CHANGES IN SODIUM-TRANSPORT DURING THE HUMAN MENSTRUAL-CYCLE AND PREGNANCY

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 da ys after menstruation and from the same women 7-9 days after ovulation . For the pregnancy part of the study, blood was drawn from eight othe r normotensive non-pregnant women and from eight age-matched normotens ive pregnant women (36-43 weeks gestation). 2. Intracellular erythrocy te and plasma Na+ and K+ concentrations were measured by flame photome try. The increase in the intracellular Na+ concentration during a 1 h 37-degrees-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<0.0 02) and the plasma K+ concentration (P<0.01) were both lower in pregna nt than in non-pregnant women. The Na+-K+ pump rate constant was highe r (P<0.02) during the luteal phase than during the follicular phase, a nd in pregnant compared with non-pregnant women. 4. We conclude that s hort-acting hormones in the plasma most probably account for the chang es in the Na+-K+ pump rate constant during the menstrual cycle.