Differential release of [3H]acetylcholine from the rat phrenic nerve-hemidiaphragm preparation by electrical nerve stimulation and by high potassium

Acetylcholine released by endothelial cells facilitates flow-mediated dilatation

KEY POINTS

The endothelium plays a pivotal role in the vascular response to chemical and mechanical stimuli. The endothelium is exquisitely sensitive to ACh, although the physiological significance of ACh-induced activation of the endothelium is unknown. In the present study, we investigated the mechanisms of flow-mediated endothelial calcium signalling. Our data establish that flow-mediated endothelial calcium responses arise from the autocrine action of non-neuronal ACh released by the endothelium.

ABSTRACT

Circulating blood generates frictional forces (shear stress) on the walls of blood vessels. These frictional forces critically regulate vascular function. The endothelium senses these frictional forces and, in response, releases various vasodilators that relax smooth muscle cells in a process termed flow-mediated dilatation. Although some elements of the signalling mechanisms have been identified, precisely how flow is sensed and transduced to cause the release of relaxing factors is poorly understood. By imaging signalling in large areas of the endothelium of intact arteries, we show that the endothelium responds to flow by releasing ACh. Once liberated, ACh acts to trigger calcium release from the internal store in endothelial cells, nitric oxide production and artery relaxation. Flow-activated release of ACh from the endothelium is non-vesicular and occurs via organic cation transporters. ACh is generated following mitochondrial production of acetylCoA. Thus, we show ACh is an autocrine signalling molecule released from endothelial cells, and identify a new role for the classical neurotransmitter in endothelial mechanotransduction.

Counter effects of higenamine and coryneine, components of aconite root, on acetylcholine release from motor nerve terminal in mice

The counter effects of higenamine and coryneine, components of aconite root, on acetylcholine (ACh) release from motor nerve terminals in the mouse phrenic nerve-diaphragm muscle preparation were studied by a radioisotope method. Both nerve-evoked release and spontaneous release of [3H]-ACh from the preparation preloaded with [3H]-choline were measured. The change in the tetanic tension of muscle was simultaneously recorded in the same preparation. Higenamine (10 microM) augmented both the nerve-evoked and spontaneous ACh releases, and the muscle tension. The effects were inhibited by pretreatment with propranolol (10 microM), a beta-adrenoceptor antagonist. Coryneine reduced the nerve-evoked release of ACh, accelerated the decay of tetanic tension (tetanic fade) at 30 microM, and it depressed the peak amplitude of tetanic tension at a higher concentration of 100 microM. These results suggest that of the two components contained in aconite root, higenamine increases ACh release via activation of beta-adrenoceptor, and conversely coryneine depresses ACh release by preferentially acting at motor nerve terminal.

Aconitine-induced increase and decrease of acetylcholine release in the mouse phrenic nerve-hemidiaphragm muscle preparation

The effect of aconitine on acetylcholine (ACh) release from motor nerve terminals in the mouse phrenic nerve-diaphragm muscle preparation was studied by a radioisotope method. Both electrical stimulation-evoked release and spontaneous release of 3H-ACh from the preparation preloaded with 3H-choline were measured. The change in the muscle tension was simultaneously recorded in the same preparation. Aconitine (0.1 microM) increased electrically evoked 3H-ACh release, while at higher concentrations (0.3-3 microM) it decreased the evoked release and muscle tension. High concentrations of aconitine (3-30 microM) caused a concentration-dependent increase in spontaneous 3H-ACh release. All these effects were suppressed by tetrodotoxin. The aconitine-induced spontaneous release consisted of two different components: a Ca(2+)-dependent phasic release that was inactivated within a few minutes and a Ca(2+)-independent, long lasting release at a low level. The depression of the Ca(2+)-dependent quantal release seems attributable to the decline of Ca2+ influx into the nerve rather than inactivation of sodium channels. We conclude that aconitine increases and then decreases electrical stimulation-evoked ACh release from the motor nerve through prolonged activation of sodium channels. Further activation of the channels enhances spontaneous release and the subsequent complete inactivation of the quantal release may be due to block of Ca2+ influx.

