Glucose and insulin exert additive ocular and renal vasodilator effects on healthy humans

Effect of DPP-4i inhibitors on renal function in patients with type 2 diabetes mellitus: a systematic review and meta-analysis of randomized controlled trials

AIMS

About 20-40% patients with type 2 diabetes mellitus (T2DM) had an increased risk of developing diabetic nephropathy (DN). Dipeptidyl peptidase-4 inhibitors (DPP-4i) were recommended for treatment of T2DM, while the impact of DPP-4i on renal function remained unclear. This study aimed to explore the effect of DPP-4i on renal parameter of estimated glomerular filtration rate (eGFR) and albumin-to-creatinine ratio (ACR) in T2DM.

METHODS

A systematic search was performed across PubMed, Embase and Cochrane Library. A fixed or random-effects model was used for quantitative synthesis according to the heterogeneity, which was assessed with I2 index. Sensitivity analysis and publication bias were performed with standard methods, respectively.

RESULTS

A total of 17 randomized controlled trials were identified. Administration of DPP-4i produced no significant effect on eGFR (WMD, -0.92 mL/min/1.73m2, 95% CI, -2.04 to 0.19) in diabetic condition. DPP-4i produced a favorable effect on attenuating ACR (WMD, -2.76 mg/g, 95% CI, -5.23 to -0.29) in patients with T2DM. The pooled estimate was stable based on the sensitivity test. No publication bias was observed according to Begg's and Egger's tests.

CONCLUSIONS

Treatment with DPP-4i preserved the renal parameter of eGFR in diabetic condition. Available evidences suggested that administration of DPP-4i produced a favorable effect on attenuating ACR in patients with T2DM. INTERNATIONAL PROSPECTIVE REGISTER FOR SYSTEMATIC REVIEW (PROSPERO) NUMBER: CRD.42020144642.

Central and peripheral changes in the retina and choroid in patients with diabetes mellitus without clinical diabetic retinopathy assessed by ultra-wide-field optical coherence tomography angiography

Background

To explore the central and peripheral retinal and choroidal changes in diabetic patients without clinical diabetic retinopathy (DM-NoDR) using ultra-wide-field swept-source optical coherence tomography angiography (UWF-SS-OCTA).

Methods

67 DM-NoDR eyes and 32 age-matched healthy eyes were recruited. Retinal and choroidal parameters, including qualitative retinal microangiopathy, vessel flow (VFD) and linear density (VLD), thickness, and volume, were measured in the central and peripheral areas of the 24 × 20 mm² UWF-SS-OCTA images.

Results

DM-NoDR eyes had significantly more nonperfusion area and capillary tortuosity than controls in the central and peripheral areas (p < 0.05). The presence of central capillary tortuosity was associated with higher levels of serum creatinine (OR 1.049, 95%CI 1.001-1.098; p = 0.044) and blood urea nitrogen (OR 1.775, 95%CI 1.051-2.998; p = 0.032) in DM-NoDR eyes. For DM-NoDR eyes versus controls, VFD in the 300-μm annulus around the foveal avascular zone, superficial capillary plexus (SCP), and full retina, and SCP-VLD significantly decreased, while VFD in the deep capillary plexus (DCP), retinal thickness, and retinal volume increased (p < 0.05). Analysis in the central and peripheral areas recapitulated all these findings, except for decreased peripheral thickness and volume and no difference in peripheral DCP-VFD. In DM-NoDR eyes, choriocapillaris-VFD, choroidal thickness, and choroidal volume increased in the central area, while VFD in the large and medium choroidal vessel layer decreased in the whole image (p < 0.05).

Conclusion

Retinal and choroidal changes already existed in the central and/or peripheral areas of DM-NoDR eyes. UWF-SS-OCTA, enabling the visualization of the peripheral fundus area, is a promising image technique for the early detection of fundus changes in DM-NoDR patients.

Acute effects of caffeine and glucose intake on retinal vessel calibres in healthy volunteers

PURPOSE

To evaluate the acute effects of caffeine and glucose intake on retinal vascular calibre of healthy adults.

METHODS

This prospective crossover study was conducted at the Centre for Eye Research Australia (Melbourne, Australia). Standardized doses of 300 mg caffeine (approximately 3 cups coffee), 30 g glucose or 300 ml of water, were each given to 19 healthy subjects on separate days. Retinal photographs and blood pressure measurements were taken at baseline, 30-, 60- and 120-min after ingestion of each solution. Central retinal artery and vein equivalents (CRAE, CRVE) and the arterio-venule ratio were measured using computer-assisted software. The mean retinal vascular calibre measurements were compared between pre- and post-ingestion images.

RESULTS

After caffeine intake, significant reductions were observed in mean CRAE of - 9.3 μm, - 10.4 μm and - 8.5 μm and CRVE of - 16.9 μm, - 18.7 μm and - 16.1 μm at 30-, 60- and 120-min after intake when compared with baseline (p ≤ 0.002 for all; paired t test). No significant changes were observed in mean retinal vascular calibre measurements after intake of either glucose or water when compared to baseline (p ≥ 0.072 for all). When controlling for baseline characteristics and blood pressure measurements, only caffeine intake had a significant effect on reducing both CRAE and CRVE at all time points post ingestion (p ≤ 0.003 for all, multiple linear regression model).

CONCLUSION

Caffeine is associated with an acute vasoconstrictive effect on retinal arterioles and venules in healthy subjects. Factors other than blood pressure-induced autoregulation play a significant role in caffeine-associated retinal vasoconstriction.

Wide-field swept-source OCTA in the assessment of retinal microvasculature in early-stage diabetic retinopathy

BACKGROUND

To perform a quantitative analysis of retinal microvasculature in patients with early-stage diabetic retinopathy (DR) using wide-field swept-source optical coherence tomography angiography (SS-OCTA).  METHODS: One hundred nineteen eyes of 119 patents (67 eyes with no DR and 52 eyes with mild-moderate nonproliferative diabetic retinopathy (NPDR)) were enrolled in this observational and cross-sectional cohort study, and an age-matched group consisting of 39 eyes of 39 non-diabetic subjects were set as the control. Each participant underwent a full ophthalmic examination, including wide-field SS-OCTA imaging. On OCTA scans (12 mm * 12 mm), the mean perfusion area (PA) and vessel density (VD) were independently measured in all 16 Early Treatment Diabetic Retinopathy Study (ETDRS) sectors. Linear regression analyses were conducted to evaluate the influences of PA.

