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Articles in PresS, published online ahead of print November 30, 2001
Am J Physiol Lung Cell Mol Physiol, 10.1152/ajplung.00356.2001
Submitted on September 4, 2001
Accepted on November 26, 2001
1 Pediatrics, Emory University, Atlanta, GA, USA
2 Physiology, Emory University, Atlanta, GA, USA; Pediatrics, Emory University, Atlanta, GA, USA
3 Pediatrics, Emory University, Atlanta, GA, USA; Physiology, Emory University, Atlanta, GA, USA
* To whom correspondence should be addressed. E-mail: deaton{at}emory.edu.
We investigated the mechanism by which cAMP increases sodium transport in lung epithelial cells. As we have previously shown, apical membranes of alveolar type-II (ATII) cells have two distinct types of amiloride-sensitive, sodium-permeable, cation channels: a non-selective cation channel (NSC) and a highly selective channel (HSC), the expression of which is determined by culture conditions. Exposure of ATII cells to cAMP, ß-adrenergic agonists, or other agents known to increase adenylyl cyclase activity, increased the activity of both types of channels, albeit by different mechanisms. In cell attached patches, the open probability (Po) of NSC channels increased several fold when exposed to terbutaline, isoproterenol, forskolin, and cAMP analogues. However, there was no change in the number of NSC channels. In contrast, pretreatment of ATII cells with terbutaline led to an increase in the number of HSC channels in cell attached patches with no significant change in the Po of individual HSC channels. For both channel types, the terbutaline effect was blocked by propranolol and H89 (a protein kinase A inhibitor) suggesting a requirement for activation of PKA for ß-adrenergic-induced changes in channel activity. Treatment with terbutaline did increase cAMP levels in ATII cells, but intracellular calcium levels also increased. Sequestration of intracellular calcium with the calcium chelator, BAPTA, blocked the ß-adrenergic-induced increase in the Po of NSC channels but did not alter the activation of HSC channels. These observations suggest that ß-adrenergic stimulation leads to an increase in intracellular cAMP and activation of PKA. PKA promotes an increase in the number of HSC channels and an increase in intracellular calcium. The increase in intracellular calcium increases the Po of NSC channels. Agents that increase cellular cAMP levels are likely to be effective in inducing lung sodium transport in physiologic and pathologic states regardless of which channel type is present.
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