Another signaling pathway that may be involved in retinal arteriolar dilation in response to adenosine is through an endothelium-dependent mechanism. The endothelium has been shown to produce three major vasodilators: NO,
41 prostaglandins,
42 and cytochrome P-450 metabolites.
43 These factors are worth investigating, because the endothelial release of the potent vasodilator NO has been shown to play a role in the dilation of coronary
10 13 and skeletal muscle
9 arterioles in response to adenosine. Because it is difficult to distinguish between possible neuronal- or vascular-mediated mechanisms of action of NO in the retina in in vivo preparations, the isolated vessel preparation provides the most appropriate approach for unambiguous identification of this signaling pathway. In the present study, it does not appear that prostaglandins or cytochrome P-450 metabolites are involved in retinal arteriolar dilation in response to adenosine because blockade of cyclooxygenase or cytochrome P-450 epoxygenase did not alter the response. In contrast, the blockade of NO synthase with
l-NAME reduced the adenosine-induced vasodilation, suggesting that NO contributes in part to the response. Likewise,
l-NAME attenuated vasodilation after application of CGS21680, indicating that the adenosine-induced NO production is mediated by A
2A receptors. In contrast to in vivo findings in the retinal circulation,
44 45 it is important to note that
l-NAME (10 μM) did not significantly increase basal tone of isolated retinal arterioles. This may be due to the absence of luminal flow in our in vitro study, since it has been shown that endothelial cells respond to increased flow (or shear stress) by releasing NO.
23 46 In this regard, it is expected that the NO component would be more pronounced in vivo (i.e., with luminal flow) compared with that in vitro (i.e., without luminal flow) in resting conditions. Therefore, the effect of
l-NAME on basal vascular tone would be less apparent in our in vitro study. The present results are consistent with our previous findings in isolated porcine coronary arterioles.
10 13 In the presence of
l-NAME, we found that the residual dilation of retinal arterioles to both adenosine and CGS21680 was further reduced by glibenclamide. These data support the idea that both endothelium-derived NO and smooth muscle K
ATP channels contribute to the adenosine receptor-activated dilation of retinal arterioles.