Megumi Toda

Canine Retinal Arterial and Arteriolar Dilatation Induced by Nipradilol, a Possible Glaucoma Therapeutic

The effects of nipradilol, an ocular hypotensive drug, on isolated canine retinal central arteries and on retinal arterioles in vivo were investigated. Nipradilol (10(-9) to 10(-5) mol/l) produced a dose-related relaxation of the arterial strips contracted with prostaglandin F2 alpha which was not influenced by timolol or indometacin. The median effective concentration of this drug was five times that of glycerol trinitrate (GTN). The nipradilol-induced relaxation in the endothelium-intact strips was not influenced by NG-nitro-L-arginine, a nitric oxide synthase inhibitor, but was abolished by oxyhemoglobin and methylene blue. Treatment with high concentrations of sodium nitroprusside abolished the response to nipradilol, as observed with that to GTN. Retinal arterial strips responded to isoproterenol with a slight relaxation which was depressed by nipradilol. In anesthetized dogs, intra-arterial injections of nipradilol dilated the retinal arterioles in the ocular fundus; the dilator potency was approximately one fifth that of GTN. It is concluded that nipradilol dilates canine retinal arteries in vitro and arterioles in vivo, possibly due to activation of soluble guanylate cyclase and increased production of cyclic guanosine monophosphate that are associated with nitric oxide liberated from the molecule itself in the tissue but not derived from the endothelium and perivascular nerve. Beta adrenoceptor blocking action was determined in the retinal artery.

Receptor subtypes involved in relaxation and contraction by arginine vasopressin in canine isolated short posterior ciliary arteries

Arginine vasopressin (AVP) produced relaxations at low concentrations (10[-11] and 10[-10] M) and contractions at higher concentrations in canine ciliary arterial strips with endothelium, partially contracted with prostglandin F2alpha. The AVP-induced relaxation was abolished or reversed to a contraction by removal of the endothelium or treatment with NG-nitro-L-arginine. The effect of this antagonist was reversed by L-arginine. The relaxant response was inhibited dose-dependently by SR49059 (10[-10]-10[-9] M), [Pmp1,Tyr(Me)2]-Arg8-vasopressin (PMP-AVP) (10[-10]-10[-9] M), V1 receptor antagonists, and OPC31260 (3 x 10[-8] M), a reported V2 receptor antagonist, but not by OPC21268 (10[-7]-10[-6] M), a reported V1 antagonist. In the endothelium-denuded strips, the AVP-induced contraction was attenuated by SR49059, PMP-AVP and OPC31260, but not by OPC21268. It is concluded that AVP in low concentrations elicits intense relaxation of canine ciliary arteries, possibly due to nitric oxide synthesized in association with activation of the endothelial V1 receptor subtype. AVP-induced contractions appear to be mediated also by the V1 receptor in smooth muscle. Antagonistic selectivities of the OPC compounds to vasopressin receptor subtypes could not be seen in this particular material.

Effects of endothelial impairment by saponin on the responses to vasodilators and nitrergic nerve stimulation in isolated canine corpus cavernosum.

Responsiveness to EDRF‐releasing substances and inhibitory nerve stimulation of canine isolated penile corpus cavernosum with and without saponin treatment were investigated. Histological studies demonstrated that saponin did not detach endothelial cells from underlying tissues, but induced degenerative changes in the endothelial cells selectively. In the cavernous strips contracted with phenylephrine, addition of acetylcholine, sodium nitroprusside, ATP and Ca2+ ionophore A23187 induced relaxations, but substance P and bradykinin did not change the muscle tone. Acetylcholine‐induced relaxation was significantly attenuated but not abolished by NG‐nitro‐L‐arginine (L‐NOARG). L‐arginine restored the response inhibited by L‐NOARG. The L‐NOARG resistant relaxation was not influenced by 1H[1,2,4]oxadiazole[4,3‐a]quinoxalin‐1‐one (ODQ) but was suppressed in the strips contracted with K+. Treatment with saponin abolished the relaxation elicited by acetylcholine and A23187 but did not influence the response to nitroprusside and ATP. The ATP‐induced relaxation was attenuated by aminophylline. Transmural electrical stimulation at 2–20 Hz produced endothelium‐independent relaxations which were abolished by tetrodotoxin and L‐NOARG but unaffected by treatment with saponin. In saponin‐treated cavernous strips, the neurogenic relaxation was not affected by acetylcholine, physostigmine, atropine and vasoactive intestinal peptide (VIP) but was abolished by ODQ. It is concluded that acetylcholine‐induced relaxations are endothelium‐dependent and mediated partly by NO and also by other substances from the endothelium. The endothelium‐independent relaxation to ATP is likely to be mediated by P1 purinoceptors. The function of nitrergic nerve does not seem to be prejunctionally modulated by acetylcholine and VIP.

Cholinergic nerve function in monkey ciliary arteries innervated by nitroxidergic nerve.

