ROLES OF NA+ AND CA2+ IN THE INHIBITION BY LOW PH OF THE HYDROOSMOTIC RESPONSE TO SEROSAL HYPERTONICITY IN TOAD BLADDER

The hydrosmotic responses to serosal hyperosmolar solutions were studied in isolated urinary bladders of toads (Bufo marinus) at different pH values to see whether the increase in watet flux obtained with serosal hypertonicity in urinary bladders of amphibia is due to non-specific mechanisms or whether it is related to specific ionic regulation. The increase in net water flux elicited by serosal hypertonicity (Ringer + 250 mM mannitol {reversed tilde equals} 430 mosM) at pH 8.2 is inhibited by lowering the pH to 6.6. The same inhibition is obtained if pH is lowered when the response is fully developed. The inhibition of the development is reversible upon return to pH 8.2, provided serosal Na+ and Ca2+ are present. The reversibility is incomplete in Ca2+-free solutions and is completely abolished in Na+-free solutions. It is concluded that the hydrosmotic response to serosal hypertonicity in the amphibian bladder is not a passive phenomenon. It is suggested that during the osmotic challenge there is an increase in a H+-sensitive Na+ permeability at the basolateral membrane which may be related to the intracellular events that lead to the permeability changes. Serosal Ca2+ could be important in the maintenance of the water permeability in the developed response to hypertonicity. A hypothesis is presented concerning the possibility of similar mechanisms in the modulation of the hydrosmotic responses to antidiuretic hormone and to serosal hypertonicity. © 1979.