Transmitter release modulated by alpha-adrenoceptor antagonists in the rabbit mesenteric artery: a comparison between noradrenaline outflow and electrical activity

Vasoconstrictor stimulus determines the functional contribution of myoendothelial feedback to mesenteric arterial tone

KEY POINTS

In isolated resistance arteries, endothelial modulation of vasoconstrictor responses to α₁ -adrenoceptor agonists occurs via a process termed myoendothelial feedback: localized inositol trisphosphate (InsP₃ )-dependent Ca²⁺ transients activate intermediate conductance Ca²⁺ -activated K⁺ (IK_Ca ) channels, hyperpolarizing the endothelial membrane potential to limit further reductions in vessel diameter. We demonstrate that IK_Ca channel-mediated myoendothelial feedback limits responses of isolated mesenteric arteries to noradrenaline and nerve stimulation, but not to the thromboxane A₂ mimetic U46619 or to increases in intravascular pressure. In contrast, in the intact mesenteric bed, although responses to exogenous noradrenaline were limited by IK_Ca channel-mediated myoendothelial feedback, release of NO and activation of endothelial small conductance Ca²⁺ -activated K⁺ (SK_Ca ) channels in response to increases in shear stress appeared to be the primary mediators of endothelial modulation of vasoconstriction. We propose that (1) the functional contribution of myoendothelial feedback to arterial tone is determined by the nature of the vasoconstrictor stimulus, and (2) although IK_Ca channel-mediated myoendothelial feedback may contribute to local control of arterial diameter, in the intact vascular bed, increases in shear stress may be the major stimulus for engagement of the endothelium during vasoconstriction.

ABSTRACT

Constriction of isolated resistance arteries in response to α₁ -adrenoceptor agonists is limited by reciprocal engagement of inhibitory endothelial mechanisms via myoendothelial feedback. In the current model of feedback, agonist stimulation of smooth muscle cells results in localized InsP₃ -dependent Ca²⁺ transients that activate endothelial IK_Ca channels. The subsequent hyperpolarization of the endothelial membrane potential then feeds back to the smooth muscle to limit further reductions in vessel diameter. We hypothesized that the functional contribution of InsP₃ -IK_Ca channel-mediated myoendothelial feedback to limiting arterial diameter may be influenced by the nature of the vasoconstrictor stimulus. To test this hypothesis, we investigated the functional role of myoendothelial feedback in modulating responses of rat mesenteric resistance arteries to the adrenoceptor agonist noradrenaline, the thromboxane A₂ mimetic U46619, increases in intravascular pressure and stimulation of perivascular sympathetic nerves. In isolated arteries, responses to noradrenaline and stimulation of sympathetic nerves, but not to U46619 and increases in intravascular pressure, were modulated by IK_Ca channel-dependent myoendothelial feedback. In the intact mesenteric bed perfused under conditions of constant flow, responses to exogenous noradrenaline were modulated by myoendothelial feedback, but shear stress-induced release of NO and activation of endothelial SK_Ca channels appeared to be the primary mediators of endothelial modulation of vasoconstriction to agonists and nerve stimulation. Thus, we propose that myoendothelial feedback may contribute to local control of diameter within arterial segments, but at the level of the intact vascular bed, increases in shear stress may be the major stimulus for engagement of the endothelium during vasoconstriction.

Selective modulation of noradrenaline release by alpha 2-adrenoceptor blockade in the rat-tail artery in vitro

The effects of blocking alpha(2)-adrenoceptors on noradrenaline (NA) and adenosine 5'-triphosphate (ATP) release from postganglionic sympathetic nerves have been investigated in rat-tail artery in vitro. Continuous amperometry was used to measure NA release and intracellularly recorded excitatory junction potentials (e.j.p.'s) were used to measure ATP release. Application of the alpha(2)-adrenoceptor antagonist, idazoxan (1 microm), increased the amplitude of NA-induced oxidation currents evoked by trains of 10 stimuli at 1 and 10 Hz. In cells deep in the media, idazoxan (1 microm) had no effect on the amplitude of e.j.p.'s evoked by trains of 10 stimuli at 1 and 10 Hz. In cells close to the adventitial - medial border, idazoxan produced a small increase in the amplitude of e.j.p.'s evoked at the end of trains of 10 stimuli at 1 Hz. In tissues pretreated with the neuronal NA uptake inhibitor, desmethylimpramine (0.3 microm), idazoxan (1 microm) markedly increased the amplitude of e.j.p.'s in cells deep in the media. The alpha(2)-adrenoceptor agonist, clonidine (0.5 microm), produced similar reductions in the amplitudes of both NA-induced oxidation currents and e.j.p.'s evoked by 10 stimuli at 1 Hz. These effects of clonidine were reversed by the subsequent addition of idazoxan (1 microm). The release of both NA and ATP is inhibited to a similar extent by activation of prejunctional alpha(2)-adrenoceptors by clonidine. In contrast, endogenously released NA more markedly inhibits NA release. These findings provide further support for the differential modulation of NA and ATP release.

Idazoxan effects upon contractile activity in the rat aorta are related to alpha adrenoceptors and L-type channels

Idazoxan is an alpha(2) adrenoceptor antagonist and alpha(1)/alpha(2) partial agonist which also blocks imidazoline receptors. Although idazoxan is widely used in pharmacological studies, its intrinsic vasoactive properties could bring about some limitations. Others have shown that in rat aorta contracted by phenylephrine idazoxan induces relaxation and that in rat small arteries it preferentially antagonizes the alpha(1)-mediated response. We further investigated this matter, using the rat aorta and focusing on the endothelium-independent effects and on L-type channels. In our study, idazoxan inhibited the contraction induced by phenylephrine, an effect which was stronger in the presence of endothelium, but did not affect the contractions induced by various other agents (high potassium, angiotensin II, prostaglandin F(2alpha)). This preferential inhibition was attenuated by 10(-4) m, but not by 10(-5) m yohimbine, and also reduced by 10(-2) m tetraethylammonium and blunted by 10(-4) m methoxyverapamil. In concentrations above 10(-5) m idazoxan induced weak contractions of the de-endothelized rings, which were prazosin- and methoxyverapamil-sensitive. Others have suggested that cyclic guanosine monophospate mediates the idazoxan-induced endothelium-dependent relaxation, but this is difficult to reconcile with our findings. Potassium efflux could play some role in the direct relaxing effect of idazoxan. The observed idazoxan effects appear as based on action upon alpha(1) receptors, but a direct interaction with L-type calcium channels could also be taken into consideration.

