A theory on heart rate control by muscular metabolic receptors

Le rôle joué par les fibres afférentes métabosensibles dans les mécanismes adaptatifs neuromusculaires

Les adaptations de l'organisme à l'exercice sont permises par l'ajustement de l'activité des neurones centraux qui est en partie régulée par l'activité des afférences I et II (mécanosensibles), par la mise en jeu des afférences III et IV (métabosensibles) et par les modifications du métabolisme musculaire au cours de l'exercice. Le rôle des afférences métabosensibles apparaît comme fondamental dans les mécanismes adaptatifs à l'exercice et dans la tolérance à la fatigue. Néanmoins, de nombreuses interrogations demeurent. Cette revue fait le bilan des connaissances concernant l'implication de ces afférences dans les boucles de rétrocontrôle sensori-motrices et les mécanismes d'adaptation neuromusculaire. Il semble désormais établi que l'activation des afférences métabosensibles soit à l'origine de l'adaptation cardiovasculaire et respiratoire à l'exercice. De plus, ces afférences seraient à l'origine d'un mécanisme de protection du muscle contre la fatigue en modulant la commande motrice centrale au niveau spinal et supraspinal.

Recommendations for resistance exercise in cardiac rehabilitation. Recommendations of the German Federation for Cardiovascular Prevention and Rehabilitation

Aerobic endurance training has been an integral component of the international recommendations for cardiac rehabilitation for more than 30 years. Notwithstanding, only in recent years have recommendations for a dynamic resistance-training program been cautiously put forward. The perceived increased risk of cardiovascular complications related to blood pressure elevations are the primary concern with resistance training in cardiac patients; recent studies however have demonstrated that this need not be a contraindication in all cardiac patients. While blood pressure certainly may rise excessively during resistance training, the actual rise depends on a variety of controllable factors including magnitude of the isometric component, the load intensity, the amount of muscle mass involved as well as the number of repetitions and/or the load duration. Intra-arterial blood pressure measurements in cardiac patients have demonstrated that that during low-intensity resistance training [40-60% maximum voluntary contraction (MVC)] with 15-20 repetitions, only modest elevations in blood pressure are revealed, similar to those seen during moderate endurance training. When properly implemented by an experienced exercise therapist, in specific patient groups an individually tailored, medically supervised dynamic resistance training program carries no inherent higher risk for the patient than aerobic endurance training. As an adjunct to endurance training, in selected patients, resistance training can increase muscle strength and endurance, as well as positively influence cardiovascular risk factors, metabolism, cardiovascular function, psychosocial well-being and quality of life. According to present data, resistance training is however not recommended for all patient groups. The appropriate training method and correct performance are highly dependent on each patient's clinical status, cardiac stress tolerance and possible comorbidities. Most studies have used middle-aged men of average normal aerobic performance capacity and with good left-ventricular (LV) function. Data are lacking for high-risk groups, women and older patients. With the current knowledge it is reasonable to include resistance training without any restraints as part of cardiac rehabilitation programs for coronary artery disease (CAD) patients with good cardiac performance capacity (i.e., revascularised and with good myocardial function). As patients with myocardial ischaemia and/or poor left ventricular function may develop wall motion disturbances and/or severe ventricular arrhythmias during resistance exercise, the following criteria are suggested for resistance training: moderate-to-good LV function, good cardiac performance capacity [>5-6 metabolic equivalents of oxygen consumption (METS)=1.4 watt/kg body weight], no symptoms of angina pectoris or ST segment depression under continued maintenance of the medical therapy. Based on available data, this article presents recommendations for risk stratification in cardiac rehabilitation programs with respect to the implementation of dynamic resistance training. Additional recommendations for specific patient groups and detailed directions showing how to structure and implement such therapy programs are presented as well.