RESULTS

In the central ring, there were no significant differences in the average PA and VD among the groups. In the 3 mm radius, the PA and VD of the no DR and mild-moderate NPDR were significantly decreased compared with the control group in superior and inferior quadrants. In the wide-field scans (9 and 12 mm radius), there was no significant difference in average PA and VD between the groups in each sectors (p > 0.05). Regression analysis found that the effect of VD on PA was statistically different (b = 1.311, p < 0.001).

CONCLUSION

Wide-field OCTA imaging is useful for evaluating peripheral capillary perfusion in eyes with early-stage DR. Decrease in PA and VD was greater in the S3 and I3 sectors, and reductions in PA and VD were uneven in wide-filed sectors (9 and 12 mm radius).

Panretinal laser photocoagulation decreases large foveal avascular zone area in non-proliferative diabetic retinopathy: A prospective OCTA study

PURPOSE

To analyze the macular microvascular changes after panretinal photocoagulation (PRP) in patients with non-proliferative diabetic retinopathy (NPDR) and a large foveal avascular zone area, using optical coherence tomography angiography.

METHODS

Twenty-four eyes of 24 patients with peripheral ischemia, superficial foveal avascular zone (FAZ) area of larger than 0.350 mm², naive severe NPDR, and no clinically significant diabetic macular edema were included in this prospective study. The PRP was applied in 360-degree in a single session. The main outcome measures of the study were the difference in best-corrected visual acuity, central macular thickness, superficial and deep vascular plexus vessel densities, FAZ features, choroidal and outer retinal flow areas at the baseline versus at one and six months after PRP treatment.

RESULTS

The study group consisted of 13 men and 11 women with a mean age of 68.11 ± 6.47 years. The baseline FAZ area was higher than at one and six months after PRP (0.416 ± 0.70, 0.399 ± 0.065 and 0.407 ± 0.066 mm²; p = 0.001 and p = 0.002, respectively). At one month after PRP, deep capillary plexus vascular density in perifoveal region was statistically significantly lower than at six months after PRP and the baseline. (45.43 ± 4.27, 47.91 ± 4.26 and 49.04 ± 5.64 %; p = 0.001 and p = 0.001, respectively).

CONCLUSION

The PRP effects retinal microvascular morphology in patients with NPDR and a large FAZ area.

Reversed Neurovascular Coupling on Optical Coherence Tomography Angiography Is the Earliest Detectable Abnormality before Clinical Diabetic Retinopathy

Diabetic retinopathy (DR) has traditionally been viewed as either a microvasculopathy or a neuropathy, though neurovascular coupling deficits have also been reported and could potentially be the earliest derangement in DR. To better understand neurovascular coupling in the diabetic retina, we investigated retinal hemodynamics by optical coherence tomography angiography (OCTA) in individuals with diabetes mellitus (DM) but without DR (DM no DR) and mild non-proliferative DR (mild NPDR) compared to healthy eyes. Using an experimental design to monitor the capillary responses during transition from dark adaptation to light, we examined 19 healthy, 14 DM no DR and 11 mild NPDR individuals. We found that the only structural vascular abnormality in the DM no DR group was increased superficial capillary plexus (SCP) vessel density (VD) compared to healthy eyes, while mild NPDR eyes showed significant vessel loss in the SCP at baseline. There was no significant difference in inner retinal thickness between the groups. During dark adaptation, the deep capillary plexus (DCP) VD was lower in mild NPDR individuals compared to the other two groups, which may leave the photoreceptors more susceptible to ischemia in the dark. When transitioning from dark to ambient light, both diabetic groups showed a qualitative reversal of VD trends in the SCP and middle capillary plexus (MCP), with significantly decreased SCP at 5 min and increased MCP VD at 50 s compared to healthy eyes, which may impede metabolic supply to the inner retina during light adaptation. Mild NPDR eyes also demonstrated DCP dilation at 50 s and 5 min and decreased adjusted flow index at 5 min in light. Our results show altered neurovascular responses in all three macular vascular plexuses in diabetic subjects in the absence of structural neuronal changes on high resolution imaging, suggesting that neurovascular uncoupling may be a key mechanism in the early pathogenesis of DR, well before the clinical appearance of vascular or neuronal loss.

Acute Hyperglycemia Reverses Neurovascular Coupling During Dark to Light Adaptation in Healthy Subjects on Optical Coherence Tomography Angiography

Purpose

To test the hypothesis that hyperglycemia perturbs neurovascular\ coupling and compromises retinal vascular response during transition from dark to light in healthy subjects using optical coherence tomography angiography (OCTA).

Methods

Ten eyes of 10 healthy subjects were tested, first during fasting and then after receiving a 75-g oral glucose solution. In both sessions, OCTA imaging was done in the dark-adapted state and at 50 seconds, 2 minutes, 5 minutes, and 15 minutes of ambient light. Parafoveal vessel density (VD) and adjusted flow index (AFI) were calculated for the superficial capillary plexus (SCP), middle capillary plexus (MCP), and deep capillary plexus (DCP), and vessel length density was calculated for the SCP. These measurements were compared among conditions after adjusting for age, refractive error, and OCTA scan quality.

Results

Hyperglycemia leads to a complete reversal of dark/light adaptation trends in VD and AFI in all layers of the inner retina. In the dark, there is significantly decreased VD in the DCP in hyperglycemia. With a transition to light in hyperglycemia, we observed decreased VD in the SCP, increased vessel density in the MCP and DCP, and decreased AFI in all three layers.

