Ultrastructural observations on myelinated fibres in experimental diabetes: effect of the aldose reductase inhibitor ponalrestat given alone or in conjunction with insulin therapy

Renal nerve ultrastructural alterations in short term and long term experimental diabetes

BACKGROUND

Despite the evidence that renal hemodynamics is impaired in experimental diabetes, associated with glomeruli structural alterations, renal nerves were not yet investigated in experimental models of diabetes and the contribution of nerve alterations to the diabetic nephropathy remains to be investigated. We aimed to determine if ultrastructural morphometric parameters of the renal nerves are affected by short term and/or long term experimental diabetes and if insulin treatment reverses these alterations. Left renal nerves were evaluated 15 days or 12 weeks (N = 10 in each group) after induction of diabetes, with a single injection of streptozotocin (STZ). Control rats (N = 10 in each group) were injected with vehicle (citrate buffer). Treated animals (N = 10 in each group) received a single subcutaneous injection of insulin on a daily basis. Arterial pressure, together with the renal nerves activity, was recorded 15 days (short-term) or 12 weeks (long-term) after STZ injection. After the recordings, the renal nerves were dissected, prepared for light and transmission electron microscopy, and fascicle and fibers morphometry were carried out with computer software.

RESULTS

The major diabetic alteration on the renal nerves was a small myelinated fibers loss since their number was smaller on chronic diabetic animals, the average morphometric parameters of the myelinated fibers were larger on chronic diabetic animals and distribution histograms of fiber diameter was significantly shifted to the right on chronic diabetic animals. These alterations began early, after 15 days of diabetes induction, associated with a severe mitochondrial damage, and were not prevented by conventional insulin treatment.

CONCLUSIONS

The experimental diabetes, induced by a single intravenous injection of STZ, in adult male Wistar rats, caused small fiber loss in the renal nerves, probably due to the early mitochondrial damage. Conventional treatment with insulin was able to correct the weight gain and metabolic changes in diabetic animals, without, however, correcting and / or preventing damage to the thin fibers caused by STZ-induced diabetes. The kidney innervation is impaired in this diabetic model suggesting that alterations of the renal nerves may play a role in the development of the diabetic nephropathy.

Phrenic nerve diabetic neuropathy in rats: unmyelinated fibers morphometry

We have demonstrated that phrenic nerves' large myelinated fibers in streptozotocin (STZ)-induced diabetic rats show axonal atrophy, which is reversed by insulin treatment. However, studies on structural abnormalities of the small myelinated and the unmyelinated fibers in the STZ-model of neuropathy are limited. Also, structural changes in the endoneural vasculature are not clearly described in this model and require detailed study. We have undertaken morphometric studies of the phrenic nerve in insulin-treated and untreated STZ-diabetic rats and non-diabetic control animals over a 12-week period. The presence of neuropathy was assessed by means of transmission electron microscopy, and morphometry of the unmyelinated fibers was performed. The most striking finding was the morphological evidence of small myelinated fiber neuropathy due to the STZ injection, which was not protected or reversed by conventional insulin treatment. This neuropathy was clearly associated with severe damage of the endoneural vessels present on both STZ groups, besides the insulin treatment. The STZ-diabetes model is widely used to investigate experimental diabetic neuropathies, but few studies have performed a detailed assessment of either unmyelinated fibers or capillary morphology in this animal model. The present study adds useful information for further investigations on the ultrastructural basis of nerve function in diabetes.

Streptozotocin induced diabetes as a model of phrenic nerve neuropathy in rats

Phrenic neuropathies are increasingly recognized in peripheral neuropathies but reports on experimental models of the phrenic nerves diabetic neuropathy are scanty. In the present study, we investigated the phrenic nerve neuropathy, due to experimental diabetes induced by streptozotocin (STZ) and the evolution of this neuropathy in diabetic rats treated with insulin. Proximal and distal segments of the left and right phrenic nerves were morphologically and morphometrically evaluated, from rats rendered diabetic for 12 weeks, by injection of STZ. Control rats received vehicle. Treated rats received a single subcutaneous injection of insulin on a daily basis. The nerves were prepared for light microcopy study by means of conventional techniques. Morphometry was carried out with the aid of computer software. The phrenic nerves of diabetic rats showed smaller myelinated axon diameters compared to controls. The g ratio was significantly smaller for myelinated fibers from diabetic rats compared to controls. Insulin treatment prevented these alterations. Histograms of size distribution for myelinated fibers and axons from control rats were bimodal. For diabetic animals, the myelinated fiber histogram was bimodal while the axon distribution turned to be unimodal. Insulin treatment also prevented these alterations. Our results confirm the phrenic nerve neuropathy in this experimental model of diabetes and suggest that conventional insulin treatment was able to prevent and/or correct the myelinated axon commitment by diabetes.