Muscle metaboreflex control of the circulation during exercise

This review covers the control of blood pressure, cardiac output and muscle blood flow by the muscle metaboreflex which involves chemically sensitive nerves located in muscle parenchyma activated by metabolites accumulating in the muscle during contraction. The efferent response to metaboreflex activation is an increase in sympathetic nerve activity that constricts the systemic vasculature and also evokes parallel inotropic and chronotropic effects on the heart to increase cardiac output. The metaboreflex elicits a significant blood pressure elevating response during exercise and functions to redistribute blood flow and blood volume. Regional specificity in the efferent response to the metaboreflex activated from either the leg or the arm is seen in the balance between signals for vasoconstriction to curtail blood flow and signals to increase cardiac output. The metaboreflex has dual functions. It can both elevate and decrease muscle blood flow depending on (1) the intensity and mode of contraction, (2) the limb in which the reflex is evoked, (3) the strength of the signal defined by the muscle mass, (4) the extent to which blood flow is redistributed from inactive vascular beds to increase central blood volume and (5) the extent to which cardiac output can be increased.

Métabosensibilité musculaire et adaptations physiologiques au cours de l’exercice

The first role played by group III (thin myelinated) and group IV (unmyelinated) afferent fibres from skeletal muscle is to transmit nociceptive information from muscle to the central nervous system. The second role of these free endings located in the interstitium of the muscle is to induce cardiovascular and respiratory adjustments during muscular exercise. These respiratory and circulatory responses during muscular exercise may be reflexly induced via muscular afferents. Indeed, static contraction of hindlimb muscles in anaesthetised mammals has been shown to reflexly increase the ventilatory function, the myocardial contractility and heart rate. The mechanical muscle deformation and the accumulation of metabolites in its intersitium are the cause of raised activity in small nerve fibres which in turn induces the physiological responses. It is also admitted that the central locomotor areas on the medullary and spinal neuronal pools control ventilatory and cardiovascular function during exercise. This mechanism is called "central command". Furthermore, adjustments of the locomotor activity during exercise is mediated by the thinly myelinated and unmyelinated fibres with endings in the working muscle. These fibres, also called "metaboreceptor" may be responsible of the coupling between the ventilation and the locomotion. Thickly myelinated muscle afferents (i.e. group I and II) appear to play little role in causing the reflex autonomic responses to contraction.

Comparison of blood pressure and heart rate responses to isometric exercise and passive muscle stretch in humans

The responses of mean arterial blood pressure (BPa) and heart rate (fc) to isometric contraction and passive stretch were compared in seven healthy male subjects at identical external forces. They were investigated in the sitting position with the hip and knee joint flexed to 90 degrees. Each subject performed two tests, separated by a day, in which the stimuli were applied in random order. After 5 min of rest they performed either 10-min static plantar flexion of one calf (200 N) or 10 min of passive calf muscle stretch at the same load. After 5-min rest, the second stimulus was applied for a further 10 min followed by 5-min rest. The second test was identical except for the sequence of the stimuli. The BPa was measured by a noninvasive and continuous method. Contraction of the vastus lateralis, gastrocnemius lateralis, and soleus muscles were determined by the myo-electric activity (electromyogram, EMG) by means of surface electrodes. The EMG activity of the vastus lateralis muscle remained at resting values throughout the experiments. Increases in EMG activity could only be detected for the triceps surae muscles during isometric contraction. During the initial 2 min of stimulation the BPa and fc responses to active contraction and passive stretch were comparable. Thereafter, both parameters showed significantly higher values during contraction. It was concluded that mechanical stress may have contributed to the early response of BPa during both passive stretch and voluntary contraction but that chemical stimuli were needed to maintain the peripheral cardiovascular drive.

Effect of graded changes in extracellular muscle volume on cardiovascular drives during static exercise