Conclusions

Our results show that hyperglycemia significantly disrupts neurovascular coupling in healthy eyes, with potential metabolic deficits affecting photoreceptor oxygen demands during dark adaptation and the inner retina during light exposure. In pathological states, such as diabetic retinopathy, where the vasculature is already attenuated, retinal neurons may be exquisitely vulnerable to intermittent hyperglycemic challenge, which should be the focus of future studies.

Improved Macular Capillary Flow on Optical Coherence Tomography Angiography After Panretinal Photocoagulation for Proliferative Diabetic Retinopathy

PURPOSE

This study evaluated the macular microvascular changes in eyes with proliferative diabetic retinopathy (PDR) following panretinal photocoagulation (PRP).

DESIGN

Using optical coherence tomographic angiography (OCTA), we prospectively studied 10 eyes of 10 subjects with high-risk PDR immediately before, at 1 month, and at 3-6 months following PRP, using a 3- × 3-mm OCTA scan at each visit.

METHODS

The following parameters were calculated for the superficial (SCP), middle (MCP), and deep capillary plexuses (DCP): parafoveal vessel density (VD), adjusted flow index (AFI), and percent area of nonperfusion (PAN). Parafoveal SCP vessel-length density (VLD) was also evaluated. We performed univariate and multivariable statistics, adjusting for age and signal strength. To model the hemodynamic effect of PRP, we also present a mathematical model based on electrical circuits.

RESULTS

We found no significant difference for the vascular density parameters following PRP, except for decreased density at the MCP at the latest timepoint in the adjusted multivariable model. PAN, a metric of nonperfusion adjusted for noise, and AFI, a surrogate metric of blood flow, showed significant increases at all capillary levels in the adjusted model. Our mathematical model explained how PRP would increase macular blood flow.

CONCLUSIONS

Using OCTA, we found an overall increase in the flow metrics of all capillary layers in the macula following PRP, unrelated to macular edema or thickening, in line with the mathematical model. Our results suggest an overall redistribution of blood flow to the posterior pole following PRP, adding a new dimension to our understanding of the complex biologic effects of PRP in PDR. NOTE: Publication of this article is sponsored by the American Ophthalmological Society.

Earliest Evidence of Preclinical Diabetic Retinopathy Revealed Using Optical Coherence Tomography Angiography Perfused Capillary Density

PURPOSE

To compare perfused capillary density (PCD) in diabetic patients and healthy controls using optical coherence tomography angiography (OCTA).

METHODS

Forty controls, 36 diabetic subjects without clinical retinopathy (NoDR), 38 with nonproliferative retinopathy (NPDR), and 38 with proliferative retinopathy (PDR) were imaged using spectral-domain optical coherence tomography. A 3 × 3-mm full-thickness parafoveal OCTA scan was obtained from each participant. Following manual delineation of the foveal avascular zone (FAZ), FAZ area, perimeter, and acircularity index were determined. Seven consecutive equidistant 200-μm-wide annular segments were drawn at increasing eccentricities from the FAZ margin. Annular PCD (%) was defined as perfused capillary area divided by the corresponding annulus area after subtraction of noncapillary blood vessel areas. Nonparametric Kruskal-Wallis testing with Bonferroni correction was performed in pairwise comparisons of group PCD values.

RESULTS

The NoDR group demonstrated consistently higher PCD compared to the control group in all 7 annuli, reaching statistical significance (36.6% ± 3.30% vs 33.6% ± 3.98%, P = .034) at the innermost annulus (FAZ margin to 200 μm out). The NPDR and PDR groups demonstrated progressively decreasing PCD. Differences in FAZ metrics between the NoDR and control groups did not reach statistical significance.

CONCLUSIONS

Relative to healthy controls, increased PCD values in the NoDR group likely represent an autoregulatory response to increased metabolic demand, while the decrease in PCD that follows in NPDR and PDR results largely from an incremental loss of capillary segments. These findings, consistent with previous studies, demonstrate the potential of OCTA as a clinical tool for earlier objective detection of preclinical diabetic retinopathy. NOTE: Publication of this article is sponsored by the American Ophthalmological Society.

Retinal oxygen extraction in individuals with type 1 diabetes with no or mild diabetic retinopathy

AIMS/HYPOTHESIS

The aim of this study was to compare retinal oxygen extraction in individuals with diabetes with no or mild non-proliferative diabetic retinopathy and healthy age- and sex-matched volunteers.

METHODS

A total of 24 participants with type 1 diabetes and 24 healthy age- and sex-matched volunteers were included in this cross-sectional study. Retinal oxygen extraction was measured by combining total retinal blood flow measurements using a custom-built bi-directional Doppler optical coherence tomography system with measurements of oxygen saturation using spectroscopic reflectometry. Based on previously published mathematical modelling, the oxygen content in retinal vessels and total retinal oxygen extraction were calculated.

RESULTS

Total retinal blood flow was higher in diabetic participants (46.4 ± 7.4 μl/min) than in healthy volunteers (40.4 ± 5.3 μl/min, p = 0.002 between groups). Oxygen content in retinal arteries was comparable between the two groups, but oxygen content in retinal veins was higher in participants with diabetes (0.15 ± 0.02 ml O₂/ml) compared with healthy control participants (0.13 ± 0.02 ml O₂/ml, p < 0.001). As such, the arteriovenous oxygen difference and total retinal oxygen extraction were reduced in participants with diabetes compared with healthy volunteers (total retinal oxygen extraction 1.40 ± 0.44 vs 1.70 ± 0.47 μl O₂/min, respectively, p = 0.03).

CONCLUSIONS/INTERPRETATION

Our data indicate early retinal hypoxia in individuals with type 1 diabetes with no or mild diabetic retinopathy as compared with healthy control individuals. Further studies are required to fully understand the potential of the technique in risk stratification and treatment monitoring.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01843114.