Angiotensin II-induced modulation of endothelium-dependent relaxation in rabbit mesenteric resistance arteries

The role of local endogenous angiotensin II (Ang II) in endothelial function in resistance arteries was investigated using rabbit mesenteric resistance arteries. First, the presence of immunoreactive Ang II together with Ang II type-1 receptor (AT1R) and angiotensin converting enzyme (ACE) was confirmed in these arteries. In endothelium-intact strips, the AT1R-blocker olmesartan (1 microM) and the ACE-inhibitor temocaprilat (1 microM) each enhanced the ACh (0.03 microM)-induced relaxation during the contraction induced by noradrenaline (NA, 10 microM). Similar effects were obtained using CV-11974 (another AT1R blocker) and enalaprilat (another ACE inhibitor). The nitric-oxide-synthase inhibitor NG-nitro-L-arginine (L-NNA) abolished the above effect of olmesartan. In endothelium-denuded strips, olmesartan enhanced the relaxation induced by the NO donor NOC-7 (10 nM). Olmesartan had no effect on cGMP production (1) in endothelium-intact strips (in the absence or presence of ACh) or (2) in endothelium-denuded strips (in the absence or presence of NOC-7). In beta-escin-skinned strips, 8-bromoguanosine 3',5' cyclic monophosphate (8-Br-cGMP, 0.01-1 microM) concentration dependently inhibited the contractions induced (a) by 0.3 microM Ca2+ in the presence of NA+GTP and (b) by 0.2 microM Ca2++GTPgammaS. Olmesartan significantly enhanced, while Ang II (0.1 nM) significantly inhibited, the 8-Br-cGMP-induced relaxation. We propose the novel hypothesis that in these arteries, Ang II localized within smooth muscle cells activates AT1Rs and inhibits ACh-induced, endothelium-dependent relaxation at least partly by inhibiting the action of cGMP on these cells.

Reduced function of endothelial prostacyclin in human omental resistance arteries in pre-eclampsia

It remains unclear in pre-eclampsia whether or not a functional change occurs in the role played by prostacyclin in endothelium-dependent relaxation in resistance arteries. We examined this using human omental resistance arteries (obtained from pre-eclamptic or normotensive pregnant women) in the presence of N(G)-nitro-L-arginine (L-NNA, an inhibitor of nitric oxide synthase). In endothelium-intact strips from both groups, 9,11-epithio-11,12-methano-thromboxane A(2) (STA(2), a thromboxane A(2) mimetic) produced a contraction. Diclofenac (an inhibitor of cyclooxygenase) enhanced the STA(2) contraction only in the normotensive pregnant group (1.4 times control, P < 0.01). In the presence of STA(2), bradykinin (0.1 microM) produced an endothelium-dependent relaxation in both groups, the relaxation being significantly smaller for the pre-eclamptic group (P < 0.002). Diclofenac significantly attenuated the bradykinin-induced relaxation only for the normotensive pregnant group (31 % inhibition, P < 0.001). The bradykinin-induced membrane hyperpolarization consisted of diclofenac-sensitive and -insensitive components. The former, but not the latter, was significantly smaller in pre-eclampsia (-4.3 vs. -2.6 mV, P < 0.05). The concentrations of 6-keto-PGF(1alpha) (a stable metabolite of prostacyclin) in these arteries were significantly lower in pre-eclampsia in both the absence and presence of bradykinin (about 0.2-0.4 times the normotensive pregnant value in each case, P < 0.01). By contrast, both the relaxation and the membrane hyperpolarization in response to beraprost (10 nM, a stable analogue of prostacyclin) were similar between the two groups. We conclude that, in pre-eclampsia, a reduced part is played by prostaglandins in the endothelium-dependent relaxation seen in resistance arteries and that this may be due to a reduced production of prostacyclin by the endothelium.

Characteristics and possible mechanisms of low-Na+ induced contractions in rat aorta

The influence of reducing external Na+ concentration ([Na+]ex) upon vascular smooth muscle contractility was investigated using the rat isolated aorta. NaCl from the physiological saline solution (PSS) was replaced with either choline-Cl, sucrose, or LiCl to give the following [Na+]ex (mM): 115, 85, 55, and 25 (115NaPSS to 25NaPSS). Small reductions in [Na+]ex (115NaPSS) induced a biphasic contraction, comparable in amplitude with the control one induced by phenylephrine 10(-6) M. Elimination of the endogenous catecholamine participation using either phentolamine 10(-5) M or guanethidine 3.10(-6) M similarly reduces these contractions to 25% (sucrose replacement). A similar relaxing effect was obtained with D600 10(-5) M, an antagonist of the voltage operated Ca2+ channels (25-30% residual tension for all the substitutes). Large reductions in [Na+]ex (25NaPSS) induced contractions comparable in amplitude and shape, but less sensitive to phentolamine and guanethidine (residual tension 65-75%, sucrose replacement) and insensitive to D600 (all the substitutes). The Na+/K+ ATPase inhibitor ouabain (10(-4) M) elicited slowly developing contractions, the amplitude being 115% of the phenylephrine 10(-6) M control. Phenylephrine further contracted the 115NaPSS precontracted preparations, but was significantly less effective in 25NaPSS, although the precontraction levels were similar for the same substitute used. The amplitude of the superimposed phenylephrine contractions exhibited [Na+]ex dependence. Phenylephrine 10(-6) M failed to further contract the ouabain 10(-4) M precontracted rings. We conclude that relatively small reductions in [Na+]ex are able to induce contractions of rat aorta primarily through release of endogenous catecholamines, probably through neural Na+/Ca2+ exchange. Larger reductions in [Na+]ex appear to cause contraction through muscular Na+/Ca2+ exchange.

Mechanisms of vasoconstriction induced by endothelin-1 in smooth muscle of rabbit mesenteric artery

1. The mechanism underlying the vasoconstriction induced by endothelin-1 (ET-1) was investigated by measuring the intracellular concentration of Ca2+ ([Ca2+]i), isometric force and phosphorylation of the myosin light chain (MLC) in endothelium-denuded unskinned and beta-escin-treated skinned smooth muscle from resistance vessels of the rabbit mesentery. The role of protein kinase C (PKC) in the action of ET-1 was studied in skinned smooth muscle using a synthetic peptide, PKC19-36, which corresponds to the autoinhibitory domain of PKC. 2. ET-1 (> 0.1 nM) induced slowly developing, maintained increases in [Ca2+]i and force. Nicardipine completely blocked the ET-1-induced increase in [Ca2+]i. BQ-123 (an inhibitor of the ETA receptor) blocked the ET-1-induced contraction but IRL 1620 (Suc-[Glu9,Ala11,15]-ET-1(8-21), an agonist of the ETB receptor) failed to induce contraction. 3. In ionomycin- and 70 mM K(+)-treated strips, ET-1 shifted the [Ca2+]i-force relationship to the left and enhanced the maximum amplitude of contraction induced by 2.6 mM Ca2+. In skinned smooth muscle treated with ionomycin, Ca2+ (0.1-3 microM) increased both force and MLC phosphorylation, in a concentration-dependent manner. ET-1 with GTP shifted both the Ca(2+)-force and Ca(2+)-MLC phosphorylation relationships to the left without significant changes in the maximum responses. ET-1 with GTP did not change the relationship between force and MLC phosphorylation. Similar effects were observed with phorbol 12,13-dibutyrate (PDBu, an activator of PKC). These results indicate that the sensitivity of MLC phosphorylation to Ca2+ is enhanced both by ET-1 with GTP and by PDBu. 4. PKC19-36 (an inhibitor of PKC) modified neither the contraction nor MLC phosphorylation induced by 0.3 microM Ca2+ but blocked the PDBu-induced enhancement of both these Ca(2+)-induced responses. However, PKC19-36 only partly inhibited the enhancement produced by ET-1 with GTP on the Ca(2+)-induced responses. PKC19-36 did not modify the relationship between force and MLC phosphorylation in the presence either of ET-1 with GTP or of PDBu. By contrast, BQ-123, neomycin and guanosine 5'-O-(2-thiodiphosphate) (GDP beta S) each abolished the ET-1-induced enhancement of the contraction induced by 0.3 microM Ca2+. 5. These results suggest that ET-1 acts on the ETA receptor and increases Ca2+ influxes through an activation of the dihydropyridine-sensitive Ca2+ channel, causing a long-lasting and maintained contraction in resistance vessels of the rabbit mesentery.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of monoamine oxidase inhibition by selegiline on concentrations of noradrenaline and monoamine metabolites in CSF of patients with Alzheimer's disease