The effects of graded changes in peripheral extracellular volume on heart rate and blood pressure during isometric exercise were studied in 12 healthy male subjects. Each subject performed four calf ergometer tests with each calf. In all tests, static plantar flexion of one foot was performed in a supine body position with the knee joint flexed to 90 degrees. After a pre-exercise period of 18 min, during which the calf volume was manipulated, the subjects had to counteract a spring force of 120 N for 8 min. In the pre-exercise period the peripheral extracellular volume of the calf muscle to be tested was manipulated in four ways. Test 1:15 min of rest in the exercise position. During the last 3.5 min, the calf volume was increased by venous congestion [80 mmHg (10.67 kPa) applied to the distal part of the thigh by pneumatic cuff]. Test 2: the same protocol as in test 1 but with 7.5-min venous congestion. Test 3:15 min of venous congestion. Test 4: the calf volume was decreased by a negative hydrostatic pressure for 15 min (calf raised about 40 cm above heart level). To clamp the changed calf volume, the thigh cuff was rapidly inflated to 300 mmHg (40.0 kPa) at the end of the volume manipulation and the subjects remained resting for a further 3 min. In test 4, the leg of the subject was passively brought into the exercise position. The occlusion was maintained until 2 min after exercise. The calf volume manipulation led to changes ranging from +105 ml (test 3) to -134 ml (test 4) as measured by water displacement plethysmography. The blood pressure response to exercise was inversely related to the calf volume changes while the heart rate response during exercise showed no clearcut relationship to the pretreatments.

Changes in extracellular muscle volume affect heart rate and blood pressure responses to static exercise

To investigate the effect of microgravity-induced peripheral extracellular fluid reductions on heart rate and blood pressure during isometric exercise, six healthy male subjects performed three calf ergometer tests with different extracellular volumes of working muscles. In all tests, body positions during exercise were identical (supine with the knee joint flexed to 90 degrees). After a pre-exercise period of 25 min, during which calf volumes were manipulated, subjects had to counteract an external force of 180 N for 5 min. During the pre-exercise period three different protocols were applied. Test A: Subjects rested in the exercise position; test B: Body position was the same as in A but calf volume was increased by venous congestion (cuffs inflated to 80 mm Hg); test C: Calf volumes were decreased by a negative hydrostatic pressure (calves about 40 cm above heart level with the subjects supine). To clamp the changed calf volumes in tests B and C, cuffs were inflated to 300 mm Hg 5 min before the onset on exercise. This occlusion was maintained until termination of exercise. Compared to tests A and B, the reduced volume of test C led to significant increases in heart rate and blood pressure during exercise. Oxygen uptake did not exceed resting levels in B and C until cuffs were deflated, indicating that exclusively calf muscles contributed to the neurogenic peripheral drive. It is concluded that changes in extracellular muscle volume have to be taken into account when comparing heart rate and blood pressure during 1g- and microgravity-exercise.

The use of isometric exercise as a means of evaluating the parasympathetic contribution to the tachycardia induced by dynamic exercise in normal man

Fourteen normal subjects were submitted to isometric exercise (IE), dynamic exercise (DE) and a combination of the two (IE + DE). The main purpose of the present study was to use IE as a means of evaluating the mechanism of the heart rate (HR) increase induced by DE. To this end, the magnitude of the IE (handgrip) was standardized so as to cause an elevation of HR almost exclusively by vagal withdrawal: IE was performed using a dynamometer strain-gauge system with a linear response at 75% of maximum voluntary contraction (MVC) for 10 s, repeated at 1 min intervals. The change in HR evoked by IE under control conditions was compared with that evoked during DE, and during the corresponding recovery period. DE was performed by the legs, with the subject in the seated position for 4 min, at workloads of 55 and 105 watts, separated by a rest period. In the combined protocol, IE was performed at the beginning of DE, as well as at 1, 2 and 3 min during DE, and at 0, 1, 2, 3 and 5 min during recovery period. The following results were obtained: (1) IE associated with DE always induced smaller increase in heart rate than IE alone, and this effect was more marked at 105 than at 55 W; this finding suggested a workload-dependent vagal withdrawal at the very beginning of DE that was sustained until the end of effort.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of muscle interstitial volume on K+-induced heart rate drive in rats