The vasodilating effects of insulin and lactate are increased in precapillary arterioles in the porcine retina ex vivo

PURPOSE

Disturbances in the diameter regulation of the larger retinal vessels are involved in the pathophysiology of a number of vision threatening retinal diseases, but the role of the smaller vessels has received less attention. Therefore, an experimental model was developed for studying diameter changes in retinal vessels at all branching levels secondary to pharmacological interventions ex vivo.

METHODS

Porcine hemiretinas were mounted in a tissue chamber that allowed the control of temperature, pH and oxygen saturation. The chamber was positioned in a fluorescence microscope, and changes in the diameter of larger arterioles, precapillary arterioles and capillaries were studied after intravascular and extravascular application of the thromboxane analogue U46619, lactate, the glutamate agonist N-Methyl-D-aspartic acid (NMDA) and insulin.

RESULTS

U46619 induced significant contraction of all vessel calibres after extravascular application, but had no significant effect on vessel diameters after intravascular application. In contracted vessels, lactate induced significant dilatation in all vessel calibres, and the effect was more pronounced after intravascular than after extravascular application where only precapillary arterioles dilated significantly. N-Methyl-D-aspartic acid (NMDA) induced similar dilatation in the three vessel types after intra- and extravascular application, whereas the dilating effect of insulin was significantly more pronounced in the precapillary arterioles than in the other vessel types.

CONCLUSION

The diameter regulation in precapillary retinal arterioles differs from that of other retinal vascular branching levels and may play a special role in the pathophysiology of retinal vascular disease.

Retinal vascular and structural dynamics during acute hyperglycaemia

PURPOSE

To compare retinal vascular dynamics during acute hyperglycaemia in patients with type 2 diabetes and healthy volunteers.

METHODS

Twenty-one patients with type 2 diabetes and 27 healthy controls were examined with fundus photographic measurement of retinal vessel diameters, retinal oximetry, macular perfusion velocities and optical coherence tomographic measurement of subfoveal choroidal thickness every 30 min during a 3-hr 75 g oral glucose tolerance test (OGTT). Patients paused antidiabetic therapy for 1 week prior to the OGTT.

RESULTS

Plasma glucose (PG) and fluctuations in PG were larger in patients with diabetes (p < 0.0001). PG increased significantly 30 min after ingestion of glucose (p < 0.0001 in both groups). With a delay of 0-120 min, the PG increase was followed by increased retinal arterial oxygen saturations and arteriovenous oxygen saturation differences, narrowed retinal veins and increased arteriovenous diameter ratios. No effect of age, gender or diabetes status was observed. Choroidal thickness was transiently reduced in controls and unchanged in patients with diabetes (p = 0.021). Macular perfusion velocities increased after 150 min in patients with diabetes but not in controls (arterial p = 0.059; venous p = 0.16). Higher age and diabetes tended to be associated with higher retinal arterial oxygen saturation.

CONCLUSION

The transition from fasting to acute hyperglycaemia is followed, with a delay of up to 2 hr, by retinal vascular changes, notably increased oxygen extraction, suggesting an effect of secondary metabolic changes. Retinal responses were similar in patients with type 2 diabetes and controls despite differences in glucose levels. It is necessary to standardize measurement conditions in studies of retinal physiology.

Acute hyperinsulinemia increases the contraction of retinal arterioles induced by elevated blood pressure

Diabetic retinopathy is accompanied by disturbances in retinal blood flow, which is assumed to be related to the diabetic metabolic dysregulation. It has previously been shown that normoinsulinemic hyperglycemia has no effect on the diameter of retinal arterioles at rest and during an increase in the arterial blood pressure induced by isometric exercise. However, the influence of hyperinsulinemia on this response has not been studied in detail. In seven normal persons, the diameter response of retinal arterioles to an increased blood pressure induced by isometric exercise, to stimulation with flickering light, and to the combination of these stimuli was studied during euglycemic normoinsulinemia (protocol N) on one examination day, and euglycemic hyperinsulinemia (protocol H) on another examination day. Isometric exercise induced significant contraction of retinal arterioles at all examinations, but during a repeated examination the diameter response was significantly reduced in the test persons following the N protocol and increased in the persons following the H protocol. Flicker stimulation induced a significant dilatation of retinal arterioles at all examinations, and the response was significantly higher during a repeated examination, irrespective of the insulin level. Repeated exposure to isometric exercise reduces contraction, whereas repeated exposure to flickering light increases dilatation of retinal arterioles in vivo. Hyperinsulinemia increases contraction of retinal arterioles induced by isometric exercise.

Pilot study of Doppler optical coherence tomography of retinal blood flow following laser photocoagulation in poorly controlled diabetic patients

PURPOSE

To investigate the effect of panretinal photocoagulation (PRP) on retinal blood flow and shear rate using Doppler Fourier-domain optical coherence tomography (FD-OCT) in poorly controlled diabetics with proliferative diabetic retinopathy (PDR).

METHODS

This was a prospective interventional pilot study in patients with a new clinical diagnosis of PDR. Retinal blood flow and vessel diameter were measured using Doppler FD-OCT according to a previously described method, immediately before PRP treatment and 7 to 8 weeks after the last PRP session.

RESULTS

Ten patients with poorly controlled PDR (mean hemoglobin A1C = 9.2 ± 2.0%) and 10 control subjects were included in the study. PDR patients had significantly lower blood flow (∼25%) than control subjects both at baseline (P = 0.01) and after PRP (P = 0.003). Compared to controls, venous and arterial velocities were significantly decreased in diabetics at baseline (∼27%; P < 0.001 and 0.017, respectively) as well as after PRP (P < 0.001 and 0.006, respectively). Compared to controls, venous and arterial shear rates were significantly reduced in diabetics at baseline (∼27%; P = 0.002, 0.03) and after PRP (P = 0.002, 0.03). PRP in this group of PDR patients did not have a statistically significant effect on retinal blood flow or vessel parameters, though there was a trend for decreased arterial diameter (P = 0.09).

CONCLUSIONS

This is the first study to use Doppler FD-OCT to quantify functional changes in retinal vascular parameters in poorly controlled PDR patients. Compared to controls, blood flow parameters in these patients were decreased at baseline, but did not decrease further following PRP, with important implications related to diabetes control, endothelial function, and therapeutic response.