We sought to determine the control of ciliary arterial tone by neurogenic acetylcholine (ACh) acting directly on smooth muscle and in conjunction with vasodilator nerves. Isolated posterior ciliary arteries from monkeys responded to ACh (10(-8)-10(-5) M) with dose-related contractions, which were endothelium independent. The response was not affected by cyclooxygenase inhibitors but was abolished by atropine. Relaxations induced at 10(-4) M ACh in the atropine-treated arterial strips were abolished by hexamethonium and NG-nitro-L-arginine (L-NNA), and L-arginine (L-Arg) reversed the response suppressed by L-NNA. Similar results were also obtained on the nicotine (10(-4) M)-induced relaxation. Contractions due to transmural electrical stimulation in the endothelium-denuded strips treated with L-NNA were potentiated by physostigmine and depressed by atropine; the remaining contraction in the presence of atropine was abolished by prazosin. Relaxations associated with electrical stimulation, sensitive to tetrodotoxin, were abolished or reversed to contractions by L-NNA and restored by L-Arg. Stimulation-induced relaxation was attenuated by exogenous ACh and physostigmine and was potentiated by atropine. ACh did not affect the relaxation caused by nitric oxide (NO). Nerve fibers and bundles containing NADPH diaphorase and acetylcholinesterase were histologically demonstrated in the adventitia of ciliary arteries. We conclude that 1) endogenous and exogenous ACh contracts monkey ciliary arteries by acting on muscarinic receptors in smooth muscle cell membranes, 2) vasodilatation elicited by nerve stimulation with electrical pulses or nicotine is mediated by NO synthesized from L-Arg, 3) neurogenic ACh seems to interfere with the nitroxidergic nerve function by acting on prejunctional muscarinic receptors, and 4) high concentrations of ACh stimulate nicotinic receptors in vasodilator nerve terminals and promote the synthesis and/or release of NO.We sought to determine the control of ciliary arterial tone by neurogenic acetylcholine (ACh) acting directly on smooth muscle and in conjunction with vasodilator nerves. Isolated posterior ciliary arteries from monkeys responded to ACh (10-8-10-5M) with dose-related contractions, which were endothelium independent. The response was not affected by cyclooxygenase inhibitors but was abolished by atropine. Relaxations induced at 10-4 M ACh in the atropine-treated arterial strips were abolished by hexamethonium and N G-nitro-l-arginine (l-NNA), andl-arginine (l-Arg) reversed the response suppressed by l-NNA. Similar results were also obtained on the nicotine (10-4 M)-induced relaxation. Contractions due to transmural electrical stimulation in the endothelium-denuded strips treated withl-NNA were potentiated by physostigmine and depressed by atropine; the remaining contraction in the presence of atropine was abolished by prazosin. Relaxations associated with electrical stimulation, sensitive to tetrodotoxin, were abolished or reversed to contractions byl-NNA and restored byl-Arg. Stimulation-induced relaxation was attenuated by exogenous ACh and physostigmine and was potentiated by atropine. ACh did not affect the relaxation caused by nitric oxide (NO). Nerve fibers and bundles containing NADPH diaphorase and acetylcholinesterase were histologically demonstrated in the adventitia of ciliary arteries. We conclude that 1) endogenous and exogenous ACh contracts monkey ciliary arteries by acting on muscarinic receptors in smooth muscle cell membranes, 2) vasodilatation elicited by nerve stimulation with electrical pulses or nicotine is mediated by NO synthesized froml-Arg, 3) neurogenic ACh seems to interfere with the nitroxidergic nerve function by acting on prejunctional muscarinic receptors, and 4) high concentrations of ACh stimulate nicotinic receptors in vasodilator nerve terminals and promote the synthesis and/or release of NO.

Desmopressin-induced dog ciliary artery relaxation

In isolated dog posterior ciliary arteries contracted with prostaglandin F2alpha, desmopressin (10(-10) to 10(-8) M), a vasopressin V2 receptor agonist, produced a concentration-related relaxation, which was reversed to a contraction by removal of the endothelium. Desmopressin was approximately 1/100 as potent as arginine vasopressin. Treatment with NG-nitro-L-arginine, a nitric oxide (NO) synthase inhibitor, reversed the desmopressin-induced relaxation to a contraction and the addition of L-arginine restored the relaxation. SR49059 ((2S)1-[(2 R3S)-(5-chloro-3-(2-chlorophenyl)-1-(3,4-methoxybenzene-s ulfony)-3-hydroxy-2,3-dihydro-1H-indole-2-carbonyl]-pyrrolidine-2-car boxamide), a selective vasopressin V1 receptor antagonist, suppressed the relaxation. In endothelium-denuded arteries, desmopressin-induced contractions were also inhibited by SR49059. It is concluded that desmopressin, although much less potent than vasopressin, relaxes ciliary arteries via a mediation of NO synthesized from L-arginine in the endothelium. Vasopressin V1-receptor Subtypes appear to be involved in the desmopressin-induced relaxation and contraction.

Regulation of Retinal Arterial and Arteriolar Tone by Nitric Oxide Derived from Endothelium and Perivascular Nerve

Nitric oxide (NO) is now recognized to be a key substance for regulating functions of not only the cardiovascular, nervous, and immune systems but also other organs and tissues of almost the entire body. The role of NO derived from the endothelium and perivascular nerve has not been well understood in the ocular circulation, possibly because detailed analyses in in vitro studies are difficult in such a small and fragile ocular artery. This chapter describes the role of endothelium-derived NO in isolated retinal central arteries and retinal arterioles in vivo. It also describes the role of NO derived from perivascular nerve in isolated retinal central, ophthalmic, and cerebral arteries and the retinal arterioles in vivo.