Contraction of epithelial (MDCK) cells in response to low extracellular calcium is dependent on extracellular sodium

Like other cells of epithelial origin, MDCK cells respond with a reversible structural transformation to a diminution in the concentration of extracel lular Ca2+. Upon deprivation of Ca2+ in the medium the cells undergo an act ive contraction mediated by the actin-myosin cytoskeleton, in parallel to d etachment of the intercellular contacts and appearance of free spaces in th e epithelium or monolayer (Castillo et al., 1998). We now present results i ndicating that the decrease of external Ca2+ plays an indirect and non-spec ific role in activating contraction, probably by allowing an influx of Na+. The omission of external Ca2+ had no effect when it was replaced by Mg2+, Ba2+ or Hg2+, and the addition of any of these divalent cations induced rel axation of cells previously contracted by exposure to low Ca2+. A null or w eak response was observed also when Ca2+ was lowered in a solution where Na + was replaced by choline or in the presence of amiloride (30 mu M), which reduces the permeability of the plasma membrane to Na+. Restitution of Naor removal of amiloride were followed by contraction in the same cultures. Li+ proved an able substitute of Na+ as requisite for cell contraction in r esponse to Ca2+ depletion. Monensin (0.1 mM) -an ionophore selective for Na +- and to a lesser extent ouabain (0.1 mM) -an inhibitor of Na+ extrusion a cross the plasma membrane- , both stimulated contraction in the presence of the normal level of external Ca2+. Decreasing by half the normal concentra tion of external K+ facilitated cell contraction, but typical responses wer e observed when K+ was increased to 40 mM by partial substitution for Na+. These findings attest that cell contraction in response to low Ca2+ is like ly due to an increase in the permeability of the plasma membrane to Na+, th ough not to membrane depolarization as such. Evidences from other motile sy stems suggest that Na+ influx might in turn cause an elevation of cytoplasm ic Ca2+, which activates the actin-myosin cytoskeleton.