Electrophysiological analysis of neurogenic vasodilatation in the isolated lingual artery of the rabbit

Neurally released ATP mediates endothelium-dependent hyperpolarization in the circular smooth muscle cells of chicken anterior mesenteric artery

The object of the present study was to clarify the neurotransmitter(s) controlling membrane responses to electrical field stimulation (EFS) in the circular smooth muscle cells of first-order branches of chicken anterior mesenteric artery.EFS (five pulses at 20 Hz, 1 ms) evoked a hyperpolarization of amplitude--21.6+/-1.2 mV, total duration 21.8+/-1.2 s and latency 641.7+/-81.9 ms. The response was tetrodotoxin-sensitive and nonadrenergic noncholinergic (NANC) in nature. The NANC response was blocked by the nonspecific purinergic antagonist, suramin, indicating that the response is mediated by the neurotransmitter adenosine 5'-triphosphate (ATP). Either desensitization or blockade of P2Y receptor with its putative agonist 2-methylthioATP (1 microM for 30 min) or with its antagonist cibacron blue F3GA (10 microM), respectively, abolished the purinergic hyperpolarization. PPADS at concentrations up to 100 microM had no effect on the EFS-induced response, indicating that this response is mediated through P2Y, but not P2X, receptor. In addition, the response was completely abolished by two specific P2Y1 receptor antagonists, namely, MRS 2179 (300 nM) and A3P5PS (10 microM). Removal of the endothelium abolished the purinergic hyperpolarization, which was converted, in some preparations, to a small depolarization, indicating that the hyperpolarizing response is endothelium-dependent. The present study suggests that in first-order branches of chicken anterior mesenteric artery, ATP released from perivascular nerves may diffuse to the endothelium-activating P2Y1 receptor to induce release of an inhibitory substance that mediates hyperpolarization in the circular smooth muscle.

Cholinesterase activity in human pulmonary arteries and veins: correlation with mRNA levels

Isolated intact human pulmonary arteries and veins were used to determine the acetylcholinesterase (AChE) or butyrylcholinesterase (BChE) activities in the absence or presence of two selective cholinesterase (ChE) inhibitors, iso-OMPA or BW284c51, respectively. These results were compared with the mRNA levels for each enzyme in human pulmonary vessels. Total ChE activities measured in presence of acetylthiocholine (ACTI, 1 mM) in intact vascular preparations were 45+/-04 and 114+/-07 mU/g tissue in human pulmonary arteries (n=14) and veins (n=14), respectively. These activities were completely abolished in presence of 10 microM neostigmine. In both types of vessels AChE and BChE activities were observed. These activities were at least 2-fold higher in human pulmonary veins when compared with arteries and were correlated with the accumulation of the corresponding transcripts (n=8). In each type of vessel, similar total ChE activities were detected in homogenized and intact preparations, while in human bronchial preparations this activity was 5-fold higher in homogenates than in intact preparations. Together these results provide evidence that the ChE activities in human pulmonary vessels may be extracellular and that the higher activity measured in veins as compared to arteries was associated with the differential accumulation of the corresponding transcripts.

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.

Medial prefrontal cortex acetylcholine injection-induced hypotension: the role of hindlimb vasodilation

The injection of acetylcholine (ACh) into the cingulate region of the medial prefrontal cortex (MPFC) causes a marked fall in arterial blood pressure which is not accompanied by changes in heart rate. The purpose of the present study was to investigate the hemodynamic basis for this stimulus-induced hypotension in Sprague-Dawley rats. The study was designed to determine whether a change in the vascular resistance of hindlimb, renal or mesenteric vascular beds contributes to the fall in arterial pressure in response to ACh injection into the cingulate cortex. Miniature pulsed-Doppler flow probes were used to measure changes in regional blood flow and vascular resistance. The results indicated that the hypotensive response was largely due to a consistent and marked vasodilation in the hindlimb vascular bed. On this basis, an additional experiment was then undertaken to determine the mechanisms that contribute to hindlimb vasodilation. The effect of interrupting the autonomic innervation of one leg on the hindlimb vasodilator response was tested. Unilateral transection of the lumbar sympathetic chain attenuated the cingulate ACh-induced vasodilation in the ipsilateral, but not in the contralateral hindlimb. These results suggest that the hypotensive response to cingulate cortex-ACh injection is caused by skeletal muscle vasodilation mediated by a sympathetic chain-related vasodilator system.

Sympathetic control of arterial membrane potential by ATP-sensitive K(+)-channels

