Aldosterone-induced glycoproteins: electrophysiological-biochemical correlation

Aldosterone induces the synthesis of a group of glycoproteins (GP65,70) in toad urinary bladders which are potential effectors of the natriferic action of this hormone. In the present study we have confirmed that aldosterone produces a two-phase electrophysiological response. During the early phase (<3 h) short-circuit current and transepithelial conductance increase in parallel, while during the late phase (>3 h) short-circuit current continues to increase without any further change in conductance. By biosynthetically labeling aldosterone-treated toad bladders with [³⁵S]methionine either during the early (h 0-2 or 1-3) or the late (h 4-6 or 7-9) phases of the natriferic response, we have demonstrated that GP65,70 is synthesized as a late effect of aldosterone. Since synthesis of GP65,70 occurs at a time when the electromotive force of the Na⁺ pump is increasing, and since GP65,70 biochemically resembles the beta subunit of Na⁺/K⁺-ATPase, studies were undertaken to examine whether GP65,70 is the beta subunit. Purified amphibian renal beta subunit was analyzed by two-dimensional polyacrylamide gel electrophoresis and was found to have an isoelectric point and M_r value similar to those of GP65,70. However, when nitrocellulose blots containing wheat germ agglutinin-purified proteins from aldosterone-treated bladders were stained with monospecific polyclonal antibodies developed against the beta subunit, GP65,70 was not recognized, whereas a group of slightly more acidic proteins of similar M_r were recognized. Thus GP65,70 is not the beta subunit of Na⁺/K⁺-ATPase. Further studies are needed to determine the cellular function of GP65,70.