Heart rate and ventilation in relation to venous [K+], osmolality, pH, PCO2, PO2, [orthophosphate], and [lactate] at transition from rest to exercise in athletes and non-athletes

Cardiac acceleration at the onset of exercise: a potential parameter for monitoring progress during physical training in sports and rehabilitation

There is a need for easy-to-use methods to assess training progress in sports and rehabilitation research. The present review investigated whether cardiac acceleration at the onset of physical exercise (HRonset) can be used as a monitoring variable. The digital databases of Scopus and PubMed were searched to retrieve studies investigating HRonset. In total 652 studies were retrieved. These articles were then classified as having emphasis on HRonset in a sports or rehabilitation setting, which resulted in 8 of 112 studies with a sports application and 6 of 68 studies with a rehabilitation application that met inclusion criteria. Two co-existing mechanisms underlie HRonset: feedforward (central command) and feedback (mechanoreflex, metaboreflex, baroreflex) control. A number of studies investigated HRonset during the first few seconds of exercise (HRonsetshort), in which central command and the mechanoreflex determine vagal withdrawal, the major mechanism by which heart rate (HR) increases. In subsequent sports and rehabilitation studies, interest focused on HRonset during dynamic exercise over a longer period of time (HRonsetlong). Central command, mechanoreflexes, baroreflexes, and possibly metaboreflexes contribute to HRonset during the first seconds and minutes of exercise, which in turn leads to further vagal withdrawal and an increase in sympathetic activity. HRonset has been described as the increase in HR compared with resting state (delta HR) or by exponential modeling, with measurement intervals ranging from 0-4 s up to 2 min. Delta HR was used to evaluate HRonsetshort over the first 4 s of exercise, as well as for analyzing HRonsetlong. In exponential modeling, the HR response to dynamic exercise is biphasic, consisting of fast (parasympathetic, 0-10 s) and slow (sympathetic, 1-4 min) components. Although available studies differed largely in measurement protocols, cross-sectional and longitudinal training studies showed that studies analyzing HRonset in relation to physical training primarily incorporated HRonsetlong. HRonsetlong slowed in athletes as well as in patients with a coronary disease, who have a relatively fast HRonsetlong. It is advised to include both HRonsetlong and HRonsetshort in further studies. The findings of this review suggest that HRonset is a potential tool for monitoring and titrating training in sports as well as in rehabilitation settings, particularly in patients with ventricular fibrillation. Monitoring HRonset in the early phase of training can help optimize the effectiveness of training and therapy. More research is needed to gain a better understanding of the mechanisms underlying HRonset in relation to their application in sports and rehabilitation settings.

Control of breathing during exercise

During exercise by healthy mammals, alveolar ventilation and alveolar-capillary diffusion increase in proportion to the increase in metabolic rate to prevent PaCO2 from increasing and PaO2 from decreasing. There is no known mechanism capable of directly sensing the rate of gas exchange in the muscles or the lungs; thus, for over a century there has been intense interest in elucidating how respiratory neurons adjust their output to variables which can not be directly monitored. Several hypotheses have been tested and supportive data were obtained, but for each hypothesis, there are contradictory data or reasons to question the validity of each hypothesis. Herein, we report a critique of the major hypotheses which has led to the following conclusions. First, a single stimulus or combination of stimuli that convincingly and entirely explains the hyperpnea has not been identified. Second, the coupling of the hyperpnea to metabolic rate is not causal but is due to of these variables each resulting from a common factor which link the circulatory and ventilatory responses to exercise. Third, stimuli postulated to act at pulmonary or cardiac receptors or carotid and intracranial chemoreceptors are not primary mediators of the hyperpnea. Fourth, stimuli originating in exercising limbs and conveyed to the brain by spinal afferents contribute to the exercise hyperpnea. Fifth, the hyperventilation during heavy exercise is not primarily due to lactacidosis stimulation of carotid chemoreceptors. Finally, since volitional exercise requires activation of the CNS, neural feed-forward (central command) mediation of the exercise hyperpnea seems intuitive and is supported by data from several studies. However, there is no compelling evidence to accept this concept as an indisputable fact.

