Respiratory, cardiovascular, and metabolic adjustments to exercise in the Hereford calf

Six Hereford steers were studied before, during, and after short exercise bouts on a motor-driven treadmill (3 degrees incline) at four speeds (1.0, 1.4, 1.8, and 2.2 m X s-1). Oxygen consumption (MO2) and carbon dioxide production (MCO2) were measured by collecting the expired gas. Arterial and mixed venous blood samples were obtained simultaneously from indwelling catheters in the aorta and pulmonary artery. A 10-fold increase was observed in MO2 and MCO2 at the highest work load. Minute ventilation increased proportionately less than MO2 and MCO2 with increasing work loads, but alveolar ventilation was found to increase in proportion to both MO2 and MCO2. The highest work load produced a threefold increase in cardiac output primarily as a result of increased heart rate. A 10-fold increase in lactate and a 63% increase in serum potassium concentration were observed at the highest work load. Plasma cortisol levels were highest at 10 min postexercise and reached levels of seven times the resting values following exercise at the highest speed. The responses to exercise in the calf are qualitatively similar to those observed in other species, but quantitative differences exist in some cardiovascular and metabolic responses which may limit this animal's ability to perform strenuous exercise.     

Hydration during exercise. Effects on thermal and cardiovascular adjustments

Five young unacclimatised subjects were exposed for 4 h at 34 degrees C (10 degrees C dew-point temperature and 0.6 m X s-1 air velocity), while exercising on a bicycle ergometer: 25 min work--5 min rest cycles for 2 hours followed by 20 min work--10 min rest cycles for two further hours. 5 experimental sessions were carried out: one without rehydration (NO FLUID) resulting in 3.1% mean loss of body weight (delta Mb), and four sessions with 20 degrees C fluid ingestion of spring water (WATER), hypotonic (HYPO), isotonic (ISO) and hypertonic (HYPER) solutions to study the effects of fluid osmolarity on rehydration. Mean final rehydration (+/- SE) after fluid intake was 82.2% (+/- 1.2). Heart rate was higher in NO FLUID while no difference among conditions was found in either delta Mb or hourly sweat rates. Sweating sensitivity was lowest in the dehydration condition, and highest in the WATER one. Modifications in plasma volume and osmolarity demonstrated that NO FLUID induced hyperosmotic hypovolemia, ISO rehydration rapidly led to plasma isoosmotic hypervolemia, while WATER led to slightly hypoosmotic normovolemia. It is concluded that adequate rehydration through ingestion of isotonic electrolyte-sucrose solution, although in quantities much smaller than evaporative heat loss, rapidly restored and expanded plasma volume. While osmolarity influenced sweating sensitivity, the plasma volume changes (delta PV) within the range -6% less than or equal to delta PV + 4% had little effect on temperature adjustments in our conditions.     

Body fluid homeostasis and cardiovascular adjustments during submaximal exercise: influence of chewing coca leaves

 The present study was undertaken to determine the haematological and cardiovascular status, at rest and during prolonged (1 h) submaximal exercise (approximately 70% of peak oxygen uptake) in a group (n = 12) of chronic coca users after chewing approximately 50 g of coca leaves. The results were compared to those obtained in a group (n = 12) of nonchewers. At rest, coca chewing was accompanied by a significant increase in heart rate [from 60 (SEM 4) TO 76 (SEM 3) beats · min⁻¹], in haematocrit [from 53.2 (SEM 1.2) to 55.6 (SEM 1.1)%] in haemoglobin concentration, and plasma noradrenaline concentration [from 2.8 (SEM 0.4) to 5.0 (SEM 0.5) μmol · l⁻¹]. It was calculated that coca chewing for 1 h resulted in a significant decrease in blood [−4.3 (SEM 2.2)%] and plasma [−8.7 (SEM 1.2)%] volume. During submaximal exercise, coca chewers displayed a significantly higher heart rate and mean arterial blood pressure. The exercise-induced haemoconcentration was blunted in coca chewers compared to nonchewers. It was concluded that the coca-induced fluid shift observed at rest in these coca chewers was not cumulative with that of exercise, and that the hypovolaemia induced by coca chewing at rest compromised circulatory adjustments during exercise. Accepted: 29 October 1996     

Influences of gravitational stress and total muscle mass on human cardiovascular adjustments during prolonged dynamic exercise

To investigate cardiovascular adjustments to combined stress of gravity and exercise during dynamic exercise prolonged over 60 minutes in upright position, the three experiments were carried out. In Experiment I, as performing moderate upright bicycling (66% VO2 max) until exhaustion (average 86 minutes) in 4 trained men, changing manners of cardiovascular adjustments were shown in three phases, which were (1) regulating set-point body temperature, (2) making to maintain suitable blood pressure, and (3) failing to control the blood pressure. In Experiment II, as performing moderate supine cycling (55% VO2 max) for 50 minutes under several LBNP conditions in 5 sedentary women, there were correspondingly similar changing manners of the cardiovascular adjustments to each of the phases given in Experiment I. In Experiment III, as examining tolerance of orthostatic circulatory regulation, the tolerance was significantly correlated to VO2 max and lean body mass (LBM) (both p less than 0.05) in 8 sedentary women. When a moderate upright exercise is performed over 60 minutes in upright position, the performance should be influenced by total muscle mass indicated by LBM as well as by VO2 max, because the muscle mass could play a valuable role in the tolerance of orthostatic circulatory regulation vs. gravity.     

