A SERIAL STUDY OF ERYTHROCYTE SODIUM-PUMP KINETICS AND SODIUM CONTENTIN THE PUERPERIUM

OBJECTIVE: Our purpose was to describe the alterations in erythrocyte sodium pump kinetics and sodium content occurring during the puerperiu m. STUDY DESIGN: Twelve healthy primigravid women were studied seriall y from late pregnancy until 20 weeks after delivery. Erythrocyte sodiu m pump rate constant, maximum velocity, and sodium affinity were calcu lated from the ouabain-sensitive sodium flux measured in whole blood a nd in erythrocytes in which sodium content had been altered with the i onophore nystatin. The Student t test was used to compare the regressi on coefficients of the values plotted against log time for specific pe riods. RESULTS: The sodium pump rate constant, maximum velocity, and s odium affinity were lower 20 weeks after delivery than in late pregnan cy (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, wherea s the decrease in maximum velocity and Michaelis-Menten constant did n ot commence until after 2 weeks. Erythrocyte sodium content was greate r 20 weeks after delivery than in late pregnancy (4.71 +/- 0.20 vs 4.1 4 +/- 0.15 mmol/kg cells) and the increase was gradual over the time s tudied. CONCLUSIONS: After delivery the rate constant of the sodium pu mp measured in plasma and the erythrocyte sodium content changed befor e any significant alteration in the maximum velocity of the pump. The return of sodium pump function to the nonpregnant state continues beyo nd 6 weeks after delivery.