Analysis of the vasodilator nerve function by nicotine in isolated dog skin artery

Nitrergic perivascular innervation in health and diseases: Focus on vascular tone regulation

For a long time, the vascular tone was considered to be regulated exclusively by tonic innervation of vasoconstrictor adrenergic nerves. However, accumulating experimental evidence has revealed the existence of nerves mediating vasodilatation, including perivascular nitrergic nerves (PNN), in a wide variety of mammalian species. Functioning of nitrergic vasodilator nerves is evidenced in several territories, including cerebral, mesenteric, pulmonary, renal, penile, uterine and cutaneous arteries. Nitric oxide (NO) is the main neurogenic vasodilator in cerebral arteries and acts as a counter-regulatory mechanism for adrenergic vasoconstriction in other vascular territories. In the penis, NO relaxes the vascular and cavernous smooth muscles leading to penile erection. Furthermore, when interacting with other perivascular nerves, NO can act as a neuromodulator. PNN dysfunction is involved in the genesis and maintenance of vascular disorders associated with arterial and portal hypertension, diabetes, ageing, obesity, cirrhosis and hormonal changes. For example defective nitrergic function contributes to enhanced sympathetic neurotransmission, vasoconstriction and blood pressure in some animal models of hypertension. In diabetic animals and humans, dysfunctional nitrergic neurotransmission in the corpus cavernosum is associated with erectile dysfunction. However, in some vascular beds of hypertensive and diabetic animals, an increased PNN function has been described as a compensatory mechanism to the increased vascular resistance. The present review summarizes current understanding on the role of PNN in control of vascular tone, its alterations under different conditions and the associated mechanisms. The knowledge of these changes can serve to better understand the mechanisms involved in these disorders and help in planning new treatments.

Recent advances in research on nitrergic nerve-mediated vasodilatation

Cerebral vascular resistance and blood flow were widely considered to be regulated solely by tonic innervation of vasoconstrictor adrenergic nerves. However, pieces of evidence suggesting that parasympathetic nitrergic nerve activation elicits vasodilatation in dog and monkey cerebral arteries were found in 1990. Nitric oxide (NO) as a neurotransmitter liberated from parasympathetic postganglionic neurons decreases cerebral vascular tone and resistance and increases cerebral blood flow, which overcome vasoconstrictor responses to norepinephrine liberated from adrenergic nerves. Functional roles of nitrergic vasodilator nerves are found also in peripheral vasculature, including pulmonary, renal, mesenteric, hepatic, ocular, uterine, nasal, skeletal muscle, and cutaneous arteries and veins; however, adrenergic nerve-induced vasoconstriction is evidently greater than nitrergic vasodilatation in these vasculatures. In coronary arteries, neurogenic NO-mediated vasodilatation is not clearly noted; however, vasodilatation is induced by norepinephrine released from adrenergic nerves that activates β1-adrenoceptors. Impaired actions of NO liberated from the endothelium and nitrergic neurons are suggested to participate in cerebral hypoperfusion, leading to brain dysfunction, like that in Alzheimer's disease. Nitrergic neural dysfunction participates in impaired circulation in peripheral organs and tissues and also in systemic blood pressure increase. NO and vasodilator peptides, as sensory neuromediators, are involved in neurogenic vasodilatation in the skin. Functioning of nitrergic vasodilator nerves is evidenced not only in a variety of mammals, including humans and monkeys, but also in non-mammals. The present review article includes recent advances in research on the functional importance of nitrergic nerves concerning the control of cerebral blood flow, as well as other regions, and vascular resistance. Although information is still insufficient, the nitrergic nerve histology and function in vasculatures of non-mammals are also summarized.

Estrogen prevents β-amyloid inhibition of sympathetic α7-nAChR-mediated nitrergic neurogenic dilation in porcine basilar arteries

AIM

β-amyloid peptides (Aβs) have been shown to block cerebral nitrergic neurogenic vasodilation by blocking sympathetic α7-nAChRs, and that oestrogen prevents Aβ-induced neurotoxicity. We examined whether Aβ-inhibition of α7-nAChR-mediated cerebral nitrergic vasodilation was prevented by oestrogen.

METHODS

Effects of Aβ and 17β-oestradiol on neurogenic nitrergic vasodilation in isolated porcine basilar arteries were examined using wire-myography. Drug effects on nicotine- and choline-induced calcium influx and inward currents in porcine cultured superior cervical ganglion (SCG) were investigated using confocal microscopy and patch-clamp techniques respectively.