During exercise heart rate is influenced by reflexogenic drives which are elicited by receptors situated in the interstitial space. Since the structure of interstitial tissue is complex (e.g. fixed negative charges of glycosaminoglycans), the situation in the immediate surrounding of the receptors might differ from the free fluid phases of blood or lymph in which the concentrations of stimulating substances are usually determined. Physiological variations of the interstitial structure may be due to changes in interstitial volume induced by exercise or the hydrostatic effects on body fluids. The objective of the present study was to investigate the effect of the interstitial volume on the relationship between heart rate and K+ stimuli applied through the muscle blood vessels. The calves of 12 male Wistar rats were artificially perfused and separated from the rest of the body with the sciatic nerve remaining intact. In these preparations the heart rate (HR) responses to low (4 mM) and high (8 mM) potassium concentrations were determined at different interstitial volumes. Expansion of the interstitial volume was obtained by reducing the colloid-osmotic pressure of the perfusate. The combination of intracellular oedema and mechanical limitation of total volume expansion (tapeing) was utilized to decrease the interstitial volume. When switching between the low and high potassium concentrations, significant heart rate responses could be observed only with reduced interstitial volume. It is suggested that the interstitial structure surrounding the muscular receptors modifies the relationship between heart rate response and the K+ stimuli determined in blood or lymph.

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.

Heart rate, metabolic and hormonal responses to maximal psycho-emotional and physical stress in motor car racing drivers

Motor car racing is representative of concentrative sporting activities, as well as instructive for mental-concentrative and psycho-emotional stress, which predominates with lower intensity, but longer duration in occupational work of today. A group of 20 car racing drivers was investigated both during car races (Formula Ford and Renault-5-Cup) and during progressive bicycle ergometry in the laboratory. Heart rate during car racing reached a mean level of 174.3 +/- 14.1 min-1 (mean +/- SD), corresponding to 90% of the maximal heart rate achieved at the end of exhaustive ergometry (n = 12). Catecholamine excretion in urine (adrenaline + noradrenaline) on average was 252.3 +/- 77.9 ng min-1 during car racing and 121.9 +/- 37.3 ng min-1 during exhaustive ergometry (n = 10). Most of the other metabolic parameters determined in blood (lactate, glucose, FFA = free fatty acids, plasma protein, insulin, HGH = human growth hormone) also showed significant differences between car racing and bicycle ergometry (n = 20). Therefore it is possible to distinguish between psychical and physical strain and the quantify their specific level. Especially blood lactate can be considered as a metabolic indicator of physical strain and FFA of psycho-emotional strain. Furthermore, significant negative correlations could be found between heart rate, FFA level, and catecholamine excretion during car racing and some measures of physical fitness determined on the bicycle ergometer (n = 12 or 10). This suggests a reduced cardiocirculatory and metabolic strain reaction in response to psychical stress situations with increased fitness. Moreover, HDL (high density lipoprotein) was found increased and oral glucose tolerance test was improved with elevated physical fitness (n = 20, respectively 16). From the results of this study it can be concluded that physical activity counteracts atherosclerosis and CHD (coronary heart disease), which are promoted by psycho-emotional and psycho-social stress.

Response of chemosensitive nerve fibers of group III and IV to metabolic changes in rat muscles

Spike recordings were obtained with preparations of group III and IV fibers from the nervus peroneus of the rat. During the recordings the muscle was stimulated by chemical substances simulating metabolic effects of static exercise: increase of [K+], enhancement of osmolality and increase of concentrations of lactic acid and inorganic phosphates. Two experimental setups were used: in series I application was performed by a perfusion of the circulatorily isolated hindleg, and in series II a single muscle of the hindleg (musculus extensor digitorum longus) was superfused by control or test solutions. Only those fiber preparations were further investigated which did not respond to pressure, tension or squeezing of the muscle. Only few fibers that were exposed to all of our stimuli responded to none of them; from the rest, about the half were selective or only preferential for one stimulus. The majority of the fibers adapted their response after 8 min while the applications still endured. A comparison of all fibers (in series II) proved that all the four stimuli elicited significant increases of activity. The greatest significant effects were found for lactic acid and potassium (in series I and II). Since the concentrations used in the test applications were characteristic for medium and heavy exercise these results support the hypothesis that metabolic muscle receptors participate in the peripheral control of circulatory and respiratory drives during static exercise.

Can lactate-evoked cardiovascular responses be used to identify muscle ergoreceptors?