Glutamatergic and GABAergic inputs to the RVL mediate cardiovascular adjustments to noxious stimulation

Stimulation of cutaneous and muscle afferents induces several cardiovascular adjustments such as hypertension, tachycardia, and muscle vasodilation. Although previous studies have demonstrated that the rostral ventrolateral medulla (RVL) mediates sympathoexcitation and pressor responses to sciatic nerve stimulation (SNS), whether it also mediates blood flow adjustments remains unclear. Therefore, in the present study, we examined the role of the RVL in the vasodilation induced by SNS and the possible neurotransmitters involved. In Urethane-anesthetized, paralyzed, and artificially ventilated rats, SNS (square pulses, 1 ms, 20 Hz, 800--1200 microA, 10 s) produced increases in blood pressure, heart rate, blood flow, and vascular conductance of the stimulated limb. Unilateral microinjection of kainic acid (2 nmol/100 nl) into the RVL contralateral to the stimulated limb abolished cardiovascular adjustments to SNS. Unilateral microinjections of kynurenic acid (2 nmol/100 nl) selectively abolished the pressor response to SNS, whereas bicuculline (400 pmol/100 nl) abolished the increases in blood flow without changing the pressor response. These results suggest that glutamatergic synapses within the RVL mediate pressor responses, whereas GABAergic synapses may mediate the vasodilation to SNS.     

Cardiovascular adjustments to rhythmic handgrip exercise: relationship to electromyographic activity and post-exercise hyperemia

The purpose of this study was to examine the association among electromyographic (EMG) activity, recovery blood flow, and the magnitude of the autonomic adjustments to rhythmic exercise in humans. To accomplish this, 10 healthy subjects (aged 23-37 y) performed rhythmic handgrip exercise for 2 min at 5, 15, 25, 40, and 60% of maximal voluntary force. Heart rate and arterial blood pressure were measured at rest (control), during each level of exercise, and for 2 min following exercise (recovery). The rectified, filtered EMG activity of the exercising forearm was measured continuously during each level of exercise and was used as an index of the level of central command. Post-exercise hyperemia was calculated as the difference between the control and the average recovery (2 min) forearm blood flows (venous occlusion plethysmography) and was examined as a possible index of the stimulus for muscle chemoreflex activation. Heart rate, arterial pressure, forearm EMG activity, and post-exercise hyperemia all increased progressively with increasing exercise intensity. The magnitudes of the increases in heart rate and arterial pressure from control to exercise were directly related to both the level of EMG activity and the degree of post-exercise hyperemia across the five exercise intensities (delta heart rate vs EMG activity: r = 0.99; delta arterial pressure vs EMG activity: r = 0.99; delta heart rate vs hyperemia: r = 0.99; and delta arterial pressure vs hyperemia: r = 0.98; all p less than 0.01). Furthermore, the level of EMG activity was directly related (r = 0.99) to the corresponding degree of hyperemia.(ABSTRACT TRUNCATED AT 250 WORDS)     

Postural specificity of cardiovascular adaptations to exercise training

The purposes of this study were to determine 1) whether posture affects the magnitude of cardiovascular adaptations to training and 2) whether cardiovascular adaptations resulting from exercise training in the supine posture transfer (generalize) to exercise in the upright posture and vice versa. Sixteen sedentary men, aged 18-33 yr, were trained using high-intensity interval and prolonged continuous cycling in the supine (STG; supine training group) or upright (UTG; upright training group) posture 4 days/wk, 40 min/day, for 8 wk, while seven male subjects served as nontraining controls. After training, maximal O2 uptake measured during supine and upright cycling, respectively, increased significantly (P less than 0.05) by 22.9 and 16.1% in the STG and by 6.0 and 14.6% in the UTG. No significant cardiovascular adaptations were observed at rest. During submaximal supine cycling at 100 W, significant increases in end-diastolic volume (21%) and stroke volume (22%) (radionuclide ventriculography and CO2 rebreathing) and decreases in heart rate, blood pressure, and systemic vascular resistance occurred in the STG, whereas only a significant decrease in blood pressure occurred in the UTG. During upright cycling at 100 W, a significant decrease in blood pressure occurred in the STG, whereas significant increases in end-diastolic volume (17%) and stroke volume (18%) and decreases in blood pressure and systemic vascular resistance occurred in the UTG. Volume of myocardial contractility, ejection fraction, and systolic blood pressure-to-end-systolic volume ratio did not change significantly after training when measured during supine and upright cycling in either training group. Blood volume increased significantly in the UTG but remained unchanged in the STG.(ABSTRACT TRUNCATED AT 250 WORDS)     

