AN IN-VITRO TEST OF THE CELL STRETCH-PROLIFERATION HYPOTHESIS OF RENAL CYST ENLARGEMENT

In renal cystic disease, fluid accumulation within cyst lumens might s tretch cyst walls and in this way stimulate cell proliferation. To tes t this idea, the effects of mechanical stretch on Madin-Darby canine k idney cells grown as cysts in a hydrated collagen gel or as monolayers on collagen-coated Flexcell membranes were examined. The percentage o f cells synthesizing DNA (labeling index) was determined by measuring bromodeoxyuridine incorporation and counting cell numbers. The distens ion of single cysts for 1 h by the intraluminal injection of saline fa iled to produce a significant increase in labeling index. The exposure of cysts for 2.5 h to 1 mM dibutyryl cAMP + 0.1 mM isobutylmethylxant hine led to a 37% increase in luminal surface area (due to stimulated fluid secretion) and a 30% increase in labeling index. The stretch (25 %) of Madin-Darby canine kidney monolayers approximately doubled the l abeling index between 12 and 24 h after starting the stretch. After 48 h, the cell population density was significantly increased (P < 0.001 ), from 41.9 +/- 0.2 (SE; N = 12) to 48.2 +/- 0.5 (N = 12) cells/10,00 0 mu m(2). The labeling index increased linearly with applied stretch, from 7.2 +/- 0.3% (N = 36) with no stretch to 16.2 +/- 1.0% (N = 6) w ith 30% stretch. Stretch had to be maintained for 8 h or more to produ ce an increase in labeling index at 18 h. No evidence was obtained for the release of a diffusible growth factor by stretched monolayers. Th e increase in labeling index induced by stretch was unaffected by 50 m u M gadolinium, a stretch-activated channel blocker, but was abolished by 5 mu g/mL cytochalasin B, an actin microfilament-disrupting agent. It was concluded that prolonged stretch stimulates renal epithelial c ells to synthesize DNA. This supports the idea that increased wall ten sion in renal cysts may stimulate cell proliferation and, thereby, may contribute to cyst enlargement.