Role of VEGFR2 in the Dilation of Retinal Arterioles to Increased Lumenal Flow

Abstract

Purpose:: Dilation of resistance arterioles to increased lumenal flow, termed flow-induced dilation, has been shown to play a key role in the regulation of local blood flow in many organ systems. However, it is unclear whether retinal arterioles also exhibit this important vasomotor behavior. Although cell culture studies suggest that flow/shear stress can activate vascular endothelial growth factor receptor 2 (VEGFR2), the involvement of VEGFR2 in flow-induced dilation in intact vessels is unknown. Herein, we employed an isolated vessel approach to specifically address these issues.

Methods:: Second-order porcine retinal arterioles were isolated, cannulated, and pressurized to 55 cmH₂O lumenal pressure by two independent reservoir systems for flow control. Diameter changes were recorded using videomicroscopic techniques. To support that the activation of VEGFR2 was capable of producing vasodilation, retinal arterioles were challenged with VEGF₁₆₅ (1 pM to 10 nM) in the absence of flow. To determine the role of nitric oxide synthase (NOS) and VEGFR2 in the flow-induced response, the flow-diameter relation was assessed at various levels of flow before and after application of a NOS inhibitor L-NAME (10 µM) and the selective VEGFR2 inhibitor SU-1498 (1 µM), respectively. Immunofluorescence analysis was employed to detect VEGFR2 in isolated retinal arterioles.

Results:: Retinal arterioles developed basal tone (55±4 µm) under zero flow conditions and dilated to VEGF₁₆₅ dose-dependently. This dilation was inhibited by L-NAME or SU-1498. A stepwise increase in lumenal flow also produced a graded dilation of retinal arterioles. Similarly, L-NAME and SU-1498 abolished the flow-induced vasodilatory response. These pharmacological blockers did not alter vasodilation to sodium nitroprusside. Immunostaining indicated that VEGFR2 was expressed in the endothelial layer of retinal arterioles.

Conclusions:: VEGFR2 is present in the endothelium of retinal arterioles and appears to mediate vasodilation in response to VEGF stimulation and elevated flow via NOS activation. Further study is required to determine the cellular/molecular signaling pathway responsible for this important vasoregulatory mechanism.

Keywords: receptors: pharmacology/physiology • signal transduction: pharmacology/physiology • vascular cells