OUABAIN-SENSITIVE NA-ATPASE ACTIVITY IN TOAD BRAIN(,K+)

Toads of the genus Bufo are highly resistant to the toxic effects of d igitalis glycosides, and the Na+,K+-ATPase of all toad tissues studied to date has been relatively insensitive to inhibition by digitalis an d related compounds. In studies of brain microsomal preparations from two toad species, Bufo marinus and Bufo viridis, inhibition of ATPase activity and displacement of [H-3]ouabain from Na+,K+-ATPase occurred over broad ranges of ouabain or bufalin concentrations, consistent wit h the possibility that more than one Na+,K+-ATPase isoform may be pres ent in toad brain. The data could be fitted to one-or two-site models, both of which were consistent with the presence of Na+,K+-ATPase acti vity with high sensitivity to ouabain and bufalin. Ki (concentration c apable of producing 50% inhibition of activity) values for ouabain in the one-site model were in the 0.2 to 3.7 mu M range, whereas Ki(1) va lues in the two-site model ranged from 0.085 to 0.85 mu M, indicating that brain ATPase was at least three orders of magnitude more sensitiv e to ouabain than B. marinus bladder ATPase (Ki = 5940 mu M). Ouabain was also an effective inhibitor of Rb-86(+) uptake in B, marinus brain tissue slices (Ki = 3.1 mu M in the one-site model; Ki(1) = 0.03 mu M in the two-site model). However, the relative contribution of the hig h ouabain sensitivity site to the total activity was 17% in the transp ort assay as compared with 63% in the Na+,K+-ATPase enzymatic assay. W e conclude that a highly ouabain-sensitive Na+,K+-ATPase activity is p resent and functional in toad brain but that its function may be parti ally inhibited in vivo. (C) 1997 Elsevier Science Inc.