Cerebral metabolism is influenced by muscle ischaemia during exercise in humans

Cerebral metabolism is influenced by muscle ischaemia during exercise in humans Mads K. Dalsgaard *, Lars Nybo, Yan Cai * and Niels H. Secher * * Department of Anaesthesia and The Copenhagen Muscle Research Centre, Rigshospitalet, and The Institute of Exercise and Sports Science, University of Copenhagen, Denmark (Manuscript received 15 August 2002; accepted 16 December 2002) Maximal exercise reduces the cerebral metabolic ratio (O 2 /(glucose + Îlactate)) to < 4 from a resting value close to 6, and only part of this decrease is explained by the intent to exercise. This study evaluated whether sensory stimulation of brain by muscle ischaemia would reduce the cerebral metabolic ratio. In 10 healthy human subjects the cerebral arterial venous differences (a v differences) for O 2, glucose and lactate were assessed before, during and after three bouts of 10 min cycling with equal workload: (1) control exercise at light intensity, (2) exercise that elicited a high rating of perceived exertion due to a 100 mmhg thigh cuff, and (3) exercise followed by 5 min of post-exercise muscle ischaemia that increased blood pressure by ~ 20 %. Control exercise did not significantly affect the a v differences. However, during the recovery from exercise with thigh cuffs the cerebral metabolic ratio decreased from a resting value of 5.4 ± 0.2 to 4.0 ± 0.4 (mean ± S.E.M.; P < 0.05) as a discrete lactate efflux from the brain at rest shifted to a slight uptake. Also, following post-exercise muscle ischaemia, the cerebral metabolic ratio decreased to 4.5 ± 0.3 (P < 0.05). The results support the hypothesis that during exercise, cerebral metabolism is influenced both by the mental effort to exercise and by sensory input from skeletal muscles. Experimental Physiology (2003) 88.2, 297 302. Brain energy consumption is provided primarily by the oxidation of glucose as demonstrated by both a local (Fox & Raichle, 1986) and a global (Scheinberg et al. 1954) uptake of O 2 and glucose in a proportion close to 6. Yet physiological stimulation of the brain reduces this cerebral metabolic ratio, as does maximal exercise (Ide & Secher, 2000). Exercise engages several brain regions and largely in proportion to its intensity (Williamson et al. 1999; Delp et al. 2001). A certain level of brain activity is presumably required for the cerebral metabolic ratio to decrease as it remains stable during submaximal exercise (Dalsgaard et al. 2002). In contrast, a maximal intent to exercise reduces the cerebral metabolic ratio, even when sensory stimulation from the muscles to the brain is kept minimal (Dalsgaard et al. 2002). A maximal intent to exercise can, however, only account for part of the reduction in the cerebral metabolic ratio (to ~ 4.9) observed in response to regular maximal exercise (to ~ 3.7). Input from exercising muscle is of importance for the blood pressure response (Strange et al. 1993), but has not been evaluated for a role in the cerebral metabolic response. The discharge of small nerve fibres, i.e. groups III and IV muscle afferents, will increase in relation to temperature and chemical as well as mechanical stimuli. Low or absent background discharge combined with the large number of these fibres imply that even a minor elevation in their firing rate has impact on the central nervous system (Gandevia, 1998). Discharge from metabolic receptors in skeletal muscles can be aggravated by muscle ischaemia. Inflation of thigh cuffs establishes a biochemical milieu within the muscle comparable to the condition following strenuous exercise and has been applied to evaluate not only the blood pressure response (Rowell et al. 1976), but also respiratory control (Asmussen et al. 1965). This study tested the hypothesis that intense sensory stimulation of the brain during post-exercise muscle ischaemia would reduce the cerebral metabolic ratio. Also, we considered that even less severe sensory feedback would reduce the cerebral metabolic ratio when the intent to exercise was enhanced concomitantly by application of ~ 100 mmhg thigh cuffs during exercise. Publication of The Physiological Society Corresponding author: madskd@tiscali.dk 2469