The increase in blood pressure and heart rate which accompanies muscular exercise is in part a reflex mediated by afferent nerve fibres in the group III and IV (small myelinated and unmyelinated) range. It has been reported that perfusion of lactate ions into hind limb muscles is an effective stimulus for these reflex responses. To investigate this hypothesis further, and to test adequacy of the controls used, a solution containing 15 mM lactic acid was perfused through a hind limb of urethane-anaesthetised rats, the leg's circulation being isolated from the rest of the body. During lactate perfusion, increases were seen in arterial blood pressure and heart rate. Denervation of the entire leg abolished the responses. To locate the receptors involved in the reflex, selective denervations of skin or muscle were performed. Clear responses were never seen when the leg was skinned or denervated by section of cutaneous nerves. On the other hand, responses to lactate perfusion were still seen following section of all nerves supplying the muscles of the leg, leaving the cutaneous innervation largely intact. It is concluded that perfusion of a hindlimb with lactate solutions is not an adequate technique to identify ergoreceptors in muscle.

The influence of furosemide on heart rate and oxygen uptake in exercising man

We recently observed that heart rate (HR) related methods for assessing physical fitness lead to an overestimation of endurance capacity in subjects treated with furosemide. To gain a more detailed description of this effect, the relationships between work load (WL), oxygen uptake (VO2), and HR were determined in the present study. To this end, nine healthy male subjects performed two incremental exercise tests (10 W increase per 30 s) on a bicycle ergometer. In one test 40 mg furosemide (Lasix) was applied orally 90 min before exercise started. Compared with control conditions, furosemide led to a change in mean blood volume of -4.5% (range: +7.8% to -11.5%). Neither the maximal VO2 (VO2max) nor the maximal work load (WLmax) were significantly altered after furosemide application. Though the WL-VO2 relationship was not significantly affected, the HR-VO2 relationship showed significant alterations which depended on both the loss of blood volume (BV) and work intensity: When the reduction in BV was less than approximately 5%, HR was found to be lowered at all workloads. When the BV reduction was greater than about 5% HR was significantly reduced only in the lower ranges of work load but significantly increased at the higher work intensities. Since BV reductions are known to increase HR during exercise, our findings suggest that, in addition to the blood volume induced changes in HR, furosemide exerts further direct or indirect effects on heart rate adjustment.

Reflex increases in heart-rate induced by perfusing the hind leg of the rat with solutions containing lactic acid

The hypothesis that metabolic receptors in skeletal muscle influence heart-rate during exercise was tested by means of a perfused preparation of the rat's hind legs. The isolated leg was connected to the body only by nerve and bone and was perfused with tyrode solution. The humoral changes of exercise were simulated by perfusing with modified tyrode solutions in which concentration of K+, osmolality, concentrations of lactic acid, and inorganic phosphate were changed to reflect to those occurring during heavy exercise. Only perfusion with a solution enriched with lactic acid elicited a significant increase in heart-rate. The response disappeared when the nerve supply to the leg was cooled or sectioned. 20-60 s after the start of perfusion with solution of high [lactic acid] heart-rate began to increase reaching a maximum (delta HR +/- SE = 20.2 +/- 8.2, n = 7) after about 2 min. The effect on heart-rate increased when the venous concentration of lactic acid was increased the range from 3 to 10 mmol/l. In further experiments, we tried to separate the effects of pH and lactate. Heart-rate responses were induced only at low pH and at low pH the extent to which heart-rate changed increased with increases in lactate concentration.

