Acidification stimulates chloride and fluid absorption across frog retinal pigment epithelium

Akt substrate of 160 kDa is essential for the calorie restriction-induced increase in insulin-stimulated glucose uptake by skeletal muscle of female rats

We evaluated effects of calorie restriction (CR; consuming 65% of ad libitum (AL) intake) for 8 weeks on female wildtype (WT) and Akt substrate of 160 kDa knockout (AS160-KO) rats. Insulin-stimulated glucose uptake (ISGU) was determined in isolated epitrochlearis muscles incubated with 0, 50, 100, or 500 µU/mL insulin. Phosphorylation of key insulin signaling proteins that control ISGU (Akt and AS160) was assessed by immunoblotting (Akt phosphorylation on Threonine-308, pAkt^Thr308 and Serine-473, pAkt^Ser473; AS160 phosphorylation on Serine-588, pAS160^Ser588, and Threonine-642, pAS160^Thr642). Abundance of proteins that regulate ISGU (GLUT4 glucose transporter protein and hexokinase II) was also determined by immunoblotting. The major results were as follows: (i) WT-CR versus WT-AL rats had greater ISGU with 100 and 500 µU/mL insulin; (ii) CR versus WT-AL rats had greater GLUT4 protein abundance; (iii) WT-CR versus WT-AL rats had greater pAkt^Thr308 with 500 µU/mL insulin; (iv) WT-CR versus WT-AL rats did not differ for pAkt^Ser473, pAS160^Ser588, or pAS160^Thr642 at any insulin concentration; (v) AS160-KO versus WT rats with each diet had lower ISGU at each insulin concentration, but not lower pAkt on either phosphosite; (vi) AS160-KO versus WT rats had lower muscle GLUT4 abundance regardless of diet; and (vii) AS160-KO-CR versus AS160-KO-AL rats did not differ for ISGU, GLUT4 abundance, pAkt on either phosphosite, or pAS160 on either phosphosite. These novel results demonstrated that AS160 expression, but not greater pAS160 on key phosphosites, was essential for the CR-induced increases in muscle ISGU and GLUT4 abundance of female rats.

Multimodal Imaging in Best Vitelliform Macular Dystrophy

Purpose

In patients diagnosed with Best vitelliform macular dystrophy (BVMD), quantitative fundus autofluorescence (qAF), near-infrared fundus autofluorescence (NIR-AF), and spectral-domain optical coherence tomography (SD-OCT) were used to elucidate pathogenic mechanisms.

Methods

Fourteen patients heterozygous for BEST1 mutations were recruited. qAF was analyzed using short-wavelength fundus autofluorescence (SW-AF) images. Mean gray levels (GL) were determined in nonlesion areas (7 to 9° eccentricity) and adjusted by GL measured in an internal fluorescent reference. NIR-AF images (787 nm; sensitivity of 96) were captured and saved in non-normalized mode. Horizontal SD-OCT images also were acquired and BVMD was staged according to the OCT findings.

Results

In the pre-vitelliform stage, NIR-AF imaging revealed an area of reduced fluorescence, whereas in the vitelliruptive stage, puncta of elevated NIR-AF signal were present. In both SW-AF and NIR-AF images, the vitelliform lesion in the atrophic stage was marked by reduced signal. At all stages of BVMD, nonlesion qAF was within the 95% confidence intervals for healthy eyes. Similarly, the NIR-AF intensity measurements outside the vitelliform lesion were comparable to the healthy control eye. SD-OCT scans revealed a fluid-filled detachment between the ellipsoid zone and the hyperreflectivity band attributable to RPE/Bruch's membrane.

Conclusions

NIR-AF imaging can identify the pre-vitelliform stage of BVMD. Mutations in BEST1 are not associated with increased levels of SW-AF outside the vitelliform lesion. Elevated SW-AF within the fluid-filled lesion likely reflects the inability of RPE to phagocytose outer segments due to separation of RPE from photoreceptor cells, together with progressive photoreceptor cell impairment.

