Changes in the density and localisation of endothelin receptors in the early stages of rat diabetic retinopathy and the effect of insulin treatment

Abstract

The endothelins (ETs) are a family of 2500 M r peptides with three distinct isoforms, endothelin-1, ±2 and ±3 (ET-1, ET-2 and ET-3) [1,2]. ET-1 is a potent vasoconstrictor produced by endothelial cells that also induces mitogenesis in vascular smooth muscle cells [1,3]. Two major types of ET receptors, termed endothelin receptor type A (ETA) and endothelin receptor type B (ETB) have been identified, cloned and sequenced in mammals [4,5]. The ETA receptor mediates most of the vasoconstrictor action of ET-1 and has selective affinity for . The ETB receptor has equal affinities for all ET isoforms and has been shown to release endothelium-derived nitric Abstract Aims/hypothesis. The endothelin system (ET system) has been implicated in the retinal blood flow abnormalities that precede the onset of diabetic retinopathy. This study was undertaken to assess whether the density and localisation of both the immunoreactive endothelin-1 and endothelin receptors in rat retina change in the early stages of diabetes and the insulin treatment would affect those changes. Methods. Untreated streptozotocin-diabetic, insulintreated streptozotocin-diabetic and age-matched control rats were killed 15, 45 and 90 days after the induction of diabetes. Binding assays were used to determine the density and proportion of endothelin receptors in neural retinal membranes. Localisation of endothelin receptors and immunoreactive endothelin-1 were analysed by microautoradiography and immunohistochemistry, respectively. Results. Fifteen days after the induction of diabetes, the neural retinal membranes of untreated streptozotocin-diabetic rats showed a statistically significant decrease in the density of both endothelin receptor subtypes when compared with age-matched control rats. At 90 days, however, the density of endothelin receptors type B was statistically significantly higher than that of control rats, and the innermost layers of the diabetic retina also showed an increase of both endothelin receptor type B receptor and immunoreactive endothelin-1 signal. Insulin treatment during 90 days up-regulated endothelin receptor type A in neural retinal membranes and in the innermost layers of the retina when compared with control retinas. Conclusion/interpretation. These results show that the endothelin system is altered in both vascular and neuronal components of the retina in early diabetic retinopathy. The up-regulation of endothelin receptor type A induced by insulin treatment suggests that insulin might be involved in retinal microangiopathy. [Diabetologia (2000) 43: 773±785]

Early cellular and molecular changes induced by diabetes in the retina

“…Even though immunohistochemical studies are not able to give precise information about the synthesis of a secreted protein, the observations are in agreement with several reports of increased endothelin mRNAs in the retina of diabetic rats [13,15,16]. The expression of endothelin receptors, detected in all retina layers as well as on vascular cells, appeared to decrease after 2 weeks of streptozotocin diabetes [14] but increase thereafter [13±15]. Studies with inhibitors of endothelin action [13,16] Radial section of human retina immunostained for von Willebrand factor to identify the vessels and counterstained with haematoxylin and eosin to show the nuclear layers.…”

“…Reduced availability of a vasodilator such as nitric oxide could contribute to the reduced retinal blood flow reported after a short duration of experimental diabetes and so compound the vascular effects of increased retinal endothelin levels. Increased endothelin-1 [13,14] and endothelin-3 [13] immunostaining has been observed in the neural cells of the inner retina 2±4 weeks after the induction of streptozotocin diabetes. Even though immunohistochemical studies are not able to give precise information about the synthesis of a secreted protein, the observations are in agreement with several reports of increased endothelin mRNAs in the retina of diabetic rats [13,15,16].…”

“…Streptozotocin-diabetic rats have shed much light on early retinal microangiopathy because they develop several of the characteristic lesions known in human diabetic retinopathy, but require further evaluation as a model for neuroglial dysfunction. Acute streptozotocin toxicity is probably not a confounding factor because the neuroglial abnormalities reported in streptozotocin-diabetic rats were prevented by intensive insulin treatment [12,14,30], noted to occur over time [23,28,33,56,57], and/or also shown in spontaneously diabetic rats or humans [15,33,57]. However, prevention by intensive insulin and time-related changes cannot rule out contributions to neuroglial abnormalities from the low growth hormone levels [38] and low thyroid state [107] that accompany severe diabetes in the streptozotocin-diabetic rats.…”

Different role of endothelin ETA and ETB receptors and endothelial modulators in diabetes-induced hyperreactivity of the rabbit carotid artery to endothelin-1

“…It is clear that many of the well-known metabolic abnormalities encountered in diabetes mellitus, such as those affecting plasma insulin, glucose, and lipid levels, contribute individually and synergistically to alterations in the release and action of endothelin-1 . The endothelin system is altered in both vascular and neuronal components of the retina in early diabetic retinopathy (De Juan et al, 2000). The increased plasma levels of endothelin-1 in diabetic patients (Sarman et al, 2000;Seligman et al, 2000;Schneider et al, 2002), together with increased free radical levels and a deficiency of prostacyclin, have been related to carotid atherosclerosis in diabetic patients (Kalogeropoulou et al, 2002).…”

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

“…Vasoactive factors, such as ET-1, NO, vascular endothelial growth factors and other factors, mediate functional and structural alterations in early diabetic retinopathy [6,21]. Further investigations will be needed to clarify the interaction between ET-1, insulin, and retinal microcirculation in the early stage of diabetes.…”