Abstract 5505: Requisite Roles of Endothelial Nitric Oxide Synthase and Smooth Muscle Phosphoinositide 3-Kinase/Rho Kinase in Vasomotor Regulation of Human Retinal Arterioles
Recent clinical evidence in the peripheral microcirculation indicates retinal arteriolar narrowing is predictive of mortality from ischemic heart disease and stroke. Although the arteriolar network is the major site of flow regulation, there is sparse information on how vasomotor function is regulated at the single arteriolar level in the human retina. Herein, we studied the vasoreactivity and its underlying mechanisms of human retinal arterioles in vitro from eyes donated by patients undergoing enucleation at the Scott & White Eye Institute. With institutional approval of the experimental protocol and patients’ consent, retinal arterioles were isolated from 4 patients with ocular melanoma and one patient with chemical burn of the cornea. The arterioles were cannulated and pressurized (55 cmH2O) by two independent reservoir systems for luminal flow control. Diameter changes were recorded using videomicroscopic techniques. Cell localization of signaling proteins was identified by immunofluorescence. All vessels (n = 10, 2/patient) developed myogenic tone (resting diameter = 44 ± 4 μm, maximum diameter = 67 ± 4 μm) under zero flow condition. Adenosine, bradykinin, and VEGF-165 evoked dose-dependent dilation with ~80% dilation at the highest doses. A stepwise increase in luminal flow produced graded dilation with ~50% dilation at the highest flow. Nitric oxide synthase (NOS) inhibitor L-NAME abolished dilations to bradykinin, VEGF and flow, but reduced the response to adenosine by ~20%. In contrast, endothelin-1 (ET-1) caused dose-dependent constriction. Both myogenic tone and ET-1-induced constriction were blocked by phosphoino-sitide 3-kinase (PI3K) inhibitor wortmannin or by Rho kinase (ROCK) inhibitor H-1152. Endothelial NOS and ROCK-1 were localized to retinal endothelial and smooth muscle layers, respectively. It appears that human retinal arterioles dilate to endogenous vasodilators and increased shear stress in an endothelial NO-dependent manner, but develop myogenic tone and constrict to ET-1 via a smooth muscle PI3K/ROCK-1 pathway. The present study provides significant insight into the activation of specific signaling pathways in endothelium and smooth muscle in mediating vasomotor regulation of human retinal arterioles.