The slow light and dark oscillation of the clinical electro-oculogram

BACKGROUND

The standing potential of the eye exhibits a slow damped oscillation under light and dark conditions that continues for at least 80 minutes. However, our understanding of the relationship between the slow dark and light oscillation has not been previously studied. The aim of this study was to explore through regression analysis a model of these oscillations in order to establish if they may have the same underlying cellular generators.

METHODS

Healthy participants undertook recordings of the standing potential using the electro-oculogram for 100 minutes. To explore the light oscillation, participants (n = 8) were dilated and performed an extended electro-oculogram protocol consisting of 15 minutes dark adaptation and 85 minutes of white light adaptation at 100 cd/m² . For the dark oscillation, participants (n = 11) undertook the electro-oculogram for 100 minutes in complete darkness. Both sessions began with pre-adaptation to 30 cd/m² of white light for five minutes. Non-parametric statistics were used to evaluate all data.

RESULTS

Ratios of the dark and light oscillations showed a significantly greater dampening of the dark oscillation compared to the light oscillation (p < 0.000). Regression analysis using a five-factor damped sine function revealed significant differences in the parameters governing the dampening (p = 0.005) and period (p = 0.009) of the functions (R² > 0.874). There were no significant differences in the dark trough amplitude.

CONCLUSION

The results support a different underlying physiological mechanism for the light and dark oscillation of the clinical electro-oculogram. Future work will need to establish how the dark oscillation and dark trough of the clinical electro-oculogram arise.

Retinal pigment epithelium in the pathogenesis of age-related macular degeneration and photobiomodulation as a potential therapy?

The retinal pigment epithelium (RPE) comprises a monolayer of cells located between the neuroretina and the choriocapillaries. The RPE serves several important functions in the eye: formation of the blood-retinal barrier, protection of the retina from oxidative stress, nutrient delivery and waste disposal, ionic homeostasis, phagocytosis of photoreceptor outer segments, synthesis and release of growth factors, reisomerization of all-trans-retinal during the visual cycle, and establishment of ocular immune privilege. Age-related macular degeneration (AMD) is the leading cause of blindness in developed countries. Dysfunction of the RPE has been associated with the pathogenesis of AMD in relation to increased oxidative stress, mitochondrial destabilization and complement dysregulation. Photobiomodulation or near infrared light therapy which refers to non-invasive irradiation of tissue with light in the far-red to near-infrared light spectrum (630-1000 nm), is an intervention that specifically targets key mechanisms of RPE dysfunction that are implicated in AMD pathogenesis. The current evidence for the efficacy of photobiomodulation in AMD is poor but its safety profile and proposed mechanisms of action motivate further research as a novel therapy for AMD.

Anatomical and Gene Expression Changes in the Retinal Pigmented Epithelium Atrophy 1 (rpea1) Mouse: A Potential Model of Serous Retinal Detachment

Purpose

The purpose of this study was to examine the rpea1 mouse whose retina spontaneously detaches from the underlying RPE as a potential model for studying the cellular effects of serous retinal detachment (SRD).

Methods

Optical coherence tomography (OCT) was performed immediately prior to euthanasia; retinal tissue was subsequently prepared for Western blotting, microarray analysis, immunocytochemistry, and light and electron microscopy (LM, EM).

Results

By postnatal day (P) 30, OCT, LM, and EM revealed the presence of small shallow detachments that increased in number and size over time. By P60 in regions of detachment, there was a dramatic loss of PNA binding around cones in the interphotoreceptor matrix and a concomitant increase in labeling of the outer nuclear layer and rod synaptic terminals. Retinal pigment epithelium wholemounts revealed a patchy loss in immunolabeling for both ezrin and aquaporin 1. Anti-ezrin labeling was lost from small regions of the RPE apical surface underlying detachments at P30. Labeling for tight-junction proteins provided a regular array of profiles outlining the periphery of RPE cells in wild-type tissue, however, this pattern was disrupted in the mutant as early as P30. Microarray analysis revealed a broad range of changes in genes involved in metabolism, signaling, cell polarity, and tight-junction organization.

Conclusions

These data indicate changes in this mutant mouse that may provide clues to the underlying mechanisms of SRD in humans. Importantly, these changes include the production of multiple spontaneous detachments without the presence of a retinal tear or significant degeneration of outer segments, changes in the expression of proteins involved in adhesion and fluid transport, and a disrupted organization of RPE tight junctions that may contribute to the formation of focal detachments.

