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Departments of 1 Internal Medicine and 2 Pediatrics, University of Iowa College of Medicine and Veterans Affairs Medical Center, Iowa City, Iowa 52242
Na+
absorption via amiloride-sensitive
Na+ channels is of critical
importance in the transition between fetal and neonatal life in several
tissues, including the colon, lung, and kidney. To characterize and
contrast the mRNA expression of each of the three epithelial
Na+ channel complex (ENaC)
subunits, we conducted RNase protection assays (RPA) and in situ
hybridization in colon and lung in fetal (17, 19, 20, and 21 days) and
postnatal (1, 3, 9, 15, and 30 days) rats (r). In the colon the 
-,
-, and 
-rENaC subunits showed quantitatively different but
qualitatively similar expression. All three subunits gradually
increased in abundance from fetal day
19 through day 30 of
life. The amount of each subunit on day 30 was approximately three times the amount at
day 1. In situ hybridization showed
that each subunit was localized to the surface epithelial cells with
minimal expression in the crypts. The lung showed a completely
different pattern. In contrast to the colon, the total amount of
-rENaC mRNA (by RPA) in the lung increased dramatically from
fetal day 19 to
21, whereas - and -rENaC showed modest prenatal increases. The amounts of all three mRNAs fell after
birth through day 9 (to about 75% of
the day 1 value). On days 15 and
30 the amount of mRNA rose to approach
the values on day 1. -rENaC mRNA
abundance always exceeded - and -rENaC, and the quantitative
expression was different for - than for - and -rENaC. In situ
hybridization studies showed that all three subunits were expressed in
epithelial cells of the bronchi, bronchioles, and alveoli and not in
blood vessels. These studies show striking developmental heterogeneity
in rENaC mRNA expression between lung and colon, probably reflecting
different developmental regulatory mechanisms in these organs.
glucocorticoid; mineralocorticoid; epithelial sodium channel complex
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