On-line electrochemical monitoring of the local noradrenaline release evoked by electrical stimulation of the sympathetic nerves in isolated rat tail artery

Myography of isolated blood vessels: Considerations for experimental design and combination with supplementary techniques

The study of the mechanisms of regulation of vascular tone is an urgent task of modern science, since diseases of the cardiovascular system remain the main cause of reduction in the quality of life and mortality of the population. Myography (isometric and isobaric) of isolated blood vessels is one of the most physiologically relevant approaches to study the function of cells in the vessel wall. On the one hand, cell-cell interactions as well as mechanical stretch of the vessel wall remain preserved in myography studies, in contrast to studies on isolated cells, e.g., cell culture. On the other hand, in vitro studies in isolated vessels allow control of numerous parameters that are difficult to control in vivo. The aim of this review was to 1) discuss the specifics of experimental design and interpretation of data obtained by myography and 2) highlight the importance of the combined use of myography with various complementary techniques necessary for a deep understanding of vascular physiology.

Differences in sympathetic neuroeffector transmission to rat mesenteric arteries and veins as probed by in vitro continuous amperometry and video imaging

As arteries are resistance blood vessels while veins perform a capacitance function, it might be expected that sympathetic neural control of arteries and veins would differ. The function of sympathetic nerves supplying mesenteric arteries (MA) and veins (MV) in rats was investigated using in vitro continuous amperometry with a carbon fibre microelectrode and video imaging. We simultaneously measured noradrenaline (NA) overflow at the blood vessel adventitial surface and vasoconstriction evoked by electrical stimulation of perivascular sympathetic nerves. Sympathetic nerve arrangement was studied using glyoxylic acid-induced fluorescence of NA. We found that: (i) there were significant differences between MA and MV in the arrangement of sympathetic nerves; (ii) frequency-response curves for NA overflow and vasoconstriction for MV were left-shifted compared to MA; (iii) the P2X receptor antagonist, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 10 microm), reduced constrictions in MA but not in MV while the alpha(1)-adrenergic receptor antagonist, prazosin (0.1 microm), blocked constrictions in MV but not in MA; (iv) NA overflow for MA was enhanced by the alpha(2)-adrenergic receptor antagonist, yohimbine (1.0 microm), and attenuated by the alpha(2)-adrenergic receptor agonist, UK 14,304 (1.0 microm), while yohimbine and UK 14,304 had little effect in MV; (v) cocaine (10 microm) produced larger increases in NA overflow in MA than in MV; (vi) UK 14,304 constricted MV but not MA while yohimbine reduced constrictions in MV but not MA. We conclude that there are fundamental differences in sympathetic neuroeffector mechanisms in MA and MV, which are likely to contribute to their different haemodynamic functions.

In vitro continuous amperometry with a diamond microelectrode coupled with video microscopy for simultaneously monitoring endogenous norepinephrine and its effect on the contractile response of a rat mesenteric artery

Continuous amperometry with a diamond microelectrode and video microscopy were used to record (in vitro) endogenous norepinephrine release simultaneously with the evoked contractile response of a mesenteric artery from a healthy Sprague Dawley rat. Norepinephrine (NE) is a vasoconstricting neurotransmitter released from sympathetic nerves that innervate the smooth muscle cell layers surrounding arteries and veins. Using these two techniques along with several drugs, the NE released at sympathetic neuroeffector junctions nearby the microelectrode was measured as an oxidation current. Key to the amperometric measurement was the use of a diamond microelectrode because of the response sensitivity, reproducibility, and stability it provided. NE release was elicited by electrical stimulation at frequencies between 1 and 60 Hz, with a maximum response seen at 20 Hz. Confirmation that the oxidation current was, in fact, associated with endogenous NE came from the results of several drugs. Tetrodotoxin (TTX, 0.3 microM), a voltage-dependent sodium channel antagonist that blocks nerve conduction, abolished both the oxidation current and the arterial constriction. The alpha(2)-adrenergic autoreceptor antagonist, yohimbine (1.0 microM), caused an increase in the oxidation current and the corresponding constriction. The addition of cocaine (10 microM), an antagonist that inhibits neuronal NE reuptake, caused both the oxidation current and the contractile response to increase. These results, combined with the fact that the hydrodynamic voltammetric E(1/2) for endogenous NE was identical to that for a standard solution, confirmed that the oxidation current was due to NE and that this compound caused, at least in part, the contractile response. The results demonstrate that continuous amperometric monitoring of NE with a diamond microelectrode and video imaging of vascular tone allow real time local measurement of the temporal relationship between nerve-stimulated NE release and arterial constriction.

