Cardiovascular reflexes during sustained handgrip exercise: role of muscle fibre composition, potassium and lactate

Consideration of absolute intensity when examining sex differences in blood pressure responses during static exercise

Low- to moderate-intensity submaximal static contractions are commonly used to study the effects of biological sex on the cardiovascular response to exercise. Under this paradigm, premenopausal females frequently demonstrate smaller blood pressure responses than age-matched males. These differences are preserved during postexercise circulatory occlusion, implicating the muscle metaboreflex as an important driver of sex differences in the blood pressure response to static exercise. The mechanisms responsible for these differences are incompletely understood but often attributed to innate sex differences in skeletal muscle fiber type distribution, muscle metabolism, and/or sympathetic control of the circulation. However, one potential confounding factor is that the majority of studies use relative intensity exercise (e.g., 30% of maximal voluntary contraction), such that on average, females are completing static contractions at a lower absolute intensity. In this review, we summarize human evidence showing that sex differences in blood pressure responses to static exercise are attenuated or abolished when controlling for absolute intensity and muscle strength, either by statistical methods or strength-matched cohorts. We highlight evidence that the effect of higher absolute contraction intensity on exercise blood pressure likely occurs through increased mechanical occlusion of skeletal muscle microvasculature, leading to greater activation of the muscle metaboreflex. These findings highlight an important need to account for absolute intensity when studying and interpreting sex differences in cardiovascular responses to exercise.

Combined inorganic nitrate/nitrite supplementation blunts α-mediated vasoconstriction during exercise in patients with type 2 diabetes

AIMS

Patients with type 2 diabetes mellitus (T2DM) have reduced vasodilatory responses during exercise partially attributable to low nitric oxide (NO) levels. Low NO contributes to greater α-adrenergic mediated vasoconstriction in contracting skeletal muscle. We hypothesized boosting NO bioavailability via 8wks of active beetroot juice (BR_A, 4.03 mmol nitrate, 0.29 mmol nitrite, n = 19) improves hyperemia, via reduced α-mediated vasoconstriction, during handgrip exercise relative to nitrate/nitrite-depleted beetroot juice (BR_P, n = 18) in patients with T2DM.

METHODS

Forearm blood flow (FBF) and vascular conductance (FVC) were calculated at rest and during handgrip exercise (20%max, 20contractions·min^-1). Phenylephrine (α₁-agonist) and dexmedetomidine (α₂-agonist) were infused intra-arterially during independent trials to determine the influence of α-mediated vasoconstriction on exercise hyperemia. Vasoconstriction was quantified as the percent-reduction in FVC during α-agonist infusion, relative to pre-infusion, as well as the absolute change in %FVC during exercise relative to the respective rest trial (magnitude of sympatholysis).

RESULTS

ΔFBF (156 ± 69 to 175 ± 73 ml min^-1) and ΔFVC (130 ± 54 to 156 ± 63 ml min^-1·100 mmHg^-1, both P < 0.05) during exercise were augmented following BR_A, but not BR_P (P = 0.96 and 0.51). Phenylephrine-induced vasoconstriction during exercise was blunted following BR_A (-17.1 ± 5.9 to -12.6 ± 3.1%, P < 0.01), but not BR_P (P = 0.58) supplementation; the magnitude of sympatholysis was unchanged by either (beverage-by-time P = 0.15). BR_A supplementation reduced dexmedetomidine-induced vasoconstriction during exercise (-23.3 ± 6.7 to -19.7 ± 5.2%) and improved the corresponding magnitude of sympatholysis (25.3 ± 11.4 to 34.4 ± 15.5%, both P < 0.05).

CONCLUSIONS

BR_A supplementation improves the hyperemic and vasodilatory responses to exercise in patients with T2DM which appears to be attributable to reduced α-adrenergic mediated vasoconstriction in contracting skeletal muscle.

Blood Pressure Responses to Static and Dynamic Knee Extensor Exercise between Sexes: Role of Absolute Contraction Intensity

PURPOSE

Males have larger blood pressure (BP) responses to relative-intensity static handgrip exercise compared with females. Controlling for absolute load (maximal voluntary contraction (MVC)) abolishes these differences. Whether similar observations exist during large muscle mass exercise or dynamic contractions, and the mechanisms involved, remains unknown.

