Zum Mechanismus der Pulsfrequenzeinstellung durch den Stoffwechsel

Contribution of the leg vasculature to hypotensive effects of an antiorthostatic posture change in humans

1. Previous results from our laboratory have shown that vasodilatation in the legs prevents mean arterial pressure (MAP) from increasing during water immersion. Therefore, we tested the hypothesis that vasodilatation in the legs is necessary for the hypotensive effects to occur during a moderate antiorthostatic posture change. 2. Ten healthy males underwent a 5 min posture change from upright seated to horizontal supine (SUP) and back to seated again with (OCCL-SUP) and without simultaneous total arterial (154 +/- 1 mmHg) thigh occlusion, and a control seated period, also with and without arterial occlusion. Cardiac output (CO) was measured by a non-invasive foreign (N2O) gas rebreathing technique. 3. MAP (brachial auscultation) decreased during SUP from 94 +/- 3 to 84 +/- 2 mmHg (P < 0.0001) and total peripheral vascular resistance (TPR = MAP/CO, n = 8) decreased by 15 +/- 4 % (P < 0.001). During OCCL-SUP, MAP decreased from 98 +/- 2 to 90 +/- 2 mmHg (P < 0.005) and TPR decreased by 14 +/- 3 % (P < 0.01). 4. In conclusion, vasodilatation in the legs is not necessary for the decrease in MAP to occur during a moderate antiorthostatic manoeuvre. Therefore, vasodilatation in more central vascular beds (e.g. abdomen) can alone account for the hypotensive effects.

Cardiovascular limitations of active recovery from strenuous exercise

Recovery from muscle fatigue after exercise is known to have two beneficial effects: improved blood lactate elimination and a central nervous recuperation of the capacity for exercise. This study indicates circulatory mechanisms that might limit active recovery. Ten subjects were seated on a cycle ergometer and performed arm cranking exercise at an anaerobic intensity which was for each individual in three periods of 6 min, alternating with recovery intervals of 14 min. In two randomly assigned tests, recovery consisted either of passive sitting (control) or cycling at 80 W for 12 min. Both tests terminated with an identical final passive rest period of 25 min. In the cycling test arm cranking led to a heart rate increase which was further elevated with each repetition, while in the control test no differences were shown among the cranking periods. No corresponding difference was found for oxygen consumption. During the 25 min of final rest, the cycling test showed arterial hypotension and elevated heart rate both of which were absent in the control tests. Venous-occlusion-plethysmography revealed a postcranking forearm hyperaemia. In the cycling test hyperaemia was markedly reduced with the onset of cycling due to vasoconstriction; this effect was absent in the control test. A reduction in blood lactate occurred faster in the cycling test, mainly at the onset of cycling. Total plasma fluid loss combined with forearm fluid uptake was accentuated and prolonged by cycling recovery. Recovery exercise performed by muscles other than those that were fatigued could have led to arterial hypotension (shock-index about 1) through both plasma fluid loss and additional vasodilatation depending on the muscle mass involved.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood pressure and heart rate during rest-exercise and exercise-rest transitions

The transients of mean arterial blood pressure (BPa) and heart rate (fc) during rest-exercise and exercise-rest transitions have been studied in six healthy sport students. After 5 min of rest in an upright position on a cycle ergometer they exercised for 15 min and remained seated for a further 5 min. The subjects exercised at four different constant intensities (40 W, 80 W, 120 W, 160 W) in random order separated by at least 24 h. The BPa was determined by a noninvasive and continuous method. During the first minute of exercise, three phases of response could be distinguished, with the first two showing no clear relationship to intensity. Phase 1 consisted of simultaneous increases in both fc and BP during the first 6 s. In phase 2, BPa decreased while fc continued to increase. During phase 3, BPa and fc approximated constant values or a linear increase. Both parameters showed no comparable intensity-independent reactions during the off-transients. In conclusion, during the first 15 s of rest-exercise transitions there seems to be a fast and uniform cardiovascular drive which overrode other influences on fc.

On the reliability of the Penaz cuff during systemic and local fingertip vasodilatation at rest and in exercise

To compare the readings of blood pressure by the Riva-Rocci (RR) method with those of peripheral arterial pressure (PAP) as recorded by the Finapres (FP) device, exercise was performed by six male subjects on a cycle ergometer at a constant exercise intensity of 140 W. In addition, forearm volume was determined by impedance plethysmography. At rest, systolic FP values exceeded RR values by greater than or equal to 10 mmHg. During 60-min exercise both values at first increased almost in parallel with each other. While RR reached a plateau after 3 min, FP then started to decrease continuously up to the 10th min and finally stabilized at 20-30 mmHg below RR. The impedance values showed a similar declining slope, indicating vasodilatation. To separate the effects of sympathetic drive from heat elicited vasodilatation, a second experimental series was performed with ischaemic static calf exercise (5 min, 90 N), since this increases the sympathetic tone but prevents systemic heat distribution. In contrast to findings reported from intra-arterial measurements, no exercise effect on the pulse pressure amplification was obtained. However, the heating of one fingertip distal to the FP-cuff led to a significant decrease in PAP compared to the control recording made simultaneously from the other hand. It was concluded that heat induced vasodilatation may make FP unrepresentative of systemic blood pressure, in particular during exercise. Moreover, the FP-cuff seemed to induce substantial vasoconstriction due to venous occlusion. The FP method would therefore be useful for monitoring continuously systemic blood pressure if no (dilative) vasomotor changes occurred or their ranges and time courses were known sufficiently well.

