handgripping to fatigue. During contralateral handgripping after stellate block, blood flow in the resting forearm increased from 6.1 +/- 0.7 to 18.7 +/- 2.2 ml dl-1 min-1 (P < 0.05). Mean arterial pressure measured concurrently increased from approximately 90 to 130 mmHg and estimated vascular conductance rose from 6.5 +/- 0.7 to 14.0 +/- 1.5 units, indicating that most of the rise in forearm blood flow was due to vasodilatation. Brachial artery administration of beta-blockers (propranolol) and the nitric oxide (NO) synthase inhibitor N G-monomethyl-L-arginine (L-NMMA) after stellate block virtually eliminated all of the vasodilatation to contralateral handgrip. Since vasodilatation was seen after stellate block, our data suggest that sympathetic dilator nerves are not responsible for limb vasodilatation seen during sympathoexcitation evoked by contralateral ischaemic handgripping to fatigue. The results obtained with propranolol and L-NMMA suggest that beta-adrenergic mechanisms and local NO release contribute to the dilatation.

Whether or not neurally mediated vasodilation contributes to the rise in skeletal muscle blood flow during exercise in humans remains unknown. Such a mechanism might serve as an important 'feed-forward' regulatory signal causing blood flow to rise prior to the development of a metabolic demand. Research in animal species has identified three neurally mediated vasodilating mechanisms with the potential to increase skeletal muscle blood flow during exercise. These include sympathetic vasodilator nerves, the potential for substances released by motor nerves to evoke vasodilation and the possibility of an 'intrinsic' vasodilator nerve system within the walls of blood vessels. In humans, sympathetic vasodilator nerves are present in several vascular beds (e.g. cutaneous). However, more recent information suggests that the human skeletal muscle is not innervated by this class of nerves. Along these lines, the vasodilator response to exercise is unaffected by sympathectomy or by blockade of the traditional transmitters associated with neurally mediated vasodilation. The possibility that spillover of substances released from motor neurones evokes vasodilation is provocative. For example, acetylcholine could produce both skeletal muscle contraction via nicotinic receptors and vasodilation via endothelial muscarinic receptors. However, in many species including humans, atropine has no effect on exercise hyperaemia. While the concept of an 'intrinsic' vasodilator pathway within the walls of the skeletal muscle vascular bed is fascinating, limited information is available on this mechanism in animals and none is available in humans. Taken together, the current information suggests that neurally mediated vasodilating mechanisms may not exist in human skeletal muscle. Additionally, even if such mechanisms exist, they do not play an obligatory role in governing the rise in muscle blood flow during exercise in humans.

This paper attempts to integrate some important concepts about the various mechanisms that are thought to cause blood flow to rise during rhythmic exercise. Mechanisms including the muscle pump, substances released by skeletal muscle, substances transported by blood, and factors released by nerves have been postulated to contribute to the rise in muscle blood flow during exercise. Additionally, the factors that initiate the dilation may not be those which sustain it. Although there is normally a close relationship between contractile activity, metabolic rate, and muscle blood flow, this relationship can be disrupted under a variety of circumstances and the active skeletal muscle overperfused. This delinking of flow and metabolism raises important questions about the nature of the vasodilating substances responsible for the rise in blood flow during exercise. We propose that understanding the mechanisms responsible for the "delinking" of flow and metabolism, along with a more synergistic view of current concepts, can provide new insight into the mechanisms which govern exercise hyperemia.

To explore the hypothesis that lower body muscle mass correlates with orthostatic tolerance, 18 healthy volunteers (age 18-48 yr; 10 men, 8 women) underwent a graded lower body negative pressure (LBNP) protocol consisting of six, 5-min stages of suction up to 60 mmHg in 10-mmHg increments. Forearm blood flow, heart rate, and blood pressure were measured, and forearm vascular resistance was calculated. Leg muscle mass was assessed by dual-energy X-ray absorptiometry. All subjects received standard intravenous hydration for at least 8 h before the study. Six men and four women completed all stages of LBNP. Four men and four women developed presyncopal symptoms, including marked bradycardia and/or hypotension, at LBNP levels of 30 mmHg (n = 2;1 man, 1 woman), 40 mmHg (n = 2;1 man, 1 woman), and 50 mmHg (n = 4;2 men, 2 women). The presyncopal subjects had leg muscle masses ranging from 19.5 to 25.2 kg in men and from 11.7 to 16.6 kg in women. In subjects who completed all stages of LBNP, leg muscle mass ranged from 17.5 to 24.1 kg in men and from 10.4 to 18.0 kg in women. Leg muscle mass did not differ between presyncopal subjects and those who completed the protocol. Furthermore, there were no differences in the hemodynamic responses to LBNP between subjects with low vs. high leg mass. These data suggest that leg muscle mass is not a critical determinant of LBNP tolerance in otherwise healthy men and women.

To test the hypothesis that acetylcholine "spillover" from motor nerves contributes significantly to skeletal muscle vasodilation during exercise, we measured the forearm blood flow responses during attempted handgripping after local paralysis of the forearm with the neuromuscular-blocking drug pipecuronium. This compound blocks postsynaptic nicotinic receptors but has no impact on acetylcholine release from motor nerves. The drug was administered selectively to one forearm by using regional intravenous drug administration techniques in five subjects. Pipecuronium reduced maximum forearm grip strength from 40.0 +/- 3.2 kg before treatment to 0.0 kg after treatment. By contrast, drug administration had no effect on maximum voluntary contraction in the untreated forearm (41.3 +/- 3.3 vs. 41.4 +/- 2.7 kg). During 2 min of attempted maximal contraction of the paralyzed forearm, the forearm blood flow increased from only 3.4 +/- 0.8 to 4.8 +/- 1.2 ml.100 ml-1.min-1 (P < 0.05). Heart rate increased from 63 +/- 3 to 73 +/- 8 beats/min (P > 0.05) during attempted contraction, and only three of five subjects showed obvious increases in heart rate. Mean arterial pressure increased significantly (P < 0.05) from 102 +/- 6 to 109 +/- 9 mmHg during attempted contractions. When these increases in flow are considered in the context of the marked (10-fold or greater) increases in flow seen in contracting forearm skeletal muscle, it appears that acetylcholine spillover from motor nerves has, at most, a minimal impact on the hyperemic responses to contraction in humans.

