Water permeability and particle aggregates in ADH-, cAMP-, and forskolin-treated toad bladder

Osmotic water flow was used to evaluate total tissue water permeability (Ptissue), and luminal membrane particle aggregates, presumed sites for transmembrane water movement, were quantified to assess luminal membrane water permeability, in bladders treated with maximally stimulating concentrations of antidiuretic hormone (ADH), adenosine 3',5'-cyclic monophosphate (cAMP), and forskolin. Aggregates were as numerous and occupied the same fractional area of the luminal membrane in response to cAMP treatment (10 mM) as treatment with ADH (20 mU/ml). Ptissue in cAMP-treated tissues, however, was only half of that induced by ADH (P less than 0.001). A similar disparity in the relationship between aggregates and Ptissue occurred for additional bladders treated with 50 microM forskolin, which is known to increase endogenous cAMP to levels much greater than caused by maximally stimulating concentrations of ADH. Although Ptissue achieved with forskolin was the same in paired bladders treated with ADH, aggregates were far more numerous (P less than 0.05) and occupied much more membrane area (P less than 0.05) with forskolin. These observations are consistent with the view that aggregate appearance in the luminal membrane is a function of intracellular cAMP. The finding that the hydrosmotic response of toad bladder to both cAMP and forskolin compared with ADH stimulation is reduced relative to measured changes in luminal membrane aggregates suggests that the effect of ADH in altering water permeability involves additional regulation via a non-cAMP-mediated mechanism. This latter event would appear to be by an effect of ADH on the permeability of a resistance at a postluminal membrane site and/or possibly on the permeability of aggregates in the luminal membrane.