Skeletal muscle-specific overexpression of SIRT1 does not enhance whole-body energy expenditure or insulin sensitivity in young mice

AIMS/HYPOTHESIS

The NAD(+)-dependent protein deacetylase sirtuin (SIRT)1 is thought to be a key regulator of skeletal muscle metabolism. However, its precise role in the regulation of insulin sensitivity is unclear. Accordingly, we sought to determine the effect of skeletal muscle-specific overexpression of SIRT1 on skeletal muscle insulin sensitivity and whole-body energy metabolism.

METHODS

At 10 weeks of age, mice with muscle-specific overexpression of SIRT1 and their wild-type littermates were fed a standard diet with free access to chow or an energy-restricted (60% of standard) diet for 20 days. Energy expenditure and body composition were measured by indirect calorimetry and magnetic resonance imaging, respectively. Skeletal muscle insulin-stimulated glucose uptake was measured ex vivo in soleus and extensor digitorum longus muscles using a 2-deoxyglucose uptake technique with a physiological insulin concentration of 360 pmol/l (60 μU/ml).

RESULTS

Sirt1 mRNA and SIRT1 protein levels were increased by approximately 100- and 150-fold, respectively, in skeletal muscle of mice with SIRT1 overexpression compared with wild-type mice. Despite this large-scale overexpression of SIRT1, body composition, whole-body energy expenditure, substrate oxidation and voluntary activity were comparable between genotypes. Similarly, skeletal muscle basal and insulin-stimulated glucose uptake were unaltered with SIRT1 overexpression. Finally, while 20 days of energy restriction enhanced insulin-stimulated glucose uptake in skeletal muscles of wild-type mice, no additional effect of SIRT1 overexpression was observed.

CONCLUSIONS/INTERPRETATION

These results demonstrate that upregulation of SIRT1 activity in skeletal muscle does not affect whole-body energy expenditure or enhance skeletal muscle insulin sensitivity in young mice on a standard diet with free access to chow or in young mice on energy-restricted diets.

CO2-induced ion and fluid transport in human retinal pigment epithelium

In the intact eye, the transition from light to dark alters pH, [Ca²⁺], and [K] in the subretinal space (SRS) separating the photoreceptor outer segments and the apical membrane of the retinal pigment epithelium (RPE). In addition to these changes, oxygen consumption in the retina increases with a concomitant release of CO₂ and H₂O into the SRS. The RPE maintains SRS pH and volume homeostasis by transporting these metabolic byproducts to the choroidal blood supply. In vitro, we mimicked the transition from light to dark by increasing apical bath CO₂ from 5 to 13%; this maneuver decreased cell pH from 7.37 ± 0.05 to 7.14 ± 0.06 (n = 13). Our analysis of native and cultured fetal human RPE shows that the apical membrane is significantly more permeable (≈10-fold; n = 7) to CO₂ than the basolateral membrane, perhaps due to its larger exposed surface area. The limited CO₂ diffusion at the basolateral membrane promotes carbonic anhydrase–mediated HCO₃ transport by a basolateral membrane Na/nHCO₃ cotransporter. The activity of this transporter was increased by elevating apical bath CO₂ and was reduced by dorzolamide. Increasing apical bath CO₂ also increased intracellular Na from 15.7 ± 3.3 to 24.0 ± 5.3 mM (n = 6; P < 0.05) by increasing apical membrane Na uptake. The CO₂-induced acidification also inhibited the basolateral membrane Cl/HCO₃ exchanger and increased net steady-state fluid absorption from 2.8 ± 1.6 to 6.7 ± 2.3 µl × cm⁻² × hr⁻¹ (n = 5; P < 0.05). The present experiments show how the RPE can accommodate the increased retinal production of CO₂ and H₂O in the dark, thus preventing acidosis in the SRS. This homeostatic process would preserve the close anatomical relationship between photoreceptor outer segments and RPE in the dark and light, thus protecting the health of the photoreceptors.

