Cytoplasmic involvement in ADH-mediated osmosis across toad urinary bladder.

Several lines of investigation have suggested that antidiuretic hormone (ADH) may have direct effects on the cytoskeletal organization of granular epithelial cells in the toad urinary bladder. To some extent, these effects are in concert with the well-established action of ADH on the hydraulic permeability of the mucosal plasma membrane, but it appears that other conformational adjustments (largely cytoplasmic) may be of comparable importance. The thrust of this review is that the hormone brings about a general restructuring of the granular cells so that the epithelium as a whole may function efficiently as an osmotic pathway. Details of cytoskeletal changes are far from clear as yet, but interference with or modulation of these particular effects infer that cytoplasmic organization is the seat of feedback control of osmotic flow rate, the basis for viability in the presence of dramatic cytosolic dilution and a major factor in the observed disparity in osmotic and diffusional permeability coefficients. In the interest of stimulating new thoughts and experiments in this area, a number of preliminary findings have been freely cited.