A reduction in the activity of placental 11-β-hydroxysteroid dehydrogenase (11-β-HSD-2), which converts cortisol to cortisone, correlates directly with low birth weight (Seckl 2001). Syncytiotrophoblast NHE activity is reduced in pregnancies that result in offspring that are small for gestational age (Johansson et al, 2002). The activity of renal epithelial NHE has been shown to be stimulated by cortisol (Ambuhl et al, 1999). In this study we hypothesised that cortisol stimulates the activity of syncytiotrophoblast NHE’s in human placenta.

Villous fragments from term human placenta (obtained as approved by Local Ethics Committee) were incubated in Tyrode’s (containing in mM; NaCl 135, KCl 5, CaCl₂ 1.8, MgCl₂ 1, MOPS 10 and glucose 5) with 1µM of the intracellular pH (pH_i) sensitive dye, BCECF (Molecular Probes), for 5 min at 37°C, washed in Tyrode’s without BCECF and incubated at 37°C for 5 min. BCECF in the syncytiotrophoblast was excited at 440 and 490 nm wavelengths and emitted light (540 nM), detected using a CoolSnap camera (Roper Scientific), with data acquisition using MetaFluor software (Universal Imaging Corporation). NHE activity was measured as the rate of Na⁺-induced recovery of pH_i following an acid load, imposed by prepulsing with 20 mM NH₄Cl, expressed as pH units/30sec, mean ± S.E.M., n = number of placentas. Carbenoxolone (100 µM), an inhibitor of 11-β-HSD-2, and/or cortisol (1 µM) were added for 3 min, in the absence of Na⁺ following the NH₄Cl pulse and in Na⁺ Tyrode’s during the recovery period.

Resting pH_i in the syncytiotrophoblast was 7.34 ± 0.05 (n = 20). Control rate of recovery from an acid load was 0.10 ± 0.02 pH units/30sec (n = 11) and this was Na⁺ dependent and inhibited by 500 µM amiloride (to 0.018 ± 0.008; n = 5, P < 0.05 Student’s t test). Neither carbenoxolone alone (0.12 ± 0.02; n = 9), nor cortisol (0.12 ± 0.02, n = 8) had an effect on basal recovery. However adding cortisol and carbenoxolone together significantly elevated the rate of recovery to 0.31 ± 0.10 (P < 0.05, ANOVA followed by Bonferroni’s post hoc test, n = 9). This elevated recovery was Na⁺ dependent and inhibited by amiloride.

In conclusion the activity of syncytiotrophoblast NHE’s was increased by acutely elevated cortisol levels. An altered ability to regulate syncytiotrophoblast pH_i may alter the activity of many transporters which are acid sensitive and important for transfer of nutrients essential for normal fetal growth (Sibley et al, 2002).

This work was supported by the Wellcome Trust