Insulin sensitivity: modulation by nutrients and inflammation

Insulin resistance is a major metabolic feature of obesity and is a key factor in the etiology of a number of diseases, including type 2 diabetes. In this review, we discuss potential mechanisms by which brief nutrient excess and obesity lead to insulin resistance and propose that these mechanisms of action are different but interrelated. We discuss how pathways that "sense" nutrients within skeletal muscle are readily able to regulate insulin action. We then discuss how obesity leads to insulin resistance via a complex interplay among systemic fatty acid excess, microhypoxia in adipose tissue, ER stress, and inflammation. In particular, we focus on the hypothesis that the macrophage is an important cell type in the propagation of inflammation and induction of insulin resistance in obesity. Overall, we provide our integrative perspective regarding how nutrients and obesity interact to regulate insulin sensitivity.

Control of chemokine gradients by the retinal pigment epithelium

PURPOSE

Proinflammatory cytokines in degenerative diseases can lead to the loss of normal physiology and the destruction of surrounding tissues. In the present study, the physiological responses of human fetal retinal pigment epithelia (hfRPE) were examined in vitro after polarized activation of proinflammatory cytokine receptors.

METHODS

Primary cultures of hfRPE were stimulated with an inflammatory cytokine mixture (ICM): interleukin (IL)-1beta, tumor necrosis factor (TNF)-alpha, and interferon (IFN)-gamma. Western blot analysis and immunofluorescence were used to determine the expression/localization of the cytokine receptors on hfRPE. Polarized secretion of cytokines was measured. A capacitance probe technique was used to measure transepithelial fluid flow (J(V)) and resistance (R(T)).

RESULTS

IL-1R1 was mainly localized to the apical membrane and TNFR1 to the basal membrane, whereas IFN-gammaR1 was detected on both membranes. Activation by apical ICM induced a significant secretion of angiogenic and angiostatic chemokines, mainly across the hfRPE apical membrane. Addition of the ICM to the basal but not the apical bath significantly increased net fluid absorption (J(V)) across the hfRPE within 20 minutes. Similar increases in J(V) were produced by a 24-hour exposure to ICM, which significantly decreased total R(T).

CONCLUSIONS

Chemokine gradients across the RPE can be altered (1) through an ICM-induced change in polarized chemokine secretion and (2) through an increase in ICM-induced net fluid absorption. In vivo, both of these factors could contribute to the development of chemokine gradients that help mediate the progression of inflammation/angiogenesis at the retina/RPE/choroid complex.

Bestrophin Cl- channels are highly permeable to HCO3-

Bestrophin-1 (Best1) is a Cl(-) channel that is linked to various retinopathies in both humans and dogs. Dysfunction of the Best1 Cl(-) channel has been proposed to cause retinopathy because of altered Cl(-) transport across the retinal pigment epithelium (RPE). In addition to Cl(-), many Cl(-) channels also transport HCO3(-). Because HCO3(-) is physiologically important in pH regulation and in fluid and ion transport across the RPE, we measured the permeability and conductance of bestrophins to HCO3(-) relative to Cl(-). Four human bestrophin homologs (hBest1, hBest2, hBest3, and hBest4) and mouse Best2 (mBest2) were expressed in HEK cells, and the relative HCO3(-) permeability (P HCO3/PCl) and conductance (G HCO3/GCl) were determined. P HCO3/PCl was calculated from the change in reversal potential (Erev) produced by replacing extracellular Cl(-) with HCO3(-). hBest1 was highly permeable to HCO3(-) (P HCO3)/PCl = approximately 0.44). hBest2, hBest4, and mBest2 had an even higher relative HCO3(-) permeability (P HCO3/PCl = 0.6-0.7). All four bestrophins had HCO3(-) conductances that were nearly the same as Cl(-) (G HCO3/GCl = 0.9-1.1). Extracellular Na+ did not affect the permeation of hBest1 to HCO3(-). At physiological HCO3(-) concentration, HCO3(-) was also highly conductive. The hBest1 disease-causing mutations Y85H, R92C, and W93C abolished both Cl(-) and HCO3(-) currents equally. The V78C mutation changed P HCO3/PCl and G HCO3/GCl of mBest2 channels. These results raise the possibility that disease-causing mutations in hBest1 produce disease by altering HCO3(-) homeostasis as well as Cl(-) transport in the retina.

