Habituation of the metabolic and ventilatory responses to cold-water immersion in humans

An experiment was undertaken to answer long-standing questions concerning the nature of metabolic habituation in repeatedly cooled humans. It was hypothesised that repeated skin and deep-body cooling would produce such a habituation that would be specific to the magnitude of the cooling experienced, and that skin cooling alone would dampen the cold-shock but not the metabolic response to cold-water immersion. Twenty-one male participants were divided into three groups, each of which completed two experimental immersions in 12 °C water, lasting until either rectal temperature fell to 35 °C or 90 min had elapsed. Between these two immersions, the control group avoided cold exposures, whilst two experimental groups completed five additional immersions (12 °C). One experimental group repeatedly immersed for 45 min in average, resulting in deep-body (1.18 °C) and skin temperature reductions. The immersions in the second experimental group were designed to result only in skin temperature reductions, and lasted only 5 min. Only the deep-body cooling group displayed a significantly blunted metabolic response during the second experimental immersion until rectal temperature decreased by 1.18 °C, but no habituation was observed when they were cooled further. The skin cooling group showed a significant habituation in the ventilatory response during the initial 5 min of the second experimental immersion, but no alteration in the metabolic response. It is concluded that repeated falls of skin and deep-body temperature can habituate the metabolic response, which shows tissue temperature specificity. However, skin temperature cooling only will lower the cold-shock response, but appears not to elicit an alteration in the metabolic response.     

Metabolic and ventilatory responses to steady state exercise relative to lactate thresholds

The metabolic and ventilatory responses to steady state submaximal exercise on the cycle ergometer were compared at four intensities in 8 healthy subjects. The trials were performed so that, after a 10 min adaptation period, power output was adjusted to maintain steady state VO2 for 30 min at values equivalent to: (1) the aerobic threshold (AeT); (2) between the aerobic and the anaerobic threshold (AeTAnT); (3) the anaerobic threshold (AnT); and (4) between the anaerobic threshold and VO2max (AnTmax). Blood lactate concentration and ventilatory equivalents for O2 and CO2 demonstrated steady state values during the last 20 min of exercise at the AeT, AeAnT and AnT intensities, but increased progressively until fatigue in the AnTmax trial (mean time = 16 min). Serum glycerol levels were significantly higher at 40 min of exercise on the AeAnT and the AnT when compared to AeT, while the respiratory exchange ratios were not significantly different from each other. Thus, metabolic and ventilatory steady state can be maintained during prolonged exercise at intensities up to and including the AnT, and fat continues to be a major fuel source when exercise intensities are increased from the AeT to the AnT in steady state conditions. The blood lactate response to exercise suggests that, for the organism as a whole, anaerobic glycolysis plays a minor role in the energy release system at exercise intensities upt to and including the AnT during steady state conditions.     

Transient ventilatory and heart rate responses to moderate nonabrupt pseudorandom exercise

Dynamic responses of inspired minute ventilation, CO2 and O2 end-tidal gas fractions, and heart rate were obtained from six normal human volunteers in response to a complex dynamic exercise challenge. Subjects pedalled a chair ergometer at constant frequency. The retarding torque applied to the ergometer pedals was controlled by a low-pass-filtered pseudorandom binary sequence (fPRBS), which provided a complex, nonanticipatory exercise stimulus containing sufficient high- and low-frequency energy to excite the small signal, broadband ventilatory response. The exercise range was chosen to produce a mean level of O2 consumption at or below 50% maximum O2 consumption. Cross-covariant analysis of the fPRBS exercise with breath-by-breath ventilation provided an estimate of the dynamic (impulse) response to exercise, which contained both fast phase 1 and slow phase 2 components. The initial, phase one, hyperpnea occurred within the same breath as the exercise transition and preceded a hypocapnic response. The phase one hyperpnea represented 26% of the total ventilatory response. The secondary, phase 2, hyperpnea was delayed several breaths from the onset of phase 1. It contained slower dynamics and followed a hypercapnic response. Heart rate increased abruptly during phase 1, peaked near the phase 1-to-2 boundary, and then decreased rapidly. The experimental protocol was designed to minimize the subjective response and provide an adequate stimulus for the faster time constants. Results obtained from these experiments were consistent with a nonhumoral induced phase 1 exercise hyperpnea.     