METHODS

BP, heart rate, muscle oxygenation (near-infrared spectroscopy), and rectus femoris EMG were recorded in 28 males and 17 females during 10% and 30% MVC static (120 s) and isokinetic dynamic (180 s; 1:2 work-to-rest ratio; angular velocity, 60°·s-1) knee extensor exercise. Static and dynamic exercises were completed on separate visits, in a randomized order. Sex differences were examined with and without statistical adjustment of MVC (ANCOVA).

RESULTS

Males had larger systolic BP responses (interaction, P < 0.0001) and muscle deoxygenation (interaction, P < 0.01) than did females during 10% static exercise, with no difference in EMG (interaction, P = 0.67). Peak systolic BP was correlated with MVC (r = 0.55, P = 0. 0001), and adjustment for MVC abolished sex differences in systolic BP (interaction, P = 0.3). BP, heart rate, muscle oxygenation/deoxygenation, and EMG responses were similar between sexes during 30% static exercise (interaction; all, P > 0.2), including following adjustment for MVC (all, P > 0.1). Males had larger systolic BP responses during dynamic exercise at 10% and 30% (interaction; both, P = 0.01), which were abolished after adjustment for MVC (interaction; both, P > 0.08). Systolic BP responses were correlated with absolute MVC and stroke volume responses during 10% (r = 0.31, P = 0.04; r = 0.61, P < 0.0001, respectively) and 30% (r = 0.48, P = 0.001; r = 0.59, P < 0.0001, respectively).

CONCLUSIONS

Absolute contraction intensity can influence systolic BP responses to 10% but not 30% MVC static, as well as 10% and 30% MVC dynamic knee extensor exercise, and should be considered in cross-sectional comparisons of exercise BP.

Greater α1-adrenergic-mediated vasoconstriction in contracting skeletal muscle of patients with type 2 diabetes

Findings presented in this article are the first to show patients with type 2 diabetes mellitus have blunted hyperemic and vasodilatory responses to dynamic handgrip exercise. Moreover, we illustrate greater α1-adrenergic-mediated vasoconstriction may contribute to our initial observations. Collectively, these data suggest patients with type 2 diabetes may have impaired functional sympatholysis, which can contribute to their reduced exercise capacity.

Influence of Sex, Menstrual Cycle, and Menopause Status on the Exercise Pressor Reflex

In this review, we highlight the underlying mechanisms responsible for the sex differences in the exercise pressor reflex (EPR), and, importantly, the impact of sex hormones and menopausal status. The EPR is attenuated in premenopausal women compared with age-matched men. Specifically, activation of the metaboreflex (a component of the EPR) results in attenuated increases in blood pressure and sympathetic vasomotor outflow compared with age-matched men. In addition, premenopausal women exhibit less transduction of sympathetic outflow to the peripheral vasculature than men. In stark contrast, postmenopausal women exhibit an augmented EPR arising from exaggerated metaboreflex-induced autonomic and cardiovascular reflexes. We propose that metaboreflex-induced autonomic and cardiovascular changes associated with menopause majorly contribute to the elevated blood pressure response during dynamic exercise in postmenopausal women. In addition, we discuss the potential mechanisms by which sex hormones in premenopausal women may impact the EPR as well as metaboreflex.

Muscle strength is a major determinant of the blood pressure response to isometric stress testing: the Asklepios population study

AIM

Maximal handgrip strength is a strong predictor of cardiovascular mortality in economically and socioculturally diverse countries, yet the main determinants of cardiovascular response to change in afterload during handgrip are not well known. We examined the blood pressure (BP) responses during submaximal handgrip (at 25% of grip strength) and the determinants of grip strength.

METHODS

We studied 2215 participants from a population-based random sample without overt clinical disease (Asklepios Study; mean age 56.2 years). Handgrip testing was performed using a modified Jamar dynamometer with direct visual feedback. Simultaneously, a validated finger plethysmographic device measured continuous BP and heart rate.