Molecular mechanisms underlying the modulation of exocytotic noradrenaline release via presynaptic receptors

The release of noradrenaline from nerve terminals is modulated by a variety of presynaptic receptors. These receptors belong to one of the following three receptor superfamilies: transmitter-gated ion channels, G protein-coupled receptors (GPCR), and membrane receptors with intracellular enzymatic activities. For representatives of each of these three superfamilies, receptor activation has been reported to cause either an enhancement or a reduction of noradrenaline release. As these receptor classes display greatly diverging structures and functions, a multitude of different molecular mechanisms are involved in the regulation of noradrenaline release via presynaptic receptors. This review gives a short overview of the presynaptic receptors on noradrenergic nerve terminals and summarizes the events involved in vesicle exocytosis in order to finally delineate the most important signaling cascades that mediate the modulation via presynaptic receptors. In addition, the interactions between the various presynaptic receptors are described and the underlying molecular mechanisms are elucidated. Together, these presynaptic signaling mechanisms form a sophisticated network that precisely adapts the amount of noradrenaline being released to a given situation.

Intermittent ATP release from nerve terminals elicits focal smooth muscle Ca2+ transients in mouse vas deferens

A confocal Ca2+ imaging technique has been used to detect ATP release from individual sympathetic varicosities on the same nerve terminal branch. Varicose nerve terminals and smooth muscle cells in mouse vas deferens were loaded with the Ca2+ indicator Oregon Green 488 BAPTA-1. Field (nerve) stimulation evoked discrete, focal increases in [Ca2+] in smooth muscle cells adjacent to identified varicosities. These focal increases in [Ca2+] have been termed 'neuroeffector Ca2+ transients' (NCTs). NCTs were abolished by alpha,beta-methylene ATP (1 microM), but not by nifedipine (1 microM) or prazosin (100 nM), suggesting that NCTs are generated by Ca2+ influx through P2X receptors without a detectable contribution from L-type Ca2+ channels or alpha(1)-adrenoceptor-mediated pathways. Action potential-evoked ATP release was highly intermittent (mean probability 0.019 +/- 0.002; range 0.001-0.10) at 1 Hz stimulation, even though there was no failure of action potential propagation in the nerve terminals. Twenty-eight per cent of varicosities failed to release transmitter following more than 500 stimuli. Spontaneous ATP release was very infrequent (0.0014 Hz). No Ca2+ transient attributable to noradrenaline release was detected even in response to 5 Hz stimulation. There was evidence of local noradrenaline release as the alpha(2)-adrenoceptor antagonist yohimbine increased the probability of occurrence of NCTs by 55 +/- 21 % during trains of stimuli at 1 Hz. Frequency-dependent facilitation preferentially occurred at low probability release sites. The monitoring of NCTs now allows transmitter release to be detected simultaneously from each functional varicosity on an identified nerve terminal branch on an impulse-to-impulse basis.

Stressful animal housing conditions and their potential effect on sympathetic neurotransmission in mice

Although the sympathetic nervous system (SNS) plays a major role in mediating the peripheral stress response, due consideration is not usually given to the effects of prolonged stress on the SNS. The present study examined changes in neurotransmission in the SNS after exposure of mice (BALB/c) to stressful housing conditions. Focal extracellular recording of excitatory junction currents (EJCs) was used as a relative measure of neurotransmitter release from different regions of large surface areas of the mouse vas deferens. Mice were either group housed (control), isolation housed (social deprivation), group housed in a room containing rats (rat odor stress), or isolation housed in a room containing rats (concurrent stress). Social deprivation and concurrent stressors induced an increase of 30 and 335% in EJC amplitude, respectively. The success rate of recording EJCs from sets of varicosities in the concurrent stressor group was greater compared with all other groups. The present study has shown that some common animal housing conditions act as stressors and induce significant changes in sympathetic neurotransmission.

