Physiological variability of fluid-regulation hormones in young women

We tested the physiological reliability of plasma renin activity (PRA) and plasma concentrations of arginine vasopressin (P-[AVP]), aldosterone (P-[AL D]), and atrial natriuretic peptide (P-[ANP]) in the early follicular phase and midluteal phases over the course of two menstrual cycles (n = 9 women, ages 25 +/- 1 yr). The reliability (Cronbach's alpha greater than or equal to 0.80) of these hormones within a given phase of the cycle was tested 1) at rest, 2) after 2.5 h of dehydrating exercise, and 3) during a rehydrati on period. The mean hormone concentrations were similar within both the ear ly follicular and midluteal phase tests; and the mean concentrations of P-[ ALD] and PRA for the three test conditions were significantly greater durin g the midluteal compared with the early follicular phase. Although Cronbach 's alpha for resting and recovery P-[ANP] were high (0.80 and 0.87, respect ively), the resting and rehydration values for P-[AVP], P-[ALD], and PRA we re variable between trials for the follicular (a from 0.49 to 0.55) and the luteal phase (oc from 0.25 to 0.66). Physiological reliability was better after dehydration for P-[AVP] and PRA but remained low for P-[ALD]. Althoug h resting and recovery P-[AVP], P-[ALD], and PRA were not consistent within a given menstrual phase, the differences in the concentrations of these ho rmones between the different menstrual phases far exceeded the variability within the phases, indicating that the low within-phase reliability does no t prevent the detection of menstrual phase-related differences in these hor monal variables.