Computational analyses of CO-rebreathing methods for estimating haemoglobin mass in humans

Measurement of haemoglobin mass (M(Hb)) is used to quantify alterations in oxygen delivery during exercise training or acclimatization to altitude. Uptake of carbon monoxide by haemoglobin is the basis of the common non-radioactive methods to determine M(Hb) in humans. This study used a validated mathematical model to simulate CO uptake during rebreathing protocols and to determine sources of errors in estimation of M(Hb). Our previously published model was validated using experimentally measured carboxyhaemoglobin levels (%HbCO) from arterial, capillary and venous blood sites of human subjects during CO-rebreathing protocols. This model was then used to simulate various CO-rebreathing protocols in 24 human subjects with known M(Hb). Using variables generated by the model, M(Hb) was estimated on the basis of assumptions typically made for calculating the volume of CO bound to myoglobin, the volume of CO exhaled and the volume of CO in the rebreathing system. It was found that inaccurate estimation of the volume of CO bound to myoglobin was the major source of error in determination of M(Hb). Additionally, the size of the error was found to depend on the site of blood sampling because of differences in %HbCO. Regression equations were developed to improve the estimation of volume of CO bound to myoglobin, and a new protocol that is less dependent on the site of blood sampling is proposed.

Relationship of potassium ions and blood lactate to ventilation during exercise

Ventilatory control during exercise is a complex network of neural and humoral signals. One humoral input that has received little recent attention in the exercise literature is potassium ions [K(+)]. The purpose of this study was to examine the relationship between [K(+)] and ventilation during an incremental cycle test and to determine if the relationship between [K(+)] and ventilation differs when blood lactate [lac-] is manipulated. Eight experienced triathletes (4 of each sex) completed 2 incremental, progressive (5-min stages) cycle tests to volitional fatigue: 1 with normal glycogen stores and 1 with reduced glycogen. Minute ventilation was measured during the final minute of each stage, and blood [lac(-)] and [K+] were measured at the end of each exercise stage. Minute ventilation and [K(+)] increased with exercise intensity and were similar between trials (p > 0.5), despite lower [lac-] during the reduced-glycogen trial. The concordance correlations (R(c)) between [lac(-)] and minute ventilation were stronger for both trials (R(c) = approximately 0.88-0.96), but the slopes of the relationships were different than the relationships between [K(+)] and minute ventilation (R(c) = approximately 0.76-0.89). The slope of the relationship between [lac-] and minute ventilation was not as steep during the reduced-glycogen trial, compared with the normal trial (p = 0.002). Conversely, the slope of the relationships between [K(+)] and minute ventilation did not change between trials (p = 0.454). The consistent relationship of minute ventilation and blood [K(+)] during exercise suggests a role for this ion in the control of ventilation during exercise. Conversely, the inconsistent relationship between blood lactate and ventilation brings into question the importance of the relationship between lactate and ventilation during exercise.

EMG signs of neuromuscular fatigue related to the ventilatory threshold during cycling exercise

We questioned whether electromyographic (EMG) signs of neuromuscular fatigue accompany the changes in respiratory variables measured at the ventilatory threshold (VTh) during exercise on a cycloergometer. This was based on the assumption that the activation of muscle afferents sensitive to accumulation of lactate and potassium is suspected to elicit both the EMG signs of fatigue and hyperventilation. In 39 subjects performing an incremental cycling, the EMG estimates of neuromuscular fatigue in vastus lateralis were a non-linear increase in root mean square (RMS), a decrease in median frequency (MF), a non-linear increase in low-frequency EMG energies (EL), and/or a decrease in high-frequency energies (EH). VTh was determined from a non-linear increase in VCO2 [VTh(VCO2 slope)] and an increased value of the respiratory equivalent for oxygen [VTh(VE/VO2)]. We measured a significant increase in venous blood concentration of lactate and potassium, and a significant pHv fall at VTh. One EMG estimate of fatigue was detected in 33/39 individuals and two EMG estimates in 17 subjects. Highly significant positive correlations were found between the oxygen uptakes corresponding to each EMG estimate and to each detection criterion of VTh. These observations suggest that the activation of muscle sensory pathways contribute to the mechanism of VTh.