The effects of hypochlorite-induced oxidative stress on presynaptic M2-receptors at sympathetic nerve endings in the rat tail artery

1 It was shown recently that stimulation of cardiac muscarinic M2-receptors revealed an enhanced negative inotropic response in isolated rat left atria after exposure to hypochlorite-induced oxidative stress. This phenomenon was not observed after stimulation of the cardiac A1-receptor, which like the M2-receptor is coupled to Gi-proteins. Since even the contractile response to M3-receptor stimulation was not amplified in the rat portal vein, we hypothesized a M2-receptor specificity of this hypochlorite-induced enhancement. 2 The present study was performed in order to investigate whether the sympathoinhibitory response to presynaptically located M2-receptor stimulation would also be modified after exposure to hypochlorite in the rat tail artery. We applied electrical field stimulation (EFS) in order to mimic sympathetic neurotransmission. 3 EFS increased the vascular tone frequency-dependently (0.3-4 Hz). EFS-induced vasoconstriction could be attenuated by acetylcholine (30 nM-1 microM) in a concentration-dependent manner. Hypochlorite (10 and 100 microM) did not affect the sympathoinhibitory effect of acetylcholine (100 nM). 4 In conclusion, in contrast to cardiac M2-receptors, hypochlorite did not amplify the sympathoinhibitory effects of presynaptic M2-receptors. The different responsiveness between neuronal and cardiac M2-receptors to hypochlorite may be explained by the different G-protein subunits involved in the activation of the underlying signalling cascade.

Three different vasoactive responses of rat tail artery to nicotine

The vasoactive effects of nicotine on isolated rat tail artery tissues were studied. Nicotine transiently contracted rat tail artery tissues (EC50, 55.6 ± 2 µM) in an extracellular Ca2+ dependent and endothelium-independent fashion. The blockade of alpha1-adrenoceptors, but not alpha2-adrenoceptors or P2X purinoceptors, inhibited the nicotine-induced contraction by 38 ± 7% (p < 0.05). Nicotine (1 mM) depolarized membrane by 13 ± 3 mV, but did not affect L-type Ca2+ channel currents, of the isolated rat tail artery smooth muscle cells. The phenylephrine-precontracted tail artery tissues were relaxed by nicotine (EC50, 0.90 ± 0.31 mM), which was significantly inhibited after the blockade of nicotinic receptors. Simultaneous removal of phenylephrine and nicotine, after a complete relaxation of the phenylephrine-precontracted tail artery strips was achieved by nicotine at accumulated concentrations (>=10 mM), triggered a Ca2+-dependent rebound long-lasting vasoconstriction (n = 20). This rebound contraction was abolished in the absence of calcium or in the presence of tetracaine in the bath solution. Pretreatment of vascular tissues with a nicotinic receptor antagonist did not affect the nicotine-induced vasoconstriction or nicotine withdrawal induced rebound contraction. The elucidation of the triphasic vascular effects of nicotine and the underlying mechanisms is important for a better understanding of the complex vascular actions of nicotine.Key words: nicotine, smokeless tobacco, vascular smooth muscles, contraction, relaxation.

Electrochemical and electrophysiological characterization of neurotransmitter release from sympathetic nerves supplying rat mesenteric arteries

1. Characteristic features of noradrenaline (NA) and adenosine 5'-triphosphate (ATP) release from postganglionic sympathetic nerves in rat small mesenteric arteries in vitro have been investigated on an impulse-by-impulse basis. NA release was measured using continuous amperometry and ATP release was monitored by intracellular recording of excitatory junction potentials (e.j.ps). 2. Electrical stimuli evoked transient increases in oxidation current. During trains of ten stimuli at 0.5 - 4 Hz there was a depression in the amplitude of oxidation currents evoked following the first stimulus in the train. 3. The neuronal NA uptake inhibitor, desmethylimipramine (1 microM), increased the amplitude of the summed oxidation current evoked by ten stimuli at 1 Hz and slowed the decay of oxidation currents evoked by trains of ten stimuli at 1 and 10 Hz. 4. The alpha2-adrenoceptor antagonist, idazoxan (1 microM), increased the amplitudes of the oxidation currents evoked during trains of ten stimuli at 0.5 - 10 Hz but had no effect on the oxidation currents evoked by the first stimulus in the train. 5. Idazoxan (1 microM) increased the amplitude of all e.j.ps evoked during trains of stimuli at 0.5 and 1 Hz. In addition, the facilitatory effect of idazoxan on e.j.ps was significantly greater than that on oxidation currents. 6. The findings indicate that NA release from sympathetic nerves supplying small mesenteric arteries is regulated by activation of presynaptic alpha2-adrenoceptors and that clearance of released NA in this tissue depends, in part, upon neuronal uptake. The different effects of idazoxan on the oxidation currents and e.j.ps may indicate that the release of NA and ATP is differentially modulated.

