4-Aminopyridine, a Voltage-Gated K+ Channel Inhibitor, Attenuates Nitric Oxide-Mediated Vasodilation of Retinal Arterioles in Rats

Proteome landscape and interactome of voltage-gated potassium channel 1.6 (Kv1.6) of the murine ophthalmic artery and neuroretina

The voltage-gated potassium channel 1.6 (Kv1.6) plays a vital role in ocular neurovascular beds and exerts its modulatory functions via interaction with other proteins. However, the interactome and their potential roles remain unknown. Here, the global proteome landscape of the ophthalmic artery (OA) and neuroretina was mapped, followed by the determination of Kv1.6 interactome and validation of its functionality and cellular localization. Microfluorimetric analysis of intracellular [K+] and Western blot validated the native functionality and cellular expression of the recombinant Kv1.6 channel protein. A total of 54, 9 and 28 Kv1.6-interacting proteins were identified in the mouse OA and, retina of mouse and rat, respectively. The Kv1.6-protein partners in the OA, namely actin cytoplasmic 2, alpha-2-macroglobulin and apolipoprotein A-I, were implicated in the maintenance of blood vessel integrity by regulating integrin-mediated adhesion to extracellular matrix and Ca2+ flux. Many retinal protein interactors, particularly the ADP/ATP translocase 2 and cytoskeleton protein tubulin, were involved in endoplasmic reticulum stress response and cell viability. Three common interactors were found in all samples comprising heat shock cognate 71 kDa protein, Ig heavy constant gamma 1 and Kv1.6 channel. This foremost in-depth investigation enriched and identified the elusive Kv1.6 channel and, elucidated its complex interactome.

L-Citrulline ameliorates the attenuation of acetylcholine-induced vasodilation of retinal arterioles in diabetic rats

In our previous study, we found that the vasodilation of retinal arterioles induced by acetylcholine and BMS-191011, a large-conductance Ca²⁺-activated K⁺ (BK_Ca) channel opener, were diminished in diabetic rats. Currently, few agents ameliorate the impaired vasodilator responses of retinal blood vessels. Our recent finding that the intravenous infusion of L-citrulline dilated retinal arterioles, suggests that L-citrulline could be a potential therapeutic agent for circulatory disorders of the retina. In this study, we determined the effect of an oral L-citrulline treatment on impaired acetylcholine- and BMS-191011-induced vasodilation in the retinal arterioles of diabetic rats. To induce diabetes, rats were administered an intravenous dose of streptozotocin (65 mg/kg) and a 5% D-glucose solution as drinking water. The L-citrulline (2 g/kg/day) and L-arginine (2 g/kg/day) treatments commenced either 15 days before or just after the streptozotocin injection and continued throughout the experimental period. A 29-day treatment with L-citrulline, but not L-arginine, significantly ameliorated the impaired acetylcholine- and BMS-191011-induced retinal vasodilation in diabetic rats without affecting their plasma glucose levels. The 2-week L-citrulline treatment tended to ameliorate the dysfunction of the acetylcholine-induced retinal vasodilation in diabetic rats. In conclusion, these results showed that the retinal blood vessel dysfunction induced by diabetes mellitus could be prevented by the long-term administration of L-citrulline and suggest that the latter could play a potentially prophylactic role in diabetic retinopathy.

Role of Epoxyeicosatrienoic Acids in Acetylcholine-Induced Dilation of Rat Retinal Arterioles in Vivo

CYP epoxygenase-derived epoxyeicosatrienoic acids (EETs) contribute to endothelium-dependent hyperpolarization (EDH)-related dilation in multiple vascular beds. The present study aimed to determine the role of EETs in the acetylcholine (ACh)-induced dilation of retinal arterioles in rats in vivo. The vasodilator responses were assessed by determining the change in diameter of the retinal arterioles on images of the ocular fundus. The intravitreal injection of 17-octadecynoic acid (1.4 nmol/eye), an inhibitor of CYP epoxygenase, and 14,15-epoxyeicosa-5(Z)-enoic acid (14,15-EE-5(Z)-E; 2 nmol/eye), an antagonist of EETs, reduced the ACh (0.3-10 µg/kg/min)-induced dilation of the retinal arterioles. The EET antagonist attenuated the vasodilator response to ACh under blockade of nitric oxide (NO) synthases and cyclooxygenases with N^G-nitro-L-arginine methyl ester (30 mg/kg) plus indomethacin (5 mg/kg). Intravitreal injection of 14,15-EET (0.5 nmol/eye) dilated retinal arterioles and the response was prevented by iberiotoxin, an inhibitor of large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels (20 pmol/eye). These results suggest that ACh stimulates the production of EETs, thereby dilating the retinal arterioles via activation of BK_Ca channels. CYP epoxygenase-derived EETs may be involved in the EDH-related component of the ACh-induced dilation of the retinal arterioles.

Involvement of Gap Junctions in Acetylcholine-Induced Endothelium-Derived Hyperpolarization-Type Dilation of Retinal Arterioles in Rats

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 (18β-glycyrrhetinic acid and carbenoxolone) reduced the ACh-induced dilation of retinal arterioles. Moreover, the retinal arteriolar response to ACh was attenuated by 18β-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 18β-glycyrrhetinic acid and iberiotoxin produced greater attenuation in the EDH-related response than that by the individual agent. Treatment with 18β-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.