The interstitial fluid content in working muscle modifies the cardiovascular response to exercise

The volume of interstitial fluid in the limbs varies considerably, due to hydrostatic effects. As signals from working muscle, responsible for much of the cardiovascular drive, are assumed to be transmitted in this compartment, blood pressure and heart rate could be affected by local or systemic variations in interstitial hydration. Using a special calf ergometer, eight male subjects performed rhythmic aerobic plantar flexions in a supine position with dependent calves for periods of 7 min. During exercise heart rate, blood pressure, oxygen uptake (VO2) and blood lactate concentrations were measured in two different tests, one before and after interstitial calf dehydration through limb elevation for 25 min, compared to the other, a control with unaltered fluid volume in a maintained working position. Impedance plethysmography showed calf volume to be stabilized in the control position. Leg elevation by passive hip flexion to 90 degrees resulted in a fast (vascular) volume decrease lasting less than 2 min, followed by a slow linear fluid loss from the interstitial compartment. Then, when returned to the control position, adjustment of vascular volume was completed within 2 min and exercise could be performed with dehydration remaining in the interstitium only. Cardiovascular response was identical at the start of both tests. However, exercising with dehydrated calves elicited a significantly larger increase in heart rate compared to the control, whereas VO2 was identical. The blood pressure response was shown to be only slightly enhanced. Structural interstitial features varying with hydration, most likely chemical or mechanical ones, may have been responsible for this amplification of signals.

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.

Reduction in extracellular muscle volume increases heart rate and blood pressure response to isometric exercise

To investigate the effect of local dehydration on heart rate and blood pressure during static exercise, six healthy male subjects performed exercise of the calf muscles with different extracellular volumes of the working muscles. Exercise consisted of 5 min of static calf muscle contractions at about 10% of maximal voluntary contraction. The body position during exercise was identical in all tests, i.e. supine with the knee joint 90 degrees flexed. During a 25-min pre-exercise period three different protocols were employed to manipulate the calf volume. In test A the subjects rested in the exercise position; in test B the body position was the same as in A but calf volumes were increased by venous congestion [cuffs inflated to 10.67 kPa (80 mmHg)]; in test C the calf volumes were decreased by lifting the 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 mmHg 5 min before the onset of exercise. This occlusion was maintained for 1 min after the 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 tests B and C until the cuffs were deflated, indicating that only calf muscles contributed to the neurogenic peripheral drive. It is concluded that extracellular muscle volume plays a significant role in adjusting heart rate and blood pressure during static exercise.

Cardiorespiratory reflex responses to static contraction of vascularly isolated hindleg muscles of the rat

One hindleg of an anasthetized rat (n = 15) was isolated from systemic blood circulation. The preparation was connected to the body only by nerve and bone. A. and V. femorales were cannulated and perfused with normoxic (PO2 = 530 mm Hg) or hypoxic (PO2 = 60 mm Hg) Tyrode solutions. Static contractions of the muscle were elicited by electrical stimulation on the sciatic nerve (2 x motor threshold, 400-800 mV, 50 s-1). A 1 s stimulus was followed by a 2 s rest period. Total test time amounted to 40 min. It was proceeded and succeeded by 20 min periods of control perfusions without stimulation. Heart rate (HR) and respiratory rate (f) were measured and cross correlated with the following outflow parameters from V. femoralis of the experimental muscle: [K+], [Na+], PO2, PCO2, pH and [lactate]. During the test period HR and f increased significantly within 20 min of the start of stimulation: HR 5.8% (p less than 0.005) and f 24.3% (p less than 0.005) for hypoxic perfusion (n = 6) and HR 3.2% (p less than 0.005) and f (p less than 0.001, ANOVA) for normoxic perfusion (n = 3). The dynamic changes of several outflow parameters were nearly simultaneous with the cardiorespiratory responses. Cross correlation analyses revealed an excellent temporal relationship between HR and PO2 or [lactate] and between f and PO2 or [lactate]. In addition PCO2 and pH correlated well with HR as well as with f. Comparison of the threshold of the cardiorespiratory response revealed an optimal relationship to pH, a good one to PCO2 and lactate concentration but no correlation to PO2.(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.

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.

Heart rate changes caused by varying the oxygen supply to isolated hind legs of rats

In a rat with an isolated hind leg circulation perfused with varying tyrode solutions, heart rate (HR) changes were studied in dependence of VO2 in the isolated hind leg and of PCO2, [K+], pH and lactic acid concentration ([Lac]) measured in the venous outflow of the isolated hind leg. In experimental series I the inflow PO2 (PiO2) was kept constantly high (either about 65 or 72 kPa). The perfusion pressure alternated between 16 and 24 kPa leading to flow rates in isolated hind legs (Qa) from 30 to 50 ml . 100 g-1 . min-1. The VO2 depended on the momentary Qa (flow-limited oxygen uptake). The [K+] and [Lac], the pH and the AVDO2 remained nearly constant while the PCO2 was lower at small flow rates. The HR decreases some 4 min after initial enhancement of Qa and VO2. Series II comprised experiments with low flow rates and a medium oxygen supply (Qa = 2.5-17.4 ml . 100 g-1 . min-1), PiO2 = 17.5-62.7 kPa). The VO2 ranged between 0.02 and 0.2 ml . 100 g-1 . min-1. The [K+] and [Lac], the PCO2 and the HR increased while the pH decreased. The [Lac] in the outflow showed a strong dependence on oxygen uptake and--at a weak oxygen supply--on the time. Cross-correlation analyses between the parameters confirmed that the HR was best temporally correlated to the [Lac] in the outflow.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.