Paired pulse analysis of ATP and noradrenaline release from sympathetic nerves of rat tail artery and mouse vas deferens: effects of K+ channel blockers

1. The paired pulse stimulus paradigm - two pulses of equal strength delivered at variable interpulse intervals was used to study the release of ATP and noradrenaline (NA) from post ganglionic sympathetic nerves of rat tail artery and mouse vas deferens. 2. Excitatory junction currents (EJCs) were used to measure the release of ATP, and differential pulse amperometry to measure that of NA. 3. At interpulse intervals of 0.1 - 1 s paired pulse stimulation caused an increase in the size of the second EJC, both in rat tail artery and mouse vas deferens. As the interpulse interval was increased to 10 s or more, the two EJCs became of equal size. 4. In both preparations the K+ channel blockers tetraethylammonium (TEA, 20 mM) and 4-aminopyridine (4-AP, 1 mM) prolonged the duration of the nerve terminal spike and greatly amplified the first EJC of the pair. 5. In the presence of TEA and 4-AP in rat tail artery paired pulse stimulation caused a dramatic depression of the second EJC without markedly affecting the nerve terminal spike. The depression of the second EJC decreased with increasing interpulse intervals, and also when external Ca2+ was reduced to 0.2 mM. In mouse vas deferens, TEA and 4-AP caused only a modest depression of the second EJC. 6. In rat tail artery in the presence of TEA and 4-AP paired pulse stimulation caused a depression of the NA oxidation current evoked by the second pulse, which was similar in magnitude and time course to that of the EJC. Similar TEA and 4-AP induced depression of the second pulse response was also observed when the purinergic and noradrenergic components of the contractile response were investigated. 7. The results show that in rat tail artery K+ channel blockers cause a dramatic paired pulse depression of the release of ATP and NA. The similarity in the depression of the EJC, the NA oxidation current, and the purinergic and noradrenergic components of the contractile response is compatible with the hypothesis that ATP and NA are released in parallel from the same neuronal sources.

Beta-adrenoceptor mediated facilitation of noradrenaline and adenosine 5'-triphosphate release from sympathetic nerves supplying the rat tail artery

1. The effects of prejunctional beta-adrenoceptor activation on electrically evoked noradrenaline (NA) and adenosine 5'-triphosphate (ATP) were studied by use of continuous amperometry and conventional intracellular recording techniques. Excitatory junction potentials (e.j.ps) were used as a measure of ATP release, and NA-induced slow depolarizations and oxidation currents as measures of NA release, from postganglionic sympathetic nerves innervating the rat tail artery in vitro. 2. Isoprenaline (0.1 microM) increased the amplitude of e.j.ps, slow depolarizations and oxidation currents evoked by short trains of stimuli at 1 to 4 Hz. The facilitatory effect of isoprenaline on e.j.ps and oxidation currents was most pronounced on responses evoked by the first stimulus in a train. 3. Isoprenaline (0.1 microM) did not detectably alter the amplitude-frequency distribution of spontaneous e.j.ps. 4. The facilitatory effect of isoprenaline on e.j.ps, slow depolarizations and oxidation currents was abolished by the beta-adrenoceptor antagonist, propranolol (0.1 microM). Propranolol alone had no effect on e.j.ps, slow depolarizations or oxidation currents. 5. Thus, activation of prejunctional beta-adrenoceptors increases the release of both NA and ATP from postganglionic sympathetic nerves. The findings are consistent with the hypothesis that NA and ATP are released from the same population of nerve terminals and presumably from the same vesicles.