A double-blind, cross-over trial with 12 patients with Alzheimer's disease (AD) was carried out primarily to test the suitability of this design in the investigation of the clinical effects of selegiline (10 mg/day) in AD. Cerebrospinal fluid (CSF) samples for the determination of concentrations of noradrenaline (NA) and several monoamine metabolites were collected at baseline and at the end of both four-week treatment periods (placebo and selegiline). The severity of dementia was assessed using Ferm's and Gottfries-Bråne-Steen (GBS) dementia scales. The concentrations of the dopamine metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC) and the NA metabolites, 3,4-dihydroxyphenylglycol (DHPG), and 3-methoxy-4-hydroxyphenyl glycol (MHPG) decreased significantly during selegiline treatment. There was a clear trend of reduction in concentrations of homovanillic acid (HVA) during selegiline treatment, whereas the concentrations of NA, 5-hydroxyindoleacetic acid (5-HIAA), and tryptophan did not differ significantly. The study design was not suitable for the analysis of the clinical results as there was a significant carry-over effect in both scales. As only the first period data could be used in the analysis, there were no significant differences in the scores of Ferm's or GBS scales, but clear positive trends could be detected in favour of selegiline.

Vasoconstriction of guinea-pig submucosal arterioles following sympathetic nerve stimulation is mediated by the release of ATP

1. The nature of the transmitter mediating vasoconstriction of guinea-pig submucosal arterioles following sympathetic nerve stimulation was studied. 2. Prazosin (0.1 microM) abolished the response to exogenously applied phenylephrine (1 microM) but had no effect on constrictions of submucosal arterioles evoked by nerve stimulation (100 pulses at 10 Hz). 3. Vasoconstrictions and excitatory junction potentials elicited by nerve stimulation were potentiated by idazoxan (0.1 microM). 4. Following reserpine treatment, catecholamine fluorescence was absent in submucosal arterioles but nerve-evoked vasoconstrictions were unaltered. 5. Vasoconstrictions and excitatory junction potentials recorded in response to sympathetic nerve stimulation, as well as constrictions evoked by exogenously applied ATP (3 microM), were abolished by the P2-purinoceptor antagonist, suramin (100 microM). Suramin had no effect on the vasoconstriction in response to noradrenaline (3 microM), or the nicotinic excitatory postsynaptic potentials (e.p.s.ps) and noradrenergic inhibitory postsynaptic potentials (i.p.s.ps) recorded from submucosal neurones. 6. We conclude that postjunctional responses of submucosal arterioles following sympathetic nerve stimulation are mediated solely through the activation of P2X-purinoceptors by ATP or a related purine nucleotide. The function of neurally released noradrenaline is to act through prejunctional alpha 2-adrenoceptors to depress transmitter release.

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.

Interactions between the effects of endothelin-1, clonidine and yohimbine on electrically-induced contractions in rat vas deferens

1. The relationship between endothelin-1(ET-1)-induced effects on the contractile responses of epididymal portion of rat vas deferens elicited by field electrical stimulation (FES: 80 V, 1 msec, 0.1 Hz) and the effects of the alpha 2-adrenoceptor agonist clonidine and the alpha 2-adrenoceptor antagonist yohimbine were studied. 2. ET-1 (0.01 nM-0.1 microM) concentration-dependently increased the FES-induced contractions. 3. ET-1 (0.1 nM-0.1 microM) reversed the inhibitory effect of clonidine on the FES-evoked contractions whereas ET-1 applied before clonidine exerted a dual effect on the clonidine-induced inhibition of the FES-evoked contractions. 4. The ET-1-induced enhancement of FES-induced contractions was potentiated in the presence of 1 microM yohimbine and was not observed at all in the presence of 10 microM yohimbine. Yohimbine, applied at concentrations of 1 and 10 microM exerted similar blocking effects on the alpha 1-adrenoceptor agonistic effects of phenylephrine. However, yohimbine at a concentration of 10 microM markedly potentiated the contractile effect of exogenous adenosine 5'-triphosphate (ATP), 30 microM. Tetrodotoxin abolished this effect of yohimbine. 5. The results presented here suggest the existence of modulating interactions between the ET-1-evoked increase of FES-induced contractions of rat vas deferens and the alpha 2-adrenoceptor drugs clonidine and yohimbine.

Membrane hyperpolarization inhibits agonist-induced synthesis of inositol 1,4,5-trisphosphate in rabbit mesenteric artery

1. Effects of membrane hyperpolarization induced by pinacidil on Ca2+ mobilization induced by noradrenaline (NA) were investigated by measuring intracellular Ca2+ concentration ([Ca2+]i), isometric tension, membrane potential and production of inositol 1,4,5-trisphosphate (IP3) in smooth muscle cells of the rabbit mesenteric artery. 2. Pinacidil (0.1-10 microM) concentration dependently hyperpolarized the smooth muscle membrane with a reduction in membrane resistance. Glibenclamide (1 microM) blocked the membrane hyperpolarization induced by 1 microM-pinacidil. NA (10 microM) depolarized the smooth muscle membrane with associated oscillations. Pinacidil (1 microM) inhibited this response and glibenclamide (1 microM) prevented the action of pinacidil on both the NA-induced events. 3. In thin smooth muscle strips, 10 microM-NA produced a large phasic and a subsequent small tonic increase in [Ca2+]i with associated oscillations. These changes in [Ca2+]i seemed to be coincident with phasic, tonic and oscillatory contractions, respectively. Pinacidil (0.1-1 microM) inhibited the increases in [Ca2+]i and in tension induced by NA, but not by 128 mM-K+. Glibenclamide inhibited these actions of pinacidil. Pinacidil (1 microM) also inhibited the contraction induced by 10 microM-NA in strips treated with A23187 (which functionally removes cellular Ca2+ storage sites), suggesting that membrane hyperpolarization inhibits Ca2+ influxes activated by NA. 4. In Ca2(+)-free solution containing 2 mM-EGTA, NA (10 microM) transiently increased [Ca2+]i, tension and synthesis of IP3. Pinacidil (over 0.1 microM) inhibited the increases in [Ca2+]i, tension and synthesis of IP3 induced by 10 microM-NA in Ca2(+)-free solution containing 5.9 mM-K+, but not in a similar solution containing 40 or 128 mM-K+. Glibenclamide (1 microM) inhibited these actions of pinacidil. These inhibitory actions of pinacidil were still observed in solutions containing low Na+ or low Cl-. These results suggest that pinacidil inhibits NA-induced Ca2+ release from storage sites through an inhibition of IP3 synthesis resulting from its membrane hyperpolarizing action. 5. In beta-escin-treated skinned strips, NA (10 microM) or IP3 (20 microM) increased Ca2+ in Ca2(+)-free solution containing 50 microM-EGTA and 3 microM-guanosine triphosphate (GTP) after brief application of 0.3 microM-Ca2+, suggesting Ca2+ is released from intracellular storage sites. Heparin (500 micrograms/ml, an inhibitor of the IP3 receptor), but not pinacidil (1 microM) or glibenclamide (1 microM), inhibited the Ca2+ release from storage sites induced by NA or IP3. These results suggest that membrane hyperpolarization is essential for the inhibitory action of pinacidil on the NA-induced Ca2(+)-releasing mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of the subtype of presynaptic alpha 2-adrenoceptors modulating noradrenaline release in cat and bovine cerebral arteries

