Renal contribution to acid-base regulation during the menstrual cycle

Menstruating women exhibit a light but sustained hypocapnia during the luteal phase. To elucidate whether the hypocapnia results primarily from a respiratory or renal mechanism, we measured the rate of urinary excretion of acid at intervals during the menstrual cycle in five subjects. The acid-base composition of arterial blood in three subjects and end-tidal PCO2 in the remaining two subjects were also determined. During the follicular phase, the acid-base composition of blood and the rate of net acid excretion remained virtually constant. After ovulation, significant decreases in PaCO2 (3.5 mmHg), [HCO3]p (2 meq/liter), and net acid excretion (2 meq/h) occurred in the first 4-6 days of the luteal phase (14 days long). Following this, net acid excretion returned to the preovulatory level. PaCO2 and [HCO3]p, however, remained decreased for 3 more days. At the end of the luteal phase, restoration of PaCO2 proceeded faster than that of [HCO3]p. The acid-base changes in blood and urine observed during the luteal phase were comparable to those occurring during adaptation and recovery from sustained hypocapnia, suggesting that hypocapnia during the luteal phase is primarily respiratory in origin.