Pharmacological studies on cutaneous inflammation induced by ultraviolet irradiation (1): quantification of erythema by reflectance colorimetry and correlation with cutaneous blood flow

This study was conducted to quantify the intensity of ultraviolet (UV) erythema in guinea pigs, a method for evaluating anti-inflammatory drugs, and to clarify any correlation of erythema with cutaneous blood flow. Skin color and cutaneous blood flow in non-administered and indomethacin-administered animals were measured by a colorimeter and a laser Doppler flowmeter over time after UV-irradiation treatment. Skin color was indicated by a XYZ colorimetric system and L*a*b* color space. In either colorimetric system, the values of two indices, x and y or a* and b*, increased along with the intensification of erythema. The increase in the chroma (C*) value calculated from a* and b* was UV-dose-dependent. This value was significantly suppressed by indomethacin 0.5-4 hr after irradiation, and it was found to be a clear and sensitive index for evaluating the suppressive effect of drugs. Cutaneous blood flow also increased with UV irradiation. Indomethacin significantly suppressed this increase 2-3 hr after UV irradiation. The changes of cutaneous blood flow correlated with those of C*. These results suggested C* was a suitable parameter to quantify UV erythema, and the change of skin color in UV erythema reflected the change of cutaneous blood flow.

Suppression by cholinesterase inhibition of a Ca(2+)-independent efflux of [3H]acetylcholine from the neuromuscular junction of the isolated rat diaphragm

Endplate preparations of the left rat hemidiaphragm were incubated with [3H]choline to label neuronal acetylcholine stores. Elevation of the concentration (13.5-135 mmol/l) of extracellular potassium chloride (KCl) stimulated the release of [3H]acetylcholine in a concentration-dependent manner. KCl (27 mmol/l) still caused a significant efflux of [3H]acetylcholine in a Ca(2+)-free medium. Inhibitors of cholinesterase (physostigmine, diisopropylfluorophosphate) suppressed by 80% this Ca(2+)-independent efflux of [3H]acetylcholine. Vesamicol (10 mumol/l), the blocker of the vesicular acetylcholine carrier, also suppressed the stimulated, Ca(2+)-independent efflux of [3H]acetylcholine. The inhibitory effect of physostigmine was not prevented by muscarine or nicotine receptor antagonists, but the inhibitory effect was lost when the stimulus strength was increased (81 mmol/l KCl). The present experiments showed cholinesterase inhibition to suppress a Ca(2+)-independent efflux of [3H]acetylcholine, probably by interference with a membrane-bound acetylcholine carrier.

Presynaptic A1-purinoceptor-mediated inhibitory effects of adenosine and its stable analogues on the mouse hemidiaphragm preparation

1. The effect of adenosine or its stable analogues (2-chloroadenosine, CADO; 5'-N-ethylcarboxamidoadenosine, NECA; and N6-cyclopentyladenosine, CPA) on the release of [3H]-acetylcholine ([3H]-ACh), and on the development of force of contraction evoked by electrical stimulation of the nerve, were studied in the mouse phrenic nerve-hemidiaphragm preparation. Evidence was obtained that the release of ACh is subject to presynaptic modulation through presynaptic A1(P1)-purinoceptors. 2. Adenosine or its stable analogues (CADO, NECA, CPA) inhibited, in a concentration-dependent manner, both the release of ACh and the force of the indirectly elicited contraction of hemidiaphragm preparation, provided in the latter case that the margin of safety was reduced by (+)-tubocurarine or magnesium. The order of potency in reducing ACh release was CPA greater than NECA greater than CADO greater than adenosine with IC50 values of 0.08 +/- 0.01, 0.74 +/- 0.05, 9.05 +/- 0.20, and 410.2 +/- 42.5 mumol/l, respectively. The order of potency in reducing twitch tension was CPA greater than NECA greater than CADO greater than adenosine with IC50 values of 0.11 +/- 0.02, 0.48 +/- 0.03, 2.07 +/- 0.49, and 240.4 +/- 20.0 mumol/l, respectively. 3. 8-Phenyltheophylline (8-PT) antagonized the inhibitory effects of the adenosine receptor agonists on ACh release and twitch tension.(ABSTRACT TRUNCATED AT 250 WORDS)

Periendothelial acetylcholine synthesis and release in bovine cerebral cortex capillaries