Nerve activity-dependent variations in clearance of released noradrenaline: regulatory roles for sympathetic neuromuscular transmission in rat tail artery

The aim of this study was to find out if clearance of noradrenaline released from sympathetic nerve terminals in rat isolated tail artery is a physiological variable and if so, to determine its role for the noradrenaline-mediated neurogenic contraction. The per pulse release of noradrenaline induced by electrical nerve stimulation and the fluctuations of the level of noradrenaline at the receptors driving the contractions were assessed from the electrochemically determined noradrenaline oxidation current at a carbon fibre electrode at the surface of the artery. Both were compared with the noradrenaline-mediated neurogenic contraction. The effects on these parameters of cocaine or desipramine, or of corticosterone, were used to assess the relative roles of neuronal and extraneuronal uptake, respectively. The effects of cocaine or desipramine, which enhance the noradrenaline level at the receptors by blocking neuronal reuptake, were compared with those of yohimbine, presumed to act exclusively by enhancing the per pulse release of noradrenaline. The results seem to support the following tentative conclusions. Clearance of released noradrenaline occurs by neuronal uptake and diffusion, while extraneuronal uptake is negligible. The noradrenaline-induced neurogenic contraction is mediated via adrenoceptors on cells near the plane of the nerve plexus; the excitation spreads from these cells throughout the syncytium. The contractile response to exogenous noradrenaline may also be mediated via receptors on the innervated key cells. Reuptake of noradrenaline into the releasing varicosities, i.e. in "active junctions", is highly efficient for single quanta but rapidly saturated by repeated release, while reuptake of noradrenaline in the "surround" of active junctions is probably rarely saturated and more independent of nerve activity. Saturation of the transporter by repeated release of quanta from the same varicosity and the consequent accumulation of "residual" noradrenaline and increased diffusion out of the junction and recruitment of noradrenaline receptors in the surround may be the cause of the rapid growth of the contraction during a high frequency train. Diffusion of released noradrenaline away from the postjunctional receptors is restricted by a local nerve activity-dependent buffering mechanism which, in spite of fading of the per pulse release, helps maintain the noradrenaline concentration at the receptors and the contraction during long high-frequency trains. Reactivation of the clearance mechanisms upon cessation of nerve activity accelerates the relaxation.(ABSTRACT TRUNCATED AT 400 WORDS)

Intermittent release of noradrenaline by single pulses and release during short trains at high frequencies from sympathetic nerves in rat tail artery

As shown by electrophysiological analysis, the release of the sympathetic co-transmitter adenosine 5'-triphosphate (ATP) from individual release sites is monoquantal and intermittent; the average release probability may be as low as 0.01. Indirect evidence from biochemical studies of noradrenaline overflow is compatible with a similar monoquantal, low probability release of noradrenaline as well. In the present study our first aim was to address this issue more directly in rat tail artery, using continuous amperometry to monitor in real time the release of noradrenaline from a relatively small number of sympathetic nerve varicosities. The results seem to provide the first direct evidence that noradrenaline, similarly to ATP, may be released intermittently during nerve stimulation at low frequency. Our second aim was to use the same technique to study the release of noradrenaline caused by nerve stimulation with single pulses or short trains (two to eight pulses) at high frequencies. The results show that during stimulation at 20 Hz the peak amplitude of the noradrenaline oxidation current response grew linearly with the train length, but at 50 Hz the curve describing this growth was sigmoid in shape.

Kinetics of ATP- and noradrenaline-mediated sympathetic neuromuscular transmission in rat tail artery

Electrophysiological, electrochemical and mechanical recordings were employed to study the kinetics of the release and clearance of adenosine 5'-triphosphate (ATP) and noradrenaline (NA) as sympathetic co-transmitters and of the neurogenic and non-neurogenic contractions in rat isolated tail artery. The life-time of ATP and NA released by a single pulse or 10 pulses at 50 Hz was brief (< 100 ms, or < 3 s, respectively); the neurogenic contractile responses occurred largely after the transmitters had been removed from the extracellular space. The ATP-induced neurogenic contractile responses to a single pulse or 10 pulses at 50 Hz were similar in time-course to the responses to direct muscle stimulation at low voltage; both seemed to be caused by activation of nifedipine-sensitive voltage-gated L-type Ca2+ channels. The alpha 1- and alpha 2-adrenoceptor-mediated components of the NA-induced neurogenic contractile response to 10 pulses at 50 Hz were more delayed and prolonged and determined by properties of the post-receptor mechanisms. The per pulse release of both ATP and NA faded rapidly during long high-frequency trains. So did the ATP level at the receptors and the ATP-induced neurogenic contraction. The NA levels and the contractile responses induced via alpha 1- and alpha 2-adrenoceptors were much better maintained during ongoing stimulation at 20 Hz but relaxed rapidly afterwards, suggesting that nerve activity suppressed, and cessation of nerve activity reactivated NA clearance.