The electro-oculogram

The retinal pigment epithelium (RPE) lying distal to the retina regulates the extracellular environment and provides metabolic support to the outer retina. RPE abnormalities are closely associated with retinal death and it has been claimed several of the most important diseases causing blindness are degenerations of the RPE. Therefore, the study of the RPE is important in Ophthalmology. Although visualisation of the RPE is part of clinical investigations, there are a limited number of methods which have been used to investigate RPE function. One of the most important is a study of the current generated by the RPE. In this it is similar to other secretory epithelia. The RPE current is large and varies as retinal activity alters. It is also affected by drugs and disease. The RPE currents can be studied in cell culture, in animal experimentation but also in clinical situations. The object of this review is to summarise this work, to relate it to the molecular membrane mechanisms of the RPE and to possible mechanisms of disease states.

Chronic food restriction enhances memory in mice--analysis with matched drive levels

We compared the effects of chronic and acute food deprivation on learning and memory using a dry-type water maze, active avoidance and passive avoidance in C57BL/6L mice. The drive level of the animals--under acute and chronic food deprivation--was matched by a progressive ratio schedule. Both deprivations led to a high degree of activity in the animals; however, the animals on an acute dietary restriction did not exhibit a significantly better performance than those on ad libitum feeding, while those on a chronic food deprivation exhibited memory enhancement. These effects were subtle and were found at a later stage of learning. These findings suggest that chronic food restriction induces memory consolidation or resistance to memory reduction in addition to increased activity.

Voltage-dependent ion channels in the mouse RPE: comparison with Norrie disease mice

We studied electrophysiological properties of cultured retinal pigment epithelial (RPE) cells from mouse and a mouse model for Norrie disease. Wild-type RPE cells revealed the expression of ion channels known from other species: delayed-rectifier K(+) channels composed of Kv1.3 subunits, inward rectifier K(+) channels, Ca(V)1.3 L-type Ca(2+) channels and outwardly rectifying Cl(-) channels. Expression pattern and the ion channel characteristics current density, blocker sensitivity, kinetics and voltage-dependence were compared in cells from wild-type and Norrie mice. Although no significant differences were observed, our study provides a base for future studies on ion channel function and dysfunction in transgenic mouse models.

The retinal pigment epithelium in visual function

Located between vessels of the choriocapillaris and light-sensitive outer segments of the photoreceptors, the retinal pigment epithelium (RPE) closely interacts with photoreceptors in the maintenance of visual function. Increasing knowledge of the multiple functions performed by the RPE improved the understanding of many diseases leading to blindness. This review summarizes the current knowledge of RPE functions and describes how failure of these functions causes loss of visual function. Mutations in genes that are expressed in the RPE can lead to photoreceptor degeneration. On the other hand, mutations in genes expressed in photoreceptors can lead to degenerations of the RPE. Thus both tissues can be regarded as a functional unit where both interacting partners depend on each other.

Chloride currents in acutely isolated Xenopus retinal pigment epithelial cells

The retinal pigment epithelium (RPE) regulates the ionic composition of the fluid surrounding the photoreceptors by transport mechanisms that utilize Cl- channels. Cl- currents in RPE cells, however, remain incompletely characterized. The purpose of this study was to identify the Cl- currents in acutely isolated Xenopus RPE cells using whole-cell patch clamp. We describe three different Cl- currents. (1) An inwardly rectifying Cl- current, ICl,ir, activates slowly with hyperpolarization (tauact = ~1 s at -80 mV, V1/2= -94 +/- 3 mV), is blocked by Zn2+ (IC50 =185 microM), is stimulated by acid (ICl,ir is 5 times larger at pH 6 than pH 8), and is blocked by DIDS in a voltage-dependent manner. ICl,ir closely resembles cloned ClC-2currents. (2) An outwardly rectifying Cl- current, ICl,Ca, is stimulated by elevated cytosolic free [Ca2+]. With 1 microM free [Ca2+]i in the patch pipette, ICl,Ca activates slowly with depolarization (tauact =325 ms at 100 mV) and deactivates upon hyperpolarization. ICl,Ca is not blocked by 1 mM Zn2+ or 10 microM Gd3+ but is blocked by DIDS. High extracellular [Ca2+] (10 mM) also activates ICl,Ca. (3) A non-rectifying current is activated by elevation of cytoplasmic cAMP with forskolin and IBMX. In addition to these three Cl- currents, Xenopus RPE cells exhibit a non-selective background current (Ibkg) which has a linear I-V relationship and is voltage insensitive. This current is blocked by Zn2+ (IC50 of 5.3 microM) or 10 microM Gd3+. This description provides new insights into the physiology of Cl- channels involved in salt and fluid transport by the retinal pigment epithelium.

