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In this review we discuss the biological significance of D-chiro-inositol, originally discovered as a component of a putative mediator of intracellular insulin action, where as a putative mediator, it accelerates the dephosphorylation of glycogen synthase and pyruvate dehydrogenase, rate limiting enzymes of non-oxidative and oxidative glucose disposal. Early studies demonstrated a linear relationship between its decreased urinary excretion and the degree of insulin resistance present. When tissue contents, including muscle, of type 2 diabetic subjects were assayed, they demonstrated a more general body deficiency. Administration of D-chiro-inositol to diabetic rats, Rhesus monkeys and now to humans accelerated glucose disposal and sensitized insulin action. A defect in vivo in the epimerization of myoinositol to chiro-inositol in insulin sensitive tissues of the GK type 2 diabetic rat has been elucidated. Thus, administered D-chiro-inositol may act to bypass a defective normal epimerization of myo-inositol to D-chiro-inositol associated with insulin resistance and act to at least partially restore insulin sensitivity and glucose disposal.

NIDDM is associated with decreased chiro-inositol excretion and decreased chiro-inositol content in muscle. These abnormalities seem to reflect the presence of insulin resistance in NIDDM:

SYNTHESISInsulin stimulates both glucose transport and glycogen synthesis; however, these actions sometimes occur in a disconnected manner. Current models for the mechanism of action for insulin, for which the dominant paradigm involves the activity of the insulin receptor Tyr kinase and its primary Tyr phosphorylated substrates-the insulin receptor substrate (IRS) family of proteins (1), are inadequate to account for these historical observations. Under certain conditions, control of glucose transport by insulin is observed in the absence of an effect on glycogen synthesis, whereas under other conditions, control of glycogen synthesis by insulin is observed in the absence of an effect on glucose transport. For example, application of insulin during perfusion of the rat heart stimulated glucose transport, but did not activate glycogen synthase (GS) (2). On the other hand, when the rat diaphragm was treated with N-ethylmaleimide, to test the effect of this sulfhydryl reagent on metabolism, no effect of insulin was observed on glucose transport, but insulin-activated GS and glycogen synthesis (3). Thus, insulin signaling proceeded along one pathway while another pathway was unaffected, suggesting the possibility that no single pathway accounts for events downstream of the IR, but parallel signaling connects the IR to activation of glucose transport and glucose metabolism. These considerations led us to the concept that a cytoplasmic second messenger was generated in parallel with the phosphorylation events initiated by the receptor Tyr kinase (4). We have emphasized the hypothesis that the phosphorylation network and the second messenger pathway operate in parallel and together are required to fully account for insulin effects on metabolic disposal of intracellular glucose (4). EVIDENCE FOR INSULIN SECOND MESSENGERSThe initial evidence to support the existence of a second messenger for insulin followed classical methods used to discover cAMP. Rats were injected with insulin and killed, and then muscle and/or liver were used to prepare heatinactivated, deproteinized extracts. The extracts from insulin-stimulated rat tissues had one or more substances that inhibited protein kinase A (PKA) and activated GS phosphatase, compared with extracts from control rats. Insulin administration maintained PKA in muscle as an inactive holoenzyme, presumably desensitized to cAMP by the soluble second D 1 6 ( 1 1 -1 2 ) 5 4 3 -5 5 1 , N Classical actions of insulin involve increased glucose uptake from the bloodstream and its metabolism in peripheral tissues, the most important and relevant effects for human health. However, nonoxidative and oxidative glucose disposal by activation of glycogen synthase (GS) and mitochondrial pyruvate dehydrogenase (PDH) remain incompletely explained by current models for insulin action. Since the discovery of insulin receptor Tyr kinase activity about 25 years ago, the dominant paradigm for intracellular signaling by insulin invokes protein phosphorylation downstream of the receptor and its primar...