Mechanisms of increased Na⁺ transport in ATII cells by cAMP: We agree to disagree and do more experiments

Existing evidence supports the presence of active transport of Na⁺ across the mammalian alveolar epithelium and its upregulation by agents that increase cytoplasmic cAMP levels. However, there is controversy regarding the mechanisms responsible for this upregulation. Herein we present the results of various patch-clamp studies indicating the presence of 25- to 27- pS, amiloride-sensitive, moderately selective Na⁺ channels (Na⁺-to-K⁺ permeability ratio = 7:1) located on the apical membranes of rat alveolar type II (ATII) cells maintained in primary culture. The addition of terbutaline to the bath solution increased the open probability of single channels present in cell-attached patches of ATII cells without affecting their conductance. A similar increase in open probability was seen after the addition of protein kinase A, ATP, and Mg²⁺ to the cytoplasmic side of inside-out patches. Measurement of short-circuit currents across confluent monolayers of rat or rabbit ATII cells indicates that terbutaline and 8-(4- chlorophenylthio)-cAMP increase vectorial Na⁺ transport and activate Cl^- channels. Currently, there is a controversy as to whether the cAMP-induced increase in Na⁺ transport is due solely to hyperpolarization of the cytoplasmic side of the ATII cell membrane due to Cl^- influx or whether it results from simultaneous stimulation of both Cl^- and Na⁺ conductive pathways. Additional studies are needed to resolve this issue.