Growing old: metabolic control and yeast aging

The metabolic characteristics of a yeast cell determine its life span. Depending on conditions, stress resistance can have either a salutary or a deleterious effect on longevity. Gene dysregulation increases with age, and countering it increases life span. These three determinants of yeast longevity may be interrelated, and they are joined by a potential fourth, genetic stability. These factors can also operate in phylogenetically diverse species. Adult longevity seems to borrow features from the genetic programs of dormancy to provide the metabolic and stress resistance resources necessary for extended survival. Both compensatory and preventive mechanisms determine life span, while epigenetic factors and the element of chance contribute to the role that genes and environment play in aging.

Molecular mechanisms of water transport in the eye

The four major sites for ocular water transport, the corneal epithelium and endothelium, the ciliary epithelium, and the retinal pigment epithelium, are reviewed. The cornea has an inherent tendency to swell, which is counteracted by its two surface cell layers, the corneal epithelium and endothelium. The bilayered ciliary epithelium secretes the aqueous humor into the posterior chamber, and the retinal pigment epithelium transports water from the retinal to the choroidal site. For each epithelium, ion transport mechanisms are associated with fluid transport, but the exact molecular coupling sites between ion and water transport remain undefined. In the retinal pigment epithelium, a H+-lactate cotransporter transports water. This protein could be the site of coupling between salt and water in this epithelium. The distribution of aquaporins does not suggest a role for these proteins in a general model for water transport in ocular epithelia. Some water-transporting membranes contain aquaporins, others do not. The ultrastructure is also variable among the cell layers and cannot be fitted into a general model. On the other hand, the direction of cotransport in symporters complies with the direction of fluid transport in both the corneal epi- and endothelium, as well as the ciliary epithelium and retinal pigment epithelium.

Inhibition of membrane-bound carbonic anhydrase decreases subretinal pH and volume

PURPOSE

The lipophilic carbonic anhydrase (CA) inhibitor acetazolamide has been shown to enhance subretinal fluid resorption, reduce subretinal pH, and can improve cystoid macular edema, but its clinical use is limited by systemic side effects. While these are most likely a result of inhibiting intracellular CA isoenzymes, retinal pigment epithelial (RPE) transport is thought to be modulated via membrane-bound CA. This study investigates whether benzolamide, a hydrophilic CA inhibitor that does not readily penetrate cell membranes, is sufficient to modulate subretinal volume and pH.

METHODS

Volume and pH were assessed in the subretinal space (SRS) of the perfused chick retina-RPE-choroid preparation by calculating these variables from data obtained with two different double-barreled, ion-selective electrodes (H+ for pH and the extracellular space marker tetramethylammonium (TMA+) for SRS volume). Light induced variations and changes in baseline measurements were recorded before and after addition of 10(-4) M acetazolamide or benzolamide to the basal perfusion.

RESULTS

Basal perfusion with either drug induced both an acidification of the SRS by 0.02-0.04 pH units, which occurred within 60 s, as well as an increase in the amplitude of the light-induced alkalinisation of the SRS. TMA+ concentration in the SRS increased steadily over a period of several minutes after basal perfusion with either of the CA inhibitors, and the calculated SRS volume was reduced by 40% within 8-10 min.

CONCLUSION

The observation that benzolamide had effects equal to acetazolamide suggests that inhibition of membrane-bound CA at the basolateral membrane of the RPE is sufficient to decrease subretinal pH and volume. This may represent a clinically important mechanism for the resorption of sub- and intraretinal fluid.

