Increased Aldosterone Plasma Protein Binding in Women on Combined Oral Contraceptives throughout the Menstrual Cycle*

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Abstract
Plasma aldosterone concentration and the percentage of the fraction bound to a specific plasma aldosterone-binding globulin (ABG) were measured throughout the menstrual cycle in 26 healthy women aged between 19 and 38 yr who were receiving estrogencontaining oral contraceptives (OC). Midafternoon upright levels of total plasma aldosterone were similar in control and OC subjects and showed normal cyclic fluctuation in both groups. The percentage of ABG-bound aldosterone was markedly higher in OC subjects than in controls at all stages of the cycle and showed a positive correlation with the mean blood pressure when all OC subjects studied were considered. In addition, when the plasma ABG levels were measured 6 months or more after the start of medication, they appeared to be related to the dose of estrogen in the OC, but not to the length of time of its administration. The identity of plasma ABG in OC subjects with that in control subjects was also established.