Alterations in retinal Na+, K(+)-ATPase in diabetes: streptozotocin-induced and Zucker diabetic fatty rats

O-Linked GlcNAcylation mediates the inhibition of proximal tubule (Na++K+)ATPase activity in the early stage of diabetes mellitus

BACKGROUND

Diabetic kidney disease (DKD) is a severe complication of diabetes mellitus (DM). It has been proposed that modifications in the function of proximal tubule epithelial cells (PTECs) precede glomerular damage during the onset of DKD. This study aimed to identify modifications in renal sodium handling in the early stage of DM and its molecular mechanism.

METHODS

Streptozotocin (STZ)-induced diabetic BALB/c mice (STZ group) and LLC-PK1 cells, a model of PTECs, were used. All parameters were assessed in the 4th week after an initial injection of STZ.

RESULTS

Early stage of DKD was characterized by hyperfiltration and PTEC dysfunction. STZ group exhibited increased urinary sodium excretion due to impairment of tubular sodium reabsorption. This was correlated to a decrease in cortical (Na++K+)ATPase (NKA) α1 subunit expression and enzyme activity and an increase in O-GlcNAcylation. RNAseq analysis of patients with DKD revealed an increase in expression of the glutamine-fructose aminotransferase (GFAT) gene, a rate-limiting step of hexosamine biosynthetic pathway, and a decrease in NKA expression. Incubation of LLC-PK1 cells with 10 μM thiamet G, an inhibitor of O-GlcNAcase, reduced the expression and activity of NKA and increased O-GlcNAcylation. Furthermore, 6-diazo-5-oxo-L-norleucine (DON), a GFAT inhibitor, or dapagliflozin, an SGLT2 inhibitor, avoided the inhibitory effect of HG on expression and activity of NKA associated with the decrease in O-GlcNAcylation.

CONCLUSION

Our results show that the impairment of tubular sodium reabsorption, in the early stage of DM, is due to SGLT2-mediated HG influx in PTECs, increase in O-GlcNAcylation and reduction in NKA expression and activity.

Photoreceptor cells and RPE contribute to the development of diabetic retinopathy

Diabetic retinopathy (DR) is a leading cause of blindness. It has long been regarded as vascular disease, but work in the past years has shown abnormalities also in the neural retina. Unfortunately, research on the vascular and neural abnormalities have remained largely separate, instead of being integrated into a comprehensive view of DR that includes both the neural and vascular components. Recent evidence suggests that the most predominant neural cell in the retina (photoreceptors) and the adjacent retinal pigment epithelium (RPE) play an important role in the development of vascular lesions characteristic of DR. This review summarizes evidence that the outer retina is altered in diabetes, and that photoreceptors and RPE contribute to retinal vascular alterations in the early stages of the retinopathy. The possible molecular mechanisms by which cells of the outer retina might contribute to retinal vascular damage in diabetes also are discussed. Diabetes-induced alterations in the outer retina represent a novel therapeutic target to inhibit DR.

Diabetic Retinopathy: The Role of Mitochondria in the Neural Retina and Microvascular Disease

Diabetic retinopathy (DR), a common chronic complication of diabetes mellitus and the leading cause of vision loss in the working-age population, is clinically defined as a microvascular disease that involves damage of the retinal capillaries with secondary visual impairment. While its clinical diagnosis is based on vascular pathology, DR is associated with early abnormalities in the electroretinogram, indicating alterations of the neural retina and impaired visual signaling. The pathogenesis of DR is complex and likely involves the simultaneous dysregulation of multiple metabolic and signaling pathways through the retinal neurovascular unit. There is evidence that microvascular disease in DR is caused in part by altered energetic metabolism in the neural retina and specifically from signals originating in the photoreceptors. In this review, we discuss the main pathogenic mechanisms that link alterations in neural retina bioenergetics with vascular regression in DR. We focus specifically on the recent developments related to alterations in mitochondrial metabolism including energetic substrate selection, mitochondrial function, oxidation-reduction (redox) imbalance, and oxidative stress, and critically discuss the mechanisms of these changes and their consequences on retinal function. We also acknowledge implications for emerging therapeutic approaches and future research directions to find novel mitochondria-targeted therapeutic strategies to correct bioenergetics in diabetes. We conclude that retinal bioenergetics is affected in the early stages of diabetes with consequences beyond changes in ATP content, and that maintaining mitochondrial integrity may alleviate retinal disease.

