Physicochemical analysis of phasic menstrual cycle effects on acid-base balance

In accordance with Stewart's physicochemical approach, the three independen t determinants of plasma hydrogen ion concentration ([H+]) were measured at rest and during exercise in the follicular (FP) and luteal phase (LP) of t he human menstrual cycle. Healthy, physically active women with similar phy sical characteristics were tested during either the FP (n = 14) or LP (n = 14). Arterialized blood samples were obtained at rest and after 5 min of up right cycling at both 70 and 110% of the ventilatory threshold (T-Vent). Me asurements included plasma [H+], arterial carbon dioxide tension (Pa-CO2), total weak acid ([A(Tot)]) as reflected by total protein, and the strong-io n difference ([SID]). The transition from rest to exercise in both groups r esulted in a significant increase in [H+] at 70% T-Vent versus rest and at 110% T-Vent versus both rest and 70% TVent. No significant between-group di fferences were observed for [H+] at rest or in response to exercise. At res t in the LP, [A(Tot)] and Pa-CO2 were significantly lower (acts to decrease [H+]) compared with the FP. This effect was offset by a reduction in [SID] (acts to increase [H+]). After the transition from rest to exercise, signi ficantly lower [A(Tot)] during the LP was again observed. Although the [SID ] and Pa-CO2 were not significantly different between groups, trends for ch anges in these two variables were similar to changes in the resting state. In conclusion, mechanisms regulating [H+] exhibit phase-related differences to ensure [H+] is relatively constant regardless of progesterone-mediated ventilatory changes during the LP.