Regulation of luminal membrane water permeability by water flow in toad urinary bladder

Intramembranous particle aggregates (presumed sites for water flow) which appear in the luminal membrane consequent to ADH treatment are derived from cytoplasmic membrane structures (now termed "aggrephores") which fuse with the luminal membrane. We have previously shown that bladders stimulated in the absence of an osmotic gradient have about twice as many aggregates and about three times as many sites of aggrephore fusion as bladders stimulated with ADH in the presence of a 175 milliosmolal gradient. The present studies show that the frequency of fused aggrephores and luminal membrane aggregates can be modified as a consequence of alterations in transmembrane water flow initiated by changing the transbladder osmotic gradient during hormone stimulation. Bladders treated with ADH for 1 hr without a gradient and then for 1 hr with a gradient had approximately 1/3 as many aggregates and fusion sites as paired bladders treated for 2 hr without a gradient. Conversely, bladders treated with ADH for 1 hr with a gradient and then for 1 hr without a gradient had approximately 2x as many aggregates and fusion sites as bladders treated for 2 hr with a gradient. In other experiments we demonstrate that the time course of hormone washout is greatly accelerated if carried out in the presence of an osmotic gradient. In paired bladders that were first stimulated with ADH for 30 min in the absence of a gradient, aggregates and fusion sites as well as osmotic water permeability determined in fixed bladders, persisted at near maximum levels for 15 min of washout in the absence of a gradient.(ABSTRACT TRUNCATED AT 250 WORDS)