Effects of PCO2 on respiratory pattern during thermal and exercise hyperventilation in domestic fowl

How important is the CO2 chemoreflex for the control of breathing? Environmental and evolutionary considerations

Haldane and Priestley (1905) discovered that the ventilatory control system is highly sensitive to CO₂. This "CO₂ chemoreflex" has been interpreted to dominate control of resting arterial PCO₂/pH (P_aCO₂/pH_a) by monitoring P_aCO₂/pH_a and altering ventilation through negative feedback. However, P_aCO₂/pH_a varies little in mammals as ventilation tightly couples to metabolic demands, which may minimize chemoreflex control of P_aCO₂. The purpose of this synthesis is to (1) interpret data from experimental models with meager CO₂ chemoreflexes to infer their role in ventilatory control of steady-state P_aCO₂, and (2) identify physiological causes of respiratory acidosis occurring normally across vertebrate classes. Interestingly, multiple rodent and amphibian models with minimal/absent CO₂ chemoreflexes exhibit normal ventilation, gas exchange, and P_aCO₂/pH_a. The chemoreflex, therefore, plays at most a minor role in ventilatory control at rest; however, the chemoreflex may be critical for recovering P_aCO₂ following acute respiratory acidosis induced by breath-holding and activity in many ectothermic vertebrates. An apparently small role for CO₂ feedback in the genesis of normal breathing contradicts the prevailing view that central CO₂/pH chemoreceptors increased in importance throughout vertebrate evolution. Since the CO₂ chemoreflex contributes minimally to resting ventilation, these CO₂ chemoreceptors may have instead decreased importance throughout tetrapod evolution, particularly with the onset and refinement of neural innovations that improved the matching of ventilation to tissue metabolic demands. This distinct and elusive "metabolic ventilatory drive" likely underlies steady-state P_aCO₂ in air-breathers. Uncovering the mechanisms and evolution of the metabolic ventilatory drive presents a challenge to clinically-oriented and comparative respiratory physiologists alike.

Afferent vagal activity during hyperthermic polypnea in the pigeon

Respiration-modulated activity in afferent vagal fibers was recorded in 10 pigeons during euthermic breathing and thermal panting. Of these fibers, 13 were identified as intra-pulmonary chemoreceptors (IPCs), that increased discharge with diminishing lung gas PCO2, and 13 as mechanoreceptors, that increased firing with lung inflation. Two types of IPC were observed that were distinct by their firing pattern during panting. Phasic IPCs displayed phasic discharge within the respiratory cycle, even at respiratory frequencies (fresp) as high as 400 min-1. Tonic IPC fired tonically and increased their discharge as fresp increased. Several IPCs were silent during euthermic breathing, but discharged tonically as fresp increased with thermal polypnea. Discharge of neither type of IPC was consistently related to PaCO2. Discharge from mechanoreceptors was phasic with respiration, up to values of fresp as high as 350 min-1. However, the average number of impulses per breath decreased as fresp increased. We conclude that discharge from phasic intrapulmonary chemoreceptors and mechanoreceptors may contribute to setting the respiratory pattern during hyperthermic polypnea.

Effect of bilateral vagotomy on arterial acid-base stability during panting in the pigeon

Bilateral, cervical vagotomy denervates, among other receptors, the known peripheral chemoreceptors in birds. To test the importance of afferent vagal input to cardiorespiratory control during thermal panting, we measured responses to an increase in body temperature (Tb) induced by ambient heating in 6 bilaterally, cervically vagotomized pigeons. These responses were compared with those we previously reported in intact pigeons (control animals). At thermoneutral conditions, respiratory frequency (fR) was lower after vagotomy compared to controls, and PaCO2 was higher. During increases in Tb in both control and vagotomized pigeons, fR, PaO2, pHa, heart rate and oxygen consumption increased, and PaCO2 decreased. However, fR rose less, PaCO2 decreased more, and pHa increased more in the vagotomized pigeons than in controls. All three were more variable than in controls. Heart rate and blood pressure were higher than controls; blood pressure was more variable. We conclude that bilateral, cervical vagotomy compromises the ability of pigeons to maintain stable respiratory and cardiovascular responses to hyperthermia.

Exercise hyperpnea in birds: evidence against a primary role for pCO2

Ventilatory responses of domestic fowl to graded intensities of treadmill exercise were compared when the birds breathed air, 3% CO2 in air or 4.2% CO2 in air. During exercise in air, increased minute ventilation resulted mainly from increased respiratory rate with little change in tidal volume. This pattern of ventilatory response was not altered when the birds respired CO2. In contrast, the pattern of ventilatory response to CO2, at given work loads, consisted of a primary increase in tidal volume with little change in respiratory rate. It is concluded that intrapulmonary pCO2 does not affect the ventilatory response to exercise.