RESULTS

During handgrip, SBP and DBP rose by, respectively, 20 ± 13 and 10 ± 6 mmHg. These changes were normally distributed and consistently higher in men. The main independent determinants of mean arterial pressure response during handgrip were: grip strength (F = 191.4; P < 0.001), baseline pulse pressure (F = 32.0; P < 0.001), height (F = 16.4; P < 0.001) and age (F = 12.8; P < 0.001). Grip strength was associated with muscle mass, better metabolic health, but also with higher baseline DBP. There was a significant graded increase in maximum pressure achieved and in the magnitude of pressure change during handgrip with increasing BP categories (P for trend <0.001).

CONCLUSION

The population BP response to handgrip is variable and its predominant determinant turned out to be grip strength itself, which should be accounted for in future analyses. Higher baseline BP, even within the normotensive range, acted as an independent and graded predictor of BP increase during handgrip.

Relationship Between BMI and Fatigability Is Task Dependent

OBJECTIVE

The objective of the current study was to determine the effect of body mass index (BMI) on fatigability of three different muscle groups at four different work intensities.

METHODS

Forty-nine normal-weight, 50 overweight, and 43 obese adults (32.1 ± 9.2 years; 50% males) performed fatiguing handgrip, shoulder flexion, and trunk extension exertions at 20%, 40%, 60%, and 80% of the associated maximum voluntary contractions.

RESULTS

Obese adults demonstrated 22% to 30% shorter endurance times than normal-weight adults, but this was only observed at lower intensities and with larger and more postural muscles of the shoulder and low back. Strength and fatigue-related strength loss remained comparable across BMI groups in both males and females in these task-specific conditions. Obesity was associated with faster progression in perception of effort at low-intensity shoulder and trunk exertions. While males were stronger than females across all muscle groups, females exhibited greater shoulder fatigue resistance than males at lower intensity levels.

CONCLUSION

Findings indicate that the relationship between obesity and fatigability is task dependent.

APPLICATION

These findings provide initial evidence on the impact of obesity on worker capacity. Future work that extends the current investigation to include more occupationally relevant scenarios are needed to facilitate occupational task (re)design and assessment practices, such that altered work capacities of two-thirds of the working population are accommodated.

The role of biceps brachii and brachioradialis for the control of elbow flexion and extension movements

How do synergistic muscles interact, when their contraction aims at stabilizing and fine-tuning a movement, which is induced by the antagonistic muscle? The aim of the study was to analyze the interaction of biceps and brachioradialis during fine-tuning control tasks in comparison to load bearing ones. The surface electromyogram of biceps, brachioradialis and triceps were examined in 15 healthy subjects in dynamic flexion and extension movements with different combinations of contraction levels, joint angles and angular velocities. The measurements were conducted in two configurations, where the torque due to an external load opposes the rotational direction of the elbow flexion (load bearing tasks) or the elbow extension (fine-tuning tasks). Whereas during load bearing control tasks, similar muscular activation of biceps and brachioradialis was observed for all joint angles, angular velocities and external loads, during fine-tuning control tasks a significant difference of the muscular activation of both flexors was observed for 1kg, F(3.639,47.305)=2.864, p=0.037, and 5kg of external load, F(1.570,21.976)=6.834, p=0.008. The results confirm the synergistic muscular activation of both flexors during load bearing tasks, but suggest different control strategies for both flexors when they comprise a fine-tuning control task.

Sweating responses and the muscle metaboreflex under mildly hyperthermic conditions in sprinters and distance runners

To investigate the effects of different training methods on nonthermal sweating during activation of the muscle metaboreflex, we compared sweating responses during postexercise muscle occlusion in endurance runners, sprinters, and untrained men under mild hyperthermia (ambient temperature, 35°C; relative humidity, 50%). Ten endurance runners, nine sprinters, and ten untrained men (maximal oxygen uptakes: 57.5 ± 1.5, 49.3 ± 1.5, and 36.6 ± 1.6 ml·kg(-1)·min(-1), respectively; P < 0.05) performed an isometric handgrip exercise at 40% maximal voluntary contraction for 2 min, and then a pressure of 280 mmHg was applied to the forearm to occlude blood circulation for 2 min. The Δ change in mean arterial blood pressure between the resting level and the occlusion was significantly higher in sprinters than in untrained men (32.2 ± 4.4 vs. 17.3 ± 2.6 mmHg, respectively; P < 0.05); however, no difference was observed between distance runners and untrained men. The Δ mean sweating rate (averaged value of the forehead, chest, forearm, and thigh) during the occlusion was significantly higher in distance runners than in sprinters and untrained men (0.38 ± 0.07, 0.19 ± 0.03, and 0.11 ± 0.04 mg·cm(-2)·min(-1), respectively; P < 0.05) and did not differ between sprinters and untrained men. Our results suggest that the specificity of training modalities influences the sweating response during activation of the muscle metaboreflex. In addition, these results imply that a greater activation of the muscle metaboreflex does not cause a greater sweating response in sprinters.