Stimulation of perivascular nerve terminals leads to a release of various neurotransmitters such as norepinephrine, epinephrine, acetylcholine, nitric oxide, and calcitonin gene-related peptide (CGRP). Because some of these substances have been shown to cause smooth muscle hyperpolarization by direct or endothelium-dependent mechanisms, we hypothesized that the liberation of 1 or more of these transmitters may lead to neurogenic hyperpolarization in arterial muscle cells. The present study was designed to determine the presence or absence of neurogenic hyperpolarization and, if present, its underlying mechanisms in isolated rat mesenteric resistance arteries, through the use of conventional microelectrode techniques. The experiments were performed under the combined blockade of alpha-adrenoceptors and purinoceptors with phentolamine and suramin to eliminate depolarizing responses to nerve stimulation. Under these conditions, perivascular nerve stimulation (5 Hz, 30 seconds) evoked smooth muscle hyperpolarization (-3.3+/-0.3 mV, n=15), which was abolished by tetrodotoxin, indicating the neurogenic origin of the response. This neurogenic hyperpolarization was resistant to atropine, nitro-L-arginine, or CGRP8-37, a CGRP antagonist, but was abolished by guanethidine and beta-blocker propranolol. This hyperpolarization was also abolished by glibenclamide, an ATP-sensitive K(+) channel (K(ATP)) blocker, but was unaffected by apamin, a Ca(2+)-activated K(+) channel blocker. In separate experiments, exogenous norepinephrine caused glibenclamide-sensitive hyperpolarization in the presence of phentolamine. On the other hand, norepinephrine-induced depolarization in the absence of phentolamine was enhanced by propranolol. These findings suggest that neurally released catecholamines cause membrane hyperpolarization through the activation of K(ATP) by beta-adrenoceptors. Such hyperpolarization may play an important role in the control of arterial membrane potential by opposing alpha-adrenergic depolarization.

ATP released from perivascular nerves hyperpolarizes smooth muscle cells by releasing an endothelium-derived factor in hamster mesenteric arteries

1. The interaction between perivascular nerves and endothelium was investigated by measuring the changes in smooth muscle membrane potentials using intracellular microelectrode techniques in hamster mesenteric thin (100-150 microm) and thick (300-350 microm) arteries. 2. In both arteries, nerve stimulation evoked excitatory junction potentials (EJPs) which were strongly inhibited by pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) (0.5-5 microM). This result indicated that the EJPs were induced by the activation of P2X receptors. 3. Transient hyperpolarizations were evoked by trains of pulses at 20 Hz in PPADS (5 microM)-pre-treated thin arteries, but not in the thick arteries. ATP (100 microM) applied to adventitial surfaces mimicked the hyperpolarizations. Both the ATP- and nerve stimulation-induced hyperpolarizations were blocked by cibacron blue F3GA (2-100 microM) and were also abolished after endothelium removal, indicating that the neurally released ATP evoked transient hyperpolarization through the activation of P2Y receptors located on the endothelium. 4. In endothelium-intact preparations, intimal application of uridine 5'-triphosphate (UTP 100 microM), a P2Y2-like receptor agonist, but not 2-methylthio ATP (7 microM), hyperpolarized the smooth muscle. The UTP-induced hyperpolarization was significantly inhibited by cibacron blue F3GA and was abolished after endothelium removal. 5. These results suggest that ATP released from the perivascular nerves may reach the endothelium and activate P2Y2-like receptors to induce the release of an endothelium-derived hyperpolarizing factor in thin arteries.

NO mediates postjunctional inhibitory effect of neurogenic ACh in guinea pig small intestinal microcirculation

The present study was designed to evaluate the role of the endothelium as an effector organ of neurally mediated inhibition of vascular tone. Acetylcholine (ACh), either released by stimulation of the submucosal ganglia or applied exogenously, inhibited phenylephrine (PE)-induced constrictions in arterioles of the guinea pig intestinal submucosa. N(G)-monomethyl-L-arginine (L-NMMA), an inhibitor of nitric oxide (NO) synthesis, attenuated the response to superfused ACh by 74% compared with 94% attenuation obtained with N(G)-nitro-L-arginine (L-NNA). L-NNA attenuated the response to neurally released ACh by 98% and that to iontophoretically applied ACh by 92%. L-Arginine reversed the effects of both L-NMMA and L-NNA. Functional integrity of the endothelium was essential for the neurally mediated inhibition of PE-induced constrictions. However, neurogenic inhibition of neurally evoked constrictions was preserved despite endothelial disruption. It was concluded that at the postjunctional level, the mechanism of action of neurally released ACh was almost exclusively via a NO-dependent pathway, with the source of NO being the vascular endothelium.

Effects of K+Channel blockers on acetylcholine-induced vasodilation in guinea-pig choroid

The purpose of this study is to clarify which K+channels contribute to the acetylcholine (ACh)-induced vasodilation from the diameter changes in arterioles of the guinea-pig choroid. The choroid was isolated from the guinea-pig eyeball, pinned flat on a silicone rubber plate and superfused with warmed oxygenated (35 degrees C) Krebs solution. Diameters of choroidal arterioles were measured using video microscopy and a computer program for analysis. The effects of K+channel inhibitors (glibenclamide, tetraethylammonium [TEA], apamin and charybdotoxin [ChTX]) on the ACh-induced vasodilation were examined in arterioles which had been constricted by either norepinephrine (NE) or high K+solution. In NE (10(-5)m)-constricted arterioles, the combination of nitroarginine (10(-4)m) and indomethacin (10(-5)m) reduced ACh (10(-6)m)-induced vasodilatation by 24%. When high K+solution was used to constrict the arterioles, ACh-induced vasodilation was abolished by nitroarginine and indomethacin. In the presence of nitroarginine and indomethacin, the ACh-induced dilatation of NE-constricted arterioles was attenuated by TEA (10(-3)m), apamin (10(-7)m), and ChTX (10(-7)m) but not by glibenclamide (2x10(-5)m). Simultaneous application of apamin and ChTX inhibited the ACh (10(-6)m)-induced dilatation by 85%. In arterioles of guinea pig-choroid, nitric oxide and prostacyclin are not main mediators in ACh-induced vasodilation. Simultaneous activation of a set of Ca2+-sensitive K+channels may take most part of ACh-induced vasodilation.

