Venous sphincters in the choroidea of spontaneously hypertensive rats

Ocular vasculopathy resulting from severe systemic hypertension affects retina, choroidea, and the optic nerve. While the pathologic changes of the arterial system, including luminal narrowing, are well documented, little is known about the ocular venous vessels in hypertension. Adult 18-week-old spontaneously hypertensive rats (SHRs) were untreated (control) or treated with lisinopril for 4 weeks; normotensive Wistar-Kyoto (WKY) rats were additional controls. The mean systemic blood pressure (MSP) was monitored chronically using telemetry. The ocular microvasculature was examined using scanning electron microscopy (SEM) of corrosion casts, histology of serial sections, and computer-based 3D reconstruction. The MSP in control SHRs (145 +/- 11.9 mmHg) was decreased to 68.1 +/- 4.9 mmHg (P < 0.001) following treatment, which was even below the baseline level of WKY (96.7 +/- 5.8) rats (P < 0.05). In addition to media thickening in arteries, the venous plexus of the choroidea in control SHRs revealed multiple tufts of smooth muscle cells (sphincters) that narrowed the lumen. Correlating to histology, SEM of casts and 3D reconstruction showed numerous constrictions and muscular tufts in veins of the choroidea, narrowing the vascular lumen up to 47%. Following antihypertensive treatment, the percentage of sphincter constriction was decreased to 6% (P < 0.001). The depth of venous sphincter contraction correlated significantly with MSP (r = 0.87). To our knowledge, these results indicate for the first time that venous sphincters occur in the choroidea of the SHRs, and that their muscularity correlates with MSP. Venous sphincters might be involved in the pathophysiology of hypertension-related ocular changes.