Dual contractile effects of ATP released by field stimulation revealed by effects of alpha,beta-methylene ATP and suramin in rat tail artery

1. The field stimulation-induced release of endogenous ATP and noradrenaline (NA) and contractile response in rat isolated tail artery were examined. The release of ATP was studied by extracellular electrophysiological recording and that of NA by a novel voltammetrical technique. The effects of the P2-purinceptor antagonist, suramin, on these parameters were compared with those of alpha,beta-methylene ATP, a P2X-purinoceptor desensitizing agent. 2. Neither alpha,beta-methylene ATP (10 microM) nor suramin (100-500 microM) had significant effects on the extracellularly recorded nerve terminal action potential but both abolished the ATP-induced excitatory junction current caused by stimulation at 0.1 Hz. Neither agent affected significantly the voltammetrically measured release of NA induced by 10 or 100 pulses at 20 Hz. 3. Combined blockade of both postjunctional alpha 1- and alpha 2-adrenoceptors by prazosin and yohimbine (both 0.1 microM) profoundly depressed the contractile response to 10 pulses at 20 Hz. The small and fast residual contraction in the presence of these agents was abolished by alpha,beta-methylene ATP (10 microM) and inhibited by suramin in a concentration-dependent manner (10-500 microM; IC50 75 microM) and was hence probably caused by ATP or a related nucleotide. 4. When added first, alpha,beta-methylene ATP (10 microM) or suramin (100-500 microM) delayed the onset and enhanced the amplitude of the neurogenic contraction. This enhanced response was abolished by further addition of prazosin and yohimbine (both 0.1 microM). 5. The K+ channel blocker, tetraethylammonium (10 mM), dramatically enhanced the contractile response to 100 pulses at 1 Hz and caused it to become diphasic. Addition of alpha,beta-methylene ATP (10 microM)or suramin (100-500 microM) abolished the large initial twitch component of this contraction and depressed the tonic phase.6. Like alpha,beta-methylene ATP, suramin (500 microM) had no effect on the contraction caused by exogenous NA (1O nM-l10 microM) or KCI (60 mM); both agents almost abolished the contraction caused by ATP(100 microM).7. In conclusion, (i) the contractile response of rat tail artery to electrical field stimulation is mediated by both ATP and NA, and is thus an expression of ATP-NA co-transmission, (ii) the released ATP exerts two opposite effects via 'P2x-like' purinoceptors, triggering the initial rapid phase of the neurogenic contraction and restricting the NA-mediated component of the contraction; and (iii) the source and possible physiological role of the ATP which causes the inhibitory effect are unknown at present.

Frequency- and train length-dependent variation in the roles of postjunctional alpha 1- and alpha 2-adrenoceptors for the field stimulation-induced neurogenic contraction of rat tail artery

The present paper examines the roles of postjunctional alpha 1- and alpha 2-adrenoceptors for the noradrenaline (NA)-induced neurogenic contractile response to field stimulation mainly with 1-100 pulses at 2 or 20 Hz, in the tail artery of adult normotensive rats. Pharmacological tools were employed to isolate and characterize the alpha 1- and alpha 2-adrenoceptor-mediated components of this response. The degree to which the drugs influenced NA release or reuptake was assessed by their effects on the electrochemically determined, stimulation-induced rise in the NA concentration at the innervated outer surface of the media. This response was unaffected by alpha,beta-methylene ATP (10 microM) or suramin (500 microM), added to desensitize or block P2-purinoceptors, respectively prazosin (0.1 microM) or SK&F 104078 (6-chloro-9-[(3-methyl-2-butenyl)oxyl]-3-methyl- 1H-2,3,4,5-tetrohydro-3-benzazepine, 0.1 microM), used to block postjunctional alpha 1- and alpha 2-adrenoceptors respectively, nifedipine (10 microM), blocker of Ca2+ influx through L-type channels, and ryanodine (10 microM), which blocks mobilization of Ca2+ from intracellular stores; it was moderately enhanced by yohimbine (0.1 microM), blocker of pre- and postjunctional alpha 2-adrenoceptors, and strongly enhanced by cocaine (3 microM) or desipramine (1 microM), blockers of NA reuptake. Judging from their inhibitory effects on the contractile responses to the alpha 1- and alpha 2-adrenoceptor agonists, phenylephrine and xylazine, prazosin (0.1 microM) and SK&F 104078 (0.1 microM) could be used to selectively block alpha 1- and alpha 2-adrenoceptors respectively, while yohimbine (0.1 microM) was less selective, strongly depressing alpha 2- and slightly depressing alpha 1-adrenoceptor-mediated responses. The alpha 1-adrenoceptor-mediated component of the contractile response to short trains at 20 Hz was fast in onset, brief in duration and abolished by ryanodine; that mediated by alpha 2-adrenoceptors was more delayed, prolonged and insensitive to ryanodine. Both components were dose-dependently depressed by nifedipine (0.1-10 microM). The small contractile responses to single pulses, or up to 50 pulses at 2 Hz, or short train (< 4 pulses) at 20 Hz, were more markedly depressed by 0.1 microM yohimbine or SK&F 104078 than by 0.1 microM prazosin and, hence, mediated mainly by alpha 2-adrenoceptors. The reverse was true of the much larger response to longer trains at 20 Hz, which thus probably was mediated mainly by alpha 1-adrenoceptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Sympathetic transmitter release in rat tail artery and mouse vas deferens: facilitation and depression during high frequency stimulation