Effect of caffeine on ventilatory responses to hypercapnia, hypoxia, and exercise in humans

The effect of oral caffeine on resting ventilation (VE), ventilatory responsiveness to progressive hyperoxic hypercapnia (HCVR), isocapnic hypoxia (HVR), and moderate exercise (EVR) below the anaerobic threshold (AT) was examined in seven healthy adults. Ventilatory responses were measured under three conditions: control (C) and after ingestion of either 650 mg caffeine (CF) or placebo (P) in a double-blind randomized manner. None of the physiological variables of interest differed significantly for C and P conditions (P greater than 0.05). Caffeine levels during HCVR, HVR, and EVR were 69.5 +/- 11.8, 67.8 +/- 10.8, and 67.8 +/- 10.9 (SD) mumol/l, respectively (P greater than 0.05). Metabolic rate at rest and during exercise was significantly elevated during CF compared with P. An increase in VE from 7.4 +/- 2.5 (P) to 10.5 +/- 2.1 l/min (CF) (P less than 0.05) was associated with a decrease in end-tidal PCO2 from 39.1 +/- 2.7 (P) to 35.1 +/- 1.3 Torr (CF) (P less than 0.05). Caffeine increased the HCVR, HVR, and EVR slopes (mean increase: 28 +/- 8, 135 +/- 28, 14 +/- 5%, respectively) compared with P; P less than 0.05 for each response. Increases in resting ventilation, HCVR, and HVR slopes were associated with increases in tidal volume (VT), whereas the increase in EVR slope was accompanied by increases in both VT and respiratory frequency. Our results indicate that caffeine increases VE and chemosensitivity to CO2 inhalation, hypoxia, and CO2 production during exercise below the AT.     

Effect of chronic pulmonary denervation on ventilatory responses to exercise

To assess the role of intrapulmonary receptors on the ventilatory responses to exercise we studied six beagle dogs before and after chronic pulmonary denervation and five dogs before and after sham thoracotomies. Each exercise challenge consisted of 6 min of treadmill exercise with measurements taken during the third minute at 3.2 km/h, 0% grade, and during the third minute at 5.0 km/h, 0% grade. Inspiratory and expiratory airflows were monitored with a low-dead-space latex mask and pneumotachographs coupled to differential pressure transducers. Both pre- and postsurgery, all dogs exhibited a significant arterial hypocapnia and alkalosis during exercise. Denervation of the lungs had no significant effect on minute ventilation at rest or during exercise, although there was a lower frequency and higher tidal volume in the lung-denervated dogs at all measurement periods. Breathing frequency increased significantly during exercise in lung-denervated dogs but to a lesser magnitude than in the control dogs. The changes that occurred in breathing frequency in all animals were due predominantly to the shortening of expiratory time. Inspiratory time did not shorten significantly during exercise following lung denervation. We conclude from these data that intrapulmonary receptors which are deafferented by sectioning the vagi at the hilum are not responsible for setting the level of ventilation during rest or exercise but are involved in determining the pattern of breathing.     

The effects of immersion and exercise on prolactin during pregnancy

Prolactin is an important hormone during pregnancy, affecting mother, fetus, and amniotic fluid volume. Immersion is known to affect prolactin levels significantly. To determine the effect of immersion and exercise on the prolactin response during pregnancy, we examined serum prolactin levels at 15, 25, and 35 weeks' gestation and 10 weeks post partum. Twelve women completed 20 min land rest, 20 min immersion in 30 degrees C water to the xiphoid, and 20 min exercise in the water at 60% VO2max. Resting prolactin levels were 1.91 +/- 0.32, 4.55 +/- 0.5, and 5.85 +/- 0.27 nmol.l-1 +/- standard error of the mean at 15, 25, and 35 weeks' gestation, respectively. Postpartum lactating women had a resting mean prolactin level of 3.95 +/- 1.6 versus 0.22 +/- 0.4 nmol.l-1 in non-lactating women. Prolactin levels declined significantly during immersion even after correction for dilution by plasma volume shifts. The immersion response was inversely related to the duration of pregnancy with 29%, 22%, and 12% drops during 15-, 25- and 35-week trials, respectively. Compared to rest, exercise prolactin levels remained depressed during the 15th and 25th week trials. We hypothesize that immersion in water caused prolactin levels to decline.     

