FIBROBLASTS CONTRACTING COLLAGEN MATRICES FORM TRANSIENT PLASMA-MEMBRANE PASSAGES THROUGH WHICH THE CELLS TAKE UP FLUORESCEIN ISOTHIOCYANATE-DEXTRAN AND CA2+

When fibroblasts contract collagen matrices, the cells activate a Ca2-dependent cyclic AMP signaling pathway. We have found that contractio n also stimulates uptake of fluorescein isothiocyanate-dextran molecul es from the medium. Our results indicate that fluorescein isothiocyana te-dextran enters directly into the cell cytoplasm through 3- to 5-nm plasma membrane passages. These passages, which reseal in less than 5 s in the presence of divalent cations, also are likely sites of Ca2+ u ptake during contraction and the first step in contraction-activated c yclic AMP signaling. The formation of plasma membrane passages during fibroblast contraction may reflect a general cellular response to rapi d mechanical changes.