Ouabain-sensitive Na+,K+-ATPase activity in toad brain

Toads of the genus Bufo are highly resistant to the toxic effects of digitalis glycosides, and the Na⁺,K⁺-ATPase of all toad tissues studied to date has been relatively insensitive to inhibition by digitalis and related compounds. In studies of brain microsomal preparations from two toad species, Bufo marinus and Bufo viridis, inhibition of ATPase activity and displacement of [³H]ouabain from Na⁺,K⁺-ATPase occurred over broad ranges of ouabain or bufalin concentrations, consistent with the possibility that more than one Na⁺,K⁺-ATPase isoform may be present 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 activity with high sensitivity to ouabain and bufalin. Ki concentration capable of producing 50% inhibition of activity) values for ouabain in the one-site model were in the 0.2 to 3.7 μM range, whereas Ki₁ values in the two-site model ranged from 0.085 to 0.85 μM, indicating that brain ATPase was at least three orders of magnitude more sensitive to ouabain than B. marinus bladder ATPase (Ki = 5940 μM). Ouabain was also an effective inhibitor of ⁸⁶Rb⁺ uptake in B. marinus brain tissue slices (Ki = 3.1 μM in the one-site model; Ki₁ = 0.03 μM in the two-site model). However, the relative contribution of the high ouabain-sensivity site to the total activity was 17% in the transport assay as compared with 63% in the Na⁺,K⁺-ATPase enzymatic assay. We conclude that a highly ouabain-sensitive Na⁺,K⁺-ATPase activity is present and functional in toad brain but that its function may be partially inhibited in vivo.