Retinal blood flow in type 1 diabetic patients with no or mild diabetic retinopathy during euglycemic clamp

OBJECTIVE

To compare total retinal blood flow in diabetic patients with no or mild nonproliferative diabetic retinopathy and healthy control subjects and to investigate in patients whether there is a difference between retinal blood flow before morning insulin and under normoglycemic conditions using a glucose clamp.

RESEARCH DESIGN AND METHODS

Twenty patients with type 1 diabetes with no or mild diabetic retinopathy were included in this open parallel-group study, and 20 healthy age- and sex-matched subjects were included as control subjects. Retinal blood flow was assessed by combining velocity measurements using laser Doppler velocimetry and diameter measurements using a commercially available dynamic vessel analyzer. Measurements were performed before and during a euglycemic clamp.

RESULTS

Total retinal blood flow was higher in diabetic patients (53 +/- 16 microl/min) than in healthy subjects (43 +/- 16 microl/min; P = 0.034 between groups). When plasma glucose in diabetic patients was reduced from 9.3 +/- 1.7 to 5.3 +/- 0.5 mmol/l (P < 0.001) retinal blood flow decreased to 49 +/- 15 microl/min (P = 0.0003 vs. baseline). Total retinal blood flow during the glucose clamp was not significantly different from blood flow in normal control subjects (P = 0.161).

CONCLUSIONS

Type 1 diabetic patients with no or only mild diabetic retinopathy have increased retinal blood flow before their morning insulin dosage. Blood flow is reduced toward normal during euglycemic conditions. Retinal blood flow may fluctuate significantly with fluctuating plasma glucose levels, which may contribute to the microvascular changes seen in diabetic retinopathy.

Intraocular pressure and aqueous humor flow during a euglycemic-hyperinsulinemic clamp in patients with type 1 diabetes and microvascular complications

BACKGROUND

Microvascular complications, including retinopathy and nephropathy are seen with type 1 diabetes. It is unknown whether functional changes in aqueous humor flow or intraocular pressure (IOP) develop in parallel with these complications. This study was designed to test the hypothesis that clinical markers of microvascular complications coexist with the alteration in aqueous humor flow and IOP.

METHODS

Ten patients with type 1 diabetes and ten healthy age- and weight-matched controls were studied. Aqueous flow was measured by fluorophotometry during a hyperinsulinemic-euglycemic clamp (insulin 2 mU/kg/min). Intraocular pressure was measured by tonometry at -10, 90 and 240 minutes from the start of the clamp, and outflow facility was measured by tonography at 240 minutes.

RESULTS

During conditions of identical glucose and insulin concentrations, mean aqueous flow was lower by 0.58 microl/min in the diabetes group compared to controls (2.58 +/- 0.65 versus 3.16 +/- 0.66 microl/min, respectively, mean +/- SD, p = 0.07) but statistical significance was not reached. Before the clamp, IOP was higher in the diabetes group (22.6 +/- 3.0 mm Hg) than in the control group (19.3 +/- 1.8 mm Hg, p = 0.01) but at 90 minutes into the clamp, and for the remainder of the study, IOP was reduced in the diabetes group to the level of the control group. Ocular pulse amplitude and outflow facility were not different between groups. Systolic blood pressure was significantly higher in the diabetes group, but diastolic and mean arterial pressures were not different.

CONCLUSIONS

We conclude that compared to healthy participants, patients with type 1 diabetes having microalbuminuria and retinopathy have higher IOPs that are normalized by hyperinsulinemia. During the clamp, a reduction in aqueous flow was not statistically significant.

Influence of exercise induced hyperlactatemia on retinal blood flow during normo- and hyperglycemia

PURPOSE

Short term hyperglycemia has previously been shown to induce a blood flow increase in the retina. The mechanism behind this effect is poorly understood. We set out to investigate whether exercise-induced hyperlactatemia may alter the response of retinal blood flow to hyperglycemia.

METHODS

We performed a randomized, controlled two-way cross over study comprising 12 healthy subjects, performed a 6-minutes period of dynamic exercise during an euglcaemic or hyperglycaemic insulin clamp. Retinal blood flow was assessed by combined vessel size measurement with the Zeiss retinal vessel analyzer and measurement of red blood cell velocities using bi-directional laser Doppler velocimetry. Retinal and systemic hemodynamic parameters were measured before, immediately after and 10 and 20 minutes after isometric exercise.

RESULTS

On the euglycemic study day retinal blood flow increased after dynamic exercise. The maximum increase in retinal blood flow was observed 10 minutes after the end of exercise when lactate plasma concentration peaked. Hyperglycemia increased retinal blood flow under basal conditions, but had no incremental effect during exercise induced hyperlactatemia.

CONCLUSIONS

Our results indicate that both lactate and glucose induce an increase in retinal blood flow in healthy humans. This may indicate a common pathway between glucose and lactate induced blood flow changes in the human retina.

Somatostatin receptor subtype 2-mediated uptake of radiolabelled somatostatin analogues in the human kidney

PURPOSE

Renal irradiation is a dose-limiting factor in peptide receptor radionuclide therapy using radiolabelled somatostatin analogues. This irradiation is mainly caused by reabsorption of radiolabelled peptides in the proximal tubule. In the human kidney, somatostatin receptors are expressed in the vasa recta, tubuli and glomeruli. It is not clear to what extent these receptors contribute to the total kidney radioactivity uptake.

METHODS

Retrospectively, [(111)In-DTPA(0)]octreotide scans of ten selected patients with carcinoids (well-differentiated gastrointestinal endocrine tumour) with liver metastases were evaluated. For each patient, two scans were obtained: one scan was performed without (control) and one during treatment with unlabelled octreotide. Kidney, tumour, spleen and liver uptake was measured in both scans.

