1. Electrolyte transport characteristics were examined in erythrocytes from 13 normal men and from two groups of women: (i) taking combined oral contraceptive preparations (O/C, n = 10), and (ii) ovulatory women (non-O/C, n = 10) pre- and post-ovulation, at the same time intervals (days 7–10 and days 15–18) during a menstrual cycle.

2. With rubidium (86Rb+) used as a potassium analogue, co-transport (ouabain-resistant, fruse-mide-sensitive 86Rb+ influx) values were found to be lowest in non-O/C women (28 ± se 2.5 nmol h−1 10−9 cells) and highest in men (56 ± 5.7, P < 0.001), with results between the two in women taking O/C (42 ± 4.2, P < 0.05 vs men, P < 0.01 vs non-O/C). Passive 86Rb leak (frusemideand ouabain-resistant) was significantly lower in men (13 ± 1.6 nmol h−1 10−9 cells) than in both groups of women (non-O/C 29 ± 1.8, P < 0.001; O/C 25 ± 1.2, P < 0.001). There was no cyclical variation within either group of women.

3. Maximum ouabain binding (number of Na+,K+-ATPase units) was the same in all groups. Na+,K+-ATPase activity, as determined by ouabain-sensitive 86Rb influx, was the same in men and non-O/C groups, but was significantly suppressed in O/C compared with both men (P < 0.01) and non-O/C women (P < 0.05).

4. The differences found were not due to alterations in either progesterone or aldosterone, but could represent an androgenic effect in vivo of the 19-nortestosterone derivatives in combined oral contraceptive preparations.

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