Number and ultrastructure of epithelial cells in crypts and villi along the streptozotocin-diabetic small intestine: a quantitative study on the effects of insulin and aldose reductase inhibition

This study has quantified the effects of insulin treatment with and without aldose reductase inhibitor (ponalrestat) on intestinal epithelial cell morphology in streptozotocin-diabetic rats. Epithelial volumes, villous and microvillous surface areas and mean volumes of cells (and their nuclei) in crypts and villi were estimated in each of four segments and in the entire intestine. We derived total numbers of cells, quantified the ultrastructural features of average cells and explored variation along the intestine and between experimental groups. In crypts, insulin and ponalrestat had significant effects on cell number (reduced towards normal values) and size (volume and apex area increased beyond normal values). There were interaction effects between insulin and ponalrestat for cell volume and apex area (insulin producing more exaggerated effects when given without ponalrestat). On villi, insulin and ponalrestat returned cell numbers towards normal values but neither treatment normalised cell size or the number and area of microvilli per cell. Indeed, ponalrestat increased microvillous number and area beyond values found in untreated diabetic animals. Again, there were interaction effects between insulin and ponalrestat. Patterns of segmental variation seen in crypts of normal rats (values tending to be higher in proximal or mid-intestinal regions) were not preserved, and only some of the segmental differences seen on villi (higher values at proximal or mid-intestinal sites) were maintained during therapy. Apart from reducing the abnormally high numbers of cells in untreated diabetic rats, these results show that insulin and ponalrestat treatment fail to restitute epithelial cell morphology in the small intestines of experimental diabetic rats.

The small intestine in experimental diabetes: cellular adaptation in crypts and villi at different longitudinal sites

Intestinal adaptation at the cellular level was examined in groups of streptozotocin-diabetic and age-matched control rats. Small intestines were removed and divided into four segments of roughly equal length. For each segment, epithelial volume, villous and microvillous surface areas and the mean volumes of epithelial cells in crypts and villi were estimated. From these data, we were able to estimate total numbers of epithelial cells in crypts and villi, assess adaptation at the level of the average cell and explore variation along the crypt-villus axis, between segments and between groups. Whilst the villus:crypt cell ratio did not change, diabetic animals contained about 80% more epithelial cells than control rats. The morphophenotype of villous epithelial cells (represented by nuclear volume, cell height, area and volume, and number and surface area of microvilli) was basically the same as that in controls. By contrast, crypt cells and their nuclei were 40-50% bigger in diabetic rats. Significant differences between segments were confined to the numbers and sizes of crypt cells and their nuclei. We conclude that experimental diabetes leads to both proliferative and hypertrophic responses within crypts. Crypt cells become fatter but not taller. Crypt hyperplasia is accompanied by an equiproportionate increase in villous epithelial cells, but these are of essentially normal morphophenotype.

Responses of enterocyte microvilli in experimental diabetes to insulin and an aldose reductase inhibitor (ponalrestat)

The small intestine of 12-week-old streptozotocin-diabetic rats was examined by light and transmission electron microscopy in order to study the effects of alternative treatments on microvillous morphology. Four groups were examined: untreated diabetic rats, insulin-treated diabetics and rats treated with an aldose reductase inhibitor (ponalrestat) given with and without insulin. Numbers and dimensions of microvilli at the apex of columnar absorptive epithelial cells (enterocytes) were estimated using stereological principles. Values were obtained for the organ as a whole as well as for different sites along its length. In the untreated diabetic intestine, the mean (standard error of mean) number of microvilli was 4.5 (0.8) x 10(12) with a total surface area of 1.9 (0.50) m2. On average, the microvilli were 1.1 (0.08) microns long, 104 (3.8) nm in diameter and packed on the villous surface at a density of 3400 (50) per 100 microns 2. Their length at least varied with intestinal location. Significant effects of insulin therapy were detected. In contrast, the study failed to find any significant effect of aldose reductase inhibition on any variable except microvillous packing density.

