SHARP INCREASE IN RAT LUNG WATER CHANNEL EXPRESSION IN THE PERINATAL-PERIOD

Three members of the water channel (aquaporin) family are expressed in adult rat lung: CHIP28 (AQP-1), MIWC (AQP-4), and AQP-5. Because wate r channels may be important in the clearance of fluid from the newborn lung, the expression of water channels just before and after birth wa s investigated using the ribonuclease (RNAse) protection assay. RNA wa s isolated from lungs, brain, and heart of prenatal rats (fetal days F 19, F20, and F21) and postnatal rats (days +1, +2, +5, +7, +21, and ad ult). Transcript expression was measured relative to a beta-actin cont rol by quantitative densitometry. Whereas beta-actin mRNA expression w as nearly constant over time, distinct expression patterns were observ ed for the three water channels. CHIP28 mRNA expression rose slowly fr om days F19 to +1, then strongly at day +2, and remained elevated over the first week. MIWC mRNA was weakly expressed prenatally, but strong ly increased just after birth. AQP-5 mRNA increased slowly and monoton ically between days F20 and +7. These patterns contrasted sharply with the developmental expression of CHIP28 in heart, which decreased over time, and MIWC in brain. Immunocytochemistry showed CHIP28 protein ex pression in capillary endothelia and MIWC in airway epithelia by day 1; quantitative immunoblot analysis showed increased CHIP28 protein ex pression over time. These findings are consistent with a role of lung water channels in perinatal fluid clearance; however, proof of physiol ogic significance will require functional measurements of air space-ca pillary water permeability.