298 M. K. Dalsgaard, L. Nybo, Y. Cai and N. H. Secher Exp Physiol 88.2 METHODS Subjects The study was approved by the Ethics Committee of Copenhagen (KF 01-369/97) and conformed to the standards set by the Declaration of Helsinki. Ten healthy subjects (3 females, 7 males; median age with range, 24 (21 26) years; height, 181 (167 197) cm; weight, 75 (55 94) kg) were studied after written informed consent was obtained. Experimental protocols On the day of the study, the participants had a light breakfast but were restricted from beverages containing caffeine. Inflatable cuffs were placed proximally around the thighs and the subjects exercised on a semi-recumbent Krogh cycle ergometer (Galbo et al. 1987) at 60 rev min _1 as dictated by a metronome. The protocol included three bouts of 10 min cycling, with the order of administration randomised: (1) control exercise at light intensity, (2) exercise with thigh cuffs that were inflated to ~100 mmhg prior to exercise and deflated immediately after cessation, and were assumed to increase both sensory stimulation of the brain and the effort to exercise, and (3) post-exercise muscle ischaemia achieved by cuff inflation to 200 mmhg 30 s before termination of exercise and maintained for 5 min postexercise to selectively enhance sensory stimulation of the brain. The workload (68 (42 103) W; median with range) was chosen to elicit a target heart rate of ~ 130 beats min _1 when the first exercise bout was with thigh cuffs and ~ 100 beats min _1 when this was not the case. The workload was maintained throughout the study. Measurements Cerebral arterial venous differences (a v differences) were obtained by means of a 2.2 mm (14 gauge) catheter in the right internal jugular vein with the tip advanced to the superior venous bulb and a 1.1 mm (20 gauge) catheter in the brachial artery of the non-dominant arm. The catheters were well tolerated by the subjects. Blood samples were drawn three times at rest, twice during exercise and several times during recovery. Samples were drawn anaerobically in pre-heparinised syringes and stored in ice water until analysis (ABL 625, Radiometer, Denmark). Figure 1 HR ( ), blood pressure (0), mean blood flow velocity in the middle cerebral artery (1), and arterial carbon dioxide tension (6) in response to control exercise, exercise with thigh cuffs and post-exercise muscle ischaemia. HR, heart rate; MAP, mean arterial pressure; MCA V mean, middle cerebral artery blood flow velocity; P a,co2, arterial carbon dioxide tension; Ex, light exercise; Ex w/ cuff; exercise with thigh cuffs of 100 mmhg; PEMI, post-exercise muscle ischaemia by 200 mmhg thigh cuffs. Values represent mean ± S.E.M. Values at rest are identical. Significant difference compared to rest: * P < 0.05; P < 0.01. Significant difference compared to control exercise: 2 P < 0.05; 3 P < 0.01.

Exp Physiol 88.2 Brain metabolism during exercise 299 Changes in cerebral perfusion were followed as the middle cerebral artery (MCA) mean blood flow velocity (V mean ) response using transcranial Doppler ultra-sound (Multidop X, DWL, Sipplingen, Germany; Ide & Secher, 2000). The MCA was located through the right posterior temporal window, and once the optimal signal-to-noise ratio was determined, the probe was secured using a customised headband and ultrasonic adhesive gel. Mean arterial pressure (MAP) was obtained by means of a Bentley transducer (Uden, Holland) positioned at heart level and connected to a patient monitor (Dialogue 2000, Danica Electronic, Copenhagen, Denmark) that also calculated heart rate (HR) from a three lead electrocardiogram. Subjects were asked to rate their perceived exertion (RPE) on a scale from 6 to 20 (Borg, 1970), where 6 and 20 correspond to the resting state and the hardest imaginable exercise, respectively. To quantify skeletal muscle ischaemia, a continuous-wave NIRS photometer was applied to determine the changes in oxy- (DHbO 2 ) and deoxyhaemoglobin (DHb) (NIRO500, Hamamatsu Phototonics, Hamamatsu, Japan). The optodes were placed on the lower leg on the lateral belly of the gastrocnemius muscle, and black rubber pads served to both attenuate the background light and to maintain a distance of 4 cm between the optodes (Madsen & Secher, 1999). The sum of DHbO 2 and DHb equals the concentration change of total haemoglobin (DHbT) with values expressed relative to rest. Values are expressed as mean ± S.E.M., or as median with range, for rating of perceived exertion and subject data. Changes occurring over time were detected by Friedman s test and located using Wilcoxon s signed ranks test. a was set at 0.05. RESULTS Cardiovascular response The HR, MAP and MCA V mean increased during control exercise and were further elevated during exercise with thigh cuffs (Fig. 1). Also, post-exercise muscle ischaemia delayed the decrease in HR and MAP compared with recovery from control exercise, but not for MCA V mean. Additionally, exercise