Ventilatory compensation for lactacidosis in ponies: role of carotid chemoreceptors and lung afferents

Ventilation and respiratory mechanics

During dynamic exercise, the healthy pulmonary system faces several major challenges, including decreases in mixed venous oxygen content and increases in mixed venous carbon dioxide. As such, the ventilatory demand is increased, while the rising cardiac output means that blood will have considerably less time in the pulmonary capillaries to accomplish gas exchange. Blood gas homeostasis must be accomplished by precise regulation of alveolar ventilation via medullary neural networks and sensory reflex mechanisms. It is equally important that cardiovascular and pulmonary system responses to exercise be precisely matched to the increase in metabolic requirements, and that the substantial gas transport needs of both respiratory and locomotor muscles be considered. Our article addresses each of these topics with emphasis on the healthy, young adult exercising in normoxia. We review recent evidence concerning how exercise hyperpnea influences sympathetic vasoconstrictor outflow and the effect this might have on the ability to perform muscular work. We also review sex-based differences in lung mechanics.

Control of breathing during exercise

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

Effect of repeated exercise and recovery on heart rate variability in elite trotting horses during high intensity interval training

REASONS FOR PERFORMING STUDY

Interval training is a commonly used training method for trotting horses. In addition, trainers are provided with efficient and inexpensive heart rate monitor devices for the management of training.

HYPOTHESIS

Since the high frequency (HF) frequency peak (fHF) of heart rate variability (HRV) corresponds to the breathing frequency in combination with stride frequency during trotting, it is hypothesised that modifications of breathing and stride frequencies induced by repeated exercise could be detected from fHF.

METHODS

RR interval time series of 7 trotting horses were recorded during an interval training session. Interval training was made up of 5 successive 800 m high-velocity trotting runs (H1, H2...H5) separated by 1 min recovery bouts at low speed (R1, R2...R5). Fast Fourier transform (FFT) and Poincaré plot analysis techniques were applied to RR series.

RESULTS

Repeated exercise had significant effects on HRV components during interval training. Despite constant trotting velocities during high-speed and recovery, repetition induced a decrease in mean RR interval (H1: 295 +/- 19 vs. H5: 283 +/- 15 msec, P<0.05) and in the root mean square of successive differences in RR series (RMSSD; H1: 6.31 +/- 1.28 vs. H5: 5.31 +/- 1.31 msec, P<0.05). Furthermore, high-speed and recovery repetitions induced an increase in fHF (H1: 1.37 +/- 0.35 vs. H5: 1.62 +/- 0.40 Hz and R1: 0.22 +/- 0.02 vs. R4: 0.64 +/- 0.38 Hz, P<0.05). Hence, recovery induced a decrease in the s.d. of the successive RR series (SDRR; R3: 10.5 +/- 3.96 vs. R5: 6.17 +/- 2.65 msecs, P>0.05) and in the long term index of Poincaré plot (SD2; R1: 43.29 +/- 28.90 vs. R5: 18.19 +/- 9.35 msecs, P<0.05).

CONCLUSIONS

The observed increase in fHF during the interval training could be induced by alterations of the coupling between breathing and stride frequency linked to the emergence of fatigue. The decrease in SD2 and SDRR during successive recovery bouts could be linked with a deterioration of the recovery pattern.

POTENTIAL RELEVANCE

HRV can provide breathing frequency data of Standardbreds during training without any respiratory device. Furthermore, HRV could provide useful makers of the emergence of fatigue states during training.

