Ouabain stimulates unidirectional and net potassium efflux in resting mammalian skeletal muscle

The present study compared ouabain-sensitive unidirectional K+ flux into (J inK) and out of (JoutK) perfused rat hindlimb skeletal muscle in situ and m ouse flexor digitorum brevis (FDB) in vitro. In situ, 5 mM ouabain inhibite d 54 +/- 4% of the total JinK in 28 +/- 1 min, and increased the net and un idirectional efflux of K+ within 4 min. In contrast, 1.8 mM ouabain inhibit ed 40 +/- 8% of the total JinK in 38 +/- 2 min, but did not significantly a ffect JoutK. In vitro, 1.8 and 0.2 mM ouabain decreased JinK to a greater e xtent (83 +/- 5%) than in situ, but did not significantly affect K-42 loss rate compared with controls. The increase in unidirectional K+ efflux (Jout K) with 5 mM ouabain in situ was attributed to increased K+ efflux through cation channels, since addition of barium (1 mM) to ouabain-perfused muscle s returned JoutK to baseline values within 12 min. Perfusion with 5 mM ouab ain plus 2 mM tetracaine for 30 min decreased JinK 46 +/- 9% (0.30 +/- 0.03 to 0.16 +/- 0.02 mu mol.min(-1).g(-1)), however tetracaine was unable to a bolish the ouabain-induced increase in unidirectional K+ efflux. In both ra t hindlimb and mouse FDB, tetracaine had no effect on JoutK. Perfusion of h indlimb muscle with 0.1 mM tetrodotoxin (TTX, a Na+ channel blocker) decrea sed JinK by 15 +/- 1%, but had no effect on JoutK; subsequent addition of o uabain (5 mM) decreased JinK a further 32 +/- 2%. The ouabain-induced incre ase in unidirectional K+ efflux did not occur when TTX was perfused prior t o and during perfusion with 5 mM ouabain. We conclude that 5 mM ouabain inc reases the unidirectional efflux of K+ from skeletal muscle through a bariu m and TTX-sensitive pathway, suggestive of voltage sensitive Na+ channels, in addition to inhibiting Na+/K+-ATPase activity.