Effect of extracellular palmitate on 2-deoxy-d-glucose uptake in muscle from Ad libitum fed and calorie restricted rats

We studied the effect of a high physiologic concentration of palmitate (1mM) on in vitro 2-deoxy-D-glucose (2DG) uptake by flexor digitorum brevis (FDB) muscle from ad libitum fed rats (AL) and rats fed 60% of ad libitum intake (CR) for 20 days. CR did not alter muscle 2DG uptake in the absence of insulin, but relative to AL, CR significantly (p<0.01) increased 2DG uptake in the presence of 20,000 microU/ml insulin. This effect of CR persisted in the presence of 1mM palmitate. The presence of 1mM palmitate significantly (p<0.01) impaired 2DG glucose uptake, both in the presence and absence of insulin, to the same extent in AL and CR muscle, despite an 18% decrease in FABPpm expression with CR. Thus, although CR profoundly affects insulin-mediated muscle glucose uptake, it does not alter the ability of extracellular fatty acid to modulate glucose utilization by skeletal muscle.

Comparison of the effects of 20 days and 15 months of calorie restriction on male Fischer 344 rats

The aim of this study was to compare, in 19-month-old male Fischer 344 rats, the influence of brief (20 days) and prolonged (approximately 15 months) calorie restriction (CR; consuming approximately 60% of ad libitum, AL, intake) on circulating levels of glucose, insulin, C-peptide, and free fatty acids (FFA); age-matched AL rats were also studied. In the prolonged CR group, there was an approximately 85% decline in fat pad masses (epididymal and retroperitoneal) compared to AL and brief CR rats (these latter groups did not differ significantly). Compared to AL levels, glucose was 15% lower with prolonged CR (p < 0.05) while the brief CR values tended to be lower (10%) than AL; the CR groups did not differ significantly. Plasma FFA levels were significantly (p < 0.05) greater (85-106%) in the brief CR group compared to each of the other groups. Plasma insulin concentrations for the CR groups were lower (p < 0.05; approximately 50-60%) than AL levels. Plasma concentrations of C-peptide (an indicator of insulin secretion) were also lower for each CR group vs AL levels, and a high correlation was found between plasma insulin and C-peptide concentrations (r2 = 0.90; p < 0.001). The C-peptide/insulin ratios for the CR groups were similar, and the value of each CR group exceeded that for the AL rats. These results demonstrate that: the CR-induced reduction in plasma insulin is attributable in large part to reduced insulin secretion; these decreases in insulin secretion and concentration are essentially undiminished when brief CR is initiated rather late in life, and the reductions are independent of substantial reductions in body fat.

Chronic ethanol feeding impairs glucose tolerance but does not produce skeletal muscle insulin resistance in rat epitrochlearis muscle

Herein, we have investigated whether male Wistar rats develop impaired glucose tolerance after ethanol feeding. Rats were fed a liquid diet providing 35% calories from ethanol (EF) or a control diet that isocalorically replaced ethanol with maltose-dextrins for 4 weeks. Intravenous glucose tolerance was impaired in EF rats compared with pair-fed (PF), but not ad libitum (AL) controls. Areas under the intravenous glucose tolerance test curve were 5476 +/- 516 mm2, 3056 +/- 421 mm2, and 4199 +/- 613 mm2 (p < 0.05) for AL, PF, and EF rats, respectively. Initial plasma insulin concentrations in EF rats were comparable with PF rats; however, 15 min after a dextrose challenge, plasma insulin levels in EF rats were 39% lower than PF rats. Because skeletal muscle is the primary sink for insulin-mediated glucose disposal, the development of skeletal muscle insulin resistance after ethanol feeding could contribute to impaired glucose tolerance. Total GLUT1 was not affected by diet in either red or white muscle. No difference in the total quantity of insulin-responsive glucose transporter, GLUT4, was observed in red muscle. In contrast, GLUT4 was 20% lower in white muscle from EF rats, compared with PF and AL rats. However, insulin-stimulated glucose transport into the epitrochlearis, a white muscle group, was not impaired with ethanol feeding. These data demonstrate that chronic ethanol feeding impairs glucose tolerance; impaired glucose tolerance was associated with an inability to maintain plasma insulin levels, rather than the development of skeletal muscle insulin resistance.