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

In Vivo Characterization of Corneal Changes in a Type 1 Diabetic Animal Model

Diabetes mellitus (DM) is a metabolic disease that affects 9% of the adult population, promoting an increase in glucose concentration that affects the corneal structure, namely, its thickness, as well as the constituents and flow of the aqueous humor. In this study, high-frequency transducers (20-MHz and 50-MHz) were used to measure and characterize changes in the corneal and aqueous humor in streptozotocin-induced type 1 diabetic rats followed over 8 weeks. Increases of 24.6 and 15.4 μm in central corneal thickness were measured with the 20-MHz and 50-MHz probes, respectively, in DM rats (p < 0.001). The increases in thickness of the different corneal layers ranged from 7% to 17%. Structural alterations of the aqueous humor were also studied by relating the amplitudes of the anterior lens and posterior cornea boundary signals, the result of which was denominated by pseudo-attenuation. The results revealed an increase of 49% at week 8 compared with the baseline values (p < 0.020, with the 50-MHz probe). This study illustrated that high-frequency ultrasound can be used to measure corneal layer thickness and study the alterations promoted by diabetes in the eye's anterior segment. Those assessments may allow early detection of DM, improving the monitoring of diabetic patients.

Src Family Kinase Links Insulin Signaling to Short Term Regulation of Na,K-ATPase in Nonpigmented Ciliary Epithelium

Insulin has been shown to elicit changes of Na,K-ATPase activity in various tissues. Na,K-ATPase in the nonpigmented ciliary epithelium (NPE) plays a role in aqueous humor secretion and changes of Na,K-ATPase activity impact the driving force. Because we detect a change of NPE Na,K-ATPase activity in response to insulin, studies were carried out to examine the response mechanism. Ouabain-sensitive rubidium (Rb) uptake by cultured NPE cells, measured as a functional index of Na,K-ATPase-mediated inward potassium transport, was found to increase in cells exposed for 5 min to insulin. The maximally effective concentration was 100 nM. An intrinsic increase of Na,K-ATPase activity evident as a >2-fold increase in the rate of ouabain-sensitive ATP hydrolysis in homogenates obtained from cells exposed to 100 nM insulin for 5 min was also observed. Insulin-treated cells exhibited Akt, Src family kinase (SFK), ERK1/2, and p38 activation, all of which were prevented by a pI3 kinase inhibitor LY294002. The Rb uptake and Na,K-ATPase activity response to insulin both were abolished by PP2, an SFK inhibitor which also prevented p38 and ERK1/2 but not Akt activation. The Akt inhibitor MK-2206 did not change the Na,K-ATPase response to insulin. The findings suggest insulin activates pI3K-dependent Akt and SFK signaling pathways that are separate. ERK1/2 and p38 activation is secondary to and dependent on SFK activation. The increase of Na,K-ATPase activity is dependent on activation of the SFK pathway. The findings are consistent with previous studies that indicate a link between Na,K-ATPase activity and SFK signaling. J. Cell. Physiol. 232: 1489-1500, 2017. © 2016 Wiley Periodicals, Inc.

Short-term stability in refractive status despite large fluctuations in glucose levels in diabetes mellitus type 1 and 2

PURPOSE

This work investigates how short-term changes in blood glucose concentration affect the refractive components of the diabetic eye in patients with long-term Type 1 and Type 2 diabetes.

METHODS

Blood glucose concentration, refractive error components (mean spherical equivalent MSE, J0, J45), central corneal thickness (CCT), anterior chamber depth (ACD), crystalline lens thickness (LT), axial length (AL) and ocular aberrations were monitored at two-hourly intervals over a 12-hour period in: 20 T1DM patients (mean age ± SD) 38±14 years, baseline HbA1c 8.6±1.9%; 21 T2DM patients (mean age ± SD) 56±11 years, HbA1c 7.5±1.8%; and in 20 control subjects (mean age ± SD) 49±23 years, HbA1c 5.5±0.5%. The refractive and biometric results were compared with the corresponding changes in blood glucose concentration.

RESULTS

Blood glucose concentration at different times was found to vary significantly within (p<0.0005) and between groups (p<0.0005). However, the refractive error components and ocular aberrations were not found to alter significantly over the day in either the diabetic patients or the control subjects (p>0.05). Minor changes of marginal statistical or optical significance were observed in some biometric parameters. Similarly there were some marginally significant differences between the baseline biometric parameters of well-controlled and poorly-controlled diabetic subjects.

CONCLUSION

This work suggests that normal, short-term fluctuations (of up to about 6 mM/l on a timescale of a few hours) in the blood glucose levels of diabetics are not usually associated with acute changes in refractive error or ocular wavefront aberrations. It is therefore possible that factors other than refractive error fluctuations are sometimes responsible for the transient visual problems often reported by diabetic patients.

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.

