Ozagrel reverses streptozotocin-induced constriction of arterioles in rat retina

Abstract

Retinal blood flow decreases early in the progression of diabetic retinopathy; however, the mediators and mechanisms responsible for this decrease have yet to be determined. In this study, diabetes was induced by streptozotocin in rats, and retinal blood flow was measured via intravital microscopy 1 or 3 weeks following the induction of hyperglycemia. Additionally, retinal arteriolar diameters and flow were measured prior to and following acute administration of the thromboxane synthase inhibitor ozagrel to investigate the potential role of thromboxane in the observed constriction. Minimal changes in the retinal diameters and flow were observed at 1 week of diabetes; however, at 3 weeks of diabetes, arteriolar constriction and decreases in blood flow were significant. Notably, the constriction occurred only in the arterioles that were in closer proximity to the venules draining the retina. Acute administration of ozagrel reversed the constriction of the closely venule-paired arterioles. In summary, the results suggest that thromboxane mediates localized, venule-dependent arteriolar constriction induced by streptozotocin-induced diabetes in rats.

1

(A) Low power (4x) intravital microscopic image of the rat retinal circulation near the optic disk. FITC-dextran has been injected into the rat and is filling the arterioles (a) before reaching the venules (v). One angle of 21° is indicated on the image. (B) Fluorescent streak in a retinal arteriole of a labeled RBC traveling 19.8 mm/s. (C) Image of a retinal arteriole filled with FITC-albumin, where the diameter of 80 µm is an average of three locations along the vessel length.

2

Distribution of arteriole-venule pairing angles in the rat retina. Values of the angle represent the average between an arteriole and the two neighboring venules.

3

Relationship between the arteriole-venule pairing angles and blood flow rate in the arterioles of the various groups as indicated in panels A–D. In the group of rats that had been diabetic for three weeks, a statistically significant correlation was found, due to a decrease in flow in the more closely paired arterioles.

4

Arteriolar diameters (A) and flow rate (B) in non-diabetic control rats, rats diabetic for 1 week, and rats diabetic for 3 weeks (before and after ozagrel treatment for the latter). For each group, the data are segregated by pairing angle into closely paired (≤ 30.2°) and distantly paired (>30.2°) arterioles. *p<0.05 and **p=0.01 between the indicated groups.

5

Relationship between the arteriole-venule pairing angles and blood flow rate in the arterioles of the various groups as indicated in panels A–D. In the group of rats that had been diabetic for three weeks, a statistically significant correlation was found, due to a decrease in flow in the more closely paired arterioles.

6

Arteriolar diameters (A) and flow rate (B) in non-diabetic control rats and rats diabetic for 3 weeks (before and after ozagrel treatment). The data are separated by pairing angle into closely paired (≤ 30.2°) and distantly paired (>30.2°) arterioles. **p<0.01 between the indicated groups.

7

Relationship between the change in arteriolar blood flow rate induced by ozagrel and the arteriole-venule pairing angle. The flow rates in (A) were obtained using fluorescently labeled platelets, and the flow rates in (B) were obtained using fluorescently labeled red blood cells (RBCs).

8

Diagram showing the working hypothesis: an elevated level of vasoconstrictor in a draining venule generates a concentration gradient extending from the venule. This gradient would induce more constriction in an arteriole closer to the venule.