In view of the lack of unanimity on the effect of long-term intake of combined oral contraceptives (OC) on external sodium-dependent lithium efflux, otherwise known as sodium-lithium countertransport (SLC), we undertook a double-blind study to investigate the possible interaction between SLC and OC in healthy women with regular menstrual cycles. In a group of 17 volunteers, aged 27.0 +/- 1.1 years (mean +/- s.e.m.) and weighing 61.4 +/- 2.0 kg, ingestion of 30 microg ethinyloestradiol + 150 microg desogestrel for 3 months caused an increase in SLC activity from a baseline value of 0.254 +/- 0.017 mmol/lcell x h to 0.274 +/- 0.017 mmol/lcell x h (P = 0.05). The activity of the transport system after 6 months treatment remained higher than at baseline (0.280 +/- 0.016 mmol/lcell x h, P < 0.025) but was comparable to that at 3 months. Blood pressure and weight remained unaltered during the study period. In a comparable group of 16 volunteers of age 26.1 +/- 1.3 years and weight 63.5 +/- 2.1 kg, control SLC activity (0.218 +/- 0.012 mmol/lcell x h) was comparable to that after 3 months (0.215 +/- 0.011 mmol/lcell x h) and 6 months (0.216 +/- 0.011 mmol/lcell x h) intake of 35 microg ethinyloestradiol + 2 mg cyproterone acetate. Body weight and blood pressure remained similarly unchanged. These results not only confirm that long-term intake of OC may cause increased SLC activity, but also suggest that the type of the progesterone in the medication used could be an important determinant of such interaction.
PIP: This double-blind study investigates the possible interaction between sodium-lithium countertransport (SLC) and oral contraceptives (OCs) in healthy women with regular menstrual cycles. 33 healthy females with regular menstrual cycles aged 18-35 years and weighing 43.5-73.0 kg were recruited. Each volunteer was randomly given either a 30 mcg ethinyl estradiol/150 mcg desogestrel combination or a 35 mcg ethinyl estradiol/2 mg cyproterone acetate combination. For each volunteer, blood was taken in the morning of days 15-18 of their menstrual cycle for biochemistry and SLC assay before treatment, and at the same stage of the cycle after 3 and 6 months. The study showed different effects of two different OC preparations in two groups of otherwise comparable volunteers. Administration of ethinyl estradiol/cyproterone acetate combinations for 6 months was not associated with alteration in any of the parameters measured. However, SLC activity increased after 3 and 6 months of ethinyl estradiol/desogestrel medication. These results not only confirm that long-term intake of OCs may cause increased SLC activity, but also suggest that the type of progesterone in the medication used could be an important determinant of such alteration in the activity of the transport system.