Choline acetyltransferase (ChAT) activity is present in isolated cerebral capillaries, where it has been considered to be a marker for perivascular cholinergic nerve terminals. However, ChAT-like immunoreactivity has been visualized in endothelial cells. This finding raised the possibility that at least part of the biochemically detected ChAT has a nonneuronal origin. To evaluate the relative contribution of endothelial cells and nerve fibers to the total acetylcholine (ACh)-synthesizing capacity of cerebral capillaries, ChAT activity and ACh release were measured in capillaries and in purified endothelial cells isolated from bovine cerebral cortex. Isolated capillaries showed ChAT activity, which was inhibited by 2-benzoylethyl trimethylammonium to the same extent as cerebral ChAT. When preincubated with [3H]choline, these capillaries presented a calcium-dependent enhancement in tritium release upon electrical field stimulation. Purified endothelial cells had minor ChAT activity and lacked the ability to release tritium in response to electrical stimulation, although the endothelial markers alkaline phosphatase, gamma-glutamyltranspeptidase, and 1,1'-dioctadecyl-1,3,3',3'-tetramethyl-iodocarbocyanide perchlorate-labeled acetylated low-density lipoprotein uptake were fully preserved. These data indicate that, within isolated cerebral capillaries, ACh is synthesized and released by a periendothelial structure. The fact that ACh release is provoked by electrical stimulation and by a calcium-dependent mechanism strongly suggests that cerebrovascular ACh has a neuronal origin.

The pre- and postjunctional components of the neuromuscular effect of antibiotics

The relative contributions of the pre- and postsynaptic components of the myoneural blocking effect of different antibiotics were studied using: (a) a radio-active method that measures selectively the Ca(2+)-dependent, stimulation evoked, quantally released, (3)H-acetylcholine ((3)H-ACh) from the mouse in vitro phrenic nerve-hemidiaphragm preparation without cholinesterase inhibition; (b) measurement of the force of contraction of the indirectly or directly stimulated muscle. The antibiotics studied (neomycin, polymyxin B and lincomycin), reduced the release of (3)H-ACh evoked by stimulation (18 trains of 40 shocks at 50 Hz) in a concentration dependent manner. While the inhibitory effect of neomycin was inversely related to [Ca(2+)](o), that of lincomycin was moderately and that of polymyxin B was not affected by increasing [Ca(2+)](o) from 0.75 to 5.0 mM. Similarly, the d-tubocurarine (d-Tc)-induced inhibition of the release of (3)H-ACh was independent of [Ca(2+)](o). The K-channel blocking agent, 4-aminopyridine (4-AP), enhanced the release of ACh in a concentration dependent manner and prevented the neuromuscular effect of neomycin. However, the neuromuscular effect of polymyxin B and of lincomycin was not affected by 4-AP. Atropine, enhanced the release of (3)H-ACh. Antibiotics, however, were still able to reduce the release of ACh when the negative muscarinic feedback mechanism of ACh release was eliminated by atropine. Our findings indicate that the antibiotics studied possess both pre- and postsynaptic effects. Presynaptically they reduce the evoked release of ACh; postsynaptically they inhibit muscle contractility. The rank order of presynaptic action is neomycin >polymyxin B >lincomycin.

Stimulation of beta 1-adrenoceptors enhances electrically evoked [3H]-acetylcholine release from rat phrenic nerve

1. The effects of isoprenaline, noradrenaline and fenoterol on the electrically evoked release of [3H]-acetylcholine from the rat phrenic nerve were investigated. 2. Isoprenaline (0.1 mumol/L) and noradrenaline (1 mumol/L) enhanced evoked [3H]-acetylcholine release by about 90%, an effect which was abolished by CGP 20712A (0.1 mumol/L), a specific antagonist at beta 1-adrenoceptors. Noradrenaline still enhanced [3H]-acetylcholine release in the presence of phentolamine (1 mumol/L). 3. The enhancing effect of both isoprenaline and noradrenaline decreased at prolonged exposure times (24-32 min). A pre-exposure of the tissue to a low concentration (0.01 mumol/L) of isoprenaline prevented the enhancing effect of 0.1 mumol/L isoprenaline. 4. Fenoterol, a specific agonist at beta 2-adrenoceptors, did not modify evoked [3H]-acetylcholine release. 5. The present results indicate the existence of facilitatory beta 1-adrenoceptors on the motor nerve. These receptors appear to be desensitized either by a high concentration of, or by a long exposure to, agonists. Under the present conditions noradrenaline enhances the release of newly synthesized transmitter mainly by stimulation of beta-adrenoceptors.