Electrophysiological and electrochemical methods were used to study the release of adenosine 5'-triphosphate (ATP) and noradrenaline (NA) from sympathetic nerves during stimulation with trains at 20 Hz (tetanus). In mouse vas deferans ATP release was mainly facilitated during the tetanus, but in rat tail artery progressively and reversibly depressed. In rat tail artery reduction of external calcium attenuated the depression and increased the facilitation during the tetanus, while increased external calcium accentuated the depression. Both ATP and NA release were depressed in parallel during the first 100 pulses of the tetanus. The depression of release was not due to action potential failure, or alpha 2-adrenoceptor-mediated autoinhibition.

Fast and local electrochemical monitoring of noradrenaline release from sympathetic terminals in isolated rat tail artery

Noradrenaline release from sympathetic nerve terminals was evoked by electrical nerve stimulation of an isolated segment of rat tail artery. This release was recorded by a carbon fiber electrode combined with differential pulse amperometry. The active part of the electrode (one carbon fiber 8 microns in diameter and 50 microns in length) was placed in close contact with the arterial surface. The oxidation current appearing at +120 mV and corresponding to the local noradrenaline concentration at the electrode surface was recorded every 0.5 s. No oxidation current was detected under resting conditions, but electrical stimulation evoked an immediate increase in this current. This response was suppressed when tetrodotoxin was added to the perfusion medium and was enhanced when noradrenaline reuptake was inhibited by cocaine. The amplitude of the response was increased with increasing stimulation frequencies (2-25 Hz) and train lengths (1-16 pulses). Finally, the time resolution of the method (0.5 s) was good enough to show that noradrenaline release precedes the postsynaptic response, i.e., the electrically evoked contraction of the artery.

Electrophysiological and electrochemical analysis of the secretion of ATP and noradrenaline from the sympathetic nerves in rat tail artery: effects of ?2-adrenoceptor agonists and antagonists and noradrenaline reuptake blockers

The aim of this study was to investigate whether or not nerve impulses release ATP and noradrenaline in parallel from the sympathetic nerve terminals of the rat tail artery. The extracellularly recorded excitatory junction current (EJC) was used to study, pulse by pulse, the release of ATP. An electrochemical method was used to study online the nerve stimulation-induced rise in the extracellular concentration of endogenous noradrenaline at the probe, a carbon fibre electrode (CF). This parameter, which does not directly represent noradrenaline release, but reflects release minus clearance, has been termed delta[NA]CF. The effects of a number of pharmacological agents on the EJCs were examined both at 0.1 and 2 Hz, and the effects on the EJC response to 100 pulses at 2 Hz compared with that on the delta[NA]CF response. Clonidine and xylazine were used as alpha 2-agonists, yohimbine and idazoxan as alpha 2-antagonists and desipramine and cocaine as blockers of noradrenaline reuptake. Most of these agents had unwanted side effects, especially at higher concentrations. However, clonidine and xylazine depressed at lower concentrations the EJC and delta[NA]CF responses to about the same extent; these effects were partially or completely reversed by yohimbine. Yohimbine or idazoxan did not affect the EJCs at 0.1 Hz but enhanced the EJC and delta[NA]CF responses to 100 pulses at 2 Hz to the same extent. All effects of desipramine (1 microM) seemed explainable as a result of block of noradrenaline reuptake, while cocaine (10 microM) in addition exerted an 'unspecific' depressant (probably local anesthetic) effect. Under control conditions, both agents depressed the EJC but dramatically enhanced the delta[NA]CF response to 100 pulses at 2 Hz. Addition of yohimbine prevented the depressant effect of desipramine on the EJCs completely and reduced that of cocaine, but increased their effects on the delta[NA]CF response. These results are compatible with the view that ATP and noradrenaline are released in parallel from the sympathetic nerve terminals of this tissue. The different, and under some conditions even opposite, effects of desipramine or cocaine on the EJC and delta[NA]CF responses are explainable in terms of the known post-secretory effects of these agents.

