The Effect of Menopause on Acid-Base Regulation and the Chemoreflex Control of Breathing during Wakefulness
Preston, Megan E.
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Acid-base regulation, as reflected by hydrogen ion concentration ([H+]), and the chemoreflex control of breathing were examined in healthy pre- (PRE; n=20) and postmenopausal (POST; n=15) women of a comparable age (45 ± 2.7 vs. 52 ± 1.8 years). [H+] behaviour was examined in both groups at rest and during exercise above the ventilatory threshold using Stewart’s physicochemical approach to acid-base analysis. Ventilatory chemoreflex characteristics were assessed using Duffin’s modified rebreathing protocol that includes 5 min of prior hyperventilation and maintenance of either hyperoxic (150 mmHg) or hypoxic (50 mmHg) iso-oxia. As expected, the ovarian hormones progesterone ([P4]) and estrogen ([E2]) were significantly lower in the POST group. [H+] was unaffected by menopausal status at rest or during exercise. At rest the POST group exhibited significantly higher PaCO2 and [SID] values relative to the PRE group. In general, the acidifying effects of increased PaCO2 were offset by the alkalizing effect of increases in [SID] (or vice versa) in the POST group such that [H+] did not differ between PRE and POST groups. The central ventilatory chemoreflex also differed between groups with the POST group exhibiting a significantly higher threshold and a lower sensitivity in the response to CO2 relative to the PRE group. [P4] was found to partially account for the significant group differences in acid-base and central ventilatory chemoreflex control characteristics supporting the role of [P4] as an important determinant of acid-base status and the chemical control of ventilation in healthy women. Findings of the current study may have potential relevance in understanding the increased occurrence of various health conditions such as osteoporosis and sleep disordered breathing in females following the onset of menopause.