Modulation of cytoskeletal organization and cytosolic granule distribution by verapamil in amphibian urinary epithelia

The present study examines the role of calcium in modulating epithelial cytomorphology by using verapamil, a calcium antagonist, and considering its effects on cytosolic granule distribution and exocytosis in toad urinary bladder. The effect of verapamil on the detection and distribution of microfilaments in toad urinary bladder using immunogold labeling techniques in toad urinary bladder epithelial cells was also examined. Verapamil, which inhibits antidiuretic hormone (ADH)-mediated water flow, increased the number, size and distribution of dense calcium-containing secretory granules in bladder epithelial cells. This calcium antagonist prevented granule exocytosis, such that, six-times the number of granules were present in verapamil-treated tissues. The normal cytomorphological changes that accompany the actions of ADH were attenuated by verapamil, including ADH-induction of microvilli. ADH increased the number of actin microfilaments as determined using protein A-gold by immunolabeling, whereas, verapamil treatment was unremarkable as compared to control. The results suggest that calcium may play a prominent role in mediating granule exocytosis and membrane fusion events that normally accompany hormone action.