Metabolomics in diabetic complications

With a global prevalence of 9%, diabetes is the direct cause of millions of deaths each year and is quickly becoming a health crisis. Major long-term complications of diabetes arise from persistent oxidative stress and dysfunction in multiple metabolic pathways. The most serious complications involve vascular damage and include cardiovascular disease as well as microvascular disorders such as nephropathy, neuropathy, and retinopathy. Current clinical analyses like glycated hemoglobin and plasma glucose measurements hold some value as prognostic indicators of the severity of complications, but investigations into the underlying pathophysiology are still lacking. Advancements in biotechnology hold the key to uncovering new pathways and establishing therapeutic targets. Metabolomics, the study of small endogenous molecules, is a powerful toolset for studying pathophysiological processes and has been used to elucidate metabolic signatures of diabetes in various biological systems. Current challenges in the field involve correlating these biomarkers to specific complications to provide a better prediction of future risk and disease progression. This review will highlight the progress that has been made in the field of metabolomics including technological advancements, the identification of potential biomarkers, and metabolic pathways relevant to macro- and microvascular diabetic complications.

Oxidative stress and light-evoked responses of the posterior segment in a mouse model of diabetic retinopathy

PURPOSE

To test the hypothesis that in a mouse model of diabetic retinopathy, oxidative stress is linked with impaired light-evoked expansion of choroidal thickness and subretinal space (SRS).

METHODS

We examined nondiabetic mice (wild-type, wt) with and without administration of manganese, nondiabetic mice deficient in rod phototransduction (transducin alpha knockout; GNAT1(-/-)), and diabetic mice (untreated or treated with the antioxidant α-lipoic acid [LPA]). Magnetic resonance imaging (MRI) was used to measure light-evoked increases in choroidal thickness and the apparent diffusion coefficient (ADC) at 88% to 100% depth into the retina (i.e., the SRS layer).

RESULTS

Choroidal thickness values were similar (P > 0.05) between all untreated nondiabetic dark-adapted groups and increased significantly (P < 0.05) with light; this expansion was subnormal (P < 0.05) in both diabetic groups. Apparent diffusion coefficient values in the SRS layer robustly increased (P < 0.05) in a light duration-dependent manner, and this effect was independent of the presence of Mn(2+). The light-stimulated increase in ADC at the location of the SRS was absent in GNAT1(-/-) and diabetic mice (P > 0.05). In diabetic mice, the light-dependent increase in SRS ADC was significantly (P < 0.05) restored with LPA.

CONCLUSIONS

Apparent diffusion coefficient MRI is a sensitive method for evaluating choroid thickness and its light-evoked expansion together with phototransduction-dependent changes in the SRS layer in mice in vivo. Because ADC MRI exploits an endogenous contrast mechanism, its translational potential is promising; it can also be performed in concert with manganese-enhanced MRI (MEMRI). Our data support a link between diabetes-related oxidative stress and rod, but not choroidal, pathophysiology.

Photoreceptors in diabetic retinopathy

Although photoreceptors account for most of the mass and metabolic activity of the retina, their role in the pathogenesis of diabetic retinopathy has been largely overlooked. Recent studies suggest that photoreceptors might play a critical role in the diabetes-induced degeneration of retinal capillaries, and thus can no longer be ignored. The present review summarizes diabetes-induced alterations in photoreceptor structure and function, and provides a rationale for further study of a role of photoreceptors in the pathogenesis of the retinopathy.

High-resolution metabolite imaging of light and dark treated retina using MALDI-FTICR mass spectrometry

MS imaging (MSI) is a valuable tool for diagnostics and systems biology studies, being a highly sensitive, label-free technique capable of providing comprehensive spatial distribution of different classes of biomolecules. The application of MSI to the study of endogenous compounds has received considerable attention because metabolites are the result of the interactions of a biosystem with its environment. MSI can therefore enhance understanding of disease mechanisms and elucidate mechanisms for biological variation. We present the in situ comparative metabolomics imaging data for analyses of light- and dark-treated retina using MALDI-FTICR. A wide variety of tissue metabolites were imaged at a high spatial resolution. These include nucleotides, central carbon metabolism pathway intermediates, 2-oxocarboxylic acid metabolism, oxidative phosphorylation, glycerophospholipid metabolism, and cysteine and methionine metabolites. The high lateral resolution enabled the differentiation of retinal layers, allowing determination of the spatial distributions of different endogenous compounds. A number of metabolites demonstrated differences between light and dark conditions. These findings add to the understanding of metabolic activity in the retina.

