Figure 7. Mutant (+/m) EDL was more sensitive than control (+/+) muscle to inhibition of force by ouabain (OB), which also exacerbated the weakness produced by elevated [K⁺]_o.

Isolated EDL muscles from 8.9 ± 0.2–month-old mutant mice or sibling controls were equilibrated in bath containing 4 mM [K⁺]_o and 1.3 mM [Ca²⁺]_o and stimulated as in Figure 6; normalized peak tetanic responses (mean ± SEM) are shown. (A) Mutant EDL was highly sensitive to 0.5–2.0 μM ouabain, which affected control muscle much more slowly. (B) Adding 0.5 μM ouabain greatly exacerbated the force reduction caused by raising [K⁺]_o to 8 mM (compare Figure 6B) and produced sustained weakness. (C) Adding 0.5 μM ouabain upon raising [K⁺]_o to 10 mM not only produced rapid paralysis that was reversible in the recovery solution but also nearly abolished the partial rebound that had occurred after 7 minutes in Figure 6C. Gray bars indicate significant differences by ANOVA (P < 0.05) between mutant (red circles) and control (open squares) responses. ANOVA was not determined in A during 40–60 minutes because the ouabain concentration was different for mutant (0 μM) and control (2 μM).