Expression of the major isoenzyme of protein kinase-C in skeletal muscle, nPKC theta, varies with muscle type and in response to fructose-induced insulin resistance.

The ability of insulin to stimulate glucose uptake by skeletal muscle varies as a function of muscle type, but the biochemical mechanisms that regulate insulin-mediated glucose transport under normal conditions and in pathological states of insulin resistance are poorly understood. We evaluated differences in the expression of nPKC theta (the major isoform of protein kinase-C in skeletal muscle) in hind limb muscles of different fiber type composition from normal Sprague-Dawley rats and rats made insulin resistant by feeding a fructose-enriched diet. In total muscle homogenates from normal rats, the amount of nPKC theta per unit total protein quantified by immunoblotting using a specific antipeptide antibody was 2.5 times higher in pure white muscle (tensor fascia latae) compared with red muscle (soleus), with two mixed muscles showing intermediate expression. The development of insulin resistance after a fructose-enriched diet was associated with significant increases in diacylglycerol and nPKC theta mass in the membrane fraction of tensor fascia latae, but fructose feeding had no effect on conventional PKC enzyme activity and immunoreactive protein. Thus, expression of nPKC theta varies as a function of muscle type, and fructose-induced insulin resistance appears to be associated with diacylglycerol-mediated isoenzyme-specific changes in nPKC theta in white muscle.