Effect of exercise intensity on mild rhythmic-handgrip-exercise-induced functional sympatholysis

This study attempts to clarify whether intensity of exercise influences functional sympatholysis during mild rhythmic handgrip exercise (RHG). We measured muscle oxygenation in both exercising and non-exercising muscle in the same arm in 11 subjects using near infrared spectroscopy (NIRS), heart rate, and blood pressure. We used the total labile signal to assess the relative muscle oxygenation by occlusion for 6 min. Subjects performed RHG (20 times/min) for 6 min at 10%, 20%, and 30% of maximal voluntary contraction (MVC) at random. We used a non-hypotensive lower body negative pressure (LBNP) of 220 mmHg for 2 min to elicit reproducible enhancement in muscle sympathetic nerve activity (MSNA) at rest and during RHG. LBNP caused decreases of 16.4% and 17.7% of the level of muscle oxygenation at rest (pre) in exercising (forearm) and non-exercising (upper arm) muscle respectively. Muscle oxygenation in non-exercising muscle with the application of LBNP during RHG did not change significantly at each intensity. In contrast, the decrease in muscle oxygenation in exercising muscle attenuated progressively as exercise intensity increased (10% MVC 8.8+/-2.8%, 20% MVC 7.1+/-2.0%, 30% MVC 4.6+/-3.0%), when LBNP was applied during RHG. The attenuation of the decrease in muscle oxygenation due to LBNP during RHG at 10%, 20%, and 30% was significantly different from that at rest (p<0.01). These findings indicate that functional sympatholysis during mild RHG might be attributed to exercise intensity.

Muscle afferent contributions to the cardiovascular response to isometric exercise

The cardiovascular response to isometric exercise is governed by both central and peripheral mechanisms. Both metabolic and mechanical stresses on the exercising skeletal muscle produce cardiovascular change, yet it is often overlooked that the afferent signal arising from the muscle can be modified by factors other than exercise intensity. This review discusses research revealing that muscle fibre type, muscle mass and training status are important factors in modifying this peripheral feedback from the active muscles. Studies in both animals and humans have shown that the pressor response resulting from exercise of muscle with a faster contractile character and isomyosin content is greater than that from a muscle of slower contractile character. Athletic groups participating in training programmes that place a high anaerobic load on skeletal muscle groups show attenuated muscle afferent feedback. Similarly, longitudinal studies have shown that specific local muscle training also blunts the pressor response to isometric exercise. Thus it appears that training may decrease the metabolic stimulation of muscle afferents and in some instances chronic exposure to the products of anaerobic metabolism may blunt the sensitivity of the muscle metaboreflex. There may be surprising parallels between the local muscle conditions induced in athletes training for longer sprint events (e.g. 400 m) and by the low-flow conditions in, for example, the muscles of chronic heart failure patients. Whether their similar attenuations in muscle afferent feedback during exercise are due to decreased metabolite accumulation or to a desensitization of the muscle afferents is not yet known.

Sustaining a constant effort by the tongue and hand: effects of acute fatigue

A constant-effort task was used previously as a potential assessment technique for fatigue. Participants sustained submaximal target effort levels with the tongue and hand against soft air-filled bulbs. For 80% of all trials, pressure decreased exponentially to a positive asymptote. In addition, pressure decreased faster when the muscles were fatigued than when they were rested. This study attempted to replicate the previous findings with new participants and to extend the findings to include surface electromyographic (EMG) data. Pressure and surface EMG signals were collected simultaneously while 10 neurologically normal young adults performed the constant-effort task at 50% of maximum pressure with the tongue and the hand. Eighty-one percent of the pressure data were modeled by a negative exponential equation with a nonzero asymptote. Seventy-three percent of the corresponding EMG data also fit this mathematical model. The pressure signals decayed more slowly than the corresponding EMG signals, particularly for the hand. After participants fatigued the tongue and hand with repeated brief maximal voluntary contractions, the time constants were reduced (rate of decay increased) for the tongue but not the hand. These results corroborate the previous finding that the time constant, determined from an exponential curve-fitting procedure, is a replicable measure. Furthermore, the reduction in the time constant after inducing acute fatigue in the tongue was replicated, although this same relationship was not replicated for the hand. The EMG data suggest that decreases in neuromuscular drive, including increased early adaptation, motor unit derecruitment, and motor unit desynchronization, contributed to the decrease in pressure during the constant-effort task, especially after acute fatigue was induced. These observations support the hypothesis that the task reflects, at least in part, central fatigue processes.

