NHE activity in human placental syncytiotrophoblast is stimulated acutely by cortisol (CORT) in the presence of the 11-β-hydroxysteroid dehydrogenase type 2 (11-β-HSD-2) inhibitor, carbenoxolone (CBNX, Speake et al. 2004a). In this study we investigated the involvement of glucocorticoid (GR) and mineralocorticoid (MR) receptors in this response and whether there is a gender effect. Villous fragments from term human placenta (obtained as approved by Local Research Ethics Committee) were incubated in Tyrode solution (containing in mM; NaCl 135, KCl 5, CaCl₂ 1.8, MgCl₂ 1, Mops 10 and glucose 5, pH 7.4) with 1μM of the intracellular pH (pH_i) sensitive dye, BCECF-AM (Molecular Probes), for 5min at 37°C, washed in Tyrode solution without BCECF and incubated at 37°C for 5min. BCECF fluorescence and NHE activity were measured as described previously (Speake et al. 2004b) and expressed as pH units/s (mean±S.E.M., n=number of placentas). CBNX (100μM) and/or CORT (1μM,) were added for 3min, in the absence of Na⁺ following the 5min, 20mM NH₄Cl pulse and in Na⁺ Tyrode solution during the recovery period. Spironolactone (1μM), an inhibitor of MR, and mefiprestone (1μM), an inhibitor of GR were added to the incubation medium during the NH₄Cl pulse and were present in all subsequent solution changes. There was no difference in the resting pH_i between male and female syncytiotrophoblast (7.34±0.03, n=7 compared to 7.36±0.05, n=10, respectively). The control rate of recovery from an acid load was 0.0050±0.0007pH units/s (n=17) and there was no difference between male or female placentas. Combined application of CBNX and CORT significantly (p<0.05 unpaired Student's| t test) increased the rate of recovery in female (0.0089±0.0015pH units/s, n=10) compared to male (0.0060±0.0012pH units/s, n=7) placentas. In female placentas the increase in recovery with the combined incubation of CBNX and CORT was significantly reduced to levels no different from controls by mefiprestone (0.0042±0.0009pH units/s, n=10 p<0.05 repeated measured ANOVA followed by Dunns’ post hoc test) and unaffected by spironolactone (0.0067±0.0012pH units/s, n=10). In conclusion the activity of syncytiotrophoblast NHE's is increased by acutely elevated CORT levels only in the presence of CBNX in female placentas only and this increase is mediated through GR. The gender dependent effect may be due to differences in 11-β-HSD-2 expression or activity (Murphy et al. 2003) and/or differential GR expression between male and female placentas (Clifton & Murphy, 2004).