The possible existence of a heterogeneous population of alpha 2-adrenoceptors (alpha 2A and alpha 2B, demonstrated by binding studies) in adrenergic nerve endings of cat and bovine cerebral arteries modulating noradrenaline release was investigated. Electrical field stimulation elicited an increase of tritium secretion from these vessels preincubated with (+/-)-[3H]noradrenaline, which was reduced by the alpha 2-agonists, clonidine (1 microM) and B-HT 920 (0.01 and 0.1 microM), in cat cerebral arteries but only by B-HT 920 in bovine cerebral arteries. This reduction was inhibited by the antagonist of the alpha 2B-subtype, prazosin, and the antagonists of alpha 2A- and alpha 2B-subtypes yohimbine and particularly rauwolscine. The effect of B-HT 920 was partially inhibited by clonidine in bovine, but not in cat cerebral arteries. In both types of arteries, prazosin, yohimbine and the alpha 1-agonist methoxamine (all at 1 microM) failed to modify the stimulated radioactivity liberation, whereas it was increased by 1 microM rauwolscine, and by yohimbine plus prazosin in cat cerebral arteries. The basal tritium release was enhanced by rauwolscine and prazosin in cat cerebral arteries but only by the latter in bovine cerebral arteries. These results suggest: (1) the existence of presynaptic alpha 2-adrenoceptors, mainly of the alpha 2B-subtype, in these vessels negatively modulating noradrenaline release, their activity being greater in cat than in bovine cerebral arteries, and (2) clonidine has no agonistic but a weak antagonistic action in the latter vessels.

Influence of neuronal uptake on pre- and postjunctional effects of alpha-adrenoceptor agonists in tissues with noradrenaline--ATP cotransmission

This study was undertaken in an attempt to explain why in some of the tissues in which noradrenaline and ATP act as co-transmitters the noradrenergic component predominates, while in others the predominant component is purinergic. Four different tissues were used: the epididymal portion of the rat vas deferens and the rabbit ear artery, tissues where the noradrenergic component predominates, and the prostatic portion of the rat vas deferens and the rabbit jejunal artery, where the purinergic component predominates. The noradrenaline content as well as the electrically-evoked release of noradrenaline were determined in all tissues. To determine the evoked release, the tissues were pretreated with pargyline (1 mmol.l-1) and then exposed to 3H-noradrenaline, washed out and transmurally stimulated (1 Hz). In addition, the influence of inhibition of neuronal uptake by desipramine (40 nmol.l-1) on pre- and postjunctional effects of adrenaline and alpha-methylnoradrenaline (and/or noradrenaline) was compared. The noradrenaline content of the tissues averaged: 17.4, 23.2, 3.1, and 4.8 micrograms.g-1 for the epididymal and the prostatic portions of the rat vas deferens and for the ear and the jejunal arteries of the rabbit, respectively. The fractional electrically-evoked release of 3H-noradrenaline was 2.02 and 2.04 x 10(-5) for the epididymal and the prostatic portions of the rat vas deferens, respectively, and 3.33 and 3.26 x 10(-5) for the ear and the jejunal arteries of the rabbit, respectively. Desipramine enhanced much more the postjunctional effect of noradrenaline, adrenaline, and alpha-methylnoradrenaline in the epididymal than in the prostatic portion of the rat vas deferens.(ABSTRACT TRUNCATED AT 250 WORDS)

Actions of dipyridamole on endogenous and exogenous noradrenaline in the dog mesenteric vein

1. In the isolated mesenteric vein of the dog, dipyridamole inhibited both the excitatory junction potential (e.j.p.) and the slow depolarization evoked by perivascular nerve stimulation, to 60-70% of control, with no change in the postjunctional membrane potential. These inhibitory actions of dipyridamole were not modified by 8-phenyltheophylline or phentolamine, suggesting that the inhibition did not involve either the actions of endogenous adenosine or the prejunctional alpha-autoregulation mechanism. 2. Dipyridamole did not produce any detectable effects on either the facilitation process of the e.j.ps or the postjunctional membrane depolarization produced by exogenously applied noradrenaline (NA). 3. Dipyridamole reduced the outflow of both the NA and the 3,4-dihydroxyphenylglycol (DOPEG) evoked by perivascular nerve stimulation to below 10% of control, the effect being much greater than that of exogenously applied adenosine (to about 90% of the control). 4. Exogenously-added NA was degraded by incubation with a segment of the vein. Dipyridamole itself produced degradation of NA and accelerated the NA-induced degradation. By contrast, pyrogallol, but not pargyline or imipramine, prevented the NA-induced degradation. 5. It is suggested that dipyridamole degrades NA directly, and also indirectly through activation of catechol-O-methyl transferase, with no alteration of the activity of monoamine oxidase or of the uptake mechanisms of NA into nerve terminals.

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.

Effect of blockade of noradrenaline re-uptake on evoked tritium overflow from mouse vasa deferentia and rat cortex slices

1. In tissues previously incubated with [3H]-noradrenaline exposure to cocaine (0.1 to 10 microM) or desmethylimipramine (0.01 to 1 microM) produced a concentration-dependent increase (up to 2 fold) in electrically evoked (3 Hz, 2 ms, 20 mA, 120s every 20 min) fractional overflow of tritium from rat brain cortex slices but not from mouse vas deferens (2.5 Hz, 2 ms, 400 mA, for 90s every 14 min). 2. Yohimbine and idazoxan (0.01 to 1 microM) increased fractional evoked overflow of tritium by up to 2 fold; in the presence of these drugs, cocaine (10 microM) produced an increase in both tissues (up to 3.5 fold over control). 3. In brain slice an increase in stimulation frequency (0.1, 0.5, 1, 3 and 6 Hz) decreased fractional evoked overflow of tritium per pulse but cocaine (10 microM) produced a significant enhancement at each frequency except 6 Hz. In vas deferens fractional tritium overflow per pulse changed little with increasing frequency and cocaine produced no effect. 4. In both tissues fractional evoked overflow of tritium was dependent on the stimulation current; cocaine (10 microM) increased fractional evoked overflow from brain slice at every current tested but was without effect in vas deferens. 5. Chromatographic separation of the released tritium showed there was little difference in the proportions of [3H]-noradrenaline and 3H-metabolites overflowing from the tissues. Cocaine increased the proportion of [3H]-noradrenaline and decreased the proportion of [3H]-DOPEG overflowing both at rest and during stimulation. 6. In brain slice an increase in electrically evoked overflow was produced by cocaine (10 microM) whether total tritium overflow (1.8 fold), overflow of [3H]-noradrenaline (1.8 fold) or overflow of unlabelled noradrenaline (1.8 fold) was measured. Evoked overflow from vas deferens was unaffected when assessed by any of these three methods. 7. The mechanism responsible for this differential effect of cocaine is unclear but may involve differences in the physical relationship between release sites, reuptake sites and presynaptic autoreceptors.

