1. We undertook a temporal study of external sodium-stimulated lithium efflux (sodium—lithium countertransport) in erythrocytes and blood pressure by measuring these two parameters in three phases of the menstrual cycle (menstrual, midcycle and luteal phases) in 22 healthy, non-medicated females with regular menstrual cycles. Plasma oestradiol and progesterone levels were also determined.

2. Sodium—lithium countertransport activity (activity in 140 mmol/1 external NaCl) in the midcycle phase (0.176 ± 0.017 mmol h−1 l−1 of cells) was lower than in the menstrual (0.192 ± 0.016 mmol h−1 l−1 of cells, P < 0.030) and luteal (0.203 ± 0.018 mmol h−1 l−1 of cells, P < 0.030) phases. The Vmax of the transporter changed similarly but the Km was unaltered.

3. The plasma oestradiol level was 628.9 ± 39.1 pmol/1 in the midcycle phase, higher than in the menstrual (232 ± 18.5 pmol/1, P < 0.001) and luteal (372.5 ± 28.1 pmol/1, P < 0.001) phases. The progesterone level was 28.6 ± 2.1 nmol/1 in the luteal phase, and values were lower in the menstrual (2.5 ± 0.3 nmol/1, P < 0.001) and midcycle (2.8 ± 0.4 nmol/1, P < 0.001) phases.

4. There was no correlation between plasma oestradiol and sodium—lithium countertransport activity or Vmax during the menstrual cycle, but plasma progesterone was positively correlated with sodium—lithium countertransport activity (r = 0.478, P < 0.025, n = 22) and Vmax (r = 0.551, P < 0.045, n = 14) in the luteal phase.

5. Systolic blood pressure did not change significantly during the menstrual cycle. However, the diastolic pressure showed variation similar to that in sodium—lithium countertransport activity/Vmax, its midcycle value of 66.6 ± 1.4 mmHg being lower than that in the luteal (71.6 ± 1.3 mmHg, P < 0.025) and menstrual (70.6 ± 1.4 mmHg, P < 0.025) phases.

6. We conclude that sodium—lithium countertransport activity exhibits catamenial variation. Therefore we suggest, given this observation, that blood sampling for the assessment of the state of activity of the transport system be standardized in relation to a phase of the menstrual cycle in future studies involving females.

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