CALCIUM-DEPENDENCE OF INTEGRITY OF THE ACTIN CYTOSKELETON OF PROXIMALTUBULE CELL MICROVILLI

To better define the role of Ca2+ in pathophysiological alterations of the proximal tubule microvillus actin cytoskeleton, we studied freshl y isolated tubules in which intracellular free Ca2+ was equilibrated w ith highly buffered, precisely defined medium Ca2+ levels using a comb ination of the metabolic inhibitor, antimycin, and the ionophore, iono mycin, in the presence of glycine, to prevent lethal membrane damage a nd resulting nonspecific changes. Increases of Ca2+ to greater than or equal to 10 mu M were sufficient to initiate concurrent actin depolym erization, fragmentation of F-actin into forms requiring high-speed ce ntrifugation for recovery, redistribution of villin to sedimentable fr actions, and structural microvillar damage consisting of severe swelli ng and fragmentation of actin cores. These observations implicate Ca2-dependent, villin-mediated actin cytoskeletal distribution in tubule cell microvillar damage under conditions conceivably present during pa thophysiological states. However, despite prior evidence for cytosolic free Ca2+ increases of the same order of magnitude and similar struct ural microvillar alterations, Ca2+- and villin-mediated events did not appear to account for the initial microvillar damage that occurs duri ng ATP depletion induced by antimycin alone or hypoxia.