A myo-inositol pool utilized for phosphatidylinositol synthesis is depleted in sciatic nerve from rats with streptozotocin-induced diabetes.

Abstract

Peripheral nerve from experimentally diabetic rats exhibits lowered levels of myo-inositol (MI) and decreased incorporation of [3H]MI into phosphatidylinositol (PI) myo-Inositol (MI) has been suggested to play an important role in the development of experimental diabetic neuropathy (1-3). Mammalian tissues maintain millimolar concentrations of MI, which in peripheral nerve is at least 40-fold higher than in plasma (4, 5). It has long been known that MI content is reduced substantially in peripheral nerve from diabetic animals (1, 4, 6). Evidence from both diabetic nerve preparations and neural cell cultures grown in the presence of elevated medium glucose concentrations indicates that the depletion of cellular MI levels is associated with increased intracellular sorbitol, alterations of inositol phospholipid metabolism, and decreased Na+,K+-ATPase activity (2,7,8), which is critical for the maintenance of resting membrane potential. Impairment of this enzyme has been linked to reduced nerve conduction velocity that is characteristic of both human and experimental diabetes (2). Furthermore, supplementing the diet of diabetic animals with MI tends to restore the cyclitol level, as well as Na',K+-ATPase activity and nerve conduction velocity, to normal (1, 9, 10).There is evidence that both Na+,K+-ATPase activity and a component of phosphatidylinositol (PI) turnover in normal tissues are inhibited by incubation in MI-free medium (11,12). Consequently, reduced MI content in diabetic nerve could reflect depletion of the MI supply that specifically serves as substrate for PI metabolism, which in turn is integral to regulation of Na+,K+-ATPase activity.To investigate this possibility, we have used propranolol, which like several other cationic amphiphilic drugs markedly enhances accumulation of phosphatidyl CMP (CMP-PA) in a number of tissues including iris muscle, pineal gland, and pancreatic islets (13)(14)(15). This occurs because propranolol redirects the metabolism of phospholipids, primarily by inhibiting PA phosphatase (16,17), thereby decreasing the formation of phosphatidylcholine and phosphatidylethanolamine and enhancing the synthesis of PI and phosphatidylglycerol. In the presence of propranolol, the accumulation of CMP-PA, the immediate precursor of the latter two phospholipids, will be determined by its rate of biosynthesis relative to its rate of utilization. The consumption of the liponucleotide is governed by the availability of endogenous MI, since its appearance is prevented by inclusion of sufficient MI in the medium (15,16 9818The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Modulation of basal nitric oxide-dependent cyclic-GMP production by ambient glucose, myo-inositol, and protein kinase C in SH-SY5Y human neuroblastoma cells.

“…Yet detailed patterns of response may vary even within the same tissue. For example, sciatic nerve from untreated streptozotocin-diabetic rats exhibits diminished total and arachidonyl-DAG (17,18), and increased activation (71) and phosphorylation (20) of (Na,K)-ATPase by exogenous PKC agonists, consistent with a diminished basal level of PKC-mediated phosphorylation of (Na,K)-ATPase. In separate studies, diminished activation of PKC (72) and decreased total and cytoplasmic intrinsic PKC activity have been noted in streptozotocin-diabetic rat sciatic nerve (16), perhaps reflecting the reported discordance between measures of cellular PKC activity and its translocation (73,74).…”

“…1 C). These findings are considered indicative of PI synthase inhibition by mass action secondary to MI depletion in other cells and tissues exposed to elevated ambient d-glucose levels (17,19,26). Exposure to 50 mM d-(but not l-) glucose for 6 d decreased both membrane-associated PKC activity 2 and the phosphorylation of MARCKS, a specific endogenous PKC substrate ( Fig.…”

“…d-glucose decreased MI and raised CDP-DG in a dosedependent fashion (17,26). Culture in MI-deficient SHTE medium reproduced and supplementation of DMEM with 500 M MI overcame the effect of 50 mM d-glucose on cGMP.…”

“…In diabetic rat peripheral nerve, NO deficiency has been hypothesized to reflect competition for NADPH cofactor by NOS and aldose reductase (AR), which is activated by D -glucose substrate in hyperglycemic states (13). Alternatively, inhibition of nerve PKC activity, which is attributed to D -glucoseinduced depletion of myo -inositol (MI) and arachidonyl DAG, could underlie NO deficiency and/or reduced blood flow in diabetic nerve (15)(16)(17)(18)(19)(20). Cultured mouse neuroblastoma cells exhibit D -glucose-induced MI depletion (21,22) and NO-mediated cGMP production (23,24).…”

Effects of glucose on sorbitol pathway activation, cellular redox, and metabolism of myo-inositol, phosphoinositide, and diacylglycerol in cultured human retinal pigment epithelial cells.

“…These glucose-induced perturbations in PPI metabolism in RPE 91 (13,14) are of particular interest because they resemble those induced by diabetes in rat peripheral nerve (26,27): when peripheral nerve tissue from diabetic rats is incubated in vitro, stimulation of PPI turnover results in abnormal CDP-DG accumulation and diminished release of arachidonoyl-containing DAG (26,27). The RPE 91 cell model also was chosen because AR2-dependent phenomena in general, and those specifically ascribed to putative secondary perturbations in cytoplasmic redox (8,9), ought to be more readily apparent in cells constitutively overexpressing AR2 (17,18).…”