1. We have studied the transport of Na+ and K+ by erythrocytes during the follicular and luteal phases of the human menstrual cycle, and in pregnant compared with non-pregnant women. Venous blood was drawn from 10 healthy young women (not taking any medication or hormones) 1-2 days after menstruation and from the same women 7-9 days after ovulation. For the pregnancy part of the study, blood was drawn from eight other normotensive non-pregnant women and from eight age-matched normotensive pregnant women (36-43 weeks gestation).
2. Intracellular erythrocyte and plasma Na+ and K+ concentrations were measured by flame photometry. The increase in the intracellular Na+ concentration during a 1 h 37°C incubation of fresh whole blood with 0.2 mmol/l ouabain (compared with no ouabain) was measured to determine the rate of active Na+ efflux. The Na+-K+ pump rate constant was calculated by dividing the active Na+ efflux rate by the intracellular Na+ concentration.
3. In fresh blood, the intracellular erythrocyte Na+ concentration (P < 0.002) and the plasma K+ concentration (P < 0.01) were both lower in pregnant than in non-pregnant women. The Na+-K+ pump rate constant was higher (P < 0.02) during the luteal phase than during the follicular phase, and in pregnant compared with non-pregnant women.
4. We conclude that short-acting hormones in the plasma most probably account for the changes in the Na+-K+ pump rate constant during the menstrual cycle.