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The effects of acute and chronic hypercapnia upon ventilation and acid-base status in the pekin duck Dodd, Graham Alan Andrew


In this study, awake Pekin ducks (Anas platyrhynchos) were exposed to periods of acute and chronic hypercapnia (0.05 F₁CO₂). Measurements were made of ventilation and acid-base status in both adult and juvenile male ducks as well as in adult female ducks. All Pekin ducks, regardless of age or sex, responded acutely to inspired CO₂ with a marked hypercapnic-hyperpnea. Inhalation of CO₂ resulted in a significant increase in arterial CO₂ tension (PaCO₂) and decrease in arterial pH (pHa). The increase (3 times (x)) in minute ventilation (V[sub E]), while primarily a function of an increase (2x) in tidal volume (V[sub T]), also involved an increase (1.5x) in breathing frequency (f[sub b]). The chronic responses of ducks to inspired CO₂, however, did differ depending upon the sex of the animal. In male ducks, the initial increase observed in V[sub E]during the first 20 minutes was reduced by 50% after 300 minutes. This partial recovery in V[sub E]resulted entirely from the complete return of f[sub b] to its control levels as V[sub T]remained both elevated and constant throughout the period of hypercapnia. In addition, the male ducks also demonstrated a significant recovery (50%) in pHa, a change that was paradoxical to the concomitant increase measured in PaCO₂. While a change in strong ion difference (SID) was not detected, the accompanying rise in calculated arterial [HCO₃⁺] suggested that metabolic compensatory processes must have alleviated the initial respiratory acidosis. The rate of metabolic compensation seen in the ducks of this study exceeds that reported for any other air-breathing vertebrate. Female ducks, on the other hand, maintained the initial increase in V[sub E]and decrease in pHa throughout the period of CO₂ exposure. The reasons for this remain unclear although it is speculated that the metabolic demands of eggshell formation may have limited the capacity of these birds to mobilize further HCO₃⁻ stores. Differences in the changes which occurred in f[sub b], V[sub T], pH and P[sub CO₂] in male and female ducks during chronic CO₂ exposure strongly suggest that the changes in f[sub b] were a singular function of changes in pHa ([H⁺]) while changes in VT were primarily a function of changes in PaCO₂. Denervation of peripheral chemoreceptors appeared to have little effect upon the overall ventilatory responses to either acute or chronic hypercapnia, suggesting that central chemoreceptors must have been predominantly responsible for the ventilatory responses observed during this study.

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