Renal response to an acute protein challenge in pregnant women with borderline hypertension

Background: The renal reserve (RR), assessed after an oral protein challenge or the intravenous administration of amino acids, is still present in healthy pregnant women (NP), although resting glomerular filtration rate (GFR) and renal plasma flow (RPF) increase progressively throughout normal gestation. No studies have addressed this issue in hypertensive gravidas; the aim of this trial was to evaluate renal response to an acute protein load (PL) in NP and pregnant women with borderline hypertension (HP). Methods: Five NP, eight HP and eight healthy non-pregnant women (CG) were evaluated. After fasting overnight, all subjects received an oral water load (20 mL/kg of body weight), the urinary output was then replaced orally with equal volumes of water. After two 30 min periods, an 80 g PL was provided. Creatinine clearance (C-Cr) was measured every 30 min from 1 h before and for 4 h following PL. Participants remained recumbent during the study, bladder emptiness was assessed by ultrasound immediately after each micturition. Baseline C-Cr was taken as the average of two 30 min periods before PL and peak C-Cr as the maximal C-Cr recorded thereafter. Results: The groups were similar with regard to age, weight or gestation age. Baseline C-Cr (NP: 118.5 +/- 6.0, HP: 127.4 +/- 6.7 and CG: 99.8 +/- 2.9 mL/min, P = 0.004 (CG vs NP and HP), increased after PL to NP: 223.5 +/- 9.8 to HP: 178.5 +/- 13 and to CG: 149.1 +/- 4.0 mL/min, P < 0.0004 (CG vs HP, CG vs NP and NP vs HP)). Peak minus baseline C-Cr was 97.3 +/- 10.1; 46.3 +/- 12.7 and 48.3 +/- 4.8 for NP, HP and CG, respectively (P < 0.006 HP vs CG and NP). The peak C-Cr was obtained significantly earlier in both pregnant groups (Period 3) compared with the healthy non-pregnant women (Period 5) (P = 0.02). The fractional proximal reabsorption of sodium (FPRNa+) at peak C-Cr was similar in the groups (NP: 0.74 +/- 0.01 HP: 0.78 +/- 0.02 and CG: 0.74 +/- 0.03, P = not significant (NS)) as was the distal delivery of sodium (DDNa+) (NP: 5.8 +/- 0.5; HP: 4.1 +/- 0.5 and CG: 4.3 +/- 0.4 meq/min, P = NS). Fractional excretion of urea (%) increased from 91.4 +/- 5.5 to 105.5 +/- 9.8%; 80.7 +/- 8.0 to 97.3 +/- 9.8; and 44.4 +/- 7.8 to 86.0 +/- 7.1 in NP, HP and CG, respectively (P = NS). There was a trend towards a poorer maternal and fetal outcome in the HP group. Conclusion: Mid-term borderline HP failed to increase CCr as much as NP did after a protein challenge, suggesting altered functional response of the nephron or lessened sensitivity of renal vasculature to additional vasodilator stimuli. These results support the interest of additional prospective studies with a larger number of patients to confirm these findings and evaluate the value of RR tests as predictors of outcome of pregnancies at risk.