Stimulation of alpha 1-adrenoceptors increases electrically evoked [3H]acetylcholine release from the rat phrenic nerve

The modulation by sympathomimetic amines of the electrically evoked [3H]acetylcholine release from the motor nerve was investigated. Phenylephrine (10 mumol/l), alpha-methylnoradrenaline (10 mumol/l) and adrenaline (1 mumol/l) enhanced the electrically evoked [3H]acetylcholine release from the rat phrenic nerve. The enhancing effect of phenylephrine was completely prevented by low concentrations of prazosin (0.1 or 0.01 mumol/l) whereas a high concentration of yohimbine (1 mumol/l) was necessary to abolish the effect of phenylephrine. The simultaneous blockade of alpha- and beta-adrenoceptors, i.e. propranolol (0.1 mumol/l) together with prazosin (0.01 mumol/l) or yohimbine (1 mumol/l), was required to abolish the enhancing effect of alpha-methylnoradrenaline. Likewise, the enhancing effect of adrenaline could be abolished only by a combination of two antagonists (propranolol together with yohimbine or prazosin). Neither clonidine nor oxymetazoline (1 or 10 mumol/l) modulated the evoked [3H]acetylcholine release. The experiments showed the existence of alpha-adrenoceptors which are present on the motor nerve and whose stimulation mediates an increase in evoked transmitter release. The different potencies found for prazosin and yohimbine indicate that an alpha 1-subtype of the receptors was involved. Increased sympathetic activity may facilitate neuromuscular transmission by stimulation of presynaptic alpha- and beta-adrenoceptors.

In favour of the vesicular hypothesis: neurochemical evidence that vesamicol (AH5183) inhibits stimulation-evoked release of acetylcholine from neuromuscular junction

1. The effects of optical isomers of vesamicol (2-(4-phenylpiperidino) cyclohexanol), an inhibitor of acetylcholine (ACh) storage, on stimulation-evoked release of [3H]-acetylcholine [( 3H]-ACh) from the neuromuscular junction have been studied in the region of the mouse hemidiaphragm which contains the motor endplates, and which can easily be loaded with [3H]-choline. This method made it possible to detect exclusively the Cao-dependent release of [3H]-ACh in response to stimulation, and therefore to test the vesicular hypothesis. 2. (-)-Vesamicol was approximately 20 times more potent than (+)-vesamicol in reducing stimulation-evoked release of [3H]-ACh. 3. 4-Aminopyridine, a potassium channel blocker, enhanced the release of ACh in response to stimulation, but failed to increase release from hemidiaphragm which had been pretreated with (-)-vesamicol. 4. The fact that (-)-vesamicol inhibited the release of [3H]-ACh in response to electrical stimulation only when it was administered prior to the loading of the tissue with [3H]-choline, and had no effect when the stores had already been filled with labelled [3H]-ACh indicates that the stimulation-evoked release of [3H]-ACh is of vesicular origin and (-)-vesamicol has no effect on the release process. This is the first neurochemical evidence for the vesicular origin of stimulation-evoked release of ACh from the neuromuscular junction.

Prejunctional modulation of acetylcholine release from the skeletal neuromuscular junction: link between positive (nicotinic)- and negative (muscarinic)-feedback modulation

1. Presynaptic receptor-mediated modulation of stimulation-evoked [3H]-acetylcholine[( 3H]-ACh) release from the neuromuscular junction was studied in the region of the mouse hemidiaphragm which contains the motor endplates, and which can easily be loaded with [3H]-choline. This method made it possible to detect exclusively the [Ca2+]0-dependent, quantal release of [3H]-ACh in response to axonal stimulation. 2. Atropine enhanced, and non-depolarizing muscle relaxants [+)-tubocurarine, pancuronium and pipecuronium) reduced, the release of [3H]-ACh evoked by high frequency trains of stimulation (50 Hz, 40 shocks) of the phrenic nerve. The effect of (+)-tubocurarine was frequency-dependent as at 5 Hz (40 shocks) it was less effective than at 50 Hz. The resting release of [3H]-ACh was not affected by these compounds. These findings indicate that ACh released into the synaptic gap by axonal firing reaches a concentration sufficient to influence its own release by a prejunctional effect. 3. The anticholinesterase, physostigmine sulphate, enhanced the release of [3H]-ACh in a concentration-dependent manner. This effect was mediated via prejunctional nicotinic receptor stimulation: (+)-tubocurarine, pancuronium and pipecuronium completely prevented the effect of physostigmine. 4. When the prejunctional nicotinic and muscarinic receptors were stimulated by a high concentration of extracellular ACh which had accumulated in the junctional gap in the presence of physostigmine, atropine did not influence the evoked release of [3H]-ACh. However, when the effect of endogenous ACh on nicotinic receptors was prevented by (+)-tubocurarine, atropine enhanced the release. 5. It is concluded that quantally-released ACh from motor endplates is subject to prejunctional automodulation: (a) ACh facilitates its own release via an effect on prejunctional nicotinic receptors (positive feedback), (b) ACh release is reduced by an action on muscarinic receptors. When the nicotinic receptor-mediated facilitation is fully operative, the muscarinic receptor-mediated negative feedback is much less effective. It is supposed that there is a link between the two feedback mechanisms possibly at the level of the second messenger system(s).