Mechanisms underlying ACh induced modulation of neurogenic and applied ATP constrictions in the submucosal arterioles of the guinea-pig small intestine

1. Role of the vascular endothelium in acetylcholine (ACh) induced modulation of neurogenic and applied ATP (adenosine 5'-triphosphate) constrictions of intestinal submucosal arterioles was investigated. 2. Arteriole constrictions, induced either by exogenous ATP or evoked by perivascular nerve stimulation, were attenuated in the presence of ACh. 100 nM ACh almost completely abolished neurogenic constrictions whereas up to 10 microM ACh reduced constrictions to exogenous ATP by only about 60%. 3. Treatment of the arterioles with 100 microM Nomega-nitro-L-arginine (NOLA) and 5 microM indomethacin, to block respectively nitric oxide (NO) and prostanoid release from the endothelium, had no effect on the ACh induced inhibition of neurogenic constrictions but significantly attenuated the inhibitory effects of ACh on constrictions to exogenous ATP. 4. Disruption of the vascular endothelium had no effect on the ACh induced inhibition of neurogenic constrictions but attenuated the inhibitory effects of ACh on applied ATP constrictions to the same extent as after treatment with NOLA and indomethacin. In comparison, endothelial disruption completely abolished the inhibitory effect of substance P (SP) on exogenously applied ATP constrictions. 5. 50 nM ACh significantly attenuated the amplitude of neurally evoked excitatory junction potentials (ejps) recorded from the vascular smooth muscle without altering the time constant of decay (taudecay) of the ejps. 6. It is concluded that ACh inhibits neurogenic constrictions by prejunctional modulation of transmitter release from the perivascular sympathetic nerves with no major role for endothelial paracrine factors. 7. Endothelial NO and/or prostanoids mediate some of the ACh induced inhibition of constrictions to exogenous ATP whereas the endothelium independent inhibitory effects of ACh are attributed to a direct action of ACh on the vascular smooth muscle. However, an indirect effect resulting from activation of vasodilator nerves cannot be ruled out.

Neuroeffector transmission in arterioles of the guinea-pig choroid

1. Using conventional microelectrode techniques, membrane potentials were recorded from smooth muscle cells of guinea-pig choroidal arterioles. 2. Transmural stimulation initiated excitatory junction potentials (EJPs) which were abolished by either guanethidine or alpha,beta-methylene-ATP but not by phentolamine, indicating that they resulted from activation of purinoceptors. 3. Trains of stimuli evoked EJPs which were followed by a slow depolarization, an inhibitory junction potential (IJP) or a biphasic membrane potential change which consisted of an IJP and a subsequent slow depolarization. 4. Slow depolarizations were abolished by either phentolamine or guanethidine, indicating that they resulted from activation of alpha-adrenoceptors. 5. IJPs were abolished by atropine but not by guanethidine, and were reduced by 50 % by apamin with the residual response being abolished by charybdotoxin, indicating that they resulted from the activation of muscarinic receptors which open two sets of Ca2+-activated K+ channels. 6. Most responses were followed by slow hyperpolarizations. These were almost abolished by L-nitroarginine, an effect which was partly overcome by L-arginine, and were abolished by glibenclamide, indicating that they resulted from the release of NO and activation of ATP-sensitive K+ channels. 7. Immunohistochemical analysis showed that arterioles were densely innervated by adrenergic nerve fibres. A population of fibres, likely to be cholinergic, was also identified. Furthermore, populations of nerve fibres immunoreactive to antibodies against either nitric oxide synthase (NOS) or substance P (SP) were also identified. 8. These findings indicate that choroidal arterioles of the guinea-pig are innervated by at least three different populations of nerves, adrenergic nerves which evoke excitatory responses, cholinergic nerves which evoke inhibitory responses and a population of nerves which cause the release of NO.

K+ channels which contribute to the acetylcholine-induced hyperpolarization in smooth muscle of the guinea-pig submucosal arteriole

