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Am J Physiol Lung Cell Mol Physiol 277: L727-L736, 1999;
1040-0605/99 $5.00
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Vol. 277, Issue 4, L727-L736, October 1999

cAMP stimulates Na+ transport in rat fetal pneumocyte: involvement of a PTK- but not a PKA-dependent pathway

Naomi Niisato, Yasushi Ito, and Yoshinori Marunaka

Lung and Cell Biology and Medical Research Council Group in Lung Development, Department of Pediatrics and Institute of Medical Science, The Hospital for Sick Children Research Institute, University of Toronto Faculty of Medicine, Toronto, Ontario, Canada M5G 1X8

To study a cAMP-mediated signaling pathway in the regulation of amiloride-sensitive Na+ transport in rat fetal distal lung epithelial cells, we measured an amiloride-sensitive short-circuit current (Na+ transport). Forskolin, which increases the cytosolic cAMP concentration, stimulated the Na+ transport. Forskolin also activated cAMP-dependent protein kinase (PKA). A beta -adrenergic agonist and cAMP mimicked the forskolin action. PKA inhibitors KT-5720, H-8, and myristoylated PKA-inhibitory peptide amide-(14---22) did not influence the forskolin action. These results suggest that forskolin stimulates Na+ transport through a PKA-independent pathway. Furthermore, forskolin increased tyrosine phosphorylation of ~70- to 80-, ~97-, and ~110- to 120-kDa proteins. Protein tyrosine kinase (PTK) inhibitors (tyrphostin A23 and genistein) abolished the forskolin action. Moreover, 5-nitro-2-(3-phenylpropylamino)benzoate (a Cl--channel blocker) prevented the stimulatory action of forskolin on Na+ transport via abolishment of the forskolin-induced cell shrinkage and tyrosine phosphorylation. Based on these results, we conclude that forskolin (and cAMP) stimulates Na+ transport in a PTK-dependent but not a PKA-dependent pathway by causing cell shrinkage, which activates PTK in rat fetal distal lung epithelial cells.

sodium transport; adenosine 3',5'-cyclic monophosphate; tyrosine phosphorylation; protein kinase A; protein tyrosine kinase; alveolar epithelium; amiloride


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