Retinal proteome alterations in a mouse model of type 2 diabetes

AIMS/HYPOTHESIS

Diabetic retinopathy is a major complication of type 2 diabetes and the leading cause of blindness in adults of working age. Neuronal defects are known to occur early in disease, but the source of this dysfunction is unknown. The aim of this study was to examine differences in the retinal membrane proteome among non-diabetic mice and mouse models of diabetes either with or without metformin treatment.

METHODS

Alterations in the retinal membrane proteome of 10-week-old diabetic db/db mice, diabetic db/db mice orally treated with the anti-hyperglycaemic metformin, and congenic wild-type littermates were examined using label-free mass spectrometry. Pathway enrichment analysis was completed with Genomatix and Ingenuity. Alterations in Slc17a7 mRNA and vesicular glutamate transporter 1 (VGLUT1) protein expression were evaluated using real-time quantitative PCR and IMMUNOFLUORESCENCE.

RESULTS

A total of 98 proteins were significantly differentially abundant between db/db and wild-type animals. Pathway enrichment analysis indicated decreases in levels of proteins related to synaptic transmission and cell signalling. Metformin treatment produced 63 differentially abundant proteins compared with untreated db/db mice, of which only 43 proteins were found to occur in both datasets, suggesting that treatment only partially normalises the alterations induced by diabetes. VGLUT1, which is responsible for loading glutamate into synaptic vesicles, was found to be differentially abundant in db/db mice and was not normalised by metformin. The decrease in Slc17a7/VGLUT1 was confirmed by transcriptomic and immunocytochemical analysis.

CONCLUSIONS/INTERPRETATION

These findings expand the knowledge of the protein changes in diabetic retinopathy and suggest that membrane-associated signalling proteins are susceptible to changes that are partially ameliorated by treatment

Update on animal models of diabetic retinopathy: from molecular approaches to mice and higher mammals

Diabetic retinopathy (DR) is the most common microvascular complication of diabetes and one of the major causes of blindness worldwide. The pathogenesis of DR has been investigated using several animal models of diabetes. These models have been generated by pharmacological induction, feeding a galactose diet, and spontaneously by selective inbreeding or genetic modification. Among the available animal models, rodents have been studied most extensively owing to their short generation time and the inherited hyperglycemia and/or obesity that affect certain strains. In particular, mice have proven useful for studying DR and evaluating novel therapies because of their amenability to genetic manipulation. Mouse models suitable for replicating the early, non-proliferative stages of the retinopathy have been characterized, but no animal model has yet been found to demonstrate all of the vascular and neural complications that are associated with the advanced, proliferative stages of DR that occur in humans. In this review, we summarize commonly used animal models of DR, and briefly outline the in vivo imaging techniques used for characterization of DR in these models. Through highlighting the ocular pathological findings, clinical implications, advantages and disadvantages of these models, we provide essential information for planning experimental studies of DR that will lead to new strategies for its prevention and treatment.

Concurrent impairment of (Na++K+)-ATPase activity in multi-organ of type-1 diabetic NOD mice

BACKGROUND

Type-1 diabetes causes serious complications. Detailed molecular pathways of type-1 diabetes-mediated organ dysfunction are not completely understood. Significantly depressed (Na(+)+K(+))-ATPase (NKA) activity has been found in erythrocytes, pancreatic β-cells, nerve cells, and muscle tissues of type-1 diabetic patients and rodent animal models. The characteristics of NKA in the development of the type-1 diabetes-mediated complications remain obscure. Here we investigated whether alterations of NKA activity in heart, kidney, and pancreas of type-1 diabetic mice occur simultaneously and whether depressed NKA activity is a universal phenomenon in major organs in the development of type-1 diabetes-induced complications.

METHODS

Female non-obese diabetic (NOD) and non-obese resistant mice were used for the study. Mice blood glucose was monitored and ouabain-sensitive NKA activity was determined.

RESULTS

Experimental results reveal that reduced NKA activity correlates with the progression of elevated blood glucose along with marked altered NKA apparent Na(+) affinity in all three organs of NOD mice. No significant changes of NKA protein expression were detected while the enzyme activity was reduced in tested mice, suggesting an inactive form of NKA might present in different tissues of the NOD mice.