Pinacidil inhibits neuromuscular transmission indirectly in the guinea-pig and rabbit mesenteric arteries

1. Effects of pinacidil were investigated on neuromuscular transmission in smooth muscle tissues of the rabbit and guinea-pig mesenteric arteries by both electrophysiological procedures and a bioassay of noradrenaline (NA) outflows. 2. Pinacidil (over 1 microM) hyperpolarized smooth muscle cell membranes in both tissues, in a concentration dependent manner. Pinacidil hyperpolarized and increased the ionic conductance of smooth muscle membrane more markedly in the rabbit mesenteric artery than in the guinea-pig. The hyperpolarization induced by pinacidil occurred in the presence or absence of endothelial cells and was blocked by glibenclamide. 3. Perivascular adrenergic nerve stimulation produced excitatory junction potentials (e.j.ps) and repetitive stimulation produced a facilitation of e.j.ps in both tissues. Pinacidil (over 1 microM) reduced the amplitude and the decay time of e.j.ps to a consistently greater extent in the rabbit mesenteric artery than in the guinea-pig. However, the facilitation process of e.j.ps was not modified following application of pinacidil (1 microM). The pinacidil-induced inhibition of e.j.ps was prevented by pretreatment with glibenclamide. 4. Pinacidil (30 microM) marginally increased the overflows of NA and its metabolite, 3,4-dihydroxyphenylglycol (DOPEG) released following repetitive perivascular nerve stimulations. 5. Pinacidil (10 microM) partly inhibited the voltage-dependent Ca channel, as estimated from the recovery process following removal of pinacidil, of action potentials evoked on e.j.ps. 6. It is concluded that pinacidil increases ionic conductance and hyperpolarizes smooth muscle cell membranes of the guinea-pig and rabbit mesenteric arteries and as a consequence, inhibits the neuromuscular transmission process occurring on adrenergic nerve stimulation with no reduction in the amount of released transmitter.

Prejunctional alpha 1-adrenoceptors modify release of [3H]noradrenaline in the guinea-pig vas deferens

1. Several alpha 1- and alpha 2-adrenoceptor agonists and antagonists were examined for effects on spontaneous and stimulus-evoked release of [3H]noradrenaline from sympathetic nerves in guinea-pig vas deferens. 2. Prazosin (0.1 and 1 microM), phentolamine (30 microM) and yohimbine (10 microM) each enhanced the stimulus-evoked release of [3H]noradrenaline. 3. Prazosin and phentolamine increased the spontaneous outflow of [3H]noradrenaline, whereas yohimbine was without effect. 4. Methoxamine (10 microM) and clonidine (0.1 microM) inhibited the stimulus-evoked release of [3H]noradrenaline, whereas only methoxamine (1 microM) decreased the spontaneous outflow of [3H]noradrenaline. 5. The identity of prejunctional alpha-adrenoceptors in the guinea-pig vas deferens is discussed.

Regulation of noradrenaline release in human cerebral arteries via presynaptic alpha 2-adrenoceptors

1. Electrical stimulation induced tritium release from branches of human middle cerebral arteries preincubated with [3H]noradrenaline (NA), which was reduced by the alpha 2-adrenoceptor agonists, clonidine and B-HT 920, and not affected by the alpha 1-agonist, methoxamine. 2. The stimulated tritium release was inhibited by yohimbine (alpha 2-antagonist), and increased by phentolamine (alpha-antagonist) and prazosin (alpha 1-antagonist). 3. The inhibitory effect of clonidine was antagonized by yohimbine. 4. NA uptake was markedly reduced when the interval between the death and the autopsy was greater than 5 hr. 5. These data indicates the existence of presynaptic inhibitory alpha 2-adrenoceptors, but not alpha 1, in human cerebral arteries, and that the adrenergic nerve endings start to degenerate from 5 hr after death.

Inhibitory actions of adenosine differ between ear and mesenteric arteries in the rabbit

In isolated ear and mesenteric arteries of rabbit, adenosine inhibited nerve-mediated contractions to a similar extent. However, the amplitude of the excitatory junction potentials evoked by perivascular nerve stimulation was increased by adenosine in the ear artery and decreased in the mesenteric artery. Outflows of noradrenaline and its metabolite, 3,4-dihydroxyphenylglycol, evoked by perivascular nerve stimulation were increased and decreased by adenosine in the ear and mesenteric arteries, respectively. Adenosine hyperpolarized the smooth muscle cells, by increasing potassium conductance of the membrane, with no relation to the endothelial cells. The hyperpolarizing action of adenosine was stronger in the ear artery than in the mesenteric artery. The inhibition of the nerve-mediated contraction by adenosine may be mainly due to postjunctional events in the ear artery and prejunctional events in the mesenteric artery.

Presynaptic dopamine DA2-receptors in rabbit jejunal arteries. An electrophysiological study

Excitatory junction potentials (e.j.ps) evoked by nerve stimulation with 15 pulses at 1 Hz were recorded from muscle cells of rabbit isolated jejunal arteries. LY 171555 1 mumol/l, SKF 38393 10 mumol/l, dopamine 10 mumol/l and clonidine 0.1 mumol/l depressed all e.j.ps in the train. The percentage inhibition was inversely related to the number of pulses. S- and R-sulpiride, 10 mumol/l, domperidone 1 mumol/l, SCH 23390 1 mumol/l and rauwolscine 1 mumol/l did not change, or even depressed the first e.j.ps. Of these compounds only S- and R-sulpiride, 10 mumol/l and rauwolscine 1 mumol/l facilitated the late e.j.ps. The percentage facilitation increased with the number of pulses until a maximum was reached; rauwolscine 1 mumol/l had the largest effect. S- and R-sulpiride, 10 mumol/l, as well as domperidone 1 mumol/l antagonized the action of LY 171555 1 mumol/l. S-Sulpiride was more potent than its R-isomer. SCH 23390 1 mumol/l and rauwolscine 1 mumol/l blunted the effect of SKF 38393 10 mumol/l. Rauwolscine 1 mumol/l slightly reduced the inhibition by dopamine 10 mumol/l; S-sulpiride 10 mumol/l was antagonistic only in the presence of rauwolscine 1 mumol/l. When rauwolscine 1 mumol/l, prazosin 0.1 mumol/l, propranolol 1 mumol/l and cocaine 10 mumol/l was added to the medium, dopamine 10 mumol/l continued to produce the same depression of e.j.ps, as in the absence of these compounds. Under such conditions S-sulpiride 10 mumol/l also counteracted dopamine 10 mumol/l. Rauwolscine 1 mumol/l prevented the effect of clonidine 0.1 mumol/l. The antagonists were not absolutely selective against only one type of agonist. We suggest that both presynaptic DA2- and postsynaptic DA1-receptors are present in rabbit jejunal arteries. The activation of either receptor-type may depress the e.j.ps. Dopamine interferes with neuroeffector transmission due to alpha 2-adrenoceptor agonist properties; its DA2-effect is unmasked only after alpha 2-adrenoceptor blockade. There was no evidence for a co-transmitter function of dopamine.