RESULTS

Precontracted endothelium-denuded basilar arteries relaxed exclusively upon transmural nerve stimulation (TNS, 8 Hz), and applications of nicotine (100 μm) or choline (1 mm), which was sensitive to nitro-L-arginine (L-NNA, 30 μm) and tetrodotoxin (0.3 μm). The relaxation induced by nicotine and choline but not that by TNS was blocked reversibly by Aβ(1-40) in a concentration-dependent manner. Aβ(1-40) also reversibly blocked nicotine- and choline-induced increase of calcium influx and inward currents in the SCG neurons. Aβ inhibition of nicotine- and choline-induced α7-nAChR-mediated nitrergic vasodilation and inward currents was prevented by 17β-oestradiol (10 μm), but not by α-oestradiol (10 μm) or testosterone (10 μm).

CONCLUSION

These results provide further evidence supporting that Aβ is an antagonist for the α7-nAChR found on post-ganglionic sympathetic adrenergic nerve terminals originating in the SCG. Aβ can cause constriction of cerebral arteries with possible decreased regional cerebral blood flow by blocking sympathetic nerve-mediated release of nitric oxide from the perivascular nitrergic nerves. This effect of Aβ can be prevented by endogenous oestrogen but not testosterone.

Control of systemic and pulmonary blood pressure by nitric oxide formed through neuronal nitric oxide synthase

Nitric oxide formed by neuronal nitric oxide synthase (nNOS) in the brain, autonomic inhibitory (nitrergic) nerves, and heart plays important roles in the control of blood pressure. Activation of nitrergic nerves innervating the systemic vasculature elicits vasodilatation, decreases peripheral resistance, and lowers blood pressure. Impairment of nitrergic nerve function, as well as endothelial dysfunction, results in systemic and pulmonary hypertension and decreased regional blood flow. Blockade of nNOS activity in the brain, particularly the medulla and hypothalamus, causes systemic hypertension. Under hypertensive states, such as those in spontaneously hypertensive and Dahl salt-sensitive rats, the expression of the nNOS gene in the brain is increased; this appears to counteract the activated sympathetic function in the vasomotor center. The present article summarizes information concerning the modulation of systemic and pulmonary hypertension through nNOS-derived nitric oxide produced in the brain and periphery.

Neurally-derived nitric oxide regulates vascular tone in pulmonary and cutaneous arteries of the toad,Bufo marinus

In this study, the role of nitric oxide (NO) in regulation of the pulmocutaneous vasculature of the toad, Bufo marinus was investigated. In vitro myography demonstrated the presence of a neural NO signaling mechanism in both arteries. Vasodilation induced by nicotine was inhibited by the soluble guanylyl cyclase (GC) inhibitor, 1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one, and the NO synthase (NOS) inhibitor, Nω-nitro-l-arginine (l-NNA). Removal of the endothelium had no significant effect on the vasodilation. Furthermore, pretreatment with N5-(1-imino-3-butenyl)-l-ornithine (vinyl-l-NIO), a more specific inhibitor of neural NOS, caused a significant decrease in the nicotine-induced dilation. In the pulmonary artery only, a combination of l-NNA and the calcitonin gene-related peptide (CGRP) receptor antagonist, CGRP(8-37), completely blocked the nicotine-induced dilation. In both arteries, the vasodilation was also significantly decreased by glibenclamide, an ATP-sensitive K+(K+ATP) channel inhibitor. Levcromakalim, a K+ATPchannel opener, caused a dilation that was blocked by glibenclamide in both arteries. In the pulmonary artery, NO donor-mediated dilation was significantly decreased by pretreatment with glibenclamide. The physiological data were supported by NADPH-diaphorase histochemistry and immunohistochemistry, which demonstrated NOS in perivascular nerve fibers but not the endothelium of the arteries. These results indicate that the pulmonary and cutaneous arteries of B. marinus are regulated by NO from nitrergic nerves rather than NO released from the endothelium. The nitrergic vasodilation in the arteries appears to be caused, in part, via activation of K+ATPchannels. Thus, NO could play an important role in determining pulmocutaneous blood flow and the magnitude of cardiac shunting.