CONCLUSION

Our study suggests that concurrent impairment of NKA function in multi-organ may serve as one of the molecular pathways participating in and contributing to the mechanism of type-1 diabetes-induced complications in NOD mice. A successful protection of NKA function from injury might offer a good intervention for controlling the progression of the disease.

Retinal ion regulation in a mouse model of diabetic retinopathy: natural history and the effect of Cu/Zn superoxide dismutase overexpression

PURPOSE

To test the hypotheses that manganese-enhanced MRI (MEMRI) is useful in evaluating intraretinal ion dysregulation in wild-type (WT) and Cu/Zn superoxide dismutase (SOD1) overexpressor mice.

METHODS

Central intraretinal ion activity and retinal thickness were measured from high-resolution data of light- and dark-adapted WT C57BL/6 mice (to gauge MEMRI sensitivity to normal visual processing in mice) and dark-adapted diabetic and nondiabetic WT and Cu/Zn superoxide dismutase overexpressor (SOD1OE) mice. Glycated hemoglobin and retinal vascular histopathology were also determined.

RESULTS

In WT mice, light adaptation reduced outer retinal manganese uptake compared with that in dark adaptation; no effect on inner retinal uptake was found. In diabetic WT mice, intraretinal manganese uptake became subnormal between 1.5 and 4 months of diabetes onset and then relatively increased. Central retinal thickness, as determined with MEMRI, decreased as a function of age in diabetic mice but remained constant in control mice. Nondiabetic SOD1OE mice had normal retinal manganese uptake but subnormal retinal thickness and supernormal acellular capillary density. At 4.2 months of diabetes, SOD1OE mice had normal manganese uptake and no further thinning; acellular capillaries frequency did not increase by 9 to 10 months of diabetes.

CONCLUSIONS

In emerging diabetic retinopathy, MEMRI provided an analytic measure of an ionic dysregulatory pattern that was sensitive to SOD1 overexpression. The potential benefit of SOD1 overexpression to inhibit retinal abnormality in this model is limited by the retinal and vascular degeneration that develops independently of diabetes.

C-Peptide effects on renal physiology and diabetes

The C-peptide of proinsulin is important for the biosynthesis of insulin and has for a long time been considered to be biologically inert. Animal studies have shown that some of the renal effects of the C-peptide may in part be explained by its ability to stimulate the Na,K-ATPase activity. Precisely, the C-peptide reduces diabetes-induced glomerular hyperfiltration both in animals and humans, therefore, resulting in regression of fibrosis. The tubular function is also concerned as diabetic animals supplemented with C-peptide exhibit better renal function resulting in reduced urinary sodium waste and protein excretion together with the reduction of the diabetes-induced glomerular hyperfiltration. The tubular effectors of C-peptide were considered to be tubule transporters, but recent studies have shown that biochemical pathways involving cellular kinases and inflammatory pathways may also be important. The matter theory concerning the C-peptide effects is a metabolic one involving the effects of the C-peptide on lipidic metabolic status.This review concentrates on the most convincing data which indicate that the C-peptide is a biologically active hormone for renal physiology.

Role of Na+-K+-Cl−cotransport and Na+/Ca2+exchange in mitochondrial dysfunction in astrocytes following in vitro ischemia

Na+-K+-Cl−cotransporter isoform 1 (NKCC1) and reverse mode operation of the Na+/Ca2+exchanger (NCX) contribute to intracellular Na+and Ca2+overload in astrocytes following oxygen-glucose deprivation (OGD) and reoxygenation (REOX). Here, we further investigated whether NKCC1 and NCX play a role in mitochondrial Ca2+(Cam2+) overload and dysfunction. OGD/REOX caused a doubling of mitochondrial-releasable Ca2+( P < 0.05). When NKCC1 was inhibited with bumetanide, the mitochondrial-releasable Ca2+was reduced by ∼42% ( P < 0.05). Genetic ablation of NKCC1 also reduced Cam2+accumulation. Moreover, OGD/REOX in NKCC1+/+astrocytes caused dissipation of the mitochondrial membrane potential (Ψm) to 42 ± 3% of controls. In contrast, when NKCC1 was inhibited with bumetanide, depolarization of Ψmwas attenuated significantly (66 ± 10% of controls, P < 0.05). Cells were also subjected to severe in vitro hypoxia by superfusion with a hypoxic, acidic, ion-shifted Ringer buffer (HAIR). HAIR/REOX triggered a secondary, sustained rise in intracellular Ca2+that was attenuated by reversal NCX inhibitor KB-R7943. The hypoxia-mediated increase in Cam2+was accompanied by loss of Ψmand cytochrome c release in NKCC1+/+astrocytes. Bumetanide or genetic ablation of NKCC1 attenuated mitochondrial dysfunction and astrocyte death following ischemia. Our study suggests that NKCC1 acting in concert with NCX causes a perturbation of Cam2+homeostasis and mitochondrial dysfunction and cell death following in vitro ischemia.