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

Exercise limitation in chronic heart failure: central role of the periphery

The symptoms of chronic heart failure (CHF) are predominantly shortness of breath and fatigue during exercise and reduced exercise capacity. Disturbances of central hemodynamic function are no longer considered to be the major determinants of exercise capacity. The two symptoms of fatigue and breathlessness are often considered in isolation. A pulmonary abnormality is usually considered to be the cause of abnormal ventilation, and increased dead space ventilation has come to be accepted as a major cause of the increased ventilation relative to carbon dioxide production seen in CHF. Rather than decreased skeletal muscle perfusion, an intrinsic muscle abnormality is considered to be responsible for fatigue. Another abnormality seen in CHF is persistent sympathetic nervous system activation, which is difficult to explain on the basis of baroreflex activation. There is increasing evidence for the importance of skeletal muscle ergoreceptors or metaboreceptors in CHF. These receptors are sensitive to work performed, and activation results in increased ventilation and sympathetic activation. The ergoreflex appears to be greatly enhanced in CHF. We put forward the "muscle hypothesis" as an explanation for many of the pathophysiologic events in CHF. Impaired skeletal muscle function results in ergoreflex activation. In turn, this causes increased ventilation, thus linking the symptoms of breathlessness and fatigue. Furthermore, ergoreflex stimulation may be responsible for persistent sympathetic activation.

Peripheral and central chemoreceptor control of ventilation during exercise in humans

The stability of arterial blood gas tensions and pH during steady-state moderate exercise has suggested an important humoral element of ventilatory control in humans. However, the involvement of central and peripheral chemoreflexes in this humoral control remains controversial. This reflects, in large part, technical and interpretational limitations inherent in currently used estimators of chemoreflex "sensitivity." Evidence suggests that the central chemoreceptors (a) contribute little during moderate exercise, given the relative stability of cerebrospinal pH, (b) constrain the hyperpnea of high-intensity exercise, consequent to the respiratory compensation for the metabolic acidemia, and (c) may play a role in the respiratory compensation during chronic metabolic acidemia. In contrast, the peripheral chemoreceptors appear to (a) exert considerable influence on ventilatory kinetics in moderate exercise, but are less important in the steady state, and (b) induce much of the respiratory compensation of high-intensity exercise.

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

Six healthy men performed sustained static handgrip exercise for 2 min at 40% maximal voluntary contraction followed by a 6-min recovery period. Heart rate (fc), arterial blood pressures, and forearm blood flow were measured during rest, exercise, and recovery. Potassium ([K+]) and lactate concentrations in blood from a deep forearm vein were analysed at rest and during recovery. Mean arterial pressure (MAP) and fc declined immediately after exercise and had returned to control levels about 2 min into recovery. The time course of the changes in MAP observed during recovery closely paralleled the changes in [K+] (r = 0.800, P < 0.01), whereas the lactate concentration remained elevated throughout the recovery period. The close relationship between MAP and [K+] was also confirmed by experiments in which a 3-min arterial occlusion period was applied during recovery to the exercised arm by an upper arm cuff. The arterial occlusion affected MAP while fc recovered at almost the same rate as in the control experiment. Muscle biopsies were taken from the brachioradialis muscle and analysed for fibre composition and capillary supply. The MAP at the end of static contraction and the [K+] appearing in the effluent blood immediately after contraction were positively correlated to the relative content of fast twitch (% FT) fibres (r = 0.886 for MAP vs % FT fibres, P < 0.05 and r = 0.878 for [K+] vs % FT fibres, P < 0.05). Capillary to fibre ratio showed an inverse correlation to % FT fibres (r = -0.979, P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Ventilatory response and arterial potassium concentration during incremental exercise in patients with chronic airways obstruction

The relationship of ventilation response (VE) to arterial potassium concentration (K+) during ramp incremental exercise was assessed in nine patients with chronic obstructive pulmonary disease (COPD), and in 10 healthy subjects. For COPD patients the maximum oxygen uptake (VOmax) was 19.6 +/- 3.8 ml kg-1 min-1 (+/- SD), and percentage of forced expired volume at 1 s (% FEV1) was 47.8 +/- 10.4%. In healthy subjects, VO2max was 44.4 +/- 7.0 ml kg-1 min-1 and FEV1 was 89.7 +/- 7.4%. Breath-by-breath determinations for VE, oxygen uptake (VO2) and carbon dioxide output (VCO2), as well as determinations for K+, partial pressure of oxygen (PO2), partial pressure of carbon dioxide (PCO2), pH and lactate in arterial blood were performed during a workout on an exercise bicycle at a ramp function work rate of 20 W min-1, preceded by a 40 min warm-up period. The major findings in the present study are: (1) that there is a linear relation between ventilation and arterial K+ concentration during ramp exercise in both healthy subjects and COPD patients; (2) that the slope of the VE-K+ relationship is significantly lower in COPD patients (16.2 +/- 7.3 l min-1 mM-1) than in normal subjects (37.4 +/- 6.9 l min-1 mM-1, P less than 0.01); and, (3) that the slope of the VE-K+ relationship is significantly related to the ability to ventilate during maximal exercise in both healthy subjects and COPD patients (P less than 0.05). It is thought that the significantly reduced slope of the VE-K+ relationship in the COPD patients could be interpreted as a reduced sensitivity to the stimulus and/or as a mechanical impairment of the ventilation.

