A serial study of erythrocyte sodium pump kinetics and sodium content in the puerperium.

ABSTRACT

OBJECTIVE: Our purpose was to describe the alterations in erythrocyte sodium pump kinetics and sodium content occurring during the puerperium. STUDY DESIGN: Twelve healthy primigravid women were studied serially from late pregnancy until 20 weeks after delivery. Erythrocyte sodium pump rate constant, maximum velocity, and sodium affinity were calculated from the ouabain-sensitive sodium flux measured in whole blood and in erythrocytes in which sodium content had been altered with the ionophore nystatin. The Student t test was used to compare the regression coefficients of the values plotted against log time for specific periods. RESULTS: The sodium pump rate constant, maximum velocity, and sodium affinity were lower 20 weeks after delivery than in late pregnancy (0.339 +/- 0.018 vs 0.399 +/- 0.016/hr, 7.02 +/- 0.08 vs 9.98 +/- 0.078 mmol/kg/hr, 2.65 +/- 0.21 vs 3.16 +/- 0.20 mmol/kg). The decrease in the rate constant commenced after 4 days of the puerperium, whereas the decrease in maximum velocity and Michaelis-Menten constant did not commence until after 2 weeks. Erythrocyte sodium content was greater 20 weeks after delivery than in late pregnancy (4.71 +/- 0.20 vs 4.14 +/- 0.15 mmol/kg cells) and the increase was gradual over the time studied. CONCLUSIONS: After delivery the rate constant of the sodium pump measured in plasma and the erythrocyte sodium content changed before any significant alteration in the maximum velocity of the pump. The return of sodium pump function to the nonpregnant state continues beyond 6 weeks after delivery.