Effect of increasing temperature on TRPV1-mediated responses in isolated rat pulmonary sensory neurons

Hyperthermia has been shown to sensitize vagal pulmonary C-fibers in anesthetized rats. However, it was not clear whether the effect was due to a direct action of hyperthermia on these sensory neurons. To answer this question, we carried out this study to determine the effect of increasing temperature on the responses to various chemical stimuli in isolated nodose and jugular ganglion neurons innervating the rat lungs. In the whole cell perforated patch-clamp study, when the temperature was increased from normal (∼36°C) to hyperthermic (∼40.6°C) level of the rat body temperature, the inward currents evoked by capsaicin, a selective activator of the transient receptor potential vanilloid type 1 (TRPV1), and 2-aminoethoxydiphenyl borate (2-APB), a nonselective activator of TRPV1–3 receptors, were both significantly increased. This potentiating effect was clearly present even at a moderate level of hyperthermia (∼39°C). However, only the slow, sustained component of acid-evoked current mediated through the TRPV1 receptor was potentiated by hyperthermia, whereas the rapid, transient component was inhibited. In contrast, the currents evoked by adenosine 5′-triphosphate and acetylcholine, neither of which is known to activate the TRPV1 channel, did not increase when the same temperature elevation was applied. Furthermore, the hyperthermia-induced potentiation of the cell response to 2-APB was significantly attenuated by either capsazepine or AMG 9810, selective TRPV1 antagonists. In conclusion, increasing temperature within the physiological range exerts a potentiating effect on the response to TRPV1 activators in these neurons, which is probably mediated through a positive interaction between hyperthermia and these chemical activators at the TRPV1 channel.

Reproducibility of relationships between human ventilation, its components and oesophageal temperature during incremental exercise

For human exercise at intensities greater than approximately 70 to 85% of maximal levels of exertion, ventilation (V E) increases proportionately to core temperature (T C) following distinct T C thresholds. This suggested T C in humans could be a modulator of exercise-induced ventilation. This study tested the reproducibility of relationships between oesophageal temperature (T oes) ventilation and its components during incremental exercise. On two nonconsecutive days, at an ambient temperature of 22.1+/-0.3 degrees C and RH of 45+/-5%, seven untrained adult males of normal physique pedaled on a seated cycle ergometer in an incremental exercise protocol from rest to the point of exhaustion. In each exercise session, ventilatory equivalents for oxygen consumption (VE.VO2 (1-)) and carbon dioxide production (VE.VCO2 (1-)) plus the components of V E, tidal volume (V T) and frequency of respiration (f), were expressed as a function of T oes. Results indicated the reproducibility criteria of Bland and Altman were met for the relationships between T oes and both (VE.VO2 (1-)) and (VE.VCO2 (1-)) as well as for relationships between T oes and each of V T and f. Intraclass correlation coefficients (R) for between-trial T oes thresholds for (R=0.91, P<0.05) and (R=0.88, P<0.05) were also high and significant. In both trials, after T oes increased by approximately 0.3 degrees C, V T demonstrated a distinct plateau point at a reproducible T oes (R=0.93, P<0.05) and f demonstrated a distinct and reproducible T oes threshold (R=0.84, P<0.05). In conclusion, the results illustrate that for humans, ventilation has a significant and reproducible relationship with core temperature during incremental exercise.

V(O2) recovery kinetics in the horse following moderate, heavy, and severe exercise

At the onset of exercise, horses exhibit O2 uptake (VO2) kinetics that are qualitatively similar to those of humans. In humans, there is a marked dissymmetry between on- and off-kinetics for VO2. This investigation sought to formally characterize the off-transient (recovery) VO2 kinetics in the horse within the moderate (M), heavy (H), and severe (S) exercise domains. Six horses were run on a high-speed treadmill at M, H, and S exercise intensities (i.e., that speed which yielded approximately 50, 85, 100% peak VO2, respectively, on the maximal incremental test). The time courses for the recovery were modeled by using a three-phase model with a single-exponential (fast component) or double-exponential (fast and slow component) phase 2. The single-exponential phase 2 model provided an excellent fit to the off-transient data, with the exception of one horse in the H domain which was best modeled by a double exponential. The time delay elicited no domain dependency (M, 18.0 +/- 1.0; H, 17.6 +/- 1.1; S, 17.8 +/- 2.0 s; P > 0.05), as was the case for the fast-component time constants (M, 16.3 +/- 2.0 s; H, 13.5 +/- 1.0 s; S, 14.6 +/- 0.3 s; P > 0.05). In the H and S (but not M) domains, the VO2 following resolution of the fast component was elevated above the preexercise baseline (H, 3.0 +/- 1.0 l/min; S, 5.7 +/- 1.1 l/min). This additional postexercise VO2 was correlated to the end-exercise increase in lactate (r = 0.94, P < 0.001) but not the end-exercise pulmonary arterial blood temperature (r = 0.45, P > 0.05). These data indicate that the time delay and subsequent kinetic response of the primary (fast-component) phase of exercise VO2 recovery in the horse is independent of the preceding exercise-intensity domain. However, in the H and S domains, the fast component resolves to an elevated baseline.