1. Membrane potentials were recorded from submucosal arterioles (diameter, 30-80 microns) of the guinea-pig small intestine, using conventional microelectrode techniques. In control solution the resting membrane potential was about -73 mV, and the addition of 0.5 mM Ba2+ depolarized the membrane to about -43 mV. 2. ACh (10 nM to 10 microM), or substance P (0.1 microM), caused a membrane hyperpolarization in preparations which had been depolarized by Ba2+ but not in control preparations. ACh produced a sustained hyperpolarization, whereas substance P produced a transient hyperpolarization, without being affected by either nitroarginine (0.1 mM) or indomethacin (10 microM). 3. In the presence of 50 microM BAPTA (acetoxymethyl ester form), the membrane potentials were not altered in the control solution or in the presence of Ba2+, but Ba2+ caused a smooth depolarization of the membrane. Following this procedure, both ACh and substance P caused membrane depolarization instead of hyperpolarization, suggesting that the ACh- and substance P-induced hyperpolarization in arteriolar smooth muscle are intracellular [Ca2+] dependent. 4. In short segments (200-500 microns) of arteriole, the time constant of electrotonic potentials produced by passing current pulses through the recording electrode was about 75 ms. The addition of Ba2+ increased both the input resistance and the time constant. 5. The hyperpolarizations produced by ACh or substance P were associated with a reduction in the amplitude and the time constant of electrotonic potential. 6. The reversal potential for the ACh-induced hyperpolarization, estimated from the current-voltage relationship, was about -86 mV, a value close to the equilibrium potential for K+. 7. In the presence of 50 nM charybdotoxin the hyperpolarization produced by ACh became transient and was reduced in amplitude: the residual response was further reduced by apamin (0.1 microM). The response produced by substance P was also reduced by 50 nM charybdotoxin: again the residual response was sensitive to 0.1 microM apamin. The hyperpolarizations produced by either ACh or substance P were insensitive to glibenclamide (10 microM) and 4-aminopyridine (1 mM). 8. It is suggested that in submucosal arterioles of the guinea-pig ileum, ACh- or substance P-induced hyperpolarizations of smooth muscle result from activation of both charybdotoxin-sensitive and apamin-sensitive K+ channels, with the former being predominant. The results are discussed in relation to the possible involvement of one or more endothelium-dependent hyperpolarizing factors in ACh- and substance P-induced hyperpolarization.

Effects of zaprinast and dissolved nitric oxide on the pulmonary circulation of fetal sheep

This study was designed to determine indirectly if the changes in ovine fetal pulmonary vascular tone caused by i.v. injections of nitric oxide-containing solutions are mediated by cGMP. We first characterized the dose-response relationship of bolus intrapulmonary injections of zaprinast (a cGMP-selective phosphodiesterase inhibitor) and nitric oxide solutions. Injections of nitric oxide solutions as well as zaprinast solutions resulted in dose-dependent decreases in pulmonary arterial pressure that were greater than reductions in systemic arterial pressure. We also evaluated the effects of simultaneous infusions of zaprinast and U46619 (a thromboxane mimetic) on the response to bolus injections of 1.0 micrograms of acetylcholine, 100 ng of endothelin-1, and 10.0 microL of ethanol saturated with nitric oxide. Zaprinast was infused at a rate of 1.5 mg/min, and the concentration of U46619 was titrated to raise mean left pulmonary arterial pressure (LPAP) to the steady state level that was present before infusing zaprinast. All bolus injections reduced left pulmonary arterial pressure more than they reduced mean systemic arterial pressure. However, neither the response magnitudes nor the response durations were affected by simultaneous infusions of zaprinast and U46619. We therefore suggest that modulation of fetal pulmonary vascular tone by endogenously produced nitric oxide may involve mechanisms other than raising smooth muscle cytoplasmic cGMP concentrations.

Vasodilatation and smooth muscle membrane potential changes in arterioles from the guinea-pig small intestine

1. Dilatation of arterioles isolated from the guinea-pig small intestine was evoked by stimulation of a submucous ganglion and the application of acetylcholine, vasoactive intestinal peptide, galanin or dynorphin A. Changes in arteriole diameter and smooth muscle membrane potential were recorded simultaneously. 2. Ganglion stimulation caused vasodilatation and smooth muscle hyperpolarization that varied in both amplitude and time course from one arteriole to another. Vasodilatation could occur without hyperpolarization. 3. Vasodilatation caused by acetylcholine was accompanied by a rapidly developing hyperpolarization that began to decline before the maximum vasodilator effect had developed. 4. Vasoactive intestinal peptide caused dilatation without any change in smooth muscle membrane potential. 5. Galanin and dynorphin caused dilatation and a hyperpolarization of similar time course to the dilatation. 6. In 48% of arterioles tested the dilatation appeared to be mediated solely by acetylcholine. In 31% there was a cholinergic component, but no evidence for the involvement of acetylcholine in the remaining 21%. When the non-cholinergic dilatation occurred without a hyperpolarization we conclude that it was due to vasoactive intestinal peptide; otherwise it may have been due to either galanin or dynorphin.

Requirement for endothelium-derived nitric oxide in vasodilation produced by stimulation of cholinergic nerves in rat hindquarters