Mechanisms of vasodilation in the dorsal aorta of the elephant fish, Callorhinchus milii (Chimaeriformes: Holocephali)

This study investigated vasodilator mechanisms in the dorsal aorta of the elephant fish, Callorhinchus milii, using anatomical and physiological approaches. Nitric oxide synthase could only be located in the perivascular nerve fibres and not the endothelium of the dorsal aorta, using NADPH histochemistry and immunohistochemistry. In vitro organ bath experiments demonstrated that a NO/soluble guanylyl cyclase (GC) system appeared to be absent in the vascular smooth muscle, since the NO donors SNP (10(-4) mol l(-1)) and SIN-1 (10(-5) mol l(-1)) were without effect. Nicotine (3 x 10(-4) mol l(-1)) mediated a vasodilation that was not affected by ODQ (10(-5) mol l(-1)), L-NNA (10(-4) mol l(-1)), indomethacin (10(-5) mol l(-1)), or removal of the endothelium. In contrast, the voltage-gated sodium channel inhibitor, tetrodotoxin (10(-5) mol l(-1)), significantly decreased the dilation induced by nicotine, suggesting that it contained a neural component. Pre-incubation of the dorsal aorta with the calcitonin gene-related peptide (CGRP) receptor antagonist, CGRP(8-37) (10(-6) mol l(-1)) also caused a significant decrease in the nicotine-induced dilation. We propose that nicotine is mediating a neurally-derived vasodilation in the dorsal aorta that is independent of NO, prostaglandins and the endothelium, and partly mediated by CGRP.

Nicotine potentiates the nitrergic relaxation responses of rabbit corpus cavernosum tissue via nicotinic acetylcholine receptors

The presence of neuronal nicotinic acetylcholine receptors in rabbit corpus cavernosum tissue and possible mechanisms underlying the potentiation of electrical field stimulation induced relaxation by nicotine were analyzed. In corpus cavernosum tissue strips nicotine (3 x 10(-5) M) and acetylcholine (10(-3) M) produced potentiation on electrical field stimulation (amplitude 50 V; frequency 4 Hz; width 0.8 ms) induced relaxation responses. This nicotine-induced potentiation was not altered by atropine (10(-6) M), guanethidine (5 x 10(-6) M) and indomethacin (10(-5) M), but abolished by hexamethonium chloride (10(-5) M) and L-nitro arginine methyl ester (10(-5) M). Nicotine did not cause any alteration on a single dose of carbachol (3 x 10(-5) M) and sodium nitroprusside (10(-5) M) induced relaxation responses. The results suggest that, nicotine-induced potentiation is NO and nicotinic acetylcholine receptor dependent but independent from prostaglandin synthesis, activation of muscarinic receptors and does not require intact adrenergic neurons. Nicotine did not affect smooth muscle and endothelium directly. In conclusion, in this study we showed for the first time that, nicotine acts on the nicotinic acetylcholine receptors located on the nitrergic nerves, thereby evoking the release of NO from these nerve terminals inducing relaxation response in rabbit corpus cavernosum tissue.

The pharmacology of nitric oxide in the peripheral nervous system of blood vessels

Unanticipated, novel hypothesis on nitric oxide (NO) radical, an inorganic, labile, gaseous molecule, as a neurotransmitter first appeared in late 1989 and into the early 1990s, and solid evidences supporting this idea have been accumulated during the last decade of the 20th century. The discovery of nitrergic innervation of vascular smooth muscle has led to a new understanding of the neurogenic control of vascular function. Physiological roles of the nitrergic nerve in vascular smooth muscle include the dominant vasodilator control of cerebral and ocular arteries, the reciprocal regulation with the adrenergic vasoconstrictor nerve in other arteries and veins, and in the initiation and maintenance of penile erection in association with smooth muscle relaxation of the corpus cavernosum. The discovery of autonomic efferent nerves in which NO plays key roles as a neurotransmitter in blood vessels, the physiological roles of this nerve in the control of smooth muscle tone of the artery, vein, and corpus cavernosum, and pharmacological and pathological implications of neurogenic NO have been reviewed. This nerve is a postganglionic parasympathetic nerve. Mechanical responses to stimulation of the nerve, mainly mediated by NO, clearly differ from those to cholinergic nerve stimulation. The naming "nitrergic or nitroxidergic" is therefore proposed to avoid confusion of the term "cholinergic nerve", from which acetylcholine is released as a major neurotransmitter. By establishing functional roles of nitrergic, cholinergic, adrenergic, and other autonomic efferent nerves in the regulation of vascular tone and the interactions of these nerves in vivo, especially in humans, progress in the understanding of cardiovascular dysfunctions and the development of pharmacotherapeutic strategies would be expected in the future.

