Effects of estrogen and progesterone on cerebrovascular responses to euoxic hypercapnia in women.

OBJECTIVES

To determine the cerebral blood flow response to step changes in end-tidal Pco(2) in premenopausal women (n = 10; mean age±standard deviation 27.0±6.4 years) during the follicular (FP), mid-cycle (MC) and luteal (LP) phases of the menstrual cycle.

METHODS

Transcranial Doppler ultrasound was used to measure beat-by-beat averaged peak blood flow velocity (V(p)) in the middle cerebral artery in response to 20 min of euoxic hypercapnia (end-tidal PO(2) = 88 Torr; end-tidal PCO(2) = 7.0 Torr above resting values). The V(p) responses to euoxic hypercapnia were fitted to a simple mathematical model that included gain terms for the on (G(on)) and off (G(off)) responses, time constants for the on (τ(on)) and off (τ(off)) responses, baseline terms and a time delay (T(d)).

RESULTS

Serum progesterone levels were significantly greater for LP compared to FP and MC (40.6±13.2 vs. 32.6±1.4 nmol/l (p < 0.001) and 8.8±3.8 nmol/l (p < 0.001), respectively). Serum estrogen concentrations were significantly lower in FP compared to MC and LP (150.9±51.2 vs. 506.5±220.5 pmol/l (p = 0.002) and 589.1±222.8 pmol/l (p < 0.001), respectively). Arterial PCO(2) was significantly greater in MC compared to LP (35.0±2.1 and 32.6±1.4 Torr, respectively; p = 0.02). There was a significant increase in G(off) during LP compared with FP and MC (3.38±0.68 vs. 2.79±0.82 cm s(-1) Torr(-1) (p = 0.021) and 2.74±0.90 (p = 0.018) cm s(-1) Torr2(1), respectively). Progesterone and the estrogen/progesterone ratio contributed to the observed differences in G(off).

CONCLUSION

There is an increase in G(off) during LP that is explained, at least in part, by increases in serum progesterone and estrogen and a decrease in arterial PCO(2).