Ventilatory effects of specific carotid body hypocapnia and hypoxia in awake dogs

Specific carotid body (CB) hypocapnia in the-10-Torr (less than eupneic) range reduced ventilation in the awake and sleeping dog to the same degree as did CB hyperoxia [CB PO2 (PCBO2); > 500 Torr; C.A. Smith, K.W. Saupe, K. S. Henderson, and J. A. Dempsey. J. Appl. Physiol. 79:689-699, 1995], suggesting a powerful inhibitory effect of hypocapnia at the carotid chemosensor over a range of PCO2 encountered commonly in physiological hyperpneas. The primary purpose of this study was to assess the ventilatory effect of CB hypocapnia on the ventilatory response to concomitant CB hypoxia. The secondary purpose was to assess the relative gains of the CB and central chemoreceptors to hypocapnia. In eight awake female dogs the vascularly isolated CB was perfused with hypoxic blood (mild, PCBO2 approximately equal to 50 Torr or severe, PCBO2 approximately equal to 36 Torr) in a background of normocapnia or hypocapnia (10 Torr less than eupneic arterial PCO2) in the perfusate. The systemic (and brain) circulation was normoxic throughout, and arterial PCO2 was not controlled (poikilocapnia). With CB hypocapnia, the peak ventilation (range 19-27 s) in response to hypoxic CB perfusion increased 48% (mild) and 77% (severe) due to increased tidal volume. When CB hypocapnia was present, these increases in ventilation were reduced to 21 and 27%, respectively. With systemic hypocapnia, with the isolated CB maintained normocapnic and hypoxic for > 70 s, the steady-state poikilocapnic ventilatory response (i.e., to systemic hypocapnia alone) decreased 15% (mild CB hypoxia) and 27% (severe CB hypoxia) from the peak response, respectively. We conclude that carotid body hypocapnia can be a major source of inhibitory feedback to respiratory motor output during the hyperventilatory response to hypoxic carotid body stimulation.

Acid:base and serum biochemistry changes in horses competing at a modified 1 Star 3-day-event

We examined the effects of participation in each of 3 modifications of Day 2 of a 3-day-event on blood and serum variables indicative of hydration, acid:base status and electrolyte homeostasis of horses. Three groups of horses - 8 European (E) horses and 2 groups each of 9 North American horses performed identical Days 1 (dressage) and 3 (stadium jumping) of a 3-day-event. E horses and one group of the North American horses (TD) performed modifications of Day 2 of a 1 Star 3-day-event and the other group of North American horses (HT) performed a Horse Trial on Day 2. Jugular venous blood was collected from each horse on the morning of Day 2 before any warm-up activity, between 4 min 55 s and 5 min 15 s after Phase D and the following morning. Eight E horses, 5 TD horses and 8 HT horses completed the trials. There were few significant differences in acid:base or serum biochemistry variables detected among horses performing either 2 variations of the Speed and Endurance day of a 1 Star 3-day-event, or a conventional Horse Trial. Failure to detect differences among groups may have been related to the low statistical power associated with the small number of horses, especially in the TD group, variation in quality of horses among groups and the different times of the day at which the E horses competed. Differences detected among time points were usually common to all groups and demonstrated metabolic acidosis with a compensatory respiratory alkalosis, a reduction in total body water and cation content, and hypocalcaemia. Importantly, horses of all groups did not replenish cation, chloride, and calcium deficits after 14-18 h of recovery.