RESULTS

The interval between the two scans per patient varied from 50 to 397 days. Octreotide treatment substantially lowered kidney [(111)In-DTPA(0)]octreotide uptake in eight out of ten patients. Kidney uptake in all patients was reduced to 82%+/-15% of control, (p < 0.01). A correlation between kidney uptake and spleen uptake was found (r=0.67, p < 0.05). Serum creatinine was unchanged. Surprisingly, tumour and liver [(111)In-DTPA(0)]octreotide uptake was not significantly influenced by unlabelled octreotide therapy, but spleen uptake was significantly lowered by treatment (30.6% of control, p < 0.002).

CONCLUSION

We conclude that the somatostatin receptor plays a role in the total renal uptake of radiolabelled somatostatin analogues. The long interval between scans might explain the finding that tumour and liver metastasis uptake of [(111)In-DTPA(0)]octreotide was unchanged. Further studies are needed to confirm and eludicate the results of this study.

Retinal arteriolar hemodynamic response to an acute hyperglycemic provocation in early and sight-threatening diabetic retinopathy

The aim was to quantify the magnitude of retinal arteriolar vascular reactivity to a hyperglycemic provocation in diabetic patients stratified by severity of retinopathy and in age-matched controls. The sample comprised 20 non-diabetic controls (Group 1), 19 patients with no clinically visible DR (Group 2), 18 patients with mild-to-moderate non-proliferative DR (Group 3) and 18 patients with diabetic macular edema (Group 4). Retinal hemodynamic measurements using the Canon Laser Blood Flowmeter (CLBF-100) were acquired before and 1 h after drinking a standardized oral glucose load drink. The magnitude of the retinal vascular response, as well as max:min velocity ratio and wall shear rate (WSR), was calculated and compared across groups. A significant change in blood glucose level was observed for all groups (p<0.05). The change in blood glucose elevation was significantly less in Group 1 compared to the other groups. No significant change in arteriolar diameter, blood velocity, blood flow, max:min velocity ratio and WSR was found in patients with diabetes and in age-matched subjects without diabetes. The results of this study indicate that retinal arteriolar blood flow is unaffected by acute elevation of blood glucose using an oral glucose load drink in patients with diabetes and in age-matched controls. This lack of a blood flow response to acute hyperglycemia in patients with early sight-threatening DR may be explained by a loss of retinal vascular reactivity.

Plasma endothelin-1 levels depress optic nerve head circulation detected during the glucose tolerance test

PURPOSE

To determine the relationship between the changes in optic nerve head (ONH) circulation and the level of plasma endothelin-1 (ET-1) during the glucose tolerance test (GTT).

METHODS

Twenty-six healthy volunteers with normal GTT and 15 patients with mild hyperglycemia and abnormal GTT were studied. The ONH circulation [square blur rate (SBR) value], blood pressure, intraocular pressure (IOP), blood glucose, blood insulin and plasma ET-1 were determined before and every hour up to 3 h after an oral intake of 75 g of glucose.

RESULTS

The SBR increased in the normal glucose tolerance group at all times during the GTT, but it decreased significantly in the abnormal glucose tolerance group (P < 0.05). Before the GTT, the plasma ET-1 level was not significantly different in the two groups; however, the level increased 1 h after the oral GTT in the abnormal glucose tolerance group (P < 0.05). No significant changes were observed in mean blood pressure or IOP.

CONCLUSIONS

ONH circulation increased after glucose intake in the normal glucose tolerance group and remained high even after the blood glucose level had returned to its baseline. The decrease in ONH circulation in the abnormal glucose tolerance group was attributed partly to the increased ET-1.

Response of retinal arteriole diameter to increased blood pressure during acute hyperglycaemia

PURPOSE

To study retinal response in terms of arteriole diameter and retinal thickness secondary to an increase in arterial blood pressure during acute hyperglycaemia.

METHODS

In a randomized, double-blinded, cross-over study, nine healthy persons were subjected to clamping of blood glucose to either 5 mmol/l or 15 mmol/l using somatostatin to control endogenous insulin secretion. The response of retinal arterioles in terms of diameter as determined with the retinal vessel analyser (RVA) and retinal thickness as assessed by optical coherence tomography (OCT) were measured after an increase in arterial blood pressure induced by isometric exercise. Arterial feeding pressure in the eye was assessed from the ophthalmic artery pressure and pulse amplitude measured by ophthalmodynamometry.

RESULTS

Isometric exercise induced a significant increase in mean arterial blood pressure and a significant contraction of the retinal arterioles. An acute increase in blood glucose from 5 mmol/l to 15 mmol/l did not affect either the diameter of retinal vessels or retinal thickness.

CONCLUSIONS

Acute hyperglycaemia per se does not change isometric exercise-induced retinal arteriolar contraction. Metabolic factors other than blood glucose are suspected to be involved in the impairment of retinal autoregulation as seen in hyperglycaemia induced by oral glucose intake.

The metabolic syndrome and uric acid nephrolithiasis: novel features of renal manifestation of insulin resistance

BACKGROUND

Uric acid nephrolithiasis primarily results from low urinary pH, which increases the concentration of the insoluble undissociated uric acid, causing formation of both uric acid and mixed uric acid/calcium oxalate stones. These patients have recently been described as exhibiting features of insulin resistance. This study was designed to evaluate if insulin resistance is associated with excessively low urinary pH in overtly healthy volunteers (non-stone formers) and if insulin resistance may explain the excessively low urinary pH in patients with uric acid nephrolithiasis.

METHODS

Fifty-five healthy volunteers (non stone-formers) with a large range of body mass index and 13 patients with recurrent uric acid nephrolithiasis underwent hyperinsulinemic euglycemic clamp, 24-hour urinary studies, and anthropometric measurements of adiposity. A subgroup of 35 non-stone formers had 2-hour timed urinary collection before and during the hyperinsulinemic phase of the clamp studies.

