Mechanical responses of single non-confluent epithelial cells to low extracellular calcium

Single non-confluent MDCK cells respond immediately to a sharp decrease in extracellular Ca2+ (< or = 5 microM) with an intense reversible retraction, along with an increase in cell height, correlating in overall rate and extent with initial cell size. Optical sectioning of individual cells by confocal microscopy showed that this structural response, observed in about 50% of the population, involves narrowing and even furrowing near the base of the cell by a thickened peripheral belt of actin filaments, which remains associated with the cortex instead of being internalized in the cytoplasm. Single cells retracted significantly in response to low Ca2+ under conditions that have been found largely inhibitory for retraction of confluent cells, such as Ca2+ replacement with Ba2+ and the substitution of Na+ with choline, a non-permeant cation. Conversely, the Na(+)-ionophore monensin applied in the normal-Ca2+ medium elicited by itself an earlier and much greater retraction in single cells than in confluent cultures. These observations indicate that single cells can retract more readily than those forming confluent cultures, suggesting that retraction in typical monolayers is resisted in part by the cell junctions. According to this view, inward actin-myosin mediated tension around the periphery of individual cells precedes and probably helps dissociation of E-cadherins in confluent cultures exposed to low Ca2+.