Sodium- and energy-dependent uptake of myo-inositol by rabbit peripheral nerve. Competitive inhibition by glucose and lack of an insulin effect.

138

Elevated cellular sorbitol levels resulting from conversion of increased glucose by aldose reductase might deplete cellular myoinositol content, which could then lower inositol phosphates (InsPs) and diacylglycerol levels, key regulators of protein kinase C (PKC). Secondary to altered PKC activity, other cellular enzymes such as (NaK)-ATPase could be affected. To test this hypothesis we examined the association between PKC activity, (Na,K)-ATPase activity, and sorbitol, myoinositol, and InsP levels in cultured bovine retinal capillary endothelial cells, a cell type prominently involved in diabetic retinopathy.Elevating glucose concentration in culture media from 100 to 400 mg/dl led to a 100% increase in sorbitol levels, which could be inhibited completely by sorbinil, an aldose reductase inhibitor. In contrast, no changes were observed in myoinositol or InsP levels. Subfractionated PKC activities showed a 100% increase in the membranous pool with a parallel decrease in the cytosolic fraction. Adding sorbinil did not affect PKC activity, whereas the PKC agonist, phorbol myristate acetate (PMA), stimulated translocation of PKC. Ouabain-inhibitable (NaK)-ATPase activity was decreased 70% by elevated glucose levels. This decrease could be prevented by adding either PMA or sorbinil. Thus, in retinal capillary endothelial cells elevated glucose concentration can affect PKC and (Na,K)-ATPase activities, probably via different mechanisms.

Section: Discussionmentioning

confidence: 95%

Section: Introductionmentioning

confidence: 99%

Differential regulation of protein kinase C and (Na,K)-adenosine triphosphatase activities by elevated glucose levels in retinal capillary endothelial cells.

Lee

MacGregor²,

Fluharty³

et al. 1989

138

“…In the experimentally diabetic rodent, increased extracellular glucose concentration reduces nerve myo-inositol (MI)' content by competitive inhibition of Na-gradient-dependent MI uptake and/or increased polyol (sorbitol) pathway activity (8,12,13). Nerve MI depletion in turn impairs nerve (Na,K)-ATPase (9,14,15), probably via decreased protein kinase C activity reflecting diminished intracellular diacylglycerol and/or inositol-1,4,5-triphosphate release from inositol phospholipids ( 16).…”

Section: Introductionmentioning

confidence: 99%

“…(Na,K)-ATPase activity: (Na,K-ATPase activity was measured enzymatically as (Na + K)-stimulated and ouabain-inhibited ATPase activities in freshly prepared crude homogenates of whole sciatic nerve as previously described in detail (8 computed by subtracting from this reaction rate the respective rates in the absence of NaCI and KCI or in the presence of 0.10 mM ouabain (8).…”

Section: Introductionmentioning

confidence: 99%

Axo-glial dysjunction. A novel structural lesion that accounts for poorly reversible slowing of nerve conduction in the spontaneously diabetic bio-breeding rat.

Sima¹,

Lattimer²,

Yagihashi³

et al. 1986

Self Cite

170

117

Biochemical abnormalities in peripher, nerve are thought to precede and condition the development of diabetic neuropathy, but metabolic intervention in chronic diabetic neuropathy produces only limited acute clinical response. The residual, metabolically unresponsive neurological deficits have never been rigorously defined in terms of either persistent metabolic derangements or irreversible structural defects because (a) human nerve tissue is rarely accessible for anatomical and biochemical study and (b) experimentally diabetic animals do not develop the structural hallmarks of human diabetic neuropathy. Detailed neuroanatomical-functional-biochemical correlation was therefore undertaken in long-term spontaneously diabetic BB-Wistar rats that functionally and structurally model human diabetic neuropathy. Vigorous insulin replacement in chronically diabetic BB rats essentially normalized both the sural nerve fiber caliber spectrum and the decreased sciatic nerve myo-inositol and (Na,K)-ATPase levels generally associated with conduction slowing in diabetic animals; yet, nerve conduction was only partially restored toward normal. Morphometric analysis revealed a striking disappearance of paranodal axo-glial junctional complexes that was not corrected by insulin replacement. Loss of these strategic junctional complexes, which are thought to limit lateral migration of axolemmal Na channels away from nodes of Ranvier, correlates with and can account for the diminished nodal Na permeability and resultant nodal conduction delay characteristic of chronic diabetic neuropathy in this animal model.

“…Elevated serum glucose also causes reduced myo-inositol and increased sorbitol in peripheral nerves of diabetic animals (8)(9)(10)(11)(12)(13). These defects are associated with reduced nerve conduction velocity (10)(11)(12) Correction of the hyperglycemia-associated sorbitol accumulation or myo-inositol decrease in nerve prevents the development of neural electrophysiological deficits in diabetic rats (10)(11)(12).…”

Section: Introductionmentioning

confidence: 99%

Treatment with aldose reductase inhibitor or with myo-inositol arrests deterioration of the electroretinogram of diabetic rats.

MacGregor¹,

Matschinsky²

1985

Biochemical abnormalities in the retinal pigment epithelium of experimentally diabetic animals include increased sorbitol and decreased myo-inositol. Diabetes also causes a progressive reduction in the amplitude of the c-wave of the electroretinogram of the pigmented rat. The c-wave is generated by the retinal pigmented epithelium. Myo-inositol administration or treatment with sorbinil, an inhibitor of aldose reductase, arrested the decline in the c-wave. Therefore, hyperglycemia-associated defects in myo-inositol or sorbitol metabolism may be involved in the pathogenesis of the electrophysiological deficit of the diabetic retina. The homogeneous cell layer of the pigment epithelium may be a useful tissue model for studying the pathogenesis of the complications of diabetes.