Impact of (cid:98) -adrenergic agonist on Na (cid:49) channel and Na -ATPase expression in alveolar type II cells

Impact of (cid:98) -adrenergic agonist on Na (cid:49) channel and Na (cid:49) -K (cid:49) -ATPase expression in alveolar type II cells. Am. J. Physiol. 275 ( Lung Cell. Mol. Physiol. 19): L414–L422, 1998. —It has been shown that short-term (hours) treatment with (cid:98) -adrenergic agonists can stimulate lung liquid clearance via augmented Na (cid:49) transport across alveolar epithelial cells. This increase in Na (cid:49) transport with short-term (cid:98) -agonist treatment has been explained by activation of the Na (cid:49) channel or Na (cid:49) -K (cid:49) ATPase by cAMP. However, because the effect of sustained stimulation (days) with (cid:98) -adrenergic agonists on the Na (cid:49) transport mechanism is unknown, we examined this question in cultured rat alveolar type II cells. Na (cid:49) -K (cid:49) -ATPase activity was increased in these cells by 10 (cid:50) 4 M terbutaline in an exposure time-dependent manner over 7 days in culture. This increased activity was also associated with an elevation in transepithelial current that was inhibited by amiloride. The enzyme’s activity was also augmented by continuous treatment with dibutyryl-cAMP (DBcAMP) for 5 days. This increase in Na (cid:49) -K (cid:49) -ATPase activity by 10 (cid:50) 4 M terbutaline was associated with an increased expression of (cid:97) 1 -Na (cid:49) -K (cid:49) ATPase mRNA and protein. (cid:98) -Adrenergic agonist treatment also enhanced the expression of the (cid:97) -subunit of the epithelial Na (cid:49) channel (ENaC). These increases in gene expression were inhibited by propranolol. Amiloride also suppressed this long-term effect of terbutaline and DBcAMP on Na (cid:49) -K (cid:49) ATPase activity. In conclusion, (cid:98) -adrenergic agonists enhance the gene expression of Na (cid:49) -K (cid:49) -ATPase, which results in an increased quantity and activity of the enzyme. This heightened expression is also associated with augmented ENaC expression. Although the cAMP system is involved, the inhibition of enhanced enzyme activity with amiloride suggests that increased Na (cid:49) entry at the apical surface plays a role in this process. PKA activity). The total PKA activity present in the sample was determined by incubating the cell lysate with 40 µM of cAMP. The data were standardized by cellular protein content measured according to Bradford’s (10) method. The cells were plated at a density of 1.5–2.0 (cid:51) 10 6 /cm 2 . The medium was changed on the second day after isolation. Monolayers of cells on ﬁlters were maintained in a humidiﬁed 5% CO incubator at 37°C 4–6 days. difference (PD) measured with an EVOM epithelial voltmeter (World Precision FL). Transepithelial current ( I ; µA/ cm 2 ) was calculated from the relationship I (cid:53) PD/ R te . Statistics . Data are presented as means (cid:54) SE. Compari-sons between groups were analyzed by paired t -test, unpaired t -test, or ANOVA and post hoc comparison (Fisher protected least signiﬁcant difference) depending on the experimental design. We accepted P (cid:44) 0.05 as indicating statistically signiﬁcant differences.