Effect of local anaerobiosis on heart rate

The isolated leg of a rat was connected to the body only by nerve and bone and was perfused with hypoxic Tyrode solution. Heart rate increased when metabolic parameters (PCO2, pH and lactate) reached values similar to those observed at the beginning of exercise. When the muscle was additionally stimulated by electric stimuli a significant temporal correlation between lactate and heart or respiratory rate was found. Metabolic changes caused by hypoxia and muscular contraction, in particular lactic acid, appear to act as chemical stimuli for metabolic muscle receptors participating in the generation of circulatory and respiratory responses to physical exercise.

Role of arterial pressure in the oedema of heart disease

The primary function of the heart is maintenance of the arterial pressure that is essential for perfusion of the tissues. Cardiac output is mainly regulated by control systems acting through the arterial pressure, whereby the heart matches its output to the perfusion of the tissues, in which blood is distributed according to local vascular resistances. When the heart is disabled, the body intensifies to an abnormal degree the renal, nervous, and endocrine mechanisms through which arterial pressure is maintained. As a result, the kidneys retain excess salt and water; the volume of extracellular liquid, including the plasma, expands; and the systems that limit the blood volume are overridden.

Respiratory and cardiac responses to exercise-simulating peripheral perfusion in endurance trained and untrained rats. II. Temporal relationships between outflow parameters and cardiac and respiratory responses

In endurance trained (TR) and untrained (UTR) rats heart rate (HR) and respiratory rate (RR) were recorded during perfusion of the circulatorily isolated hind leg of the rat with exercise simulating modified tyrode solutions (TR:n = 10, UTR:n = 10; compare part I). During the 20 min test period and the preceding and succeeding periods of control perfusions with an unmodified tyrode solution, [lactate], pH, [K+], [Na+], PO2 and PCO2 were measured in the outflow of the femoral vein. In 3 experimental series: (1) hypoxic tyrode solution enriched with lactic acid (15 mmol.l-1), (2) normoxic solution with lactic acid, (3) hypoxic solution without lactic acid, were applied. The outflow parameters were cross correlated with both HR and RR. The analysis revealed a significant temporal relationship between [lactate], pH, PO2, PCO2 and [K+] and both HR and RR. In the trained rats no temporal correlation between either of the outflow and reflex parameters could be determined. This result was not due to low [lactate], but was also found during perfusion with lactic acid. In all 3 test conditions [lactate] in untrained individuals was best correlated with both HR and RR. Although the correlation peaks of the respiratory response, but not of the HR response were definitely lower in normoxic lactic and perfusion than in the two other experimental conditions, both inter- and intraindividual correlation analyses revealed a high degree of interdependence between respiratory and cardiac responses.

Congestive cardiac failure: central role of the arterial blood pressure

A review of the history of our knowledge and understanding of the peripheral oedema of congestive cardiac failure points to the conclusion that an inability of the heart to maintain the arterial pressure is of central importance in this condition. Although the function of the circulation is to perfuse the tissues, the body monitors the adequacy of its perfusion, not not through metabolic messengers carried from the tissues in the blood stream, but by sensing the arterial pressure; and the mechanisms evoked act to maintain the arterial pressure. In the short term this is achieved by autonomic regulation of the heart and blood vessels; in the longer term the arterial pressure is maintained through an increase in the blood volume by a retention of salt and water by the kidney. To support the latter process, intrinsic renal mechanisms are successively magnified by the renin-angiotensin-aldosterone system and by the activities of the sympathetic system and vasopressin. The natriuretic influence mediated through volume receptors and the release of atrial peptide is overruled by the arterial baroreceptors, so that the body maintains the arterial pressure at the expense of an increase in blood volume. In these ways the syndrome of congestive cardiac failure may be regarded as one which arises when the heart becomes chronically unable to maintain an appropriate arterial pressure without support.