RESULTS

For the non-stone former population, low insulin sensitivity measured as glucose disposal rate significantly correlated with low 24-hour urinary pH (r= 0. 35; P= 0.01). In addition to the previously described acidic urine pH and hypouricosuria, patients with recurrent uric acid nephrolithiasis were found to be severely insulin resistant (glucose disposal rate: uric acid stone-formers vs. normals; 4.1 +/- 1.3 vs. 6.9 +/- 2.1 mg/min/kg of lean body mass, P= 0.008). Acute hyperinsulinemia was associated with higher urinary pH (6.1 +/- 0.7 at baseline to 6.8 +/- 0.7 during hyperinsulinemia; P < 0.0001), urinary ammonia excretion (2.7 +/- 1.6 mEq/2 hr at baseline and 4.0 +/- 2.6 mEq/2 hr P= 0.002) and urinary citrate excretion (48 +/- 33 mg/2 hr at baseline and 113 +/- 68 mg/2 hr P < 0.0001).

CONCLUSION

We conclude that one renal manifestation of insulin resistance may be low urinary ammonium and pH. This defect can result in increased risk of uric acid precipitation despite normouricosuria.

Free fatty acids exert a greater effect on ocular and skin blood flow than triglycerides in healthy subjects

BACKGROUND

Free fatty acids (FFAs) and triglycerides (TGs) can cause vascular dysfunction and arteriosclerosis. Acute elevation of plasma FFA and TG concentration strongly increase ocular and skin blood flow. This study was designed to discriminate whether FFA or TG independently induce hyperperfusion by measuring regional and systemic haemodynamics.

METHODS

In a balanced, randomized, placebo-controlled, double-blind, three-way, crossover study nine healthy subjects received either Intralipid (Pharmacia and Upjohn, Vienna, Austria) with heparin, Intralipid alone or placebo control. Pulsatile choroidal blood flow was measured with laser interferometry, retinal blood flow and retinal red blood cell velocity with laser Doppler velocimetry, and skin blood flow with laser Doppler flowmetry during an euglycaemic insulin clamp.

RESULTS

A sevenfold increase of FFA during Intralipid/heparin infusion was paralleled by enhanced choriodal, retinal, and skin blood flow by 17 +/- 4%, 26 +/- 5% (P < 0.001), and 47 +/- 19% (P = 0.03) from baseline, respectively. In contrast, a mere threefold increase of FFA by infusion of Intralipid alone did not affect outcome parameters, despite the presence of plasma TG levels of 250-700 mg dL(-1); similar to those obtained during combined Intralipid/heparin infusion. Systemic haemodynamics were not affected by drug infusion.

CONCLUSIONS

Present findings demonstrate a concentration-dependent increase in ocular and skin blood flow by FFA independently of elevated TG plasma concentrations. As vasodilation of resistance vessels occur rapidly, FFA may play a role in the development of continued regional hyperperfusion and deteriorate microvascular function.

Reduced response of retinal vessel diameters to flicker stimulation in patients with diabetes

BACKGROUND/AIM

Stimulation of the retina with flickering light increases retinal arterial and venous diameters in animals and humans, indicating a tight coupling between neural activity and blood flow. The aim of the present study was to investigate whether this response is altered in patients with insulin dependent diabetes mellitus.

METHODS

26 patients with diabetes mellitus with no or mild non-proliferative retinopathy and 26 age and sex matched healthy volunteers were included in the study. Retinal vessel diameters were measured continuously with the Zeiss retinal vessel analyser. During these measurements three episodes of square wave flicker stimulation periods (16, 32, and 64 seconds; 8 Hz) were applied through the illumination pathway of the vessel analyser.

RESULTS

In retinal arteries, the response to stimulation with diffuse luminance flicker was significantly diminished in diabetic patients compared to healthy volunteers (ANOVA, p<0.0031). In non-diabetic controls flicker stimulation increased retinal arterial diameters by +1.6% (1.8%) (mean, p<0.001 v baseline), +2.8% (SD 2.2%) (p<0.001) and +2.8% (1.6%) (p<0.001) during 16, 32, and 64 seconds of flicker stimulation, respectively. In diabetic patients flicker had no effect on arterial vessel diameters: +0.1% (3.1%) (16 seconds, p = 0.9), +1.1% (2.7%) (32 seconds, p = 0.07), +1.0% (2.8%) (64 seconds, p = 0.1). In retinal veins, the response to flicker light was not significantly different in both groups. Retinal venous vessel diameters increased by +0.7% (1.6%) (16 seconds, p<0.05), +1.9% (2.3%) (32 seconds, p<0.001) and 1.7% (1.8%) (64 seconds, p<0.001) in controls during flicker stimulation. Again, no increase was observed in the patients group: +0.6% (2.4%), +0.5% (1.5%), and +1.2% (3.1%) (16, 32, and 64 seconds, respectively).

CONCLUSION

Flicker responses of retinal arteries and veins are abnormally reduced in patients with IDDM with no or mild non-proliferative retinopathy. Whether this diminished response can be attributed to altered retinal vascular reactivity or to decreased neural activity has yet to be clarified.

Hyperglycemia affects flicker-induced vasodilation in the retina of healthy subjects

Flickering light stimulation of retinal photoreceptors induces retinal vessel dilation in humans. In the present study the effect of high blood glucose levels on this neuro-vascular mechanism was investigated in 12 healthy young male subjects. Blood glucose levels were consecutively increased during 30 min to 100, 200 and 300 mg/dl and kept at the respective level for the following 30 min using hyperglycemic insulin clamps. Eight Hertz flickering light was applied to the fundus at the end of each glucose plateau during continuous retinal vessel diameter measurements with the Zeiss retinal vessel analyser (RVA). During normoglycemia (100 mg/dl) flickering light induced a significant vasodilation of retinal arteries (+2.8+/-0.4%, p<0.0001) and veins (+2.6+/-0.4%, p<0.0001). At 300 mg/dl blood glucose the flicker response in retinal veins was significantly decreased by 55% (p=0.015 versus 100 mg/dl). The modified RVA employed in the present study provides high sensitivity and is capable of studying flicker-induced retinal vasodilation. Using this technique the present study confirms that flickering light stimulation of the human retina induces vasodilation in retinal vessels in healthy subjects. In addition, our data indicate that the retinal vessel response to flickering light stimulation is significantly reduced during hyperglycemia in humans. The relevance of this finding for diabetes-related eye disease remains to be shown.

