Takayuki Kamiya

Dilation of porcine retinal arterioles to cilostazol: roles of eNOS phosphorylation via cAMP/protein kinase A and AMP-activated protein kinase and potassium channels.

PURPOSE Cilostazol, a selective inhibitor of phosphodiesterase 3, has antiplatelet aggregation and peripheral vasodilation effects. We examined the effects of cilostazol on the retinal microvascular diameter to determine its dependence on the endothelium and/or smooth muscle to reveal the signaling mechanisms involved in this vasomotor activity. METHODS Porcine retinal arterioles were isolated, cannulated, and pressurized without flow in vitro. Video microscopic techniques recorded the diametric responses to cilostazol. RESULTS The retinal arterioles dilated in response to cilostazol in a dose-dependent (100 pM-10 μM) manner; the dilation decreased by 60% after endothelial removal. The nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), inhibited cilostazol-induced vasodilation comparable to denudation. Inhibition of soluble guanylyl cyclase and blockade of protein kinase A (PKA) were comparable to L-NAME. Compound C, an AMP-activated protein kinase (AMPK) inhibitor, partially inhibited cilostazol-induced vasodilation, which exhibited a weaker inhibitory effect on cilostazol-induced vasodilation than blockade of PKA. The large-conductance Ca²⁺-activated K channel (BK(Ca) channel) blocker, iberiotoxin, also inhibited cilostazol-induced vasodilation. The residual vasodilation decreased further with co-administration of L-NAME and iberiotoxin. CONCLUSIONS Cilostazol elicits endothelium-dependent and -independent dilation of the retinal arterioles mediated by NO release and BK(Ca) channel activation, respectively. Endothelial nitric oxide synthase (eNOS) phosphorylation via the cAMP/PKA and AMPK pathways and consequent activation of the soluble guanylyl cyclase/cyclic guanosine monophosphate pathway might play an important role in cilostazol-induced vasodilation of the retinal arterioles.

Role of Ca2+-dependent and Ca2+-sensitive mechanisms in sphingosine 1-phosphate-induced constriction of isolated porcine retinal arterioles in vitro

Although sphingosine 1-phosphate (S1P), a bioactive lipid derived from activated platelets, has a variety of physiologic effects on vessels, no reports have described the effect of S1P on the retinal circulation. We examined the effect and underlying mechanism of the vasomotor action of S1P on porcine retinal arterioles. The porcine retinal arterioles were isolated, cannulated, and pressurized without flow for in vitro study. S1P-induced diameter changes were recorded using videomicroscopic techniques. S1P elicited concentration-dependent (1 nM-10 μM) vasoconstriction of the retinal arterioles that was abolished by the S1P receptor 2 (S1PR2) antagonist JTE-013. S1P-induced vasoconstriction was abolished by the Rho kinase (ROCK) inhibitor H-1152 and was inhibited partly by the protein kinase C (PKC) inhibitor Gö-6983. The inhibition of phospholipase C by U73122 and L-type voltage-operated calcium channels (L-VOCCs) by nifedipine inhibited S1P-induced vasoconstriction; a combination of both inhibitors abolished S1P-induced vasoconstriction. Furthermore, inhibition of myosin light chain kinase (MLCK) by ML-9 significantly blocked S1P-induced vasoconstriction; further coadministration of ML-9 with H-1152 or Gö-6983 abolished S1P-induced vasoconstriction. The current data suggest that S1P elicits vasoconstriction of the retinal arterioles via S1PR2 in vascular smooth muscle cells and this vasoconstriction may be mediated by the Ca2+ -sensitive pathway via activation of PKC leading to activation of ROCK and the Ca2+ -dependent pathway via activation of L-VOCCs resulting in activation of MLCK.

Fenofibrate, an anti-dyslipidemia drug, elicits the dilation of isolated porcine retinal arterioles: role of nitric oxide and AMP-activated protein kinase.

PURPOSE Although recent clinical trials have demonstrated that fenofibrate is effective for treating diabetic retinopathy, the mechanism of this beneficial effect remains unclear. In the current study, we examined the effect of the vasomotor action of fenofibrate on porcine retinal arterioles. METHODS Porcine retinal arterioles (internal diameter, 60-90 μm) were isolated, cannulated, and pressurized (55 cmH(2)O) without flow in vitro. Video-microscopic techniques recorded the diameter responses to fenofibrate. RESULTS The retinal arterioles dilated in a dose-dependent manner in response to fenofibrate (10 nM to 30 μM). This vasodilation significantly decreased after the endothelium was removed. N(ω)-nitro-L-arginine methyl ester (a nitric oxide [NO] synthase inhibitor), 1H-(1,2,4)oxadiazole(4,3-alpha)quinoxaline-1-one (a soluble guanylyl cyclase inhibitor), wortmannin (a phosphatidylinositol [PI] 3-kinase inhibitor), and compound C (an AMP-activated protein kinase inhibitor) attenuated the effect of fenofibrate-induced vasodilation to an extent comparable to that produced by denudation. Pretreatment with GW6471, a peroxisome proliferator-activated receptor-α blocker, did not significantly inhibit fenofibrate-induced vasodilation. CONCLUSIONS Fenofibrate primarily elicited endothelium-dependent dilation of the retinal arterioles. The current findings suggested that fenofibrate-induced endothelium-dependent vasodilation is mediated by the release of NO, which probably mediates dilation via activation of guanylyl cyclase, the PI3-kinase pathway, and the AMP-activated protein kinase pathway. Understanding the vasodilatory effect of fenofibrate on the retinal microvasculature may improve potential therapy for diabetic retinopathy.

