ABSTRACT
An electrical communication between the endothelial and smooth muscle cells via gap junctions, which provides the signaling pathway known as endothelium-dependent hyperpolarization (EDH), plays a crucial role in controlling the vascular tone. In this study, we investigated the role of gap junctions in the acetylcholine (ACh)-induced EDH-type dilation of rat retinal arterioles in vivo. The dilator response was evaluated by measuring the diameter of retinal arterioles. Intravitreal injection of gap junction blockers (18ß-glycyrrhetinic acid and carbenoxolone) reduced the ACh-induced dilation of retinal arterioles. Moreover, the retinal arteriolar response to ACh was attenuated by 18ß-glycyrrhetinic acid under treatment with a combination of NG-nitro-L-arginine methyl ester (a nitric oxide (NO) synthase inhibitor; 30 mg/kg) and indomethacin (a cyclooxygenase inhibitor; 5 mg/kg). The NO- and prostaglandin-independent, EDH-related component of ACh-induced dilation of retinal arterioles was prevented by intravitreal injection of iberiotoxin, which inhibits large-conductance Ca2+-activated K+ channels. Furthermore, the combination of 18ß-glycyrrhetinic acid and iberiotoxin produced greater attenuation in the EDH-related response than that by the individual agent. Treatment with 18ß-glycyrrhetinic acid revealed no significant effect on NOR3 (an NO donor)-induced retinal vasodilator response. These results suggest that gap junctions contribute to the ACh-induced, EDH-type dilation of rat retinal arterioles in vivo.
Subject(s)
Acetylcholine/pharmacology , Arterioles/drug effects , Endothelium, Vascular/drug effects , Gap Junctions , Retina/drug effects , Retinal Vessels/drug effects , Vasodilation , Animals , Dilatation , Endothelium-Dependent Relaxing Factors , Male , Muscle, Smooth, Vascular , Nitric Oxide/metabolism , Rats, Wistar , Signal Transduction , Vasodilator Agents/pharmacologyABSTRACT
Nitric oxide (NO) is an important regulator of the retinal blood flow. The present study aimed to determine the role of voltage-gated K+ (KV) channels and ATP-sensitive K+ (KATP) channels in NO-mediated vasodilation of retinal arterioles in rats. In vivo, the retinal vasodilator responses were assessed by measuring changes in the diameter of retinal arterioles from ocular fundus images. Intravitreal injection of 4-aminopyridine (a KV channel inhibitor), but not glibenclamide (a KATP channel blocker), significantly attenuated the retinal vasodilator response to the NO donor (±)-(E)-4-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamide (NOR3). Intravitreal injection of indomethacin (a non-selective cyclooxygenase inhibitor) also reduced the NOR3-induced retinal vasodilator response. The combination of 4-aminopyridine and indomethacin produced a greater reduction in the NOR3-induced response than either agent alone. 4-Aminopyridine had no significant effect on pinacidil (a KATP channel opener)-induced response. These results suggest that the vasodilatory effects of NO are mediated, at least in part, through the activation of 4-aminopyridine-sensitive KV channels in the retinal arterioles of rats. NO exerts its dilatory effect on the retinal vasculature of rats through at least two mechanisms, activation of the KV channels and enhancement of prostaglandin production.
Subject(s)
4-Aminopyridine/pharmacology , Arterioles/drug effects , Nitric Oxide/physiology , Potassium Channel Blockers/pharmacology , Retinal Vessels/drug effects , Vasodilation/drug effects , Animals , Arterioles/physiology , Indomethacin/pharmacology , Male , Rats, Wistar , Retinal Vessels/physiology , Vasodilation/physiologyABSTRACT
The soluble guanylyl cyclase/cGMP system plays an important role in the vasodilator response to nitric oxide (NO) in various vascular beds. However, in rat retinal arterioles, the cyclooxygenase-1/cAMP-mediated pathway contributes to the vasodilator effects of NO, although the specific prostanoid involved remains to be elucidated. In the present study, we investigated the role of prostaglandin I2 and its receptor (prostanoid IP receptor) system in NO-induced vasodilation of rat retinal arterioles in vivo. Fundus images were captured using a digital camera that was equipped with a special objective lens. Changes in diameter of retinal arterioles were assessed. The NO donor (±)-(E)-4-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamide (NOR3) increased the diameter of retinal arterioles but decreased systemic blood pressure in a dose-dependent manner. Treatment of rats with indomethacin, a non-selective cyclooxygenase inhibitor, markedly attenuated the retinal vasodilator, but not depressor responses to NOR3. The prostanoid IP receptor antagonist 4,5-dihydro-N-[4-[[4-(1-methylethoxy)phenyl]methyl]phenyl]-1H-imadazol-2-amine (CAY10441), and the prostaglandin I2 synthase inhibitor 9α,11α-azoprosta-5Z,13E-dien-1-oic acid (U-51605), both showed similar preventive effects against the NOR3-induced retinal vasodilator response. Neither CAY10441 nor U-51605 showed any significant effects on the depressor response to NOR3. NOR3 enhanced the release of prostaglandin I2 from cultured human retinal microvascular endothelial cells and the NOR3-induced prostaglandin I2 release was almost completely abolished by the cyclooxygenase-1 inhibitor SC-560, but not by the cyclooxygenase-2 inhibitor NS-398. However, NOR3 did not increase the release of prostaglandin I2 from human intestinal microvascular endothelial cells. These results suggest that NO exerts its dilatory effect via cyclooxygenase-1/prostaglandin I2/prostanoid IP receptor signaling mechanisms in the retinal vasculature.