The contributions of acetylcholine and/or nitric oxide (NO) to the rapid changes in human forearm blood flow (FBF) at the onset and recovery from mild exercise were studied in eight subjects. Rhythmic handgrip contractions were performed during brachial artery infusions of saline (2 ml/min; control), atropine (0.2 mg over 3 min), to block acetylcholine binding to muscarinic receptors, or atropine + NG-monomethyl-L-arginine (L-NMMA; 4 mg/min for 4 min), to additionally inhibit NO synthase. Brachial artery mean blood velocity (MBV; pulsed Doppler ultrasound) and diameter (echo Doppler) were measured continuously, and FBF was calculated. Atropine reduced acetylcholine-induced increases in FBF by approximately 71% (P < 0.05). FBF at rest was reduced by atropine and further reduced with atropine + L-NMMA. Both drug conditions reduced FBF during exercise by approximately 10% compared with control, with no difference between drug treatments. Brachial artery diameter was unchanged from rest by exercise, recovery, and drug treatments. Neither drug treatment altered the rate or magnitude of the increase in FBF above rest. Peak FBF after exercise was reduced by atropine and atropine + L-NMMA. Total FBF during 5 min of recovery was reduced with atropine + L-NMMA compared with control and atropine. The results suggest that 1) acetylcholine and NO mechanisms additively contribute to FBF levels at rest, 2) a cholinergic mechanism adjusts the absolute FBF levels during exercise, 3) neither acetylcholine nor NO is essential to observe the normal time course or magnitude of the exercise response, and 4) NO contributes to the FBF response during recovery from exercise.

The role of skeletal muscle mass in the age-associated decline in maximal O2 uptake (VO2max) is poorly defined because of confounding changes in muscle oxidative capacity and in body fat and the difficulty of quantifying active muscle mass during exercise. We attempted to clarify these issues by examining the relationship between several indexes of muscle mass, as estimated by using dual-energy X-ray absorptiometry and treadmill VO2max in 32 chronically endurance-trained subjects from four groups (n = 8/group): young men (20-30 yr), older men (56-72 yr), young women (19-31 yr), and older women (51-72 yr). VO2max per kilogram body mass was 26 and 22% lower in the older men (45.9 vs. 62.0 ml.kg-1.min-1) and older women (40.0 vs. 51.5 ml.kg-1.min-1). These age differences were reduced to 14 and 13%, respectively, when VO2max was expressed per kilogram of appendicular muscle. When appropriately adjusted for age and gender differences in appendicular muscle mass by analysis of covariance, whole body VO2max was 0.50 +/- 0.09 l/min less (P < 0.001) in the older subjects. This effect was similar in both genders. These findings suggest that the reduced VO2max seen in highly trained older men and women relative to their younger counterparts is due, in part, to a reduced aerobic capacity per kilogram of active muscle independent of age-associated changes in body composition, i.e., replacement of muscle tissue by fat. Because skeletal muscle adaptations to endurance training can be well maintained in older subjects, the reduced aerobic capacity per kilogram of muscle likely results from age-associated reductions in maximal O2 delivery (cardiac output and/or muscle blood flow).

During vasovagal syncope, profound bradycardia and hypotension occur. Atropine administration can prevent the bradycardia but not the hypotension, suggesting that marked peripheral vasodilation is a major cause of the fall in arterial pressure. This concept has been confirmed since vasovagal syncope can be seen in patients who have undergone heart transplantation and also in patients subject to cardiac pacing. In both cases, there is no bradycardia but hypotension during the syncopal attacks. The major site of the vasodilation is in skeletal muscle and muscle sympathetic nerve activity is suppressed just prior to and during vasovagal attacks, indicating that sympathetic withdrawal contributes to the dilation. However, the skeletal muscle vasodilation seen during syncope is greater than that caused by sympathetic withdrawal alone, and it is absent in limbs that have undergone surgical sympathectomy, or local anesthetic nerve block. These observations suggest a role for neurally mediated "active" vasodilation during syncope. The afferent neural pathways that evoke the profound vasodilation during vasovagal attacks remain the subject of debate. The neural pathways responsible for the active component of the dilation are also unknown. Recent evidence has demonstrated that cholinergic, beta-adrenergic, and nitroxidergic (nitric oxide) vasodilator mechanisms are not essential to observe the dilation, demonstrating that the mechanisms responsible for it remain a continuing conundrum.

The current concept is that the vasodilation which contributes to fainting (vasovagal syncope) is caused entirely by withdrawal of sympathetic vasoconstrictor tone (i.e. passive vasodilation). This concept has supplanted the idea that an active, sympathetically mediated component to the vasodilation exists in humans. We have several lines of evidence suggesting that there can be sympathetically mediated active vasodilation in humans. We speculate that this active vasodilation may be linked to the release of the recently identified vasodilator nitric oxide. Along these lines, we have experimental evidence consistent with neurally mediated nitric oxide release during several types of sympathoexcitatory maneuvers in humans. We have also observed forearm vasodilation during a vasovagal response after alpha-adrenergic blockade of the forearm under study. These observations indicate that the potential role of active vasodilation during fainting in humans should be revisited.