Benzo(e)pyrene Inhibits Endothelium-Dependent NO-Mediated Dilation of Retinal Arterioles via Superoxide Production and Endoplasmic Reticulum Stress

Purpose To investigate whether benzo(e)pyrene (B(e)P), a toxicant in cigarette smoke, affects the endothelium-dependent nitric oxide (NO)-induced vasodilation of the retinal arterioles, and whether oxidative stress, distinct protein kinase signaling pathways, and endoplasmic reticulum (ER) stress are associated with the B(e)P-induced effect on the retinal arterioles. Methods In this in vitro study, porcine retinal arterioles were isolated, cannulated, and pressurized without flow. These vessels were treated with intraluminal administration of B(e)P or B(e)P plus blockers for 180 minutes. Diametric changes to agonists were recorded by videomicroscopy. Results Intraluminal treatment with 100 μM B(e)P for 180 minutes significantly reduced the arteriolar vasodilation caused by the endothelium-dependent NO-mediated agonists bradykinin and A23187 but not that caused by endothelium-independent NO donor sodium nitroprusside. The adverse effects of B(e)P on the vasodilatory action of bradykinin were prevented by the superoxide scavenger 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL), the nicotinamide adenine dinucleotide phosphate-oxidase (NADPH oxidase) inhibitor apocynin, the c-Jun N-terminal kinase (JNK) inhibitor SP600125, the p38 mitogen-activated protein kinase inhibitor SB203580, genistein, resveratrol (RSV), and the ER stress inhibitor 4-phenylbutyrate (4-PBA). The xanthine oxidase inhibitor allopurinol did not alter the effect of B(e)P on the vasodilatory action induced by bradykinin. Conclusions B(e)P decreases the endothelium-dependent NO-induced vasodilation in the retinal arterioles through the production of superoxide from NADPH oxidase, which is linked to JNK and p38 kinase. The results suggested that ER stress is instrumental in B(e)P-induced endothelial dysfunction and that genistein and RSV might preserve endothelial function.

Histamine-Induced Dilation of Isolated Porcine Retinal Arterioles: Role of Endothelium-Derived Hyperpolarizing Factor.

PURPOSE Although endothelium-dependent nitric oxide (NO)-mediated dilation of retinal arterioles has been well described, the role of endothelium-derived hyperpolarizing factor (EDHF) in the retinal arteriolar response remains unclear. In the current study, we examined the contribution of EDHF to the retinal arteriolar dilation to the inflammatory agent histamine and investigated the signaling mechanisms underlying this vasomotor activity. METHODS Porcine retinal arterioles were isolated, cannulated, and pressurized without flow for functional study by using video microscopic techniques. The immunohistochemical staining was performed to determine histamine receptor subtypes. RESULTS Histamine (0.1-30 μM) produced concentration-dependent dilation of retinal arterioles in a manner sensitive to H1- and H2-receptor antagonists chlorpheniramine and famotidine, respectively. Histamine-induced vasodilation was almost abolished after endothelial removal. In the intact vessels, vasodilation to histamine was partially inhibited by the inhibitors of cyclooxygenase (indomethacin), NO synthase (NG-nitro-L-arginine methyl ester, L-NAME), or Ca2+ -activated K+ (KCa) channels (apamin plus charybdotoxin). Combination of the above inhibitors abolished histamine-induced vasodilation. Residual vasodilation in the presence of indomethacin and L-NAME was further reduced by the cytochrome P450 enzyme inhibitor sulfaphenazole but not by the gap junction inhibitor carbenoxolone or the hydrogen peroxide scavenger catalase. Immunohistochemical signals for H1- and H2-receptor expression were found only in the endothelium. CONCLUSIONS The endothelium plays an essential role in the dilation of porcine retinal arterioles to histamine via H1- and H2-receptor activation. The EDHF derived from cytochrome P450 contributed in part to this vasodilation via KCa channel activation, in addition to the endothelial release of NO and prostanoids.

Beraprost sodium, a stable prostacyclin analogue, elicits dilation of isolated porcine retinal arterioles: roles of eNOS and potassium channels.