1. We aimed to determine whether nitric oxide (NO) and/or the endothelium is involved in cholinergic neurogenic vasodilatation in the rat isolated hindquarters. 2. The abdominal aorta was cannulated for perfusion of the rat hindquarters with Krebs bicarbonate solution containing phenylephrine, to induce basal constrictor tone. In the presence of noradrenergic neurone blockade with guanethidine (200 mg kg-1, i.p.) electrical stimulation of peri-aortic nerves induced frequency-dependent decreases in hindquarters perfusion pressure, indicating vasodilatation. Both the endothelium-dependent vasodilator, acetylcholine (ACh) and the endothelium-independent vasodilator, sodium nitroprusside (SNP) induced dose-dependent decreases in perfusion pressure. In each experiment, responses to either nerve stimulation, ACh or SNP were recorded before and after treatment with saline vehicle, atropine (1 microM), NG-nitro-L-arginine (L-NOARG, 100 microM), L-arginine (1 mM), L-arginine plus L-NOARG, or 3-3 cholamidopropyl dimethylammonio 1-propanesulphonate (CHAPS, 30 mg). Hindquarters dilatation after each treatment was expressed as a percentage of the control response. 3. Following treatment with saline, responses to nerve stimulation and ACh were 99 +/- 9% and 107 +/- 10% of control, respectively demonstrating the reproducibility of these responses. Nerve stimulation-induced dilation was abolished by atropine (0 +/- 0% of control, P < 0.05) or reduced to 14 +/- 10% of control by NO synthase inhibition with L-NOARG (P < 0.05). Dilator responses to ACh were also abolished by atropine (0 +/- 0% of control, P < 0.05) or inhibited by L-NOARG (59 +/- 10% of control, P < 0.05), indicating that the neurogenic dilatation is cholinergic and is mediated by NO. The administration of the NO precursor, L-arginine, prevented the inhibitory effect of L-NOARG on dilator responses to nerve stimulation and ACh (L-arginine plus L-NOARG: 89 +/- 13% and 122 +/- 24% of control, respectively). In addition CHAPS, which removes endothelial cells, inhibited responses to both nerve stimulation (0 +/- 0% of control, P <0.05) and ACh (33 +/- 8% of control, P <0.05). In contrast,no treatment significantly reduced the vasodilator responses to SNP.4. These observations suggest that cholinergic neurogenic vasodilatation in the rat isolated hindquarters requires the synthesis and release of NO from the endothelium.

Acetylcholine-induced endothelium-independent relaxations in monkey isolated superior and inferior caval veins

1. We examined the effects of acetylcholine (ACh), isoprenaline (Isop) and Ca-ionophore, A23187 on monkey isolated superior (SCV) and inferior caval veins (ICV) with and without intact endothelium, which had been partially contracted by 2 x 10(-6)-5 x 10(-6) M prostaglandin F2 alpha (PGF2 alpha). 2. Low concentrations of ACh (10(-10)-10(-9) M) produced a dose-dependent relaxation in the precontracted venous segments with endothelium. ACh at concentrations more than 10(-7) M elicited a transient contraction followed by a relaxation in these segments. 3. An addition of 5 x 10(-7) M A 23187 induced about 60% of maximum relaxation produced by 10(-5) M sodium nitroprusside (SNP) in each venous segment with endothelium. 4. Isop (10(-10)-10(-5) M) caused a dose-related relaxation in the precontracted caval veins with intact endothelium. 5. Removal of endothelium caused no significant effect on the ACh-induced dual responses but a significant inhibition of the A23187-induced relaxation. 6. Pretreatment with atropine antagonized competitively the ACh-induced relaxations in the endothelium-intact and endothelium-denuded caval veins. The Schild plot analysis showed that the pA2 values of the segments with and without endothelium were 9.72 +/- 0.14 (n = 5) and 10.01 +/- 0.23 (n = 6) in the ICV; and 9.95 +/- 0.20 (n = 5) and 9.70 +/- 0.10 (n = 5) in the SCV, respectively. 7. Pretreatment with 5 x 10-5M aspirin, 3 x 10-5M N0-nitro-L-arginine methylester, 1 mM tetraethylammonium,or 3 x 10-6 M glibenclamide caused no significant effect on the basal tone, ACh induced transient contraction, and ACh;.induced relaxation in the precontracted venous segments with and without endothelium.8. Pretreatment with 10-5 M methylene blue produced a significant reduction of the ACh- and SNP induced relaxations in the precontracted venous segments with and without endothelium. The pretreatment with the same concentration of methylene blue, however, caused no significant effect on the Isop-induced relaxation in venous segments with endothelium.9. The results suggest that ACh acts directly on the venous smooth muscle cells via a high-affinity muscarinic receptor subtype to accumulate cellular cyclic GMP producing endothelium-independent relaxation in the monkey caval veins.

A study of cholinergic and beta-adrenergic components in the regulation of blood flow in the tooth pulp and gingiva in man

In 10 subjects, laser Doppler flowmetry was used to study whether cholinergic or beta-adrenergic pathways are involved in the control of tooth pulp blood flow (PBF) in response to isometric hand grip and the cold pressor test. We also examined if differences exist between the regulation of blood flow in the tooth pulp and the nearby gingiva (GBF). Isometric hand grip (35% of maximum force, 2 min) and the subsequent ischaemia (2 min) induced a brief rise in PBF and a more long-lasting rise in GBF. Atropine increased heart rate about by 40% and changed the pulpal response to a fall in flow, without altering gingival flow. Propranolol, causing a 20% reduction in heart rate, had no effect on either flow during the actual test, but induced a rise in GBF after the ischaemic period. The cold pressor test (2 min at 0.5 degrees C) resulted in a reduction in PBF and GBF, unaffected by the blocking drugs. With atropine, however, PBF increased immediately after this test. The relative changes in arterial pressure and heart rate were unaffected by the drugs. Our study has demonstrated the existence of cholinergic nervous vasodilation in vessels serving the tooth pulp. Non-adrenergic non-cholinergic mechanisms probably contribute to the evoked rise in GBF during exercise. Beta-adrenoceptors are involved in the control of GBF immediately after isometric exercise. While the two tests under control conditions evoked mostly parallel changes in PBF and GBF, the use of blocking agents showed that blood flow is controlled by different mechanisms in the two adjacent vascular beds.