Ultrastructural alterations in capillaries of the diabetic hypertensive rat retina: protective effects of ACE inhibition

AIMS/HYPOTHESIS

The ACE inhibitor cilazapril was administered to diabetic hypertensive rats to evaluate its ability to influence the development of retinal capillary alterations.

METHODS

Normotensive (strain: Wistar Kyoto) and genetically hypertensive (strain: spontaneously hypertensive) rats were rendered diabetic by intravenous injections of streptozotocin. Half of the diabetic animals received cilazapril with their daily food. At 20 weeks of diabetes, endothelial cells, pericytes and extracellular matrix were assessed by ultrastructural morphometry. Each experimental group consisted of seven animals.

RESULTS

Cilazapril normalised systolic arterial pressure in diabetic hypertensive rats (137+/-2 mm Hg compared with 188+/-16 mm Hg in non-medicated diabetic hypertensive rats, p<0.001). The number of endothelial intercellular junctions was reduced in untreated diabetic hypertensive rats (0.15+/-0.05, p<0.02, vs 0.47+/-0.20 in non-diabetic normotensive rats). In diabetic hypertensive animals treated with cilazapril, this loss was attenuated (0.32+/-0.16, p<0.05). The significant thickening of the basement membrane observed in the diabetic normotensive (132.8+/-19.4 nm) and diabetic hypertensive (150.3+/-20.2 nm) groups was decreased by cilazapril in the diabetic hypertensive group (116.7+/-11.0 nm, p<0.01), but was unaffected in the normotensive (131.9+/-17.3 nm) group. No protective effect of the drug was observed in either group on pericytes.

CONCLUSIONS/INTERPRETATION

Long-term administration of an effective antihypertensive therapy normalises endothelial alterations and basement membrane thickness in diabetic hypertensive conditions, and thus may account for the well-known improvement of the blood-retinal barrier observed during antihypertensive treatment.

Corneal changes in magnesium-deficient rats

PURPOSE

The purpose of the current study is to investigate the cornea in magnesium (Mg) deficiency and elucidate the local function of trace elements.

METHODS

After delivery, mother Wistar Kyoto rats were fed a low Mg diet containing 0.1 mg Mg/100 g diet with all other nutrients and distilled and deionized water. Infant rats were suckled by their mothers for 21 days and then fed the same Mg-deficient diet. Control mother rats were fed commercial rat pellets containing 24 mg Mg/100 g diet and all other nutrients. The corneas were examined by electron microscopy at 6 weeks of age.

RESULTS

In the Mg-deficient rats, serum Mg levels were significantly lower and calcium (Ca) levels higher than in the control rats. The corneas of Mg-deficient rats showed decreased microvilli and microplicae in the epithelial cells of the most superficial layer, increased mitochondria with abnormal shapes in the basal cells in the epithelium, condensed chromatin in the nuclei of the basal cells, and high density deposits and macrophage-like cells in the subepithelium of the stroma. Mg-deficient rats had pentagonal and square endothelial cells.

CONCLUSION

Since Mg2+ has biologic functions including structural stabilization of protein, nucleic acids, and cell membranes, Mg deficiency may induce changes in the corneal surface and nuclei of corneal epithelial and endothelial cells. These disturbances may interfere with protection from infections, foreign bodies, dryness, and direct exposure to air. Thus, Mg is essential for the cornea to maintain normal structure and function.