PURPOSE Prostacyclin (PGI2) is usually described as an endoEDRFsthelium-derived relaxing factor, but the vasoreactivity to PGI2 in the retinal arterioles and the underlying mechanisms are not fully understood. We examined the effects of PGI2 on the retinal microcirculation using beraprost sodium (BPS), a stable PGI2 analogue, and the signaling mechanisms involved in this vasomotor activity. METHODS Porcine retinal arterioles were isolated, cannulated, and pressurized without flow in vitro. Video microscopic techniques recorded the diametric responses to BPS. RESULTS Beraprost sodium elicited dose-dependent (0.1 pM-0.1 μM) vasodilation of the retinal arterioles that was abolished by the PGI2 receptor (IP) antagonist CAY10441. Beraprost sodium-induced vasodilation decreased by 50% after the endothelium was removed and was inhibited by the nitric oxide (NO) synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) comparable with denudation. Inhibition of soluble guanylyl cyclase by 1H-1,2,4-oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) and blockage of protein kinase A (PKA) by Rp-8-Br-cAMPS were comparable to L-NAME. Beraprost sodium-induced vasodilation was also inhibited by the nonselective potassium channel inhibitor, tetraethylammonium, and the adenosine triphosphate-sensitive potassium (KATP) channel blocker, glibenclamide. Residual vasodilation in the presence of glibenclamide decreased further with subsequent application of ODQ. CONCLUSIONS Beraprost sodium, a stable PGI2 analogue, causes vasodilation of the retinal arterioles mediated via the IP receptor. The current findings suggest that BPS elicits endothelium-dependent and -independent dilation of the retinal arterioles mediated by NO induced by activation of PKA in the endothelium and the KATP channel activation in the vascular smooth muscle, respectively.

Thrombin-Induced Responses via Protease-Activated Receptor 1 Blocked by the Endothelium on Isolated Porcine Retinal Arterioles

ABSTRACT Purpose: Thrombin, a serine protease, causes organ-specific responses to vessels. However, the mechanism by which thrombin affects the retinal microcirculation remains unclear. We examined the effects of thrombin on the retinal microvasculature and signaling mechanisms. Methods: Porcine retinal arterioles were isolated, cannulated, and pressurized (55 cmH2O) without flow in this in vitro study. Videomicroscopy techniques recorded changes in diameter in the retinal arterioles in response to thrombin at concentrations ranging from 0.001 to 20 mU/ml. Results: Extraluminal administration of thrombin induced concentration-dependent vascular responses, that is, vasoconstriction at low concentrations less than 5 mU/ml and vasorelaxation with high concentrations greater than 5 mU/ml. However, intraluminal administration of thrombin (5 mU/m) did not constrict the retinal arterioles; in denuded vessels, intraluminal administration constricted the retinal arterioles. Thrombin-induced vasoconstriction was significantly (p < 0.01) suppressed by pretreatment with a protein kinase C (PKC) inhibitor and a protease-activated receptor (PAR)-1 inhibitor but not by PAR-2 and PAR-4 inhibitors or denudation. A rho kinase (ROCK) inhibitor also suppressed thrombin-induced vasoconstriction (5 mU/ml) compared with sodium nitroprusside. Endothelial denudation and pretreatment with an endothelial nitric oxide (NO) synthase inhibitor suppressed vasorelaxation caused by a high concentration of thrombin. Conclusions: A low concentration of thrombin causes vasoconstriction of smooth muscles via PAR-1, PKC, and ROCK, and a high concentration of thrombin possibly causes vasorelaxation of the retinal arterioles via nitric oxide synthase activation in the endothelium. The vascular endothelium might block signaling of thrombin-induced vasoconstriction in the retinal arterioles when administered intraluminally.

Beraprost Sodium, a Stable Prostacyclin Analogue, Elicits Dilation of Isolated Porcine Retinal Arterioles: Roles of eNOS and Potassium ChannelsBPS-Induced Vasodilation in Retinal Arterioles

PURPOSE Prostacyclin (PGI2) is usually described as an endoEDRFsthelium-derived relaxing factor, but the vasoreactivity to PGI2 in the retinal arterioles and the underlying mechanisms are not fully understood. We examined the effects of PGI2 on the retinal microcirculation using beraprost sodium (BPS), a stable PGI2 analogue, and the signaling mechanisms involved in this vasomotor activity. METHODS Porcine retinal arterioles were isolated, cannulated, and pressurized without flow in vitro. Video microscopic techniques recorded the diametric responses to BPS. RESULTS Beraprost sodium elicited dose-dependent (0.1 pM-0.1 μM) vasodilation of the retinal arterioles that was abolished by the PGI2 receptor (IP) antagonist CAY10441. Beraprost sodium-induced vasodilation decreased by 50% after the endothelium was removed and was inhibited by the nitric oxide (NO) synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) comparable with denudation. Inhibition of soluble guanylyl cyclase by 1H-1,2,4-oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) and blockage of protein kinase A (PKA) by Rp-8-Br-cAMPS were comparable to L-NAME. Beraprost sodium-induced vasodilation was also inhibited by the nonselective potassium channel inhibitor, tetraethylammonium, and the adenosine triphosphate-sensitive potassium (KATP) channel blocker, glibenclamide. Residual vasodilation in the presence of glibenclamide decreased further with subsequent application of ODQ. CONCLUSIONS Beraprost sodium, a stable PGI2 analogue, causes vasodilation of the retinal arterioles mediated via the IP receptor. The current findings suggest that BPS elicits endothelium-dependent and -independent dilation of the retinal arterioles mediated by NO induced by activation of PKA in the endothelium and the KATP channel activation in the vascular smooth muscle, respectively.