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Department of Physiology, College of Medicine, The University of Tennessee Health Science Center, Memphis, Tennessee 38163
In isosmotic
conditions, insulin stimulation of PI 3-K/Akt and p38 MAPK pathways in
skeletal muscle inhibits
Na+-K+-2Cl
cotransporter (NKCC)
activity induced by the ERK1,2 MAPK pathway. Whether these signaling
cascades contribute to NKCC regulation during osmotic challenge is
unknown. Increasing osmolarity by 20 mosM with either glucose or
mannitol induced NKCC-mediated 86Rb uptake and water
transport into rat soleus and plantaris skeletal muscle in vitro. This
NKCC activity restored intracellular water. In contrast to mannitol,
hyperosmolar glucose increased ERK1,2 and p38 MAPK phosphorylation.
Glucose, but not mannitol, impaired insulin-stimulated phosphorylation
of Akt and p38 MAPK in the plantaris and soleus muscles, respectively.
Hyperosmolarity-induced NKCC activation was insensitive to insulin
action and pharmacological inhibition of ERK1,2 and p38 MAPK pathways.
Paradoxically, cAMP-producing agents, which stimulate NKCC activity in
isosmotic conditions, suppressed hyperosmolar glucose- and
mannitol-induced NKCC activity and prevented restoration of muscle cell
volume in hyperosmotic media. These results indicate that NKCC activity
helps restore muscle cell volume during hyperglycemia. Moreover,
hyperosmolarity activates NKCC regulatory pathways that are insensitive
to insulin inhibition.
hyperglycemia; adenosine 3',5'-cyclic monophosphate; Akt; Na+-K+-2Cl
cotransporter; phosphatidylinositol 3-kinase
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