Electrochemical and electrophysiological analysis of the effects of SK&F 104078 on prejunctional alpha 2-adrenoceptors

SK&F 104078 is proposed to be a selective postjunctional alpha 2-adrenoceptor antagonist. We examined its effects on the electrically evoked release of endogenous noradrenaline (NA) from the rat tail artery and of ATP from the mouse vas deferens, as determined electrochemically or by extracellular recording, respectively. The alpha 2-adrenoceptor antagonist yohimbine (0.1 and 1 microM) increased NA release at both 2 and 20 Hz; the effect of yohimbine was further enhanced by the NA uptake blocker cocaine. SK&F 104078 (0.01-1 microM) did not change NA release at 2 or 20 Hz in the absence or presence of cocaine. NA release at 2 Hz was dose dependently depressed by the alpha 2-adrenoceptor agonist xylazine, an effect reversed by yohimbine (1 microM) but unaffected by SK&F 104078 (0.1 and 1 microM). Similarly, the inhibitory effect of xylazine on ATP release at 0.1 Hz was not influenced by SK&F 104078 (1 microM) but partially reversed by yohimbine (1 microM). The results show (i) that prejunctional alpha 2-adrenoceptors mediate autoinhibition of the release of endogenous NA and ATP from sympathetic nerves in the rat tail artery and in the mouse vas deferens, respectively, and (ii) that SK&F 104078 does not block these receptors in the concentration range 0.01-1 microM.

Neurotransmitters and pre- and post-junctional receptors involved in the vasoconstrictor response to sympathetic nerve stimulation in rat tail artery

The study was prompted by the need to re-evaluate, in view of the complexity of the evidence in the literature, the relative roles of different sympathetic transmitters and receptors in the contractile response of the tail artery of adult normotensive rats to electrical field stimulation. By the pharmacological approach employed, noradrenaline and adenosine 5'-triphosphate appeared to contribute to this response; the possible roles of other putative transmitters such as neuropeptide Y could not be examined due to lack of specific antagonists. Noradrenaline, clearly the main mediator, exerted both excitatory and inhibitory effects, acting in part via different receptors depending on the stimulus parameters. Thus, yohimbine and prazosin (alpha 2- and alpha 1-adrenoceptor antagonists, respectively) were about equally effective as inhibitors of the noradrenaline-mediated contractile response to stimulation with short trains and/or at low frequency, but the response caused by long trains and/or high-frequency stimulation was much more strongly inhibited by prazosin than by yohimbine. As expected, yohimbine enhanced the [3H]noradrenaline overflow response to long but not to short stimulus trains, presumably because in the latter case the noradrenaline concentration in the relevant biophase was too low to activate the pre-junctional alpha 2-adrenoceptors. Finally, propranolol, an unselective beta-adrenoceptor antagonist, enhanced the neurogenic contraction, indicating that noradrenaline restricts this response by effects via post-junctional beta-adrenoceptors. Adenosine triphosphate appeared to exert dual, excitatory as well as inhibitory, post-junctional effects. Thus, the P2x-purinoceptor desensitizing agent, alpha, beta-methylene adenosine triphosphate, abolished the initial phase, but enhanced the amplitude of the neurogenic contraction, without affecting the nerve stimulation-induced overflow of [3H]noradrenaline. The results indicate that noradrenaline and adenosine triphosphate, the main mediators of the neurogenic contraction of this preparation, act in a more complex fashion than earlier thought; they argue against a significant direct contribution by other putative transmitters but do not exclude that such agents may act indirectly as modulators of this response.