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

Abstract

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 a custom-made device. Na(+) pump activity was subsequently assessed by ouabain-inhibitable uptake of (86)Rb(+), 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 Gd(3+), 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.