Adrenergic modulation of potassium metabolism during exercise in normal and diabetic humans

The effect of acute and chronic beta- and alpha-adrenergic blockade on potassium homeostasis during moderate intensity exercise (40% VO2max) was investigated in control and insulin-dependent diabetic subjects. In protocol I, subjects were studied during exercise alone, exercise plus intravenous propranolol, and exercise plus intravenous phentolamine. In both the control and diabetic groups, exercise alone produced a modest increase in the plasma potassium concentration (0.31 +/- 0.06 meq/l), while propranolol exacerbated this hyperkalemic response. In contrast, the increment in plasma potassium during phentolamine was similar to exercise alone in normals but was 26% (P less than 0.05) lower in the diabetic group. In protocol II, the effect of chronic (5 days) beta-adrenergic blockade on potassium homeostasis was examined. Subjects participated in three studies: exercise alone, exercise plus propranolol (beta 1/beta 2-antagonist), and exercise plus metoprolol (beta 1 antagonist). In the nondiabetic group, both propranolol and metoprolol were associated with a 40% greater increase in potassium compared with exercise alone. In the diabetic group, propranolol, but not metoprolol, was associated with a deterioration in potassium tolerance. In no study could the alterations in potassium homeostasis be explained by a change in urinary potassium excretion. In summary, alpha-adrenergic blockade ameliorates exercise-induced hyperkalemia in diabetic but not in control subjects, nonspecific beta-adrenergic blockade causes a greater increment in potassium when compared with exercise alone, and specific beta 1-adrenergic blockade exacerbates exercise-induced hyperkalemia in control, but not in diabetic subjects. These results indicate that both alpha- and beta-adrenergic regulation of extrarenal potassium metabolism is altered in insulin-dependent diabetes mellitus.

Influence of inspired oxygen concentration on the dynamics of the exercise hyperpnoea in man

In order to determine the role of the carotid bodies on the ventilatory control characteristics during the non-steady-state phase of exercise in man, six normal males performed cycle ergometry with four repetitions of a 6 min, constant-load work bout at inspired O2 fractions (FI,O2) of 0.12, 0.15, 0.21, 0.30 and 1.00. Each test began with unloaded pedalling; this was followed by a constant load which was 90% of the subject's anaerobic threshold at FI,O2 = 0.12. Ventilation (VE), CO2 output (VCO2) and O2 uptake (VO2) were determined breath-by-breath during the test and the time constants of response (tau VE, tau VCO2 and tau VO2) were established by least-squares techniques, following interpolation (1 s), temporal alignment and averaging of the four responses. In each subject, tau VE and tau VCO2 increased as functions of increasing FI,O2, and were inverse functions of the proportional contribution to VE of peripheral chemoreceptor drive (as estimated from hyperoxic-transition or 'Dejours' tests). tau VE averaged 40 s at FI,O2 = 0.12 and 112 s at FI,O2 = 1.00, each response being well fitted by a single exponential. However, tau VO2 was not significantly affected by the alterations in FI,O2. Although there was no discernible peripheral chemosensitivity at FI,O2 = 0.30 or 1.00, the tau VE increased appreciably between these inspirates. We therefore conclude that the peripheral chemoreceptors are important, but not exclusive determinants of the exponential response characteristics during the non-steady-state phase of the exercise hyperpnoea in man. This supports the contention of a component of the control being humorally mediated even during moderate exercise.

Blood flow, PO2, PCO2 and pH during progressive working contractions in a whole muscle group

Alterations in blood flow during progressive working contractions were examined to elucidate their relation to work rate in a predominantly glycolytic muscle group, i.e., m. gastrocnemius and m. plantaris, in rabbits anesthetized with urethane and chloralose. In one series of animals, the sciatic nerve was stimulated to induce plantar flexions of constant length at 2, 5 and 8% of an afterload at which only isometric tension could be developed. Another series was exercised at 30 and 50% of this value, and a third group served as non-exercised controls. Each experimental session consisted of a series of 5 min non-exercise periods followed by 6 min exercise periods, and a 10 min post-exercise period. Femoral venous blood was obtained just before the first exercise period, during the final minute of each exercise period, and 10 min after the final exercise period. The composition of venous blood samples from control animals did not change during the experimental session. Blood flow in the exercising limb increased at the lowest workload, and attained a maximum flow rate at the 5% workload. Blood gases were altered to a similar extent at all afterloads, averaging: PO2 = 4.0 +/- 0.2 kPa and PCO2 = 7.5 +/- 0.3 kPa. pH, in contrast, was lower at the heaviest afterloads (X = 7.144 +/- 0.03) compared to the lighter afterloads (X = 7.245 +/- 0.03). The blood flow and pH patterns are consistent with the glycolytic fiber type composition of this muscle group. Venous PO2 indicates that O2 delivery was adequate, even at the highest afterload.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of exercise training on the ventilatory threshold of patients with coronary heart disease