Greater serum GH response to arm than to leg exercise performed at equivalent oxygen uptake

The aim of this study was to provide information concerning the mechanism of exercise-induced stimulation of growth hormone (GH) release in human subjects. For this reason serum GH as well as some hemodynamic variables and blood concentrations of noradrenaline (NA), insulin (IRI), lactate (LA), glucose (BG), and free fatty acids (FFA) were determined in seven healthy male subjects exercising on a bicycle ergometer with arms or legs and running on a treadmill at equivalent oxygen consumption levels. Significantly greater increases in serum GH concentration accompanied arm exercises than those observed during the leg exercises. This was accompanied by greater increases in heart rate, blood pressure, and plasma NA and blood lactate concentrations. Serum IRI decreased during both leg exercises and did not change during the arm exercise. There were no differences in BG and plasma FFA concentrations between the three types of exercise. The role of humoral and neural signals responsible for the greater GH response to arm exercise is discussed. The findings are consistent with the hypothesis that neural afferent signals sent by muscle "metabolic receptors" participate in the activation of GH release during physical exercise. It seems likely that the stimulation of these chemoreceptors is more pronounced when smaller muscle groups are engaged at a given work load. However, a contribution of efferent impulses derived from the brain motor centres to the control system of GH secretion during exercise is also possible.

Mechanism of sympathetic activation during prolonged physical exercise in dogs. The role of hepatic glucoreceptors

It seems likely that depletion of body carbohydrates may account for the rise in the sympathetic activity during prolonged exercise, since glucose given during or before exercise reduces the increase in plasma catecholamines. The aim of the present study was to find out whether the increase in plasma noradrenaline (NA) in response to exercise can be reduced by 1. increasing of the amount of carbohydrate available for metabolism without producing hyperinsulinemia and 2. by inhibition of afferent activity from hepatic glucoreceptors. The study was performed on dogs which exercised whilst receiving either the intravenous fructose infusion (2.2 mmol/min) or a slow glucose infusion (0.25 mmol/min) which was given either via the portal or a peripheral vein. Fructose infusion reduced the muscle glycogen depletion during exercise and reduced the increase in plasma NA and glycerol concentrations without altering the blood glucose or insulin levels. The exercise-induced increases in plasma NA and glycerol concentrations were significantly smaller with intraportal than with peripheral glucose infusion but there were no differences between these two cases in the concentration of glucose in the systemic circulation. These findings indicate that the reduction of the plasma NA response to physical effort under conditions of increased carbohydrate availability cannot be attributed to the inhibitory effect of insulin on sympathetic activity and provide evidence for the participation of hepatic glucoreceptors in the control of the sympathetic activity during exercise.

Responses in muscle afferent fibres of slow conduction velocity to contractions and ischaemia in the cat

The aim of the study was to find out to what extent muscle receptors with slowly conducting afferent fibres (group III and IV) are activated by muscular contractions of moderate force, and what kind of muscle afferents could mediate the pain of ischaemic exercise. In chloralose-anaesthetized cats, the impulse activity of single afferent units from the triceps surae muscle was recorded from dorsal root filaments during muscular contractions with intact blood supply and after occlusion of the muscle artery. Two types of responses were observed to contractions without muscular ischaemia. One was characterized by sudden onset and a graded response amplitude to contractions of increasing force. In most cases stretching the muscle was also an effective stimulus. Units showing this response behaviour were labelled c.s.m (contraction-sensitive with mechanical mechanism of activation). The other response type had a more delayed onset and often outlasted the exercise period; because of the unknown mechanism of activation, units of this kind were labelled c.s.x. The proportion of c.s.m receptors was significantly higher amongst group III than amongst group IV units. During ischaemic contractions of comparable force the c.s.m and c.s.x receptors exhibited an unchanged or a decreased response amplitude. Under these conditions another receptor type (N, for nociceptive) was activated which did not respond to contractions with intact blood supply. Vigorous activations during ischaemic work were only observed in group IV receptors. The majority of the 131 group III and IV units tested did not respond to contractions at all. These contraction-insensitive (c.i.) endings probably comprised different receptor populations (nociceptors, thermoreceptors, low-threshold mechanoreceptors). It is concluded that the various central nervous effects of muscular exercise without ischaemia which are known to be due to raised activity in thin muscle afferents (e.g. cardiopulmonary adjustments, spinal locomotor reflexes) are probably produced by the c.s.m and c.s.x types. The pain of ischaemic contractions is most likely mediated by the N receptors most of which possess non-myelinated afferent fibres.

