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|Title: ||Vasomodulatory actions of des-asp-angiotensin I and angiotensin 1-7 in resistance vessels of hypertensive and diabetic rat models|
|Authors: ||Murugan, Dharmani Devi|
Diabetic rat models
|Issue Date: ||2007 |
|Publisher: ||University Malaya|
|Abstract: ||A number of anatomical and functional disturbances of the vascular endothelium are observed in diabetes and hypertension. Angiotensin II (Ang II), a major effector peptide of the rennin angiotensin system (RAS), acts primarily on angiotensin AT1 receptor. The oligopeptide plays a key role in the initiation and amplification of pathobiological events underlying several vascular diseases. The discovery of other angiotensin peptides i.e. des-Asp-angiotensin I (DAA-I) and angiotensin 1-7 (Ang 1-7) with contrasting vascular actions has led to further investigations to determine the roles of these peptides.
The aim of this study was to evaluate the direct and modulatory actions of these angiotensin peptides in the isolated renal and mesenteric vascular bed preparations from Wistar-Kyoto (WKY) rats (as normotensive), Spontaneously Hypertensive rats (SHR) (as hypertensive) and streptozotocin (STZ)-induced diabetic rats (as diabetic model). In the isolated renal vascular bed, Ang II pressor response was increased in SHR and reduced in STZ-induced diabetic rats. However, in the mesenteric vasculature, contractile action of Ang II was unaltered in diabetic but decreased in the hypertensive animals. DAA-I appears to have a smaller vasoconstrictor actions than Ang II in both vasculatures. In the mesenteric vasculature, the contractile actions of the nanopeptide were not significantly altered in hypertensive and diabetic conditions. In contrast, Ang 1-7 did not exhibit constrictor actions in either vascular beds.
DAA-I attenuated the Ang II pressor action in normotensive and hypertensive rat kidneys, however, this action was absent in the STZ-induced diabetic rat model. A similar pattern of DAA-I action was observed in the mesenteric vasculature. The vasodepressor action of DAA-I appeared not to involve AT2 receptor or cyclooxygenase byproducts. Nitric oxide appeared to be involved in the modulatory actions of DAA-I in hypertensive animals. Similarly, Ang 1-7 attenuated Ang II-induced vasoconstriction in both normotensive and hypertensive rat kidneys and this action was absent in the STZdiabetic rat model. In the mesenteric vascular bed, Ang 1-7 attenuated Ang II-induced vasoconstrictions in all three animal groups. In the kidney, Ang 1-7 action was mediated via the Ang 1-7 receptor in WKY and SHR, and involved the release of Prostaglandins and nitric oxide. In the mesentery, the vasodepressor action appeared to be mediated by Ang 1-7 receptor and involved vasodilator prostaglandins and nitric oxide in SHR, and only nitric oxide in the normotensive and diabetic rats.
The involvement of AT1 receptor in the actions of DAA-I and Ang 1-7 actions was investigated in the renal vasculature. Receptor binding assay revealed that the rat kidney homogenate contained mainly the AT1 receptor subtype. The AT1 receptor density was found to be significantly increased in hypertensive rat kidney. This increase may explain the hyperresposiveness to Ang II observed previously in the perfused kidney.
RT-PCR and Western blot analysis were also in agreement with the results from the receptor binding assay. In contrast to SHR, the AT1 receptor density was decreased in the diabetic rat kidney. This reduction is in tandem with the reduced responsiveness to Ang II observed in the perfused kidney. The results from RT-PCR and Western blot analysis were also in agreement with the binding and functional studies.
In kidneys perfused with 10-9 M DAA-I, receptor binding data demonstrated a reduced AT1 receptor density. This suggests that the DAA-I vasodepressor actions in WKY and SHR, may be partly due to downregulation of AT1 receptors. However, the findings from the RT-PCR and Western blot analysis showed no changes in the AT1 receptor gene and protein expression. Alternatively, the reduced AT1 receptor density may be due to receptor internalization of AT1 receptors. On the other hand, in kidney homogenates perfused with lower concentrations of DAA-I, no significant changes in AT1 receptor were seen (density and expression). In diabetic rat model, no changes in AT1 receptor density or expression were seen in kidneys perfused with DAA-I. This supports the earlier finding where DAA-I had no effect on Ang II-induced pressor action in STZ-induced diabetic rat kidney. A slightly lower receptor affinity was seen in STZ induced diabetic untreated and DAA-I treated compared to WKY. This suggests that DAA-I could bind to other binding sites than AT1 receptor and DAA-I binding ability may be altered in diabetes.
Receptor binding assay from kidney homogenate perfused with Ang 1-7 (10-7M) demonstrated a reduced AT1 receptor density in WKY and SHR. This showed that Ang 1-7 vasodepressor action is partially modulated via AT1 receptor, possibly by receptor downregulation. However, RT-PCR and Western blot analysis ruled out receptor downregulation, suggesting that Ang 1-7 may internalize the renal AT1 receptor similarly like the DAA-I. Pre-treatment of Ang 1-7 in STZ-induced diabetic kidney did not affect AT1 receptor (density and expression). This finding agrees well with the isolated perfused kidney study where Ang 1-7 vasodepressor action was compromised in diabetic condition. Mas receptor has been shown to be the endogenous Ang 1-7 receptor and alteration on this receptor in diabetic condition may explain the diminished Ang 1-7 vasodepressor action. Preliminary study of isolated perfused kidney from neonatal (n-STZ) induced diabetic rat as a model for type 2 diabetes mellitus demonstrated a similar pattern as type I diabetes mellitus rat model (STZ-induced) to the pressor responses to Ang II, DAA-I and Ang 1-7. Also similar are the actions of DAA-I and Ang 1-7 on Ang II-induced pressor response. This indicates that in both type of diabetic models, these peptides facilitate the ongoing renal vascular changes in a similar manner.
These findings provide further information on the actions of angiotensin peptides especially in resistance vasculatures and their roles in diseases such as hypertension and diabetes. The data also demonstrates the ability of both DAA-I and Ang 1-7 in reducing Ang II actions especially in hypertensive condition and this may enable targeted therapies to be developed. This study also acts as a first step toward understanding the compromised vasodepressor actions of DAA-I and Ang 1-7 in diabetes mellitus.|
|Description: ||Thesis (PhD) -- Faculty of Medicine, University of Malaya, 2009.|
|Appears in Collections:||PhD Theses: Medicine|