Is sympathetic neural vasoconstriction blunted in the vascular bed of exercising human muscle?

Sympathetic vasoconstriction of muscle vascular beds is important in the regulation of systemic blood pressure. However, vasoconstriction during exercise can also compromise blood flow support of muscle metabolism. This study tested the hypothesis that local factors in exercising muscle blunt vessel responsiveness to sympathetic vasoconstriction. We performed selective infusions of three doses of tyramine into the brachial artery (n = 8) to evoke endogenous release of noradrenaline (norepinephrine) at rest and during moderate and heavy rhythmic handgrip exercise. In separate experiments, tyramine was administered during two doses of adenosine infusion (n = 7) and two doses of sodium nitroprusside (SNP) infusion (n = 8). Vasoconstrictor effectiveness across conditions was assessed as the percentage reduction in forearm vascular conductance (FVC), calculated from invasive blood pressure and non-invasive Doppler ultrasound blood flow measurements at the brachial artery. Tyramine evoked a similar dose-dependent vasoconstriction at rest in all three groups, with the highest dose resulting in a 42-46 % reduction in FVC. This vasoconstriction was blunted with increasing exercise intensity (e.g. tyramine high dose percentage reduction in FVC; rest -43.4 +/- 3.7 %, moderate exercise -27.5 +/- 2.3 %, heavy exercise -16.7 +/- 3.6 %; P < 0.05). In contrast, tyramine infusion resulted in a greater percentage reduction in FVC during both doses of adenosine vs. rest (P < 0.05). Finally, percentage change in FVC was greater during low dose SNP infusion vs. rest (P < 0.05), but not different from rest at the high dose of SNP infusion (P = 0.507). A blunted percentage reduction in FVC during endogenous noradrenaline release in exercise but not vasodilator infusion indicates that sympathetic vasoconstriction is blunted in exercising muscle. This blunting appears to be exercise intensity-dependent.

Attenuated cardiovascular adjustment to sustained static exercise after carbohydrate loading

We examined whether a higher plasma lactic acid (LA) concentration resulting from carbohydrate (CHO)-loading affects the cardiovascular responses to exercise through a greater activation of LA-induced metaboreflex. Before and after CHO-loading, LA concentration, mean arterial blood pressure (MAP), heart rate (HR), cardiac output (CO), rating perceived exertion (RPE), and integrated electromyogram (iEMG) of the vastus lateralis were studied during a sustained static knee extension of single legs in 16 volunteers. The CHO-loading comprised an exhaustive bout of one-legged cycling (73+/-3% of maximal oxygen uptake for 130-160 min) and consuming a low-CHO diet for 2-3 days and a high-CHO diet for the next 3 days. In the leg that performed the exercise (the experimental leg), the LA concentration after CHO-loading was significantly increased, but the magnitude of MAP, HR, and CO responses during static exercise was significantly decreased in parallel with a significant reduction of RPE and iEMG. In the control leg, there were no changes in the variables before and after CHO-loading. These results suggest that the increased LA concentration resulting from CHO-loading did not affect the cardiovascular adjustment to the sustained exercise. Other mechanisms related to the reduction of RPE and iEMG seem to be responsible for the attenuated cardiovascular responses observed in the experimental leg after CHO-loading.

Myocardial dysfunction and adrenergic cardiac innervation in patients with insulin-dependent diabetes mellitus

BACKGROUND

Insulin-dependent diabetes mellitus (IDDM) is associated with an increased incidence of heart failure due to several factors, and in some cases a specific cardiomyopathy has been suggested.