The effect of pancreatic islet transplantation on experimental diabetic neuropathy

Quantitative light and electronmicroscopical morphometric techniques were used to determine the effect of pancreatic islet transplantation on experimental diabetic neuropathy. Groups of STZ-diabetic rats were given islet transplants at 3 weeks after diabetes onset (prevention) and at 6 months after diabetes onset (reversal). Comparisons were made with onset controls, age-matched non-diabetic controls and untreated diabetic controls 6 months later (n = 8 for all groups). Euglycaemia and normal levels of glycosylated haemoglobin were achieved in both groups of diabetics after islet transplantation. Loss of body weight in diabetic animals was prevented by early islet transplantation, but was only partially reversed following delayed islet transplantation. Normal growth of myelinated fibres and axons during development was retarded in untreated diabetics, but was normal in rats given islet transplants soon after the onset of diabetes (cross-sectional perimeter and area). Diabetics transplanted with islets after a delay had myelinated fibres and axons with diminished calibre. Teased fibre preparations of nerves from diabetics which had received islet transplants showed no excess of abnormalities. This study has shown that the development of certain structural abnormalities of peripheral nerve fibres is prevented in diabetic rats which receive transplants of islets of Langerhans soon after the onset of diabetes. However, once established abnormal fibre morphology can not be completely ameliorated merely by achieving and sustaining euglycaemia through delayed islet transplantation.

Preventive effect of long-term aldose reductase inhibition (ponalrestat) on nerve conduction and sural nerve structure in the spontaneously diabetic Bio-Breeding rat

To test the hypothesis that aldose reductase inhibition may prevent or delay the development of functional and structural neuropathy in the insulin-deficient diabetic Bio-Breeding rat (BB-rat), hyperglycemic rats were begun on the aldose reductase inhibitor (ARI) ponalrestat 25 mg/kg body wt soon after the onset of diabetes and followed for 4 or 6 mo. Ponalrestat treatment completely prevented the characteristic nerve conduction slowing and structural abnormalities of the node of Ranvier for 4 mo despite only partial preservation of axonal integrity. Ponalrestat treatment for 6 mo achieved a partial but significant prevention of nerve conduction slowing, axoglial dysjunction, and axonal degenerative changes. This incomplete but significant prevention of neuropathy by ponalrestat suggests that additional mechanisms besides polyol-pathway activation may be of importance in the pathogenesis of diabetic neuropathy. Alternatively, the dosage used in the present study may not have been sufficient to achieve a complete prevention. Despite the only partial protective effect of ARI treatment on degenerative peripheral nerve changes in hyperglycemic BB-rats, 6 mo of treatment resulted in a more than threefold increase in regenerating nerve fibers. These data suggest that prophylactic ARI treatment may be efficacious in delaying the development of diabetic neuropathy.

Relative growth and maturation of axon size and myelin thickness in the tibial nerve of the rat. 2. Effect of streptozotocin-induced diabetes

The relative changes in the growth and maturation of axon size and myelin thickness were studied in the medial plantar division of the tibial nerve in the lower leg and in the motor branches of the tibial nerve to the calf muscles in rats in which diabetes mellitus had been induced with streptozotocin at the time of weaning. Observations were made at 6 weeks and 3, 6, 9 and 12 months of diabetes for comparison with age-matched controls. Similar changes were observed in both nerves. Growth in body weight and skeletal growth was severely retarded from the time of induction of diabetes but at the 6-week stage axon size was not reduced, suggesting that neural growth may initially be relatively protected. At later stages axon size was consistently reduced in the diabetic animals as compared with the controls and showed an absolute reduction at 12 months, as compared with 9 months, that was greater than in the controls. Myelin thickness became reduced earlier and was more severely affected than axon size so that the fibers were relatively hypomyelinated. The myelin changes were greater in larger than in smaller fibers. The index of circularity of axons was reduced in the diabetic nerves. These results show that induction of diabetes in prepubertal rats produces effects on peripheral nerve fibers which differ from those resulting from diabetes induced in adult animals. The effects also differ between large and small nerve fibres. These observations may explain some of the disparate findings obtained in previous studies on experimental diabetes in rats.

Small intestinal morphology in experimental diabetic rats: a stereological study on the effects of an aldose reductase inhibitor (ponalrestat) given with or without conventional insulin therapy

The gross and microscopical dimensions of small intestines from 12-week old streptozotocin-diabetic rats receiving no therapeutic intervention were compared with those from animals receiving insulin alone or in conjunction with the aldose reductase inhibitor, ponalrestat. Four regions along each intestine were analysed stereologically. Insulin had significant beneficial effects on body weight as well as on intestinal length, width, surface area and volume. In contrast, ponalrestat did not improve body weight deficits and was associated with crypt hypertrophy and a reduced villous surface/crypt volume ratio. There were interaction effects between insulin and ponalrestat for intestinal length and primary mucosal surface area. All groups displayed significant regional differences in surface area of primary mucosa and volume of muscularis externa. Only untreated diabetic rats failed to reveal regional variation in the surface area and volume of villi. Ratios of villous surface area/crypt volume varied from region-to-region in insulin-treated diabetic rats but not in other groups. The study fails to reveal any beneficial effect of aldose reductase inhibition on the changes in intestinal morphology seen in experimental diabetes.