Muscarinic receptors--characterization, coupling and function

At least five muscarinic receptor genes have been cloned and expressed. Muscarinic receptors act via activation of G proteins: m1, m3 and m5 muscarinic receptors couple to stimulate phospholipase C, while m2 and m4 muscarinic receptors inhibit adenylyl cyclase. This review describes the localization, pharmacology and function of the five muscarinic receptor subtypes. The actions of muscarinic receptors on the heart, smooth muscle, glands and on neurons (both presynaptic and postsynaptic) in the autonomic nervous system and the central nervous system are analyzed in terms of subtypes, biochemical mechanisms and effects on ion channels, including K+ channels and Ca2+ channels.

Acetylcholine released from guinea-pig submucosal neurones dilates arterioles by releasing nitric oxide from endothelium

1. The role of the endothelium as an effector of the neurogenic cholinergic vasodilatation in submucosal arterioles of the guinea-pig ileum was investigated by measuring changes in arteriolar diameter in response to exogenous application of muscarine or electrical stimulation of the submucosal ganglia. 2. NG-Monomethyl-L-arginine (L-NMMA), an inhibitor of nitric oxide (NO) synthesis, competitively inhibited the vasodilatation produced by muscarine in arterioles which had been preconstricted with the prostaglandin analogue U46619. L-Arginine (10 mM), but not D-arginine (10 mM), prevented the inhibition by L-NMMA. 3. Neither tetrodotoxin (TTX, 1 microM), nor the cyclo-oxygenase inhibitor, indomethacin (10 microM), altered the muscarinic vasodilatation or the inhibitory effect of L-NMMA. 4. Sodium nitroprusside (SNP), an activator of the soluble guanylate cyclase, dilated the arterioles in a concentration-dependent manner. This vasodilatation was unaffected by L-NMMA but was abolished by the guanylate cyclase inhibitor, methylene blue (10 microM). In addition, methylene blue antagonized the muscarinic vasodilatation to a similar degree as did L-NMMA. 5. The vasodilatation produced by ganglionic stimulation (10 Hz, 10 s) was blocked by TTX and the muscarinic receptor antagonist, 4-diphenylacetoxy-N-methyl-piperidine methiodide (4-DAMP, 1 microM). The neurally evoked vasodilatation was inhibited by 70% in the presence of L-NMMA; this inhibition was prevented by L-arginine. Methylene blue inhibited the neurogenic vasodilatation to the same extent as did L-NMMA. 6. These results show that arteriolar vasodilatation by muscarine is mediated mainly through the release of NO formed from L-arginine; the origin of the L-arginine appears to be the endothelium. These results also demonstrate that acetylcholine released from submucosal nerves onto submucosal blood vessels reaches the endothelium to cause the release of NO formed from L-arginine; the endothelial-derived NO dilates the arteriole.

Properties of sympathetic neuromuscular transmission and smooth muscle cell membranes in vascular beds

In vascular smooth muscle tissues, the cycle of contraction-relaxation is mainly regulated by the cytosolic Ca, and many other factors, such as substances released from endothelial cells and perivascular nerve terminals (mainly sympathetic nerves). In this article, we introduce regional differences in specific features of ionic channels in vascular smooth muscle membranes (mainly on features of Ca, Na and K channels) in relation to mobilization of the cytosolic Ca. In many vascular tissues, neurotransmitters released from sympathetic nerve terminals activate post-junctional receptors, and subsequently modify ion channels (receptor-activated cation channel and voltage-dependent Ca channel), whereas in some tissues, ionic channels are not modified by receptor activations (pharmaco-mechanical coupling). However, activation of receptors, with or without modulation of ionic channels, regulates the cytosolic Ca through synthesis of second messengers. In addition, receptors distributed on prejunctional nerve terminals positively or negatively regulate the release of transmitters. Roles of neurotransmitters (mainly ATP and noradrenaline) are also discussed in relation to the generation of excitatory junction potentials.

Vasodilatation of arterioles by acetylcholine released from single neurones in the guinea-pig submucosal plexus

