Signalling mechanisms underlying the rapid and additive stimulation of NKCC activity by insulin and hypertonicity in rat L6 skeletal muscle cells

We have investigated the expression and regulation of the Na⁺-K⁺-2Cl^- cotransporter (NKCC) by insulin and hyperosmotic stress in L6 rat skeletal muscle cells. NKCC was identified by immunoblotting as a 170 kDa protein in L6 myotubes and mediated 54% of K⁺ (⁸⁶Rb⁺) influx based on the sensitivity of ion transport to bumetanide, a NKCC inhibitor. The residual ⁸⁶Rb⁺ influx occurred via the Na⁺,K⁺-ATPase and other transporters not sensitive to bumetanide or ouabain. NKCC-mediated ⁸⁶Rb⁺ influx was enhanced significantly (∼1.6-fold) by acute cell exposure to insulin, but was inhibited significantly by tyrosine kinase inhibitors, wortmannin and rapamycin, consistent with a role for the insulin receptor tyrosine kinase, phosphoinositide 3 (PI3)-kinase and mTOR, respectively, in cotransporter activation. In contrast, the hormonal activation of NKCC was unaffected by inhibition of the classical Erk-signalling pathway. Subjecting L6 myotubes to an acute hyperosmotic challenge (420 mosmol l^-1) led to a 40% reduction in cell volume and was accompanied by a rapid stimulation of NKCC activity (∼2-fold). Intracellular volume recovered to normal levels within 60 min, but this regulatory volume increase (RVI) was prevented if bumetanide was present. Unlike insulin, activation of NKCC by hyperosmolarity did not involve PI3-kinase but was suppressed by inhibition of tyrosine kinases and the Erk pathway. While inhibition of tyrosine kinases, using genistein, led to a complete loss in NKCC activation in response to hyperosmotic stress, immunoprecipitation of NKCC revealed that the cotransporter was not regulated directly by tyrosine phosphorylation. Simultaneous exposure of L6 myotubes to insulin and hyperosmotic stress led to an additive increase in NKCC-mediated ⁸⁶Rb⁺ influx, of which, only the insulin-stimulated component was wortmannin-sensitive. Our findings indicate that L6 myotubes express a functional NKCC that is rapidly activated in response to insulin and hyperosmotic shock by distinct intracellular signalling pathways. Furthermore, activation of NKCC in response to hyperosmotic-induced cell shrinkage represents a critical component of the RVI mechanism that allows L6 muscle cells to volume regulate.