OBJECTIVES

This study sought to assess the mechanisms of exercise-induced left ventricular (LV) dysfunction in asymptomatic patients with IDDM in the absence of hypertensive or coronary artery disease.

METHODS

Fourteen consecutive patients with IDDM were enrolled (10 men, 4 women; mean [+/- SD] age 28.5 +/- 6 years); 10 healthy subjects matched for gender (7 men, 3 women) and age (28.5 +/- 3 years) constituted the control group. LV volume, LV ejection fraction (LVEF) and end-systolic wall stress were calculated by two-dimensional echocardiography at rest and during isometric exercise. LV contractile reserve was assessed by post-extrasystolic potentiation (PESP) obtained by transesophageal cardiac electrical stimulation and dobutamine infusion. Myocardial iodine-123 metaiodobenzylguanidine (MIBG) scintigraphy was performed to assess adrenergic cardiac innervation.

RESULTS

Diabetic patients were classified into group A (n = 7), with an abnormal LVEF response to handgrip (42 +/- 7%), and group B (n = 7), with a normal response (72 +/- 8%). Baseline LVEF was normal in both group A and B patients (60 +/- 6% vs. 61 +/- 7%, p = NS). In group A patients, the LV circumferential wall stress-LVEF relation showed an impairment in LVEF disproportionate to the level of LV afterload. No significant changes in LVEF occurred during dobutamine (60 +/- 6% vs. 64 +/- 10%, p = NS), whereas PESP significantly increased LVEF (60 +/- 6% vs. 74 +/- 6%, p < 0.001); PESP at peak handgrip normalized the abnormal LVEF (42 +/- 7% vs. 72 +/- 5%, p < 0.001); and MIBG uptake normalized for body weight or for LV mass was lower than that in normal subjects (1.69 +/- 0.30 vs. 2.98 +/- 0.82 cpm/MBq per g, p = 0.01) and group B diabetic patients (vs. 2.79 +/- 0.94 cpm/MBq per g, p = 0.01). Finally, a strong linear correlation between LVEF at peak handgrip and myocardial MIBG uptake normalized for LV mass was demonstrated in the study patients.

CONCLUSIONS

Despite normal contractile reserve, a defective blunted recruitment of myocardial contractility plays an important role in determining exercise LV dysfunction in the early phase of diabetic cardiomyopathy. This abnormal response to exercise is strongly related to an impairment of cardiac sympathetic innervation.

Influence of posture and training on the endurance time of a low-level isometric contraction

Classically, the critical force of a muscle (the relative force below which an isometric contraction can be maintained for a very long time without fatigue) is comprised of between 15 and 20% of its maximum voluntary contraction (MVC). However, some authors believe that the value is below 10% MVC. If such is the case, signs that accompany the establishment of muscle fatigue (EMG changes, continuous increase in systolic blood pressure [SBP] and heart rate [HR]) would have to appear more rapidly and with a higher intensity if the muscle is already partially fatigued at the start of maintaining a contraction at 10% MVC. Twelve healthy untrained participants carried out two isometric contractions with the digit flexors: one (test A) began with a maximum contraction sustained for 4 min followed without interruption by a contraction at 10% MVC for 61 min; the other (test B) was a contraction maintained at 10% MVC for 65 min. For test B, after an initial increase of 4 bpm with respect to at rest, HR remained stable until the end of contraction, SBP progressively increased by 24 mm Hg in 28 min, then remained unchanged until the end, and there were no significant changes in EMG (absence of spectral deviation towards low frequencies). For test A, in spite of the initial maximum contraction, changes in the parameters being studied (total maintenance time, HR, SBP, EMG) during maintenance at 10% MVC were identical to those for test B. The results show that (1) when the number and intensity of the co-contractions are minimized by applying an appropriate posture, it is possible to sustain an isometric contraction at 10% MVC for at least 65 min without the appearance of signs of muscle fatigue; (2) the critical force of the digit flexors is higher than 10% MVC.