Renal and metabolic effects of insulin lispro in type 2 diabetic subjects with overt nephropathy

OBJECTIVE

To assess whether the insulin analog lispro may antagonize the renal effects of IGF-1, a mediator of glomerular hyperfiltration involved in the progression of diabetic and nondiabetic chronic nephropathies.

RESEARCH DESIGN AND METHODS

In a randomized crossover study, we compared the renal and metabolic responses to regular or lispro insulin (0.1 units/kg body wt) administered after a euglycemic clamp and 5 and 30 min before a standard meal to 11 type 2 diabetic patients with macroalbuminuria.

RESULTS

Two- and four-hour postprandial changes (vs. preprandial euglycemia) in glomerular filtration rate (GFR) followed a significantly different trend (5.8 +/- 5.0 vs. -6.3 +/- 4.7, P < 0.05; and 11.0 +/- 6.8 vs. 0.7 +/- 5.1%, P < 0.05) after regular insulin and lispro injection, respectively. After lispro, postprandial GFR changes were negatively correlated (r = -0.48, P = 0.0001) with plasma insulin concentration. After regular insulin, renal plasma flow increased in parallel with a decrease in renal vascular resistances. Both changes were fully prevented by lispro. Postprandial blood glucose maximum concentration (278 +/- 16 vs. 240 +/- 16 mg/dl, P < 0.01) and area under the curve (79,381 +/- 19,237 vs. 72,810 +/- 16,211 mg/dl per min, P < 0,05) were significantly lower after insulin lispro than after regular insulin injection, respectively, despite comparable postprandial insulin profiles. Changes in total and gluconeogenic amino acids followed a similar trend. Changes in blood glucose and plasma amino acids did not correlate with concomitant changes in GFR.

CONCLUSIONS

In overt nephropathy of type 2 diabetes, lispro prevents glomerular hyperfiltration and offsets the renal effects of meal or meal-associated hyperglycemia by mechanisms possibly related to IGF-1 antagonism.

Renal hemodynamic effects of somatostatin are not related to inhibition of endogenous insulin release

BACKGROUND

Somatostatin inhibits endocrine and exocrine secretions and exerts renal vasoconstriction. The mechanism underlying somatostatin's vascular effects is unknown. Since insulin can cause vasodilation, we hypothesized that removal of basal insulin release by somatostatin may contribute to somatostatin-induced renal vasoconstriction.

METHODS

The study was conducted in different protocols comprising forty-six healthy male volunteers. Randomized studies were performed to compare the effects of somatostatin alone (0.1 microg/kg/min) to the effects of somatostatin + low dose insulin (0.1 mU/kg/min), the effects of somatostatin + low dose insulin to the effects of somatostatin + high dose insulin (1.5 mU/kg/min), and the effects of insulin (1.5 mU/kg/min) + somatostatin. Renal plasma flow (RPF) and glomerular filtration rate (GFR) were measured with the para-aminohippurate (PAH) and the inulin clearance technique, respectively. Blood pressure and pulse rate were measured non-invasively.

RESULTS

Somatostatin alone decreased GFR (-14 +/- 6%, P < 0.001) and RPF (-16 +/- 7%, P < 0.001) whereas systemic hemodynamics were unchanged. Preceding or concomitant infusion of insulin at high doses (insulin plasma concentration of 127 +/- 25 or 144 +/- 17 microU/mL) but not co-infusion with low dose insulin (insulin plasma concentration of 11 +/- 3 microU/mL) mitigated or reversed the vasoconstrictive actions of somatostatin on GFR and RPF.

CONCLUSIONS

Somatostatin induces marked renal vasoconstriction and exogenous restoration of fasting insulin concentrations does not influence the renal vascular effects. Therefore, it is unlikely that somatostatin-induced vasoconstriction is due to removal of basal insulin. Plasma insulin concentrations in the high postprandial range can reverse somatostatin-induced renal vasoconstriction, suggesting functional antagonism.

Acute effects of insulin on aqueous humor flow in patients with type 1 diabetes

PURPOSE

Previous studies reported reduced aqueous humor flow through the anterior segment of the eye in patients with type 1 diabetes. This study investigates whether reduced flow is the result of the diabetic state or of alterations in glucose or insulin concentrations.

METHODS

A cross-sectional study, involving patients with type 1 diabetes and healthy controls, measured aqueous flow at different insulin concentrations. Eleven patients with type 1 diabetes (hemoglobin A1C = 7.0 +/- 0.3% [mean +/- SEM], normal < 6.5) with no microvascular complications and 17 controls were prospectively studied. Controls were studied fasting and during a hyperinsulinemic-euglycemic clamp (insulin 2 mU/kg per minute). Patients with type 1 diabetes were similarly studied during two euglycemic clamp procedures (insulin 0.5 and 2.0 mU/kg per minute). Aqueous flow was measured by fluorophotometry. Pulsatile ocular blood flow and intraocular pressure were measured with a Langham flow probe.

RESULTS

Control subjects had no change in aqueous flow during fasting and hyperinsulinemic conditions (3.0 +/- 0.1 vs 2.8 +/- 0.1 microl per minute). In the patients with type 1 diabetes, aqueous flow was not decreased with hyperinsulinemia, compared with the low insulin state (P =.7). Compared with control subjects, patients with type 1 diabetes had lower aqueous flow during hyperinsulinemia (2.4 +/- 0.1 microl per minute, P =.03) and at lower insulin conditions (2.6 +/- 0.1 microl per minute, P <.05). No differences in intraocular pressure or pulsatile ocular blood flow were noted between groups or between insulin states within groups.

CONCLUSIONS

Aqueous flow is decreased in patients with type 1 diabetes under euglycemic conditions of high and relatively low insulin concentrations, despite the absence of microvascular complications.