with thigh cuffs resulted in a higher RPE (17 (15 19)) than did control exercise (10 (7 12); P < 0.01). The arterial carbon dioxide tension (P a,co2 ) did not change significantly during control exercise, but it decreased during exercise with thigh cuffs and it remained low during the early recovery except for a transient normalisation at the time of cuff release. In addition, P a,co2 was reduced during post-exercise muscle ischaemia as compared with both rest and recovery from control exercise. Muscle oxygenation Control exercise increased DHbO 2 (3.1 ± 1.9 mmol l _1 ) and DHb (3.4 ± 0.6 mmol l _1 ) (Fig. 2). During exercise with thigh cuffs the influence on muscle oxygenation was illustrated by an increase in DHb of 16.4 ± 4.2 mmol l _1 which was different from control exercise (P < 0.05), but DHbO 2 did not change significantly. Post-exercise muscle ischaemia was demonstrated in that DHbO 2 became negative (_13.3 ± 6.6 mmol l _1 ) and the increase in DHb (26.6 ± 5.5 mmol l _1 ) was greater than that following control exercise (P < 0.05). During the recovery from exercise with thigh cuffs, muscle oxygenation was not significantly different from control. Cerebral metabolic response At rest the cerebral metabolic ratio of O 2 /(glucose + Îlactate) was relatively low at 5.4 ± 0.2 and there was a slight lactate efflux from the brain of _0.04 ± 0.02 mmol l _1 (Fig. 3). Control exercise did not change the a v differences for O 2, glucose and lactate significantly. However, following exercise with thigh cuffs the cerebral metabolic Figure 2 Muscle oxygenation of the lower leg in response to control, exercise with thigh cuffs and post-exercise muscle ischaemia. CON, control exercise; Ex w/ cuff, exercise with thigh cuffs of 100 mmhg; PEMI, post-exercise muscle ischaemia by 200 mmhg thigh cuffs; DHbO 2, changes in oxyhaemoglobin; DHb, changes in deoxyhaemoglobin. Values are mean ± S.E.M. Significant difference compared to rest: * P < 0.05; P < 0.01. Significant difference compared to control exercise: 2 P < 0.05.

300 M. K. Dalsgaard, L. Nybo, Y. Cai and N. H. Secher Exp Physiol 88.2 ratio was reduced after 1 and 3 min of the recovery (to 4.0 ± 0.4 and 4.4 ± 0.3, respectively). This decrease manifested as there was a slight cerebral uptake of lactate (0.1 ± 0.1 mmol l _1 ) that remained during the early recovery (P < 0.01). Equally, following post-exercise muscle ischaemia there was a brief reduction in the cerebral metabolic ratio to 4.5 ± 0.3 (P < 0.05). DISCUSSION The main finding of the study is that the cerebral metabolic ratio for the brain was influenced both by exercise with thigh cuffs and by post-exercise muscle ischaemia. In contrast, light exercise did not affect the cerebral metabolic ratio significantly. Post-exercise muscle ischaemia was applied to enhance the sensory stimulus from the muscles. The fact that DHbO 2 became negative and DHb therefore entirely responsible for the increase in DHbT implies that the muscles were ischaemic, as also indicated by an elevated blood pressure (Rowell et al. 1976). Despite the enhanced cardiovascular response to post-exercise muscle ischaemia the MCA V mean did not change significantly. Exercise with thigh cuffs was in addition to an aggravated sensory feedback to the brain taken to enhance the mental effort to exercise. Accordingly, RPE was higher than during control exercise. During exercise with thigh cuffs the influence on leg perfusion was demonstrated by an increase in DHbT of ~ 200 % and an increase of the fraction comprised by DHb suggesting that venous outflow from the legs was partially obstructed. Furthermore, during exercise with thigh cuffs the cerebral integration of a concomitantly increased mental effort and sensory stimulation was illustrated by an enhanced MCA V mean and cardiovascular response. In fact, the elevation in MCA V mean occurred while P a,co2 was low, which in itself would be expected to reduce blood flow (and in turn MCA V mean ) by constriction of the cerebral vasculature of smaller diameter than the MCA (Serrador et al. 2000). The reduction in the cerebral metabolic ratio under the influence of exercise with thigh cuffs and post-exercise muscle ischaemia was not as pronounced as that seen in response to exhaustive exercise (Ide et al. 2000; Dalsgaard Figure 3 The cerebral metabolic ratio ( ) and arterial to internal jugular vein (AV) differences for O 2 (0), glucose (1) and lactate (6). Conditions and abbreviations as in Fig. 1. Values represent mean ± S.E.M. and are identical at rest. Significant difference compared to rest: * P < 0.05; P < 0.01.

