Figure 7.

Functional expression of WT or mutant rat α1-subunits in stably transfected OK cells. ⁸⁶Rb (K) uptake was measured under initial rate in OK cells preincubated in the absence or presence of increasing concentrations of ouabain. The Na⁺,K⁺-ATPase-mediated ⁸⁶Rb uptake was obtained by subtraction of ouabain-insensitive ⁸⁶Rb (K) uptake measured in the presence of 5 × 10⁻³ M ouabain. Results from three or four independent experiments are expressed as percent of control (absence of ouabain) ± SEM. OK cells expressing the WT (WT) rat α1-subunit or its tyrosine phosphorylation site mutants (Y10A and Y10E) exhibited a bimodal ouabain inhibition pattern. The IC₅₀ values of Na⁺,K⁺-ATPase with high ouabain sensitivity were WT, 2.2 ± 0.9 × 10⁻⁸ M; Y10A, 3.8 ± 1.5 × 10⁻⁸ M; and Y10E, 3.8 ± 2.5 × 10⁻⁸ M. The IC₅₀ values of Na⁺,K⁺-ATPase with low ouabain sensitivity were WT, 4.3 ± 0.9 × 10⁻⁵ M; Y10A, 8.7 ± 2.7 × 10⁻⁵ M; and Y10E, 3.0 ± 1.6 × 10⁻⁵ M. The percentages of ⁸⁶Rb (K) uptake mediated by the ouabain-resistant Na⁺,K⁺-ATPase were WT, 57.1 ± 2.6%; Y10A, 47.6 ± 3.0%; and Y10E, 45.8 ± 5.7%.