Biochemical and ultrastructural studies in the neural retina and retinal pigment epithelium of STZ-diabetic rats: effect of captopril

We measured the activities of total Na+, K+-ATPase (Na, K-ATPase), its alpha1 and alpha2/alpha3 isoforms and the angiotensin-converting enzyme (ACE) in the microvascular and neural compartments of the retina, and/or retinal pigment epithelium (RPE) of streptozotocin (STZ)-diabetic rats. The effect of captopril, an ACE inhibitor on Na, K-ATPase activities was also determined and correlated to morphological changes. Insulin-dependent diabetes mellitus was induced by a single intraperitoneal injection of STZ (60 mg/kg) in male Long-Evans rats. ACE activity was inhibited by captopril (10 mg/kg given in the drinking water) for 1 month. Na, K-ATPase activity was measured spectrophotometrically or by a radioassay (32P-labeled ATP). The activity of ACE was determined by a radioassay using tritiated benzoyl-gly-gly-gly as substrate. Both the alpha1 and alpha2/alpha3 isoforms of Na, K-ATPase were present in the microvascular and neural compartments of retinas, whereas only one isoform, the alpha2/alpha3, was found in the RPE. In 2-month diabetic rats, the activity of the alpha2/alpha3 isoform was reduced in both the microvascular and neural compartments of retinas, while the activity of the alpha1 isoform was reduced only in the neural isolates. ACE activity was significantly decreased in the retinal neural compartment and unaltered in the microvascular compartment from 2-month diabetic rats. In 5-month diabetic rats, Na, K-ATPase activity was moderately but not significantly reduced in RPE preparations. Ultrastructural studies revealed a significant deepening of basal infoldings in the RPE and a noticeable increase in the size of the extracellular space between the basal infoldings of 5-month diabetic animals. Captopril stimulated Na, K-ATPase activity in the neural retina, but not in the RPE. Diabetes-induced morphological changes in the RPE were not improved by captopril. An enlargement of intercellular space between the RPE cells was a frequent finding in the treated group. In conclusion, captopril stimulated Na, K-ATPase activity in the neural retina of diabetic rats. This stimulation seems to be beneficial to the neural retina. ACE inhibition, however, did not improve RPE morphological changes. Although the clinical significance of increased intercellular spacing between RPE cells in treated animals is not clearly established, we speculate that it might contribute to an increased alteration of their barrier function. Additional studies are necessary to assess both the desirable and adverse effects of captopril and other ACE inhibitors in the retinas of diabetic patients.

Gamma-linolenic acid restores renal medullary thick ascending limb Na(+),K(+)-ATPase activity in diabetic rats

In diabetes, the activity of Delta-6 desaturase, which converts linoleic acid (LA) into gamma-linolenic acid (GLA), the first step of arachidonic acid (AA) synthesis, is decreased, leading to alterations in membrane phospholipid composition. On the other hand, 12 wk after the onset of diabetes, Na(+),K(+)-ATPase activity is reduced in many organs, including the kidney. The medullary thick ascending limb (MTAL) reduced Na(+),K(+)-ATPase activity, whereas the sodium load secondary to glomerular hyperfiltration was increased. The aim of our study was to examine whether the changes in membrane fatty acid composition resulting from the inhibition of Delta-6 desaturase may be involved in the decreased Na(+),K(+)-ATPase activity observed in the outer MTAL after 12 wk of diabetes. GLA is a fatty acid that by-passes the Delta-6 desaturase step. We measured the membrane fatty acid composition and the Na(+),K(+)-ATPase activity in the renal outer medulla of control and streptozotocin (STZ)-induced diabetic rats 12 wk after the induction of diabetes. Measurements were performed after supplementation of control rats with sunflower oil (SO) or GLA for 12 wk, and supplementation of 12 wk diabetic rats with SO for 12 wk or with GLA for 6 or 12 wk. Supplementation with GLA not only prevented the decrease in Na(+),K(+)-ATPase activity observed after 12 wk of diabetes but also time dependently stimulated Na(+),K(+)-ATPase activity in the outer medulla. The changes in Na(+),K(+)-ATPase activity were related to parallel changes in the amount of Na(+),K(+)-ATPase alpha(1) subunit protein. In addition, in diabetic rats only, Na(+),K(+)-ATPase activity was positively correlated with the amount of AA present in cell membranes (r = 0.92, P < 0.05). Our results indicate that nutritional GLA supplementation increases Na(+),K(+)-ATPase activity and expression in diabetic rats. In addition, the positive correlation between AA content and Na(+),K(+)-ATPase activity suggests that in diabetic rats, alterations in membrane fatty acid composition contribute to the decreased Na(+),K(+)-ATPase activity in outer medulla.

