The reversibility of the effects of ACTH and cytochalasin B on the ultrastructure and steroidogenic activity of adrenocortical tumor cells in vitro. 
We have demonstrated previously that the steroidogenic activity of ACTH on cultured adrenal tumor cells is associated with cell rounding and a rearrangement of microfilaments.  Cytochalasin B (CB) also induces cell rounding, but changes the conformation of microfilaments and severely inhibits steroidogenesis.  ACTH and CB may have different modes of action on the contractile machinery which are related to their opposing actions on steroidogenesis.  To investigate this possibility further, we have examined the reversibility of the morphological and functional effects of these agents.  Cultures were incubated for 1 hr, with and without ACTH (10 microU/ml of media), or with CB (50 micrograms/ml), or with both agents simultaneously.  After a media wash, the cultures were incubated for 1 hr, with and without ACTH.  The steroid production of the cells during pre- and post-washout incubations was determined, and some cultures were fixed for electron microscopy at the end of both incubation periods.  The three- to ten-fold increases in steroidogenic activity of ACTH-stimulated cells declined during recovery incubations, but remained well above basal values.  These cells nearly reflattened and began to regain stress fibers which had been 'pulled apart'.  The 'washed out' ACTH-stimulated cells were often refractory to restimulation.  Cells recovering from CB also reflattened.  Masses of filamentous felt induced by the drug disappeared from the cytoplasm, lost microvilli reappeared and stress fibers reformed.  The 20-50% inhibition of basal steroidogenesis by CB was completely reversed.  When ex-CB-treated cells were incubated with ACTH, their morphology and steroid production were typical of acutely stimulated cells.  The recovery behavior of cells incubated with ACTH and CB simultaneously reflected the observation that there were cell-specific responses to one agent or the other during initial incubations.  The persistence of heightened steroidogenic activity following a washout of ACTH and the rapid reversal of the effects of CB strongly support the concept that regulated actomyosin interactions are an integral part of the steroidogenic process.