Na+-K+-ATPase activity in alveolar epithelial cells increases with cyclic stretch

Na+-K+-ATPase pumps (Na+ pumps) in the alveolar epithelium create a transepithelial Na+ gradient crucial to keeping fluid from the pulmonary air space. We hypothesized that alveolar epithelial stretch stimulates Na+ pump trafficking to the basolateral membrane (BLM) and, thereby, increases overall Na+ pump activity. Alveolar type II cells were isolated from Sprague-Dawley rats and seeded onto elastic membranes coated with fibronectin or 5-day-conditioned extracellular matrix. After 2 days in culture, cells were uniformly stretched for 1 h in acustom-made device. Na+ pump activity was subsequently assessed by ouabain-inhibitable uptake of Rb-86(+),a K+ tracer, and BLM Na+ pump abundance was measured. In support of our hypothesis, cells increased Na+ pump activity in a "dose-dependent" manner when stretched to 12, 25, or 37% change in surface area (DeltaSA), and cells stretched to 25% DeltaSA more than doubled Na+ pump abundance in the BLM. Cells on 5-day matrix tolerated higher strain than cells on fibronectin before the onset of Na+ pump upregulation. Treatment with Gd3+, a stretch-activated channel blocker, amiloride, a Na+ channel blocker, or both reduced but did not abolish stretch-induced effects. Sustained tonic stretch, unlike cyclic stretch, elicited no significant Na+ pump response.