Effect of osmotic gradient on ADH-induced intramembranous particle aggregates in toad bladder

Paired toad urinary bladders were prepared without or with an osmotic gradient (175 mosm) across them, stimulated for 2.5 (n=6), 5 (n=6), 30 (n=6) or 60 (n=6) min with ADH (20 mU/ml), and studied by freeze-fracture electron microscopy. Water permeability at these times was assessed in additional bladders (n=6 for each case) after tissue fixation according to the technique of Eggena. After both 60 and 30 min of ADH stimulation, the presence of a gradient compared with the absence of one was associated with fewer aggregates (242±35vs. 382±14 ×235 μm−2 at 60 min,P<0.01; 279±36vs. 470±51 ×235 μm−2 at 30 min,P<0.01) and lower water permeability (8.4±1.1vs. 18.8±1.8μg×min−1×cm−1 ×mosm −1 at60min,P<0.005; 9.2±1.0vs. 22.0±2.1 μg ×min−1×cm−2×mosm −1 at 30 min,P<0.001). In addition, with a gradient both maximum water permeability and maximum aggregate frequency were reached nearly together; a similar correspondence occurred without a gradient. We conclude that in the presence of an osmotic gradient both the ADH-associated aggregates and the water permeability response to ADH are prevented from reaching the higher levels observed in bladders not exposed to a gradient.