Exp Physiol 88.2 Brain metabolism during exercise 301 et al. 2002). During exercise, such reduction of the cerebral metabolic ratio incorporates metabolism in several brain regions. For instance, localised elevation of brain blood flow differentiates between areas responsible for the regulation of cardiovascular variables ( central command : motor cortex and the insula; Williamson et al. 1999), volitional ventilation (Thornton et al. 2001), maintenance of equilibrium and integration of motor output and sensory input (anterior and dorsal cerebellar vermis, and vestibular nuclei, respectively; Delp et al. 2001), and integration of the effort sense (insula and thalamic regions; Williamson et al. 2001). Individual contribution to the global cerebral metabolism is demonstrated since an isolated augmentation of intent to exercise to a maximal level yields a cerebral metabolic ratio of 4.9 (Dalsgaard et al. 2002). Thus, this feed-forward aspect of exercise accounts for only a part of the reduction following regular maximal exercise. In the present study, impact on the brain by sensory input from working skeletal muscle was evident as the cerebral metabolic ratio decreased after post-exercise muscle ischaemia to ~ 4.5. Moreover, combining enhanced mental effort and a sensory stimulation, i.e. exercise with thigh cuffs, reduced the cerebral metabolic ratio to ~ 4.2. For the brain as a whole, this indicates an additive nature of areas engaged during exercise for decreasing its metabolic ratio. A criticism to the applied method is that the arterial blood sample was obtained from the brachial rather than from the carotid artery. Therefore, there may be some mismatch between the arterial and the venous blood sampling with respect to the brain of importance especially when blood concentrations change rapidly. Such influence may have caused some inconsistency in the cerebral metabolic ratio when the arterial cuff was released, i.e. two values were low, while the value obtained at 2 min was not reduced significantly. Prior investigators of physiological activation of the human brain have taken careful measures to ensure that subjects were deprived of sensory stimulation at rest (Madsen et al. 1995). Yet, strenuous exercise was performed with normal vision and with encouragement, and consequently, we let subjects rest without blocked vision or hearing, so that the evaluation of whole brain metabolism was not biased by additional engagement at the transition to exercise. This might explain why the cerebral metabolic ratio at rest was 5.4 and thus lower than the commonly reported value of ~ 6. This investigation emphasised the role played by metabolic receptors, although exercise also excites mechanoreceptors with a potential influence on brain metabolism (Williamson et al. 1997). In fact, regional cerebral perfusion appears to be linked more closely to stimuli from mechano- than from metaboreceptors (Jorgensen et al. 1992; Williamson et al. 1996). Lactate uptake is included in the cerebral metabolic ratio as the human brain extracts lactate from blood during maximal exercise (Ide et al. 2000). In the immediate recovery from exercise with thigh cuffs the reduction in the cerebral metabolic ratio coincided with a cerebral uptake of lactate. That the brain took up lactate when the cuffs were released may signify that the uptake is a consequence of increased availability. Against this notion is the argument that lactate can be metabolised by neurons in vitro (Larrabee, 1996) and that infusion of lactate abolishes symptoms in hypoglycaemic subjects (Veneman et al. 1994). Moreover, a lactate uptake is likely to depend on cerebral activation (Ide et al. 2000). 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