Control of transmitter release from the motor nerve by presynaptic nicotinic and muscarinic autoreceptors

Until recently, release studies have failed to indicate the existence of autoreceptors on motor nerves. Ignaz Wessler now reports on a refinement of the technique - the measurement of newly synthesized [3H]acetylcholine released from the phrenic nerve - which provides clear evidence in support of release-modulating autoreceptors. Presynaptic nicotinic receptors mediate a positive feedback mechanism, can rapidly be desensitized and appear to differ in their pharmacological profile from the postsynaptic receptors. In addition, inhibitory and facilitatory muscarinic receptors appear to be involved in the presynaptic control of transmitter release from the phrenic nerve.

Beta-adrenoceptor stimulation enhances transmitter output from the rat phrenic nerve

1. Neurally-evoked output of newly synthesized [3H]-acetylcholine from the rat phrenic nerve was measured in the absence of cholinesterase inhibitors. 2. Noradrenaline and isoprenaline enhanced neurally-evoked transmitter output markedly. Moreover, immediately after the application of noradrenaline the basal tritium efflux increased significantly. 3. Pretreatment with propranolol (0.1 mumol l-1) or atenolol (0.3 mumol l-1) completely prevented the stimulatory effect of noradrenaline and isoprenaline on evoked transmitter output. 4. The facilitatory effect of isoprenaline declined, when the exposure time was increased. This observation supports the assumption that beta-adrenoceptors can be desensitized or inactivated during continued exposure to agonists. 5. It was shown for the first time that stimulation of beta-adrenoceptors enhances transmitter output from the motor nerve. It is proposed that these beta-adrenoceptors are of the beta 1-subtype and are localized on the endings of motor nerves. Circulating catecholamines may facilitate neuromuscular transmission by stimulation of presynaptic beta-adrenoceptors.

Muscarine receptors on the rat phrenic nerve, evidence for positive and negative muscarinic feedback mechanisms

Neuronal transmitter stores of the rat phrenic nerve were labelled by incubation with [3H]choline. Release of [3H]acetylcholine was elicited by electrical nerve stimulation (100 or 1,500 pulses, 5 or 25 Hz) or by high potassium (27 mmol/l) and the effects of the muscarine receptor agonist oxotremorine and the antagonist scopolamine were investigated. Neither oxotremorine nor scopolamine affected the basal tritium efflux. A low concentration of oxotremorine (10 nmol/l) enhanced and a high concentration of oxotremorine (1 mumol/l) reduced the electrically evoked [3H]acetylcholine release. Likewise, the high potassium-evoked [3H]acetylcholine release was reduced by a high concentration of oxotremorine. Both effects of oxotremorine, increase and decrease, were abolished by a pretreatment (30 min before the first stimulation period) with 0.1 mumol/l scopolamine. Scopolamine (0.1 mumol/l) alone, enhanced [3H]acetylcholine release evoked by 100 pulses (5 Hz) or by high potassium. Scopolamine, however, reduced [3H]acetylcholine release evoked by 1,500 pulses (5 Hz or 25 Hz). The concentration-response curves obtained for scopolamine under these latter stimulation conditions were flat-running and biphasic which might indicate the involvement of two opposite effects (increase and decrease) of scopolamine under the present stimulation conditions. Both effects of scopolamine were reduced in the presence of 10 mumol/l neostigmine.(ABSTRACT TRUNCATED AT 250 WORDS)