Bunazosin, an alpha 1-adrenoceptor blocker, differentially releases co-transmitters in dog mesenteric vessels

The effects of bunazosin on the electrical and mechanical responses of smooth muscle cells elicited by exogenously applied noradrenaline (NA) and by perivascular nerve stimulation were studied in the isolated mesenteric artery and vein of the dog. NA (above 10(-7) M in the artery and above 3 X 10(-8) M in the vein) depolarized the membrane. Perivascular nerve stimulation evoked an excitatory junction potential (e.j.p.) and slow depolarization in both vessels. Bunazosin and prazosin inhibited the NA-induced depolarization and slow depolarization in the artery but not in the vein. The NA actions in the vein were inhibited by yohimbine. Bunazosin (above 10(-6) M) increased the amplitude of the e.j.p. but decreased the outflow of NA during nerve stimulation. The amplitude and conduction velocity of the compound action potential of perivascular nerves were inhibited by higher concentrations of bunazosin (above 10(-5) M). The results provide evidence that bunazosin has selective inhibitory actions at alpha 1-adrenoceptors. This drug exerted differential effects on the release of co-transmitters which generate the e.j.p. and the slow depolarization, as bunazosin increased the former and decreased the latter. This suggests that e.j.p. is generated by a substance other than NA.

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.

Modulation of noradrenaline release from cat cerebral arteries by presynaptic alpha 2-adrenoceptors. Effect of chronic treatment with desipramine and cocaine

1. Field electrical stimulation elicited an increase of the tritium efflux over the basal level from cat cerebral arteries previously incubated with (+/-) [3H]noradrenaline ([3H]NA). 2. This efflux was: (a) reduced by clonidine, NA or B-HT 920; (b) unaffected by methoxamine, prazosin and yohimbine (10(-6) M); (c) reduced by yohimbine (5 x 10(-6) M), and (d) increased by phentolamine. 3. The effect of clonidine was blocked by yohimbine. 4. The daily treatment with the neuronal uptake blockers desipramine (10 mg/kg, i.p.) or cocaine (10 mg/kg, i.p.) [during 12 days], antagonized the inhibitory action of clonidine totally or partially, respectively. 5. These results suggest: (1) the existence of presynaptic alpha 2-adrenoceptors in these arteries, which modulate the NA release, and (2) that chronic treatment with desipramine or cocaine induces a subsensitivity of these alpha 2-receptors, which facilitates the NA release.

Presynaptic P1-purinoceptors in jejunal branches of the rabbit mesenteric artery and their possible function

1. Excitatory junction potentials (EJPs) evoked by nerve stimulation with fifteen pulses at 1 Hz were recorded from smooth muscle cells of the rabbit isolated mesenteric artery. The effects of P1-purinoceptor agonists and antagonists, as well as of substances which interfere with the inactivation of endogenous adenosine, were tested. 2. Adenosine and its analogues depressed the EJPs in the train in a concentration-dependent manner. The percentage inhibition of the first EJP and that of the later ones was similar; some early EJPs, however, were inhibited more markedly. The rank order of potency of the agonists was (-)-N6-(R-phenylisopropyl)-adenosine (R-PIA) congruent to 5'-N-ethylcarboxamidoadenosine (NECA) greater than (+)-N6-(S-phenylisopropyl)-adenosine (S-PIA) greater than adenosine. The respective IC40 values (the concentrations producing 40% inhibition of the first EJP in the train) were 0.018, 0.028, 0.83 and 4.7 mumol/l. 3. Three methylxanthines, namely 8-phenyltheophylline (1, 10 mumol/l), 8-cyclopentyltheophylline (0.1, 1 mumol/l) and 8-(p-sulphophenyl)-theophylline (100 mumol/l), antagonized the effect of R-PIA (0.1 mumol/l). When given alone they also enhanced the amplitudes of all EJPs in the train. The percentage facilitation of the first EJP and that of the later ones was similar. Some early EJPs, however, were potentiated more markedly. 8-Phenyltheophylline was less potent than 8-cyclopentyltheophylline both in preventing the action of R-PIA and in enhancing the EJPs. A concentration (100 mumol/l) of 8-(p-sulphophenyl)-theophylline, which strongly antagonized the R-PIA effect, produced only a moderate facilitation of EJPs. 4. S-(p-nitrobenzyl)-6-thioguanosine (10 mumol/l) both depressed the EJPs, and enhanced the inhibitory effect of adenosine. Adenosine deaminase (10 micrograms/ml) caused some potentiation of EJPs; this action was prevented by a concentration (10 mumol/l) of deoxycoformycin, which had no effect of its own. AH21-132 (10 mumol/l) enhanced all EJPs in the train. 5. None of the above substances influenced the resting membrane potential of the smooth muscle cells. In addition, R-PIA (0.1 mumol/l) did not change the depolarization induced by noradrenaline (3 mumol/l). 6. We suggest that the axon terminals of postganglionic sympathetic neurones in the rabbit mesenteric artery possess P1-purinoceptors of the A1-type. The activation of these presynaptic receptors by endogenous adenosine may inhibit the release of the main neuroeffector transmitter, which is probably ATP.(ABSTRACT TRUNCATED AT 400 WORDS)

Plasma dihydroxyphenylglycol and the intraneuronal disposition of norepinephrine in humans

We examined plasma levels of the sympathetic neurotransmitter norepinephrine (NE) and its deaminated metabolite dihydroxyphenylglycol (DHPG) during supine rest in healthy human subjects and in sympathectomized patients, during physiological (tilt) or pharmacological (yohimbine, clonidine) manipulations known to affect sympathetically mediated NE release, during blockade of neuronal uptake of NE (uptake-1) using desipramine, and during intravenous infusion of NE. Healthy subjects had a mean arteriovenous increment in plasma DHPG in the arm (10%, P less than 0.05), whereas sympathectomized patients had a mean arteriovenous decrement in DHPG in the affected limb (mean decrease 21%, P less than 0.05 compared with healthy subjects). Tilt and yohimbine, which stimulate, and clonidine, which inhibits, release of endogenous NE, produced highly correlated changes in plasma NE and DHPG (r = 0.94). Pretreatment with desipramine abolished DHPG responses to yohimbine while enhancing NE responses. To attain a given increase in plasma DHPG, about a tenfold larger increment in arterial NE was required during NE infusion than during release of endogenous NE. When plasma NE was markedly suppressed after administration of clonidine, plasma DHPG decreased to a plateau level of 700-800 pg/ml. The results indicate that (i) plasma DHPG in humans is derived mainly from sympathetic nerves; (ii) increments in plasma DHPG during stimulation of NE release result from uptake of NE into sympathetic nerve endings and subsequent intraneuronal conversion to DHPG; (iii) plasma DHPG under basal conditions probably is determined mainly by net leakage of NE into the axonal cytoplasm from storage vesicles; and (iv) increments in NE concentrations at neuronal uptake sites can be estimated by simultaneous measurements of DHPG and NE during NE infusion and NE release. Measurement of NE and DHPG provides unique clinical information about sympathetic function.

Modulation by noradrenaline and yohimbine of noradrenergic transmission in the guinea-pig mesenteric artery

In the guinea-pig mesenteric artery, transmitter release modulated by noradrenaline (NA) or yohimbine was estimated from changes in amplitude of the excitatory junction potential (e.j.p.) recorded from smooth muscle cells. NA decreased the amplitude of the e.j.p. with no change in the facilitation. Yohimbine antagonized the effect of NA on the e.j.p. amplitude and enhanced the facilitation of e.j.p.; the latter action was not antagonized by NA. TTX-resistant e.j.p.s evoked by stronger intensity of stimuli were not affected by NA or yohimbine. It is concluded that NA inhibits and yohimbine enhances the release of transmitter, and that the latter event involves prejunctional alpha-adrenoceptor-dependent and -independent processes.

