Effects of Sex Hormones on Central Chemoreflex Function

Abstract

Women in the low hormone phase of the menstrual cycle (LH) have greater light headedness compared to when they are in the high hormone phase (HH). Both cerebrovascular reactivity to CO2 and central chemoreflex activity might play a role since CO2 can elicit both cerebrovascular dilation and activation of sympathetic nerve activity (SNA). Research has shown that women in the HH phase demonstrate higher SNA and may also have higher cerebrovascular reactivity. We investigate the effect of hypercapnia on ventilatory and cerebrovascular responses in women throughout the menstrual cycle. Women (n=8) breathed five minutes of 5% CO2 during LH (day 2–5 or placebo pill) and HH (day 18-24 or maximal dose pilQ phases). Ventilation (Ve; Pneumotachometer), blood flow velocity through the middle cerebral artery (MCA; Transcranial Doppler), and blood pressure (BP; ccNexFin) were measured. Ve equals tidal volume x breathing rate. Cerebrovascular conductance index (CVCi) equals MCA mean + BP. Paired t-tests were used to compare the change due to CO2. There was no effect of phase on the Ve response to CO2 (LH: +0.09± 0.081/min; HH: +0.10± 0.091/min; p; 0.57) though there was a tendency for a greater increase of CVCi in the HH phase during CO2 inhalation (LH: +0.11±0.08cm/s/mmHg; HH: +0.19±0.09 cm/s/mmHg; p; Q,08). Furthermore, there was no effect of phase on the BP response to CO2 (LH: +2.16±2.94mmHg; HH: +1.81±2.20mmHg; p; Q.63). Our results suggest that the presence of estrogen and progesterone may not affect the central chemoreflex but tends to increase cerebrovascular reactivity.