Contribution of acid-base changes to control of breathing during exercise

The mechanisms mediating the exercise hyperpnea remain controversial; there is no unequivocal evidence that any of numerous proposed mechanisms mediates the hyperpnea. However, a great deal has been learned including the potential role of changes in PCO2, [H+], strong ion differences (SID), weak acids, or any other acid-base component. The contribution of acid-base changes to the hyperpnea during exercise is likely through known or postulated chemoreceptors. Two of these, pulmonary and intracranial chemoreceptors, do not appear critical for the ventilatory adjustments to meet the metabolic demands of exercise. A third, the carotid chemoreceptors, appear to fine-tune alveolar ventilation during exercise to minimize disruptions in arterial blood gases. The role of the fourth chemoreceptors, those within skeletal muscles, is least clear. However, there is evidence that they do contribute to the hyperpnea, and it is quite clear that a muscle chemoreflex contributes to the exercise muscle pressor reflex; thus the contribution of these chemoreceptors to the exercise hyperpnea requires additional study.

Hypoxic response is inversely related to degree of exercise hyperventilation

The Dejours hyperoxic test has been used to quantitate peripheral chemoreceptor contribution to the hyperpnea of exercise. The strength of this drive, measured by the percent reduction in ventilation, varies among individuals and is lacking in chemodenervated humans, who also fail to manifest a hyperventilatory response in heavy exercise. We reasoned that greater hyperventilation in exercise above the anaerobic threshold ought to be associated with greater hypoxic (carotid body) drive. The present study tested this hypothesis. In 17 naive subjects, carotid body O2 chemosensitivity was tested repeatedly during exercise above the ventilatory anaerobic threshold (VAT) using 2 breaths of O2. The response to these transients was quantitated by the percentage change in ventilation, and exercise hyperventilation was quantitated by VE in excess of VCO2 predicted from the slope of delta VE/delta VCO2 below VAT in incremental exercise. Contrary to expectations, there was an inverse relation between the degree of exercise hyperventilation and the percentage reduction in exercise ventilation in response to O2. The significance of this observation and its integration with current thinking of the role of the peripheral chemoreceptor in mediating hyperventilation of heavy exercise is discussed.

Do carotid chemoreceptors inhibit the hyperventilatory response to heavy exercise?

In this paper two types of evidence are presented which question the commonly presumed role of carotid chemoreceptor stimulation as the primary mediator of the hyperventilatory response to heavy exercise. First, carotid-body denervation in ponies increases their hyperventilatory response to heavy exercise. Second, the awake dog and the goat at rest show an immediate and substantial depression of tidal volume and of ventilation when their isolated carotid chemoreceptors are made hypocapnic. Accordingly, it is proposed that during heavy exercise the carotid chemoreceptors are inhibitory to respiratory motor output and that the cause of the hyperventilatory response originates from extrachemoreceptor, locomotor-linked, feed-forward stimuli.

Effects of potassium and lactic acid on ventilation in anaesthetized cats

Intravenous infusions of lactic acid alone, KCl alone and both together were administered to anaesthetized, spontaneously breathing cats. Ventilation (VI), end-tidal PO2 (PETO2), end-tidal PCO2 (PETCO2), arterial blood pressure and heart rate were recorded continuously. [K+]a and pHa were monitored using intravascular ion selective catheter electrodes. The increase in VI during infusion of KCl and lactic acid together was greater than that observed during infusion of lactic acid alone. The increment in VI produced by the addition of an infusion of KCl to the lactic acid infusion was greater than the increment produced by the addition of KCl to a control infusion of normal saline. The reduction in PaCO2 which occurred when KCl was added to the lactic acid infusion was similar to that when KCl was infused with NaCl. Thus the inhibition of respiration secondary to reduced PaCO2 was similar in both circumstances. These results suggest that the combined respiratory stimulant effect of elevation of [K+]a and acute lactic acidosis is more than additive.