Unique pharmacological interactions of taurine and chelerythrine in the retina

The effects of taurine and chelerythrine (CHT) on ATP-dependent calcium uptake and the phosphorylation of the approximately 20 kDa phosphoprotein in the retina were compared. In the absence of the CHT, taurine stimulated ATP-dependent calcium uptake and attenuated the phosphorylation of the approximately 20 kDa phosphoprotein. On the other hand, CHT produced the opposite results in the absence of taurine. When the two agents were used simultaneously, it was found that CHT non-competitively inhibited the action of taurine to stimulate calcium uptake, while taurine non-competitively inhibited the action of CHT to stimulate the phosphorylation of the approximately 20 kDa phosphoprotein. The data present an unusual pharmacological mechanism for controlling the signal transduction pathway involving the two distinct cellular processes being studied. Given the unique data, a control system is proposed in which the function of the approximately 20 kDa phosphoprotein is linked to the stimulation of ATP-dependent calcium uptake.

Changes in the redox state in the retina and brain during the onset of diabetes in rats

Diabetic retinopathy is thought to result from chronic changes in the metabolic pathways of the retina. Hyperglycemia leads to increased intracellular glucose concentrations, alterations in glucose degradation and an increase in lactate/pyruvate ratio. We measured lactate content in retina and other ocular and non-ocular tissues from normal and diabetic rats in the early stages of streptozotocin-induced diabetes. The intracellular redox state was calculated from the cytoplasmic [lactate]/[pyruvate] ratio. Elevated lactate concentration were found in retina and cerebral cortex from diabetic rats. These concentrations led to a significant and progressive decrease in the NAD+/NADH ratio, suggesting that altered glucose metabolism is an initial step of retinopathy. It is thus possible that tissues such as cerebral cortex have mechanisms that prevent the damaging effect of lactate produced by hyperglycemia and/or alterations of the intracellular redox state.

Effect of taurine on chelerythrine inhibition of calcium uptake and ATPase activity in the rat retina

Taurine potentiates calcium uptake in whole retinal homogenates as well as in rod outer segments and mitochondrial fractions. The aim of this study was to correlate taurine potentiation of calcium uptake with its effects on other cellular processes through the use of chelerythrine (CHT), a modulator of protein kinase C (PKC), ATPase activity, and, as recently shown, of retinal protein phosphorylation. CHT inhibited calcium uptake only when ATP was present, and inhibition increased significantly in conditions of ATP excess. Taurine potentiated ATP-dependent calcium uptake but decreased the potency of ATP to induce uptake activity. CHT inhibition of calcium uptake exhibited similar potencies in the presence and absence of taurine, and this inhibition seemed to be independent of PKC inhibition. Because of the ATP-dependence of the observed effect, total ATPase activity was studied using similar treatments. In the absence of taurine, CHT inhibited ATPase activity with the same potency (IC50 approximately 59.3 microM) as with calcium uptake inhibition (IC50 approximately 87.9 microM), presenting a possible mechanism of action of CHT. In the presence of taurine, no such correlation was observed, suggesting an ATPase-independent mechanism of action. In fact, taurine did not potentiate ATPase activity, but rather it decreased the potency of CHT inhibition of ATPase, effects incongruent with the effects of taurine on calcium uptake and on CHT inhibition of calcium uptake. Enzyme kinetic experiments provided more supporting data. Taurine was found to cause an increase in the affinity of the ATP substrate for the ATPase enzyme, contradicting the aforementioned effect of taurine to decrease the potency of ATP to induce calcium uptake. Thus, taurine seems to increase calcium uptake through a hitherto unreported mechanism distinct from its modulation of ATPase activity.

