Relation of ADH effects to altered membrane fluidity in toad urinary bladder

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Abstract
Membrane fluidity, urea permeability and osmotic water permeability in toad urinary bladder are regularly enhanced by antidiuretic hormone (ADH). Organized intramembranous particle aggregates, which correlate specifically with hormonally stimulated water permeability, are found in granular cell luminal membranes consequent to ADH stimulation. ADH-stimulated changes in urea and osmotic water permeability and luminal membrane aggregates at room temperature (24.8 .+-. 0.4.degree. C) and in the cold (10.6 .+-. 0.2.degree.) were compared with corresponding changes in membrane fluidity, as assessed by n-butyramide permeability. Although a critical level of membrane fluidity is undoubtedly required, the occurrence of aggregates in the luminal membrane is independent of an accompanying hormonally induced change of membrane fluidity. ADH-stimulated osmotic water permeability in toad bladder is also independent of the coincident change in membrane fluidity, and as a process almost certainly involves membrane channels, not a solubility-diffusion process through membrane lipids. For ADH-stimulated transbladder urea movement, channels seem to be involved as well, and the change induced in membrane fluidity by ADH could be an underlying factor in their formation.