K+ Transport and Volume Regulatory Response by NKCC in Resting Rat Hindlimb Skeletal Muscle

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Abstract
This study tested the hypothesis that the NKCC is involved in volume regulation, specifically regulatory volume increase (RVI), in resting skeletal muscle. Neurally and vascularly isolated rat hindlimbs were perfused with a bovine erythrocyte perfusate containing 42K or 86Rb as markers of unidirectional K+ flux across the sarcolemma. Compared to controls, perfusion with 120 µM bumetanide (a specific inhibitor of the NKCC) decreased JinK by 15±2%, indicating the functional presence of the NKCC. Experiments with ouabain (to block active K+ transport by the Na,K ATPase) showed that the bumetanide-sensitive component of JinK comprised 35% of the total ouabain-sensitive JinK. Inhibition of NKCC resulted in a net loss of water by muscle. When hindlimbs were perfused with hypertonic (380 mOsm/L by addition of sucrose) perfusate for 20 min, after initially blocking K+ channels with 1 mM barium, JinK rapidly (2-3 min) increased 2-fold followed by a rapid decline. This rapid, transient increase in JinK was abolished with bumetanide, confirming that perfusion with hypertonic perfusate stimulated NKCC activity and RVI. The hypertonic perfusate also resulted in temporally associated decreases in net water uptake by muscle. It is concluded that a functional NKCC is present in mammalian skeletal muscle and that it is involved in cell volume regulation.

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