The nervous control of arterioles in the guinea-pig submucosal plexus was studied. Outside diameters of arterioles were recorded using a video-monitoring system. Changes in arteriolar diameter in response to electrical stimulation of single neurones or ganglia in the plexus were measured. 2. When the arteriole was pre-constricted with the prostaglandin analogue U46619 or with phenylephrine, electrical stimulation (2-20 Hz, 10 s) of a ganglion dilated the blood vessel. This vasodilatation was abolished by tetrodotoxin or by cutting the fine nerve strands running between the ganglion and the arteriole. 3. The vasodilatations caused by ganglionic stimulation were blocked by the muscarinic antagonists atropine, pirenzepine, (11[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5,11-dihydro-6H- pyrido[2,3-b][1,4]benzodiazepine-6-)-one (AFDX-116), 4-diphenylacetoxy-N-methyl-piperidine methiodide (4-DAMP) and hexahydrosilodifenidol (HSDF). IC50 values for the inhibition of nerve-evoked vasodilatation by pirenzepine, AFDX-116 and HSDF were 500 nM, 4 microM and 25 nM respectively. Physostigmine (1 microM) increased the dilatation by 90%. 4. Muscarine dilated all submucosal arterioles; the concentration causing half-maximum effects was 200 nM. Muscarinic vasodilatations were inhibited by pirenzepine, AFDX-116, and HSDF in a competitive manner; dissociation equilibrium constants determined by Schild analyses were 125 nM, 1.3 microM and 4 nM respectively. 5. Gossypol, an irreversible inhibitor of the production of endothelium-derived relaxing factor (EDRF), did not reduce the vasodilatation produced by either ganglionic stimulation or muscarine in submucosal arterioles. 6. Intracellular recordings were made from submucosal neurones and action potentials were elicited by depolarizing current pulses (10 ms duration, 10 Hz/10 s). In seven neurones vasodilatation was associated with intracellularly evoked action potentials; this vasodilatation was blocked by pirenzepine. Cell bodies of reidentified vasodilator neurones were subsequently shown to contain immunoreactive choline acetyltransferase. 7. These results show that cholinergic neurones in the submucosal plexus project to submucosal arterioles and that they release acetylcholine onto muscarinic receptors to produce vasodilatation. The muscarinic receptor activated by nerve-released acetylcholine is the M3 subtype and its location appears to be on the vascular smooth muscle rather than the endothelium.

Dihydropyridine inhibition of single calcium channels and contraction in rabbit mesenteric artery depends on voltage

1. The effects of membrane potential and the dihydropyridine calcium channel inhibitor, nisoldipine, on single calcium channels in the presence of Bay K 8644 and contraction in the presence and absence of Bay K 8644 were examined in the rabbit mesenteric artery. 2. Membrane depolarization decreased the peak average single calcium channel current that could be elicited by a test pulse to 0 mV. The steady-state inactivation relationship could be described by the Boltzmann equation, [1 + exp[Vm-V0.5)/k)]-1, with a steepness factor, k, of 7.1 mV. Nisoldipine shifted the steady-state inactivation curve to more negative potentials by increasing the fraction of test pulses without openings. 3. The degree of nisoldipine inhibition of average single calcium channel currents increased with membrane depolarization. Depolarization of the holding potential from -100 to -55 mV decreased the concentration of nisoldipine needed for 50% inhibition (Kapp) from 12.1 to 1.9 nM in the presence of 1 microM-Bay K 8644. 4. Membrane depolarization by external potassium (K+) of the intact artery in the presence of nisoldipine decreased contractions evoked by depolarizing test pulses. The relationship between membrane potential and contraction could be empirically described by the Boltzmann equation, with a steepness factor, k, of 7.1 mV. Increasing the nisoldipine concentration from 0.25 to 2.0 nM shifted the mid-point of this relationship from -20.5 to -33.0 mV, without affecting the steepness factor. 5. Nisoldipine inhibition of contraction increased with membrane depolarization. Membrane depolarization from -68.6 to -30.0 mV decreased the Kapp of nisoldipine for contractions from 3.02 to 0.69 nM. Bay K 8644 (1 microM) elevated Kapp about 9.3-fold at 5 mM-K+. In the presence of Bay K 8644, membrane depolarization from -68.6 to -30.0 mV reduced Kapp from 28.4 to 4.0 nM. 6. In the presence of nisoldipine, the effect of membrane depolarization on the time course of development of inhibition was examined. In 3 nM-nisoldipine, after membrane depolarization with 20 mM-K+, the time course of development of inhibition of force could be described by a single exponential with a time constant of 16.5 min. Membrane depolarization to a more positive potential accelerated the development of inhibition. 7. The results were interpreted by a model in which nisoldipine binds with higher affinity to the inactivated state than to the resting state of calcium channels in the mesenteric artery. The approach presented here can be used to estimate the properties of steady-state calcium channel inactivation and dihydropyridine interactions in smooth muscle cells in the intact artery under physiological conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Some electrical properties of the endothelium-dependent hyperpolarization recorded from rat arterial smooth muscle cells

