Na(+)-H+ exchange activity throughout pregnancy: the proper experimental approach

Pregnancy is associated with a 30-50% rise in cardiac output and a close to 50% increase in blood volume. The Na(+)-H+ exchanger is a key mediator of tubular NaCl absorption and a stimulus-response coupling mediator. We measured erythrocyte Na(+)-H+ exchange activity over the course of normal pregnancy in 18 healthy pregnant women (mean age 32 +/- 4 years) at 14, 24 and 33 weeks of gestation and 15 nonpregnant healthy women (mean age 32 +/- 9 years). No pregnancy was complicated by hypertension. Serum urea, creatinine and sodium did not change through gestation, while serum potassium slightly but significantly decreased. Urinary excretion rates of both sodium and potassium remained unchanged. Urea and creatinine clearances were constantly elevated in pregnant vs non-pregnant control women. Erythrocyte Na(+)-H+ exchanger reached the highest activity at about the 14th week of gestation, when cardiac output also peaked. Thereafter, it tended to decrease, yet remaining above the normal values until the 34th week. Conceivably, the observed hyperactivity of the transporter may be a contributing factor to the hemodynamic adjustments attending to normal pregnancy.