Pb2+ inhibition of sympathetic alpha 7-nicotinic acetylcholine receptor-mediated nitrergic neurogenic dilation in porcine basilar arteries

Chronic exposure to inorganic lead (Pb2+) has been shown to facilitate peripheral vasoconstriction causing hypertension. Effect of lead on cerebral vascular function has not been reported. We have suggested in isolated porcine cerebral arteries that alpha 7-nicotinic acetylcholine receptors (alpha 7-nAChRs) on perivascular sympathetic nerves mediate calcium influx in these neurons, resulting in release of norepinephrine. The released norepinephrine then acts on presynaptic beta2-adrenoceptors located on the neighboring nitrergic nerve terminals, causing nitric oxide (NO) release and vasodilation. Because Pb2+ has been shown to inhibit alpha 7-nAChR-mediated responses in the central nervous system, effects of Pb2+ on alpha 7-nAChR-mediated nitrergic neurogenic dilation in isolated porcine basilar arteries and calcium influx in cultured superior cervical ganglion (SCG) cells of the pig were examined using in vitro tissue bath and confocal microscopic techniques. The results indicated that Pb2+ (but not Cd2+, Zn2+, or Al3+) in a concentration-dependent manner blocked relaxation of endothelium-denuded basilar arterial rings induced by nicotine (100 microM) and choline (1 mM) without affecting relaxation induced by sodium nitroprusside or isoproterenol. Furthermore, significant calcium influx in cultured SCG cells induced by choline and nicotine was attenuated specifically by Pb2+ with IC50 values comparable with those from tissue bath study. These results provide evidence supporting that lead is a likely antagonist for alpha 7-nAChRs that are found on postganglionic sympathetic adrenergic nerve terminals of SCG origin. Furthermore, these results indicate that lead can attenuate dilation of cerebral arteries by blocking sympathetic nerve-mediated release of NO from the perivascular nitrergic nerves.

Nicotine modulates nitric oxide in rat brain

Nicotine exerts its central actions by regulating cationic fluxes through nicotinic acetylcholine receptors (nAChRs). By this effect, the drug likely also modifies events occurring beyond the nAChR, including the regulation of nitric oxide (NO) synthesis. The present study was undertaken to assess the effects of acute and chronic nicotine administration (0.4 mg/kg, s.c.) on levels of NO(-)(2)+NO(-)(3), stable metabolites of NO, in brain regions of male and female rats. Nicotine increased levels of the metabolites, and therefore presumably of NO, with sex differences in the degree of stimulation, the brain regions affected, and the variance between the effects of acute and chronic administration. Prior inhibition of NO synthase eliminated the effect of nicotine in all regions studied. While nicotine appeared to increase NO indirectly via glutamate receptors in the cortex and hippocampus, this was not true of the corpus striatum, where blocking NMDA-type glutamate receptors with MK-801 had no effect. The findings support the view that NO is likely involved in some of the central effects of nicotine.

Functional studies on blockade by neosurugatoxin of nicotinic receptors in nitroxidergic and sensory nerve terminals and intramural ganglionic cells

In isolated canine cerebral and cutaneous arteries and duodenum, effects of neosurugatoxin (NSTX) on the response to nicotine were compared. Nicotine-induced relaxations are mediated by nitric oxide (NO) from vasodilator nerves in cerebral arteries and by calcitonin gene-related peptide (CGRP) in cutaneous arteries treated with NO synthase inhibitors. Duodenal relaxation to nicotine is mediated by NO from inhibitory nerves. Cerebral arterial strips without endothelium responded to nicotine with relaxations that were inhibited by NSTX (3 x 10(-10) to 3 x 10(-9) M) concentration-dependently. Relaxations to nicotine of duodenal strips were attenuated by NSTX and abolished by tetrodotoxin, whereas those induced by K+ and electrical stimulation were not influenced. In cutaneous arteries treated with N(G)-nitro-L-arginine, nicotine-induced relaxations were attenuated by NSTX as low as 10(-10) M, but unaffected by tetrodotoxin. The inhibitory potency of NSTX was in the order of cutaneous artery > duodenum > cerebral artery. It is concluded that NSTX selectively antagonizes actions on nicotinic receptors in nitroxidergic nerves of cerebral arteries, CGRP-mediated sensory nerves of cutaneous artery and ganglionic cells of the duodenum, but the affinity of this toxin to nicotinic receptors in sensory nerves is the highest.

