Ouabain stimulates unidirectional and net potassium efflux in resting mammalian skeletal muscle
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abstract
The present study compared ouabain-sensitive unidirectional K+flux into (JinK) and out of (JoutK) perfused rat hindlimb skeletal muscle in situ and mouse flexor digitorum brevis (FDB) in vitro. In situ, 5 mM ouabain inhibited 54 ± 4% of the total JinK in 28 ± 1 min, and increased the net and unidirectional efflux of K+within 4 min. In contrast, 1.8 mM ouabain inhibited 40 ± 8% of the total JinK in 38 ± 2 min, but did not significantly affect JoutK. In vitro, 1.8 and 0.2 mM ouabain decreased JinK to a greater extent (83 ± 5%) than in situ, but did not significantly affect42K loss rate compared with controls. The increase in unidirectional K+efflux (JoutK) with 5 mM ouabain in situ was attributed to increased K+efflux through cation channels, since addition of barium (1 mM) to ouabain-perfused muscles returned JoutK to baseline values within 12 min. Perfusion with 5 mM ouabain plus 2 mM tetracaine for 30 min decreased JinK 46 ± 9% (0.30 ± 0.03 to 0.16 ± 0.02 µmol·min1·g1), however tetracaine was unable to abolish the ouabain-induced increase in unidirectional K+efflux. In both rat hindlimb and mouse FDB, tetracaine had no effect on JoutK. Perfusion of hindlimb muscle with 0.1 mM tetrodotoxin (TTX, a Na+channel blocker) decreased JinK by 15 ± 1%, but had no effect on JoutK; subsequent addition of ouabain (5 mM) decreased JinK a further 32 ± 2%. The ouabain-induced increase in unidirectional K+efflux did not occur when TTX was perfused prior to and during perfusion with 5 mM ouabain. We conclude that 5 mM ouabain increases the unidirectional efflux of K+from skeletal muscle through a barium and TTX-sensitive pathway, suggestive of voltage sensitive Na+channels, in addition to inhibiting Na+/K+-ATPase activity.Key words: cardiac glycoside, Na,K pump, K+channels, Na+channels, perfused rat hindlimb, flexor digitorum brevis, TTX, barium, tetracaine.