Aqueous flow in galactose-fed dogs

Dogs fed galactose develop diabetes-like ocular complications that include keratopathy, cataracts, and retinopathy. The purpose of this study was to investigate whether galactosemic dogs display reduced aqueous flow similar to that observed in patients with insulin-dependent diabetes mellitus. Twelve male beagles at 9 months of age were divided into three groups of four. The Galactose group was fed diet containing 30% galactose for 97 months and the Reversal group was fed the galactose diet for an initial 38 months then standard dog diet for the remaining period. The Control group was fed standard dog diet for 97 months. Aqueous flow was determined by fluorophotometry in one eye per dog at 96 and 97 months after the initiation of galactose feeding. Intraocular pressure (IOP) was measured once in the morning by pneumatonometry. Anterior chamber depth was measured by A-scan. At the end of the experiment, eyes were enucleated and processed for histological examination. Dogs fed galactose diet for 97 months had significantly (p<0.05) increased body weights but similar IOP and anterior chamber depth compared to the other groups, and significantly (p=0.05) reduced aqueous flow compared to the control group (4.4+/-2.2 vs. 6.8+/-2.4 microl/min, mean+/-standard deviation, respectively). Additionally, aqueous flow decreased in the Reversal group to 3.1+/-1.3 microl/min (p=0.002). This decrease correlated with morphological changes of the ciliary processes. Like patients with insulin-dependent diabetes mellitus, galactose-fed dogs demonstrate reduced aqueous flow. This reduction was irreversible and independent of the retinopathy present. This animal model may be useful for the study of aqueous humor dynamics in diabetes.

Insulin secretion by rat lacrimal glands: effects of systemic and local variables

To understand the secretory mechanisms and physiological role of insulin in the tear film, the present study examined 1) the time course of insulin secretion in the tear film under glucose intravenous stimulation, 2) the glucose- and carbachol-induced insulin secretion from isolated lacrimal gland (LG), 3) the effect of insulin on glucose consumption by the cornea, and 4) the expression of insulin, pancreatic duodenal homeobox-1 (PDX-1), and glucose transport proteins (GLUTs) in LG tissue. The insulin level in the tear film of 8-wk-old male Wistar rats increased from 0.6 +/- 0.45 to 3.7 +/- 1.3 ng/ml in the initial minutes after glucose stimulation. In vitro assays demonstrated that higher glucose concentrations from 2.8 to 16.7 mM, 200 microM carbachol, or 40 mM KCl significantly increased insulin secretion from lacrimal glands compared with controls, but did not detect C-peptide as measured by RIA. Glucose consumption by corneal tissue, evaluated by radiolabeled D-[U-14C]glucose uptake, was 24.07 +/- 0.61 and was enhanced to 31.63 +/- 3.15 nmol x cornea(-1) x 2 h(-1) in the presence of 6 nM insulin (P = 0.033) and to 37.5 +/- 3.7 nmol x cornea(-1) x 2 h(-1) in the presence of 11.2 mM glucose (P = 0.015). Insulin and PDX-1 mRNA was detected in LG. Insulin was located in the apical areas of acinar cells by immunoperoxidase and the expression of GLUT-1, but not PDX-1, was confirmed by Western blot. These findings suggest that insulin secretion in the tear film is influenced by local stimuli such as nutrient and neural inputs and that this hormone plays a metabolic role in ocular surface tissues. These data also indicate that under normal conditions the insulin secreted by LG is stored, but it is not clear that is locally produced in the LG.

Determination of the Aqueous Humor Volume by Three-Dimensional Mapping of the Anterior Chamber

In this study involving 12 patients planned for routine cataract surgery, we used the topography of the anterior chamber depth and the corneal diameter obtained from Orbscan II data to calculate the aqueous humor volume. Prior to the surgical procedure, a small amount of fluorescein was injected into the anterior chamber and an aqueous humor sample was taken, from which the aqueous humor volume could be calculated by fluorometry. The volumes obtained from Orbscan II data were validated by the fluorometric measurements and compared to three theoretical formulas for aqueous humor volume calculation. The aqueous humor volume calculations based on the Orbscan II data aligned better to the fluorometric values (R(2) = 0.890) than the values obtained by Heim's formula (R(2) = 0.677), Brubaker's formula (R(2) = 0.671), and Schenker's formula (R(2) = 0.585), or the assumption of a constant aqueous humor volume.

Non-invasive glucose measurement technologies: an update from 1999 to the dawn of the new millennium

There are three main issues in non-invasive (NI) glucose measurements: namely, specificity, compartmentalization of glucose values, and calibration. There has been progress in the use of near-infrared and mid-infrared spectroscopy. Recently new glucose measurement methods have been developed, exploiting the effect of glucose on erythrocyte scattering, new photoacoustic phenomenon, optical coherence tomography, thermo-optical studies on human skin, Raman spectroscopy studies, fluorescence measurements, and use of photonic crystals. In addition to optical methods, in vivo electrical impedance results have been reported. Some of these methods measure intrinsic properties of glucose; others deal with its effect on tissue or blood properties. Recent studies on skin from individuals with diabetes and its response to stimuli, skin thermo-optical response, peripheral blood flow, and red blood cell rheology in diabetes shed new light on physical and physiological changes resulting from the disease that can affect NI glucose measurements. There have been advances in understanding compartmentalization of glucose values by targeting certain regions of human tissue. Calibration of NI measurements and devices is still an open question. More studies are needed to understand the specific glucose signals and signals that are due to the effect of glucose on blood and tissue properties. These studies should be performed under normal physiological conditions and in the presence of other co-morbidities.