Female sex steroids modulate alveolar epithelial sodium transport
Alveolar fluid clearance from the distal airspaces is mainly driven by vectorial sodium transport, where sodium enters the alveolar epithelial cells through apically located sodium channels (ENaC) and is extruded by basolaterally operating Na-K-ATPases. Hence sodium transport plays a crucial role in the immediate postnatal alveolar fluid clearance or the reabsorption of pulmonary edema. Women with acute respiratory distress have higher alveolar fluid clearance and higher survival than males and female preterm infants have higher survival rates and less pulmonary disease than males. It has been assumed that female sex steroids are responsible for these gender-related differences in morbidity and mortality. The female sex steroids estrogen (E2) and progesterone (P) are involved in many developmental processes including the acquisition of a proper lung function. To analyze the effects of E2 and P on epithelial sodium transport isolated alveolar epithelial cells from 18–19-day gestational age rat fetuses, grown in serum-free media supplemented with E2 and P were studied. The estrogen receptor (ER)-β mRNA was found in the isolated cells, but ER-α mRNA was not detected. Real-Time PCR revealed an increase of α- and β-ENaC mRNA in the media supplemented with E2 and P. α-ENaC mRNA expression showed an up to 3-fold higher expression in the presence of high P concentrations. The β-ENaC mRNA expression revealed an almost 2-fold increase compared to controls and was almost equal in the different E2 and P containing media. The mRNA-level of γ-ENaC was not altered, but the Na-K-ATPase-β1 subunit was elevated by about 50% under the influence of E2 and P. The CFTR-mRNA expression was also increased by E2 and P supplementation. Single-channel Patch Clamp analysis showed the existence of highly-selective and non-selective ENaC in the examined cells. The percentage of responsive patches increased from 45% in non-supplemented media to 91% in the presence of E2 and P. Short-circuit current (I SC in µA/cm2) measurement employing Ussing chambers showed that the baseline, amiloride- and ouabain-sensitive I SC was elevated by E2 and P in a dose-dependent manner. The baseline I SC was increased from 10.05±0.25 to 12.09±0.69, amiloride-sensitive I SC from 7.80±0.20 to 9.87±0.50 and ouabain-sensitive I SC from 8.18±0.22 to 10.24±0.51. These results demonstrate the impact of female sex steroids on epithelial sodium transport, revealing an increase of vectorial sodium transport by stimulating the activity and expression of the participating ion transporters.