Out of 156 patients with stable coronary heart disease randomized to either an exercise intervention group or a control group, 41 had complete gas analysis data. Continuous gas exchange data, including the ventilatory threshold, and selected heart rates were determined initially and at 1 year. The mean attendance for the exercise group was 2.2 +/- 0.7 days a week at an intensity of 60 +/- 9% of estimated peak oxygen uptake for 1 year of the study. Statistically significant differences (p less than 0.05) were observed between the exercise group (n = 19) and the control group (n = 22) for peak oxygen uptake (L/min), total treadmill time, and supine rest and submaximal heart rates after 1 year. The most remarkable change was a 16% increase in treadmill time. There was no difference between groups for the ventilatory threshold expressed either as an absolute oxygen uptake or as a percentage of peak oxygen uptake at 1 year. However, there was a significant correlation (r = 0.45; p less than 0.05) between the absolute change in peak oxygen uptake and the absolute change in the ventilatory threshold. These results indicate that a moderate exercise program is inadequate to alter the ventilatory threshold in patients with coronary heart disease and that changes in ventilatory threshold do not explain the increase in treadmill time that usually occurs.

Changes in venous blood content from active and inactive hindlimb during isotonic exercise

Venous blood samples were obtained from either exercising (n = 9) or nonexercising (n = 8) hindlimb during a progressive isotonic exercise in rabbits anesthetized with urethane and chloralose. Each experimental session consisted of 5-min nonexercise periods alternated with 6-min exercise periods, followed by a 10-min postexercise period. During each exercise period, stimulation of the distal stump of the right sciatic nerve at 1 Hz induced plantar flexions which lifted loads comparable to 2, 5, 8, 30, or 50% of an afterload at which only an isometric tension developed. Free-flowing venous blood samples were obtained before the first exercise period, during the last minute of each exercise period, and 10 min following the last exercise session. Increases in [Na+], [K+] and lactate concentration were obtained in blood from active limbs. Only lactate concentration increased in blood from nonexercising limbs, while [K+] decreased slightly. Inferences concerning the vascular volume response to this protocol would be quite different depending on the blood sampling site. Changes in blood from inactive tissue, further, may indicate only saturation of homeostatic mechanisms which normally compensate for vascular volume alterations initiated in active tissue.

Influence of exposure to moderate altitude on the plasma concentraton of cortisol, aldosterone, renin, testosterone, and gonadotropins

The influence of 11 days at moderate altitude (2,000 m) combined with exercise on plasma concentration of testosterone, FSH (follicle-stimulating hormone), LH (luteinizing hormone), cortisol, aldosterone, and renin activity was studied in ten healthy subjects. Within 48 h of arrival at moderate altitude a significant increase in testosterone was found whereas FSH had decreased significantly and LH showed a tendency to decrease. Cortisol increased significantly at the beginning and reached a maximum at the end of altitude exposure. The plasma aldosterone level rose continuously and on the last day of altitude was significantly elevated. Plasma renin activity showed a tendency to decrease. On return to low land all measured parameters returned to base line values within 2 days. The findings of increases in plasma levels of aldosterone and testosterone (and serum T3 and T4, as reported by others) are in contrast to the previously found decrease of urinary excretion of all these hormones. This appears to be a distinct dissociation of serum levels of adrenal (and thyroid) hormones from their urinary excretion. The observed increase in plasma aldosterone is probably mediated through ACTH and the rise in plasma potassium, since plasma renin activity showed an opposite trend. The rise in plasma testosterone is probably of adrenal origin since plasma gonadotropins declined simultaneously. The increase of plasma levels of glucocorticoids, mineralocorticoids, and androgens after an ascent from 600 m to 2,000 m above sea level is compatible with an ACTH-mediated stimulation of the entire adrenal cortex and/or a diminished elimination of adrenal steroids: The concomitant fall of FSH, LH, and plasma renin would then be a consequence of a direct negative feedback inhibition of these hormones.