Diurnal variations in ventilatory and cardiorespiratory responses to submaximal treadmill exercise in females

Diurnal variations in ventilatory and cardiorespiratory responses to submaximal treadmill exercise were analysed in 11 eumenorrhoeic women and in 10 women using monophasic oral contraceptives. Subjects performed submaximal treadmill exercise at three intensities averaging 7, 8, and 9 km x h(-1), each for 4 min at 0800, 1300 and 1700 hours, assigned randomly on 3 separate days. Rectal temperature was measured before (T(rec(b))) and after (T(rec(a))) exercise. Cardiac frequency (f(c)), ventilation (V(E)), oxygen uptake (VO(2)), carbon dioxide output (VCO(2)), and respiratory exchange ratio (R) were assessed in the last minute of each stage of the exercise. Both T(rec(b)) and T(rec(a)) increased from 0800 to 1700 hours (P < 0.001). For a given submaximal work rate, VO(2) and VCO(2) were higher in the afternoon compared to the morning. Similarly, R was increased at 1700 hours compared to 0800 hours during the recovery period following exercise (P < 0.05). However, V(E) did not vary significantly during the day at any of the running intensities. No significant interactions (group x time of day) were observed in any of the studied parameters. In contrast to ventilation, the VO(2) and VCO(2) of the females during submaximal exercise were both affected by the time of day, without any differences between eumenorrhoeic women and users of oral contraceptives.     

Thermoregulatory responses during exercise and a hot water immersion and the affective responses to peripheral thermal stimuli

Tympanic (Tty), mean skin (¯Tsk) and mean body (¯Tb) temperatures and heart rate (HR) increased more in low Vo₂ max group (LG) than in high Vo₂ max group (HG) during exercise. The regression coefficient of body temperatures (Tty and ¯Tb) on HR and the increased rate of heat storage were larger in LG than in HG during exercise. The local sweat rate (per min/cm²) during a hot water bath exhibited a considerable large quantity in comparison with the amount during exercise. Internal and skin temperatures during a hot water bath increased more immediately than those during exercise. The levels of comfort sensation during the preovulatory phase in women and pre-exercise period in men were higher at 40C than at 20C as peripheral thermal stimulus. The levels during the postovulatory and post-exercise phases in the same subjects were higher with the cool stimuli than with the warm stimuli. Above results suggest that thermoregulatory responses during submaximal exercise are different according to physical fitness and that these responses are different from those during hot water immersion. In addition, these suggest that the scores of thermal sensation with warm and cool stimuli are different during the pre- and post-ovulatory phases and the pre- and post-exercise periods.     

Initial ventilatory and circulatory responses to dynamic exercise are slowed in the elderly.

To elucidate the characteristics of ventilatory and circulatory responses at the onset of brief and light exercise in the elderly, 13 healthy, elderly men, aged 66.8 yr (mean), exerted bilateral leg extension-flexion movements for only 20 s with a weight around each ankle, with each weight being approximately 2.5% of their body mass. Similar movements were passively performed on the subjects by the experimenters. These results were compared with those of 13 healthy, young men (22.9 yr). Minute ventilation increased at the onset of voluntary exercise and passive movements in both groups but showed a slower increase in the elderly. Heart rate also increased in both groups but showed less change in the elderly. Mean blood pressure temporarily decreased in both groups but less in the elderly. The magnitude of relative change (gain) of heart rate in the elderly was significantly smaller than that in the young, whereas the increasing rate to reach one-half of the gain (response time) of ventilation in the elderly was significantly slower than that in the young. Similar tendencies were observed in the passive movements. It is concluded that the elderly show slower ventilatory response and attenuated circulatory response at the onset of dynamic voluntary exercise and passive movements.     

Breath-to-breath “noise” in the ventilatory and gas exchange responses of children to exercise

The purposes of this investigation were to quantify the noise component of child breath-by-breath data, investigate the major determinants of the breath-to-breath noise, and to characterise the noise statistically. Twenty-four healthy children (12 males and 12 females) of mean (SD) age 13.1 (0.3) years completed 25 min of steady-state cycle ergometry at an exercise intensity of 50 W. Ventilatory and gas exchange variables were computed breath-by-breath. The mean (SD) oxygen consumption (VO2) ranged from 0.72 (0.16) to 0.92 (0.26) l x min(-1); mean (SD) carbon dioxide production (VCO2) ranged from 0.67 (0.20) l x min(-1) to 0.85 (0.16) l x min(-1); and mean (SD) minute ventilation ranged from 17.81 (3.54) l x min(-1) to 24.97 (5.63) l x min(-1). The majority of the breath-to-breath noise distributions differed significantly from Gaussian distributions with equivalent mean and SD parameters. The values of the normalised autocorrelation functions indicated a negligible breath-to-breath correlation. Tidal volume accounted for the majority of the VO2 (43%) and VCO2 (49%) variance. The breath-to-breath noise can be explained in terms of variations in the breathing pattern, although the large noise magnitude, together with the relatively small attainable response amplitudes in children reduces the certainty with which ventilatory and gas exchange kinetics can be measured.     