Electrophysiological evidence for an alpha 2-adrenergic inhibitory control of transmitter release in the rabbit mesenteric artery

Excitatory junction potentials (e.j.p.s) evoked by nerve stimulation with 15 pulses at 1 Hz were recorded from muscle cells of the rabbit isolated mesenteric artery. Clonidine and B-HT 933 depressed all e.j.p.s in the train. The percentage inhibition was inversely related to the number of pulses. Yohimbine, rauwolscine and tolazoline reduced the early e.j.p. amplitudes but enhanced the later ones. The percentage facilitation of e.j.p.s increased with the number of pulses until a maximum was reached. Prazosin and corynanthine did not influence the first few e.j.p.s but potentiated the subsequent ones; their effects were less pronounced than those of yohimbine and rauwolscine. All the drugs antagonized the inhibition by clonidine but the effects of yohimbine and rauwolscine were more marked than those of prazosin and corynanthine. Phenylephrine, St 587 and noradrenaline depressed the e.j.p.s. Yohimbine diminished the effects of these substances and was a stronger antagonist of phenylephrine than prazosin. We suggest that, in the rabbit mesenteric artery, noradrenaline and the neuroeffector transmitter (probably ATP) are co-released from the terminals of postganglionic sympathetic nerves. Noradrenaline activates presynaptic alpha 2-adrenoceptors and thereby depresses transmitter release. The degree of presynaptic inhibition depends on the number of pulses applied, i.e. on the biophase concentration of noradrenaline.

Identification of the neuroeffector transmitter in jejunal branches of the rabbit mesenteric artery

Vasoconstriction or excitatory junction potentials (e.j.ps) evoked by nerve stimulation (15 field pulses at 2 Hz every 3 min) were recorded in rabbit isolated jejunal arteries. The resting diameter of the arteries and its decrease in response to stimulation was measured by a photoelectric method. Vasoconstriction was insensitive to prazosin 0.1 or 1 mumol/l. Yohimbine 1 mumol/l considerably enhanced, whereas alpha,beta-methylene ATP (alpha,beta-meATP) 1 mumol/l abolished the contractile response. In order to test the effect of exogenously applied transmitter candidates, noradrenaline (0.1-1 mumol/l) and ATP (10-30 mumol/l) were added in concentrations which evoked a vasoconstriction comparable to that induced by electrical stimulation. The action of noradrenaline was prevented by prazosin 0.1 mumol/l, but was unaffected by both yohimbine 1 mumol/l and alpha,beta-meATP 1 mumol/l. Alpha,beta-meATP 1 mumol/l depressed the effect of ATP. The e.j.ps evoked by a train of 15 pulses showed facilitation up to the third response and thereafter depression; a partial summation was also observed. Prazosin 0.1 mumol/l did not change the e.j.p. amplitudes. By contrast, when yohimbine 0.1 or 1 mumol/l was added to the prazosin-containing medium, both the late e.j.ps in the train and the summation were enhanced in a concentration-dependent manner. Alpha,beta-meATP 1 mumol/l almost abolished the e.j.ps. In conclusion, in rabbit jejunal arteries, stimulation of postganglionic sympathetic nerves may release noradrenaline together with ATP which is probably the sole neuroeffector transmitter under our conditions. Transmitter release seems to be modulated by the activation of presynaptic alpha 2-adrenoceptors. Under the stimulation conditions of the present experiments the released transmitter does not activate postsynaptic alpha 1-adrenoceptors.

Pre- and post-junctional effects of adenosine triphosphate on noradrenergic transmission in the rabbit ear artery

1. The effects of adenosine triphosphate (ATP), 5'-adenylylimidodiphosphate (AMP-PNP) or alpha,beta-methylene ATP (mATP) on the excitatory junction potential (e.j.p.) and slow depolarization evoked by perivascular nerve stimulation were studied in smooth muscle cells of the rabbit ear artery. 2. ATP (above 10(-6) M), AMP-PNP (above 10(-6) M) and mATP (above 10(-8) M) transiently (10-15 min) depolarized the membrane. The membrane remained depolarized after prolonged exposure (over 20 min) to ATP (above 3 X 10(-5) M), AMP-PNP (above 10(-5) M) or mATP (above 3 X 10(-8) M). 3. ATP (above 10(-5) M), AMP-PNP (above 5 X 10(-6) M) or mATP (above 3 X 10(-8) M) decreased the membrane resistance. Increasing the external K+ concentration (K+o) to 10.1 mM also decreased the membrane resistance, with an associated depolarization. 4. ATP (10(-6)-5 X 10(-5) M) or AMP-PNP (over 10(-6) M) transiently decreased and then increased amplitudes of the e.j.p. and of the slow depolarization, the latter component increasing more than the former. 5. Depolarization of the membrane by 10.1 mM-K+o solution or mATP (10(-7) M) decreased the amplitude of e.j.p.s, with no change in the facilitation, and the slope of the relationship between amplitude of e.j.p. and that of slow depolarization decreased with mATP but not with 10.1 mM-K+o solution. 6. The outflows of noradrenaline and 3,4-dihydroxyphenylglycol (DOPEG) induced by perivascular nerve stimulation increased with ATP (above 10(-6) M) or AMP-PNP (above 10(-5) M), while there was no change with mATP (10(-8)-10(-5) M) or 10.1 mM-K+o solution. 7. Pre-treatment with mATP inhibited the ATP-induced increase in the outflow of noradrenaline and DOPEG, and also the ATP-induced enhancement of the amplitude of the e.j.p. 8. Therefore ATP and AMP-PNP have predominantly excitatory actions on both pre- and post-junctional membranes, while mATP has an excitatory action on the post-junctional membrane but antagonizes the facilitatory action of ATP on release of noradrenaline from the nerve terminal.

Pre- and postjunctional alpha-adrenoceptor-mediated effects of prazosin, methoxamine and 6-fluoronoradrenaline in blood-perfused canine skeletal muscle in situ

Pre- and postjunctional effects of the alpha 1-selective adrenoceptor antagonist prazosin and the alpha 1- and alpha 2-selective adrenoceptor agonists methoxamine and 6-fluoronoradrenaline, respectively, were studied in skeletal muscle in situ. Prazosin reduced the vasoconstriction and enhanced the overflow of endogenous noradrenaline elicited by sympathetic nerve stimulation (1-4 Hz, 2 min); the threshold concentration was 10-100 times lower for postjunctional than for prejunctional alpha-adrenoceptor blockade. The enhancement of noradrenaline overflow by prazosin was not inversely frequency-dependent, as shown elsewhere for alpha 2-adrenoceptor antagonists. Thus, different mechanisms may be involved. Inhibition of prostaglandin synthesis by diclofenac did not alter the stimulation-evoked noradrenaline overflow, indicating a minor importance of prostaglandin-mediated transjunctional mechanisms in the modulation of noradrenaline overflow. Methoxamine and 6-fluoronoradrenaline elevated the basal vascular tone and, at higher concentrations, reduced the stimulation-evoked noradrenaline overflow. Methoxamine was 20 times more selective than 6-fluoronoradrenaline for postjunctional receptors. Our results are compatible with a pre- and postjunctional localization of alpha 2-adrenoceptors and a predominantly, but not exclusively, postjunctional localization of alpha 1-adrenoceptors. The postjunctional selectivity for prazosin was less marked than previously reported from in vitro studies. Hence, care should be taken when extrapolating in vitro findings to the more complex in vivo situation.