1. Electrical responses produced by acetylcholine (ACh) and histamine were recorded from smooth muscle cells of the intralobular small pulmonary artery (SPA), main pulmonary artery (MPA) and thoracic aorta of rats. 2. In MPA and SPA, ACh and histamine produced a transient hyperpolarization of the membrane, and the potential decayed exponentially with a time constant of 2-3 min. In aorta, ACh produced a sustained and histamine produced a transient hyperpolarization. 3. The ACh- and histamine-induced hyperpolarizations were blocked by atropine and mepyramine, respectively, or by removing the endothelial cells. 4. The amplitude of the hyperpolarization was increased in low [K+]o solutions and decreased in high [K+]o solutions. The ionic conductance of the membrane was increased during the hyperpolarization, suggesting an involvement of the increased potassium conductance. 5. A reproducible amplitude of hyperpolarization was generated when ACh or histamine was applied at intervals of over 10 or 30 min, respectively. 6. In aorta, after the transient hyperpolarization had ceased during continued application of histamine, ACh again produced a hyperpolarization, i.e. the transient nature of the hyperpolarization was not due to desensitization of the receptor upon which the hyperpolarizing substance acted, assuming histamine and ACh release the same hyperpolarizing substance. 7. ACh and histamine relaxed the tissues from SPA, MPA and aorta during the noradrenaline (NA)- or high [K+]o solution-induced contraction, in a concentration-dependent manner, only when the endothelial cells were intact. Both ACh and histamine were potent relaxants in MPA and aorta, but showed weak relaxing actions in SPA. 8. In aorta, ACh and histamine produced a sustained relaxation for up to 10 min, and Methylene Blue diminished and altered it to a transient relaxation (for histamine) or an initial large, followed by a small sustained (for ACh), relaxation. 9. In the presence of NA and NA plus Methylene Blue, ACh and histamine also produced a hyperpolarization similar to that seen in the control. 10. It is concluded that in arteries of the rat, ACh and histamine release a hyperpolarizing substance from the endothelial cells. This substance may be different from the endothelium-derived relaxing factor (EDRF), and is released mainly transiently. The hyperpolarization is generated by an increase in potassium conductance of the membrane, and this has some contribution to the endothelium-dependent relaxation.

Mechanisms of inhibitory noradrenergic transmission in the rabbit facial vein

In isolated buccal segment of the rabbit facial vein, electrical responses produced by perivascular nerve stimulation and exogenously applied noradrenaline (NA) were recorded from the smooth muscle cells using microelectrode. Perivascular nerve stimulation hyperpolarized the smooth muscle cell membrane. The hyperpolarization was converted to depolarization after application of the beta-adrenoceptor antagonist, propranolol, and the depolarization was blocked by alpha 2-adrenoceptor antagonists, yohimbine. These responses elicited by nerve stimulation were blocked by tetrodotoxin or guanethidine, but not by atropine. Exogenously applied NA mimicked the responses elicited by nerve stimulation. The amplitude of the beta-adrenoceptor-mediated hyperpolarization was increased in low potassium solution, decreased in high potassium solution, but unaltered by low sodium or low chloride solution, i.e., the hyperpolarization may be generated by an increase in potassium conductance of the membrane. An involvement of the apamin-sensitive (Ca-dependent) potassium channel or sodium-potassium ATPase in the hyperpolarization was ruled out.

Heterogeneous distribution of muscarinic receptors in the rabbit saphenous artery

1. The properties of the muscarinic receptors in the rabbit saphenous artery were determined from electrical and mechanical responses of smooth muscle cells produced by acetylcholine (ACh). The inhibitory action of atropine and pirenzepine on the ACh-induced responses was also studied. 2. ACh produced a transient hyperpolarization of the membrane and inhibited the noradrenaline (NA)-induced contraction. These effects of ACh were apparent only when the endothelial cells were intact. 3. The ACh-induced transient hyperpolarization was antagonized by atropine or pirenzepine, with similar potencies (the ID50 values were about 2 x 10(-8) M for both antagonists). 4. The ACh-induced inhibition of the contraction to NA was antagonized by atropine more preferentially than by pirenzepine (the ID50 values were 2 x 10(-8) M for atropine and 10(-6) M for pirenzepine). 5. The excitatory junction potential (e.j.p.) evoked by perivascular nerve stimulation was inhibited by ACh (above 10(-8) M). The ACh-induced inhibition of the e.j.p. was antagonized by atropine more preferentially than by pirenzepine (the ID50 values were 3 x 10(-8) M for atropine and 6 x 10(-6) M for pirenzepine). 6. It is concluded that in the rabbit saphenous artery, two subtypes of muscarinic receptor (M1 and M2) are located on the endothelial cells. Stimulation of each subtype releases a different substance, i.e., a hyperpolarizing substance (M1-subtype) or a relaxant substance (M2-subtype). In prejunctional nerve terminals, the muscarinic receptors responsible for inhibiting the release of transmitter substances are of the M2-subtype.

Atropine potentiates neurogenic vasodilatation of the feline infraorbital artery: possible mechanisms

After treatment with guanethidine to inactivate sympathetic nerves, the feline infraorbital artery (IOA) relaxes in response to activation of periarterial nerves in vitro. This response was 60-65% greater in magnitude and 50% longer in duration in the presence of atropine, thus revealing a significant non-adrenergic, non-cholinergic (NANC) dilator response which is potentiated by blockade of muscarinic receptors. Nerve-mediated dilations and the potentiating effect of atropine were endothelial cell-independent. In the presence of atropine the resting membrane potential (-51 +/- 2 mV) of infraorbital vascular smooth muscle cells was not changed by activation of nerves, nor by exogenously applied vasoactive intestinal polypeptide (VIP). Electrical stimulation caused release of VIP from this artery, but atropine did not measurably enhance the degree of release of VIP. Therefore, although presynaptic, muscarinic inhibition of release of a NANC transmitter probably occurs in the IOA, either VIP is not the transmitter involved in this response or the changes in release of VIP are too slight to be detected by the in vitro techniques employed in this study.