Different [3H]ouabain binding characteristics of fast and slow skeletal muscles in IDDM and NIDDM diabetic rats

The aim of this study was to determine the ouabain receptor density, sodium content and contractile properties of skeletal muscles in rats with insulin-dependent (IDDM) and non-insulin-dependent (NIDDM) diabetes mellitus induced by streptozotocin treatment. These parameters were compared in isolated soleus (SOL) and extensor digitorum longus (EDL) muscles of diabetic animals and their age-matched controls. Reversibility of the diabetes-induced changes was studied by insulin or thyroxine substitution. In IDDM SOL muscles both the maximum [3H]ouabain binding capacity (Bmax) and dissociation constant (Kd) were decreased, the sodium content of the muscles increased and the velocity of contraction and relaxation decreased. Similar changes (except for the reduction of Bmax) were observed in diabetic EDL muscles; however, in this case these differences were less prominent. All of these changes were reversed by insulin substitution, whereas thyroxine treatment normalized only the changes in Bmax and velocity of contraction. In contrast to the changes observed in IDDM, NIDDM increased both Kd and Bmax values. Linear correlation was observed between the velocity of contraction or relaxation and the density of [3H]ouabain binding sites in the SOL muscles of IDDM rats. It is concluded that IDDM and NIDDM induce opposite alterations in the density and ouabain sensitivity of the Na(+)-K+ pump in rat skeletal muscle. These diabetic changes are fully reversible with insulin substitution, they are variable in size according to the type of muscle, and are also reflected in the sodium content and, ultimately, in the contractile parameters of the muscles.

Dopamine inhibits mammalian photoreceptor Na+,K+-ATPase activity via a selective effect on the alpha3 isozyme

The rat retina contains dopaminergic interplexiform cells that send processes to the outer plexiform layer where dopamine is released in a light-dependent manner. We report herein that physiologically relevant concentrations of dopamine inhibited ouabain-sensitive photoreceptor oxygen consumption in dark- and light-adapted rat retinas and inhibited Na+,K+-ATPase specific activity (EC 3.6.1.37) in a rat rod outer-inner segment preparation. Experiments with the selective D1 agonist fenoldopam or D2 agonist quinpirole and experiments with dopamine plus either the D1 antagonist SCH23390 or D2/D4 antagonist clozapine showed that the inhibition of oxygen consumption and enzyme activity were mediated by D2/D4-like receptors. The amphetamine-induced release of dopamine, monitored by the inhibition of oxygen consumption, was blocked by L-2-amino-4-phosphonobutyric acid and kynurenic acid. Pharmacological and biochemical experiments determined that the IC50 values of ouabain for the alpha1-low and alpha3-high ouabain affinity isozymes of photoreceptor Na+,K+-ATPase were approximately 10(-5) and approximately 10(-7) M, respectively, and that the D2/D4-like mediated inhibition of Na+,K+-ATPase was exclusively selective for the alpha3 isozyme. The dopamine-mediated inhibition of alpha3 first occurred at 5 nM, was maximal at 100 microM (-47%), had an IC50 value of 382 +/- 23 nM, and exhibited negative cooperativity (Hill coefficient, 0.27). Prior homogenization of the rod outer-inner segment completely prevented the long-lasting inhibition, suggesting that the effect was coupled to a second messenger. Although the physiological significance of our findings to photoreceptor function is unknown, we hypothesize that these results may have relevance for the temporal tuning properties of rods.

Effect of hyperglycemia duration on rabbit corneal thickness and endothelial ATPase activity

This study compares corneal thickness and corneal endothelial Na,K ATPase activity in normal and age-matched diabetic rabbits with up to 10 weeks of hyperglycemia. Corneal endothelial Na,K ATPase activity in normal rabbits increased between 10 and 20 weeks of age. Corneal thickness increased after 5 weeks of hyperglycemia. Decreased corneal endothelial homogenate Na,K ATPase activity was first seen after 10 weeks of hyperglycemia, though an earlier onset was possible due to sampling restrictions. Corneal endothelial homogenate Mg++ ATPase activity showed a transient increase after 2 to 5 weeks of hyperglycemia. This study shows that hyperglycemia in rabbit is associated with complex dynamic interactions between corneal thickness and corneal endothelial Mg++ ATPase and Na,K ATPase activities.

Defective activity of Na+,K(+)-ATPase in peripheral nerve of diabetic rats is independent of the axonal transport of the enzyme

This study addressed the question as to whether the reduced activity of Na+,K(+)-ATPase reported to occur in diabetic nerves and to play a crucial role in the pathogenesis of diabetic neuropathy could be due to derangements in the axonal transport of the enzyme. A micromethod was developed to evaluate the ATPase accumulation in individual segments of ligated sciatic nerves from streptozotocin-induced diabetic rats. The results confirmed a approximately 40% decrease in the background activity, but showed that the enzyme was transported at similar rates in both anterograde and retrograde directions, suggesting that the decrease in its activity does not depend on an altered delivery along the axons.