The relationship between maximum breath hold time in air and the ventilatory responses to immersion in cold water.

Eight subjects performed maximum breath holds in air and naked head-out immersions of 2 min duration in stirred water at 5, 10 and 15 degrees C. Analysis of the respiratory data collected in air and on immersion revealed a significant (P less than 0.05) inverse relationship between the maximum breath hold time (tbh,max) of subjects in air and their frequency of breathing and inspiratory volumes on immersion. No such relationship was identified between tbh,max in air and tidal volumes on immersion. It is concluded that the tbh,max of individuals in air may provide an indication of the magnitude of some of their respiratory responses to immersion. This information may be of use when personnel are being selected for activities with a high risk of immersion in cold water.     

Cardiac and ventilatory responses ofCrangon crangon to cadmium,copper and zinc

The acute (30 min) responses of heart and scaphognathite activities ofCrangon crangon on exposure to concentrations of 1–20 mg Cd, Cu or Zn 1⁻¹ are increased beat frequencies. The relative magnitude of response(Δ f) is linearly related to immediate pretreatment frequency(f) and standardised responses(Δ fs) are given forf values of 70 and 100 beats min⁻¹ for hearts and scaphognathites, respectively.Δ fs values for each organ are also linearly related to test concentration for each metal. Qualitative changes to organ activities described include an increased incidence of scaphognathite reversals in concentrations of 5.0 mg Cu 1⁻¹ and in 20.0 mg Zn 1⁻¹. Chronic (13 days) exposure to incipient lethal levels of the test metals produced increases in scaphognathite rates of the Cd-treated animals and in heart and scaphognathite rates of the coppertreated animals. The general applicability of these methods to studies of pollution stress in decapods is discussed.     

Naloxone and the ventilatory response to exercise in man

Endogenous opiate peptides are known to exert a depressant action on ventilation (VE), and their plasma levels have been shown to be elevated during a variety of exercise protocols. We investigated whether they might modulate the control of the hyperpnea of short-term constant-load (CLE) and incremental (IE) cycle-ergometer exercise. Four healthy subjects performed CLE tests at ca. 80% of the anaerobic threshold (theta an) for 5 min following a period of unloaded pedaling, and IE tests (10 or 20 W min-1) to the limit of tolerance. Normal saline (3 ml) or the opiate antagonist naloxone (1.2 mg in 3 ml) were administered intravenously prior to each test. Naloxone elicited no discernible effect on VE, alveolar gas tensions, or heart rate throughout the entire range of work rates; neither were theta an nor the maximum work rate affected. It is concluded that, for short-term exercise ranging in intensity from moderate to severe, the role played by endogenous opiate peptides in the control of the exercise hyperpnea appears to be negligible in man.     

Effects of altered CSF [H+] on ventilatory responses to exercise in the awake goat

We investigated the effects of selective large changes in the acid-base environment of medullary chemoreceptors on the control of exercise hyperpnea in unanesthetized goats. Four intact and two carotid body-denervated goats underwent cisternal perfusion with mock cerebrospinal fluid (CSF) of markedly varying [HCO-3] (CSF [H+] = 21-95 neq/l; pH 7.68-7.02) until a new steady state of alveolar hypo- or hyperventilation was reached [arterial PCO2 (PaCO2) = 31-54 Torr]. Perfusion continued as the goats completed two levels of steady-state treadmill walking [2 to 4-fold increase in CO2 production (VCO2)]. With normal acid-base status in CSF, goats usually hyperventilated slightly from rest through exercise (-3 Torr PaCO2, rest to VCO2 = 1.1 l/min). Changing CSF perfusate [H+] changed the level of resting PaCO2 (+6 and -4 Torr), but with few exceptions, the regulation of PaCO2 during exercise (delta PaCO2/delta VCO2) remained similar regardless of the new ventilatory steady state imposed by changing CSF [H+]. Thus the gain (slope) of the ventilatory response to exercise (ratio of change in alveolar ventilation to change in VCO2) must have increased approximately 15% with decreased resting PaCO2 (acidic CSF) and decreased approximately 9% with increased resting PaCO2 (alkaline CSF). A similar effect of CSF [H+] on resting PaCO2 and on delta PaCO2/VCO2 during exercise also occurred in two carotid body-denervated goats. Our results show that alteration of the gain of the ventilatory response to exercise occurs on acute alterations in resting PaCO2 set point (via changing CSF [H+]) and that the primary stimuli to exercise hyperpnea can operate independently of central or peripheral chemoreception.