Mechanism of the exercise hyperkalemia: an alternate hypothesis

A progressive hyperkalemia is observed as exercise intensity increases. The current most popular hypothesis for the hyperkalemia is that the Na+-K+ pump cannot keep pace with the K+ efflux from muscle during the depolarization-repolarization process of the sarcolemmal membrane during muscle contraction. In this report, we present data that suggest an alternate hypothesis to those previously described. Because phosphocreatine (PCr) is a highly dissociated acid and creatine is neutral at cell pH, the concentration of nondiffusible anions decreases, and an alkaline reaction takes place when PCr hydrolyzes. This creates a state of cation (K+) excess and H+ depletion in the cell. To examine the balance of K+ and H+ for exercising muscle during the early period of exercise when PCr changes most rapidly, catheters were inserted into the brachial artery and femoral vein (FV) in five healthy subjects who performed two 6-min cycle ergometer exercise tests at 40 and 85% of peak oxygen uptake. FV blood was sampled every 5 s during the first 2 min, then every 30 s for the remaining 4 min of exercise and the first 3 min of recovery, and then less frequently for the next 12 min. Arterial sampling was every 30 s during exercise and simultaneous with FV sampling during recovery. Arterial K+ concentration ([K+]) increase lagged FV [K+] increase. The hyperkalemia observed during early exercise results from K+ release from skeletal muscle. FV [K+] increased by 5 s of the start of exercise and followed the rate of H+ loss from the FV blood for the first 30 s of exercise. FV lactate and Na+ kinetics differed from K+ kinetics during exercise and recovery. As predicted from the PCr hydrolysis reaction, the exercising limb took up H+ and released K+ at the start of exercise (first 30 s) at both exercise intensities, resulting in a FV metabolic alkalosis. K+ release was essentially complete by 3 min, the time at which oxygen uptake (and, presumably, PCr) reached its asymptote. These findings lead us to hypothesize that the early K+ release by the cell takes place with H+ exchange and that the major mechanism for the exercise hyperkalemia is the reduction in nondiffusible intracellular anions in the myocyte as PCr hydrolyzes.

The relationship between the pressor response to involuntary isometric exercise and the contractile protein profile of the active muscle in man

The present investigation examined the relationship between the pressor response during electrically evoked isometric ankle plantar flexion and the contractile protein profile of the active muscle in seven young men [mean (SD) age, 26 (6) years] and five older men [70 (4) years]. Muscle biopsy samples were taken from lateral gastrocnemius (LG) and soleus (SOL) of each subject. These were analysed for isomyosin composition using non-denaturing pyrophosphate polyacrylamide gel electrophoresis. The degree of association was examined between the cardiovascular changes and the fast isomyosin content of LG and SOL individually and in combination (SOL/LG). In the total subject group there was no association between the heart rate response or the change in systolic blood pressure (BP) and the fast isomyosin composition. However, the change in diastolic BP was significantly associated with the fast isomyosin composition of SOL/LG (delta diastolicBP = 0.31 + 0.045% FM SOL/LG, r = 0.65, P = 0.029). These findings suggest that the magnitude of the peripheral reflex mediated pressor response to isometric exercise and the fast isomyosin content of the active muscle are related.

Differences in muscle sympathetic nerve response to isometric exercise in different muscle groups

The aim of this study was to examine the effects of muscle fibre composition on muscle sympathetic nerve activity (MSNA) in response to isometric exercise. The MSNA, recorded from the tibial nerve by a microneurographic technique during contraction and following arterial occlusion, was compared in three different muscle groups: the forearm (handgrip), anterior tibialis (foot dorsal contraction), and soleus muscles (foot plantar contraction) contracted separately at intensities of 20%, 33% and 50% of the maximal voluntary force. The increases in MSNA relative to control levels during contraction and occlusion were significant at all contracting forces for handgrip and at 33% and 50% of maximal for dorsal contraction, but there were no significant changes, except during exercise at 50%, for plantar contraction. The size of the MSNA response correlated with the contraction force in all muscle groups. Pooling data for all contraction forces, there were different MSNA responses among muscle groups in contraction forces (P = 0.0001, two-way analysis of variance), and occlusion periods (P = 0.0001). The MSNA increases were in the following order of magnitude: handgrip, dorsal, and plantar contractions. The order of the fibre type composition in these three muscles is from equal numbers of types I and II fibres in the forearm to increasing number of type I fibres in the leg muscles. The different MSNA responses to the contraction of different muscle groups observed may have been due in part to muscle metaboreflex intensity influenced by their metabolic capacity which is related to by their metabolic capacity which is related to the fibre type.