Extracellular fluid volume and central circulation after long lasting exercise and dehydration in conscious dogs

Two aspects of the recovery period after endurance exercise were investigated: a) the fluid distribution between the intra- and extravascular parts of the extracellular fluid volume (ECFV) induced by exercise dehydration, b) the cardiovascular response pattern [blood pressure (BP), heart rate (HR), cardiac output (CO), total peripheral resistance (TPR), and central venous pressure (CVP)] to the heat load which results from the preceding exercise. Seven conscious dogs performed endurance exercise in a cool environment (16 degrees C) on a horizontal treadmill till 4% of the body weight was lost. It was found that about 70% of the total fluid loss of the body came from intracellular water. During exercise sodium and chloride concentrations rose by 6 mMol and 7 mMol respectively (P less than 0.005) and remained elevated throughout the early recovery period indicating a fluid loss of about 100-200 ml out of the ECFV. Direct measurements of the ECFV as sulfate space confirmed these values. Since the plasma volume remained unchanged, this fluid loss was carried totally by the interstitial fluid volume. Immediately after exercise body temperature was elevated by 1.5 degrees C and returned towards control within 90 min. Cardiac output was above control level for 2 h after the end of exercise, at first due to an increased HR and thereafter to an elevated stroke volume (SV) (P less than 0.02). CVP and TPR were below control levels for at least 2 h (P less than 0.01). A linear correlation was found between CVP and TPR. A close correlation existed between the body temperature and the cardiovascular parameters. It can be concluded that even long after exercise the cardiovascular system has to serve thermoregulatory needs.

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

To evaluate to what metabolci event in contracting muscles heart rate (HR) and VE are related, time courses of femoral and cubital venous [K=], osomolality (OSM), pH, POC2, PO2, [lactate], and [orthophosphate] ([Pi]) at onset of exercise were studied in athletes (TR) and non-athletes (UT) and compared to time courses of HR and VE. During ischaemic work with the calf muscles it could be shown that most of these blood constituents were only released from contracting muscles. Thus their time courses reflected the metabolic events in working muscles being not essentially disturbed by non-working parts of the body. Ischaemic work induced, however, substantial increases of HR and VE. In the course of non-ischaemic bicycle work HR and VE rose more rapidly in TR than in UT but were lower in TR during the steady state. During non-ischaemic work only the increased of femoral venous [K=1 closely mimicked the cardiorespiratory transients in TR as well as in UT. None of the other femoral venous substances showed such a rapid change or such typical variations between TR and UT. Cubital venous [K=1 and [Pi] approached femoral venous concentrations only in second minute after start whereas pH, PCO2, and OSM increased mainly in venous outflow from contracting muscles. PO2 decreased in femoral venous blood of TR and UT, but in cubital venous blood it remained depressed only in UT. It was discussed that the cardiorespiratory adjustment during the initial stages of work was related to K+ release in working muscles and not to O2 consuming or H+ producing processes, nor to release of Pi or increase of OSM.

Relationships of femoral venous [K+], PO2, osmolality, and [orthophosphate) with heart rate, ventilation, and leg blood flow during bicycle exercise in athletes and non-athletes

The relationship of femoral venous [K+], [H+], osmolality (OSM), PO2, and [inorganic phosphate] ([Pi]) with heart rate (HR), ventilation (VE), and calculated leg blood flow (Q) were investigated during bicycle exercise in endurance trained (TR) and untrained (UT) test subjects. At a given VO2 the increases of [K+], OSM, [Pi] and the decrease of PO2 were significantly lower in TR than in UT. In the same proportion the increases of HR, VE, and Q were diminished. Thus in TR and UT identical and highly significantly correlated regression lines of [K+], [H+], OSM, [Pi] and PO2 with HR, VE, and Q were obtained. These constituents changed in the same proportion as the relative VO2 in TR and UT. No relationships with [Na+], [Ca++], and [ Mg++] were found. By means of a multiple regression analysis the partial influence of K+, H+, OSM, PO2, and Pi upon the total change of HR, VE and Q was estimated to compare with data from infusion experiments. The findings were discussed in view of the hypothesis that these candidates may provide linkage between metabolic events, circulatory, and ventilatory adjustments during work.