Respiratory, cardiovascular, and metabolic adjustments to hypoxemia during sleep in piglets

The influence of sleep on ventilation, metabolic rate, cardiovascular function, and regional distribution of blood flow during hypoxemia (PaO2 of 45-50 mm Hg (1 mm Hg = 133.3 Pa)) was studied in piglets at 6+/-1 and 34+/-5 days (mean+/-SD). Measurement of ventilation and metabolic rate was done in a metabolic chamber, and blood flow was measured using the microsphere technique. A subgroup of animals was instrumented for cardiac output measurement (dye-dilution technique) and continuous monitoring of the hemoglobin saturation in oxygen (SaO2). We found that although sleep did not influence the metabolic and cardiac output response to hypoxemia, it affected the ventilatory response as well as the brain and the respiratory muscle blood flows. During active sleep in the older animals, the ventilatory response to hypoxemia was smaller than in the other two states; marked drops in SaO2 occurred with changes in the breathing pattern; and that state was associated with the highest rate of brain blood flow. As well, age affected the ventilatory and metabolic response, but not the cardiovascular response to hypoxemia. The age-dependent ventilatory changes with hypoxemia (smaller ventilatory response in the young than in the older animals) were related to the different levels of oxygen consumption. In summary, active sleep was responsible for all the sleep-dependent changes in the response to a moderate degree of hypoxemia.     

Delay time adjustments to minimize errors in breath-by-breath measurement of VO2 during exercise

Proctor, David N., and Kenneth C. Beck. Delay timeadjustments to minimize errors in breath-by-breath measurement of O₂ during exercise.J. Appl. Physiol. 81(6):2495-2499, 1996.If the delay time between gas concentration andflow signals is not adequately corrected during breath-by-breathanalysis of expired gas, an error in calculation of oxygen consumption(O₂) will result. Toexamine the frequency and delay time dependences of errors inO₂ measurement, six healthymen exercised at 100, 200, and 250 W on a cycle ergometer whilebreath-by-breath assessment ofO₂ was made simultaneouslywith collection of expired air. Subjects breathed first at normal rates(15-30 breaths/min) and then at 70 breaths/min. Each subjectperformed each level of exercise twice by using erroneous values forthe delay time between gas concentration and flow signals. At normalbreathing frequencies, errors inO₂ measurement were±10% over the full range of delay times used, and the errors werenot tightly correlated with variations in delay times from optimum.However, at 70 breaths/min, errors approached ±30% as thevariations in delay times deviated ±0.1 s from the optimal, and theerrors were highly correlated with the variations in delay times. Weconclude that there is greater potential for errors inO₂ measurement withincorrect delay time at higher breathing frequencies. These findingssuggest that the optimal delay time for breath-by-breath systems shouldbe adjusted by using high breathing frequencies.

     

Central dopaminergic activity influences rats ability to exercise

Rats were run to exhaustion on a motor driven treadmill. Apomorphine given to intact rats prolonged the time to exhaustion. Apomorphine given to 6-hydroxydopamine lesioned rats also prolonged the time to exhaustion. Intracerebroventricular 6-hydroxydopamine alone reduced the time to exhaustion. Clonidine given to these animals had no effect. We suggest that central dopaminergic activity influences rats ability to exercise.     

Central neural control of sympathetic nerve activity in heart failure following exercise training

Typical characteristics of chronic congestive heart failure (HF) are increased sympathetic drive, altered autonomic reflexes, and altered body fluid regulation. These abnormalities lead to an increased risk of mortality, particularly in the late stage of chronic HF. Recent evidence suggests that central nervous system (CNS) mechanisms may be important in these abnormalities during HF. Exercise training (ExT) has emerged as a nonpharmacological therapeutic strategy substitute with significant benefit to patients with HF. Regular ExT improves functional capacity as well as quality of life and perhaps prognosis in chronic HF patients. The mechanism(s) by which ExT improves the clinical status of HF patients is not fully known. Recent studies have provided convincing evidence that ExT significantly alleviates the increased sympathetic drive, altered autonomic reflexes, and altered body fluid regulation in HF. This review describes and highlights the studies that examine various central pathways involved in autonomic outflow that are altered in HF and are improved following ExT. The increased sympathoexcitation is due to an imbalance between inhibitory and excitatory mechanisms within specific areas in the CNS such as the paraventricular nucleus (PVN) of the hypothalamus. Studies summarized here have revealed that ExT improves the altered inhibitory pathway utilizing nitric oxide and GABA mechanisms within the PVN in HF. ExT alleviates elevated sympathetic outflow in HF through normalization of excitatory glutamatergic and angiotensinergic mechanisms within the PVN. ExT also improves volume reflex function and thus fluid balance in HF. Preliminary observations also suggest that ExT induces structural neuroplasticity in the brain of rats with HF. We conclude that improvement of the enhanced CNS-mediated increase in sympathetic outflow, specifically to the kidneys related to fluid balance, contributes to the beneficial effects of ExT in HF.