Mechanisms underlying constrictor and dilator responses to perivascular nerve stimulation in canine lingual arteries

In isolated canine lingual arteries denuded of the endothelium, transmural electrical stimulation (2-20 Hz) produced a frequency-related contraction which was not significantly influenced by prazosin but which was reversed to a relaxation by alpha,beta-methylene ATP. The responses were abolished by tetrodotoxin. The stimulation-induced relaxation was abolished by treatment with NG-nitro-L-arginine (L-NA, 10(-6) M) and restored by the addition of L-arginine. Neurogenic relaxation resistant to L-NA was not observed after electrical stimulation, even though the pulse width and stimulus intensity were raised. Under treatment with prazosin, alpha,beta-methylene ATP and indomethacin, the arterial strips responded to nicotine (10(-4) M) with a marked relaxation that was abolished by hexamethonium. The relaxation was significantly inhibited but not abolished by L-NA (10(-5) M), and raising the concentration of the inhibitor to 10(-4) M, did not produce additional inhibition. In the strips treated with L-NA, the nicotine-induced relaxation was abolished or markedly reduced under desensitization with vasoactive intestinal peptide (VIP) or calcitonin gene-related peptide (CGRP) and by treatment with high concentrations of beraprost, a stable analog of prostaglandin I2, but was unaffected by CGRP or VIP receptor antagonists. Relaxant responses to a low concentration of nicotine (5 x 10(-6) M) were abolished by L-NA and restored by L-arginine. Histochemical study demonstrated many nerve fibers and bundles containing NADPH diaphorase in the adventitia of the arteries. It is concluded that the neurogenic arterial contraction is induced mainly by ATP via stimulation of P2X purinoceptors, and that the relaxation induced by electrical stimulation or a low concentration of nicotine is mediated by nitric oxide (NO) released from perivascular nerves. In high concentrations, nicotine elicits marked relaxations possibly due to the liberation of NO from the nerve and also vasodilator substances that increase the content of cyclic AMP in the tissue. CGRP and VIP are unlikely to be involved.

Monkey corpus cavernosum relaxation mediated by NO and other relaxing factor derived from nerves

Isolated monkey corpus cavernosum muscle strips contracted with prostaglandin F2 alpha and treated with prazosin responded to transmural electrical stimulation with frequency-related relaxations that were abolished by tetrodotoxin. The nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine (L-NNA) significantly attenuated but did not abolish the response; L-arginine reversed the inhibition. The neurogenic relaxation was not influenced in the strips treated with atropine or calcitonin gene-related peptide (CGRP)-(8-37), a CGRP-receptor antagonist, and those desensitized to vasoactive intestinal polypeptide (VIP) or pituitary adenylate cyclase-activating polypeptide (PACAP). Nerve fibers containing NADPH diaphorase were histochemically demonstrated in cavernous tissues. The relaxant response resistant to the NO synthase inhibitor was abolished by high K+ and tetrabutylammonium but was unaffected by glibenclamide, charybdotoxin, apamin, ouabain, SKF-525a, a cytochrome P-450 inhibitor, and oxyhemoglobin. It is concluded that neurogenic relaxations of monkey corpus cavernosum muscle is associated partly with NO released as a neurotransmitter and that other relaxing factor(s) possibly responsible for K+ channel opening also participates; however, the type of K+ channel involved is not determined. Acetylcholine, VIP, CGRP, PACAP, and the Na+ pump do not seem to be involved in the neurogenic relaxation.

Mechanisms underlying the neurogenic relaxation in dog isolated hepatic arteries

In canine hepatic arterial strips responding to nicotine with contraction, prazosin abolished the response or reversed it to a relaxation. Mechanisms underlying the relaxation were analyzed in hepatic and coronary arterial strips denuded of the endothelium and treated with prazosin and indomethacin. In the hepatic arterial strips precontracted with prostaglandin (PG) F2alpha, nicotine-induced relaxations were not influenced by atropine but were inhibited by timolol and abolished by hexamethonium. Treatment with [8-37] calcitonin gene-related peptide ([8-37] CGRP), a selective CGRP1-receptor antagonist, also attenuated the nicotine-induced relaxation, but a vasoactive intestinal polypeptide antagonist was without effect. Combined treatment with timolol and [8-37] CGRP depressed the response to a greater extent than either antagonist alone. The slight relaxation remaining under the combined treatment was abolished by NG-nitro-L-arginine (L-NA) and restored by L-arginine. In coronary arterial strips precontracted with PGF2alpha, nicotine produced a moderate relaxation, which was abolished or markedly inhibited by treatment with hexamethonium or timolol but was unaffected by L-NA. It is concluded that the nicotine-induced relaxation is mediated by norepinephrine, CGRP, and NO released from perivascular nerves in dog hepatic arterial strips; the responses associated with activations of beta-adrenoceptors and CGRP1 receptors are predominant over those to NO. The coronary arterial relaxation seems to be mediated by neurogenic norepinephrine but not by NO.