Effects of bunazosin on electrical responses of smooth muscle cells of the guinea-pig mesenteric artery and vein to perivascular nerve stimulation and to noradrenaline

The noradrenaline-induced depolarization of smooth muscle cell membrane was blocked by bunazosin in the mesenteric artery but not in the mesenteric vein. Bunazosin enhanced the excitatory junction potential (e.j.p.) evoked in the mesenteric artery but did not modulate the slow depolarization evoked in the mesenteric vein. Application of noradrenaline decreased the amplitude of e.j.p. enhanced by bunazosin but not by yohimbine. It was concluded that bunazosin is a highly selective alpha 1-adrenoceptor blocker in vascular tissues.

Presynaptic opioid delta-receptors in the rabbit mesenteric artery

Excitatory junction potentials (e.j.p.s) evoked by nerve stimulation were recorded from muscle cells of the rabbit isolated mesenteric artery. At 0.03 Hz the e.j.p. amplitudes were stable. When a train of fifteen pulses was applied at 0.25 Hz or at higher frequencies (0.5, 1 and 2 Hz), e.j.p.s showed an initial facilitation followed by depression. [Met5]enkephalin 0.1 and 1 mumol/l, [D-Ala2,D-Leu5]enkephalin 0.1 and 1, but not 0.01 mumol/l, and [D-Pen2, L-Pen5]enkephalin 3 mumol/l all depressed the e.j.p.s evoked by trains of fifteen pulses at 1 Hz. When more than one concentration was used ([Met5]enkephalin, [D-Ala2,D-Leu5]enkephalin), the inhibition was concentration dependent. It was always greater for the first few e.j.p.s than for the later ones in a train. [Met5]enkephalin 1 mumol/l reduced the first e.j.p. at 1 Hz and the e.j.p.s evoked by 0.03 Hz to a similar extent. The inhibitory effect of [Met5]enkephalin 1 mumol/l on e.j.p.s persisted in the presence of yohimbine 0.3 mumol/l. Naloxone 1 mumol/l did not interfere with the effect of [Met5]enkephalin 1 mumol/l. Naloxone 10 mumol/l depressed some e.j.p.s and prevented the inhibition by [Met5]enkephalin 1 mumol/l. Neither ICI 154129 10 mumol/l nor ICI 174864 0.3 mumol/l had any effect of their own and both compounds antagonized the action of [Met5]enkephalin 1 mumol/l. Normorphine 10 mumol/l, fentanyl 1 mumol/l, ethylketocyclazocine 0.1 mumol/l, and dynorphin A(1-13) 1 mumol/l were all ineffective. Ethylketocyclazocine 1 mumol/l did not change the e.j.p.s either, but antagonized [Met5]enkephalin 1 mumol/l. [Met5]enkephalin 1 mumol/l failed to influence both the resting membrane potential of the muscle cells and the depolarizing effect of noradrenaline 3 and 30 mumol/l. We suggest that the axon terminals of post-ganglionic sympathetic neurones in the rabbit mesenteric artery possess opioid delta-, but not mu- or kappa-receptors. The activation of presynaptic delta-receptors inhibits the release of the neuroeffector transmitter. There is no evidence for any effect of co-released endogenous opioid peptides under our experimental conditions.

Inositol 1,4,5-trisphosphate activates pharmacomechanical coupling in smooth muscle of the rabbit mesenteric artery

To clarify the nature of the noradrenaline (NA)-induced contraction, the effects of NA on inositol phospholipid metabolism and the actions of inositol 1,4,5-trisphosphate (InsP3) on skinned muscle of the rabbit mesenteric artery were investigated. NA, in concentrations over 1 nM, reduced the amount of phosphatidylinositol 4,5-bisphosphate (PI-P2) and increased the amount of phosphatidic acid (PA). The maximum reduction in the amount of PI-P2 and the maximum increase in the amount of PA were observed in the presence of 1 microM-NA. With prolonged application of NA, the PI-P2 was gradually restored to near the control level, but with repeated applications of NA separated by rinses with Krebs solution, there was a consistent reduction of PI-P2. The NA-induced PI-P2 breakdown was inhibited by the alpha 1-adrenoceptor blocking agent, prazosin. After incubation of the tissue with radioactive inositol-containing solution, NA transiently increased the amount of radioactive InsP3 which was followed by increases in the amount of inositol 1,4-bisphosphate (InsP2) and inositol monophosphate (InsP). After accumulation of Ca by saponin-treated muscle cells of the dog mesenteric artery dispersed by collagenase, InsP3 released Ca stored in cells but InsP2 did not. A23187 (5 microM) but not InsP3 (up to 10 microM), depleted Ca accumulated in the presence of ATP. In saponin-treated skinned muscle tissues, InsP3 in concentrations over 0.3 microM, produced contraction following accumulation of Ca into the store site. InsP3 released Ca from the same source as caffeine. The release of Ca by InsP3 appeared in a concentration-dependent manner and this release also depended on the amount of Ca stored in cells (the median effective dose (ED50) was 3.0 microM in 0.1 microM-Ca and 1.0 microM in 0.3 microM-Ca). We concluded that NA activates alpha 1-adrenoceptors, thus hydrolysing PI-P2 and synthesizing InsP3. This product can release Ca stored in cells as estimated from the contraction in skinned muscle tissues, and also reduces the residual amount of Ca stored in skinned dispersed muscle cells. Contraction evoked by NA through pharmacomechanical coupling can be explained as a function of InsP3.

Noradrenaline and adenosine triphosphate as co-transmitters of neurogenic vasoconstriction in rabbit mesenteric artery

The largest rami caecales of the ileocolic artery, which is a branch of the mesenteric artery, were perfused at a constant rate of flow. Either vasoconstriction (as an increase in perfusion pressure) or the release of previously incorporated [3H]-noradrenaline was measured. Noradrenaline and ATP, but not carbachol, serotonin, adenosine, Arg-vasopressin and neuropeptide Y, caused marked vasoconstriction. When the sympathetic vasoconstrictor axons in the arterial wall were stimulated by electrical field pulses (either 5 pulses at 10 Hz or 100 pulses at 5 Hz; 0.3 ms pulse width, 200 mA current strength), the ensuing vasoconstriction was at best slightly reduced by phentolamine, prazosin and phenoxybenzamine. The response to 100 pulses, 5 Hz was even enhanced by phentolamine and yohimbine. All antagonists except yohimbine blocked the effect of exogenous noradrenaline. Prazosin did not change the effect of exogenous ATP. alpha,beta-Methylene-ATP (3-15 mumol/l) elicited transient vasoconstriction. Subsequently, responses to ATP as well as to electrical stimulation were reduced and recovered slowly. The response to noradrenaline was not changed. That part of the electrically induced vasoconstriction that remained after alpha,beta-methylene-ATP was almost abolished by phentolamine or prazosin. Pre-treatment of the animals with reserpine decreased but did not prevent the electrically evoked contraction of their arteries. The reserpine-resistant response was not changed by prazosin but was abolished by alpha,beta-methylene-ATP. The vasoconstriction elicited by electrical pulses was not affected by atropine or methysergide but was entirely blocked by tetrodotoxin, guanethidine or exposure to 6-hydroxydopamine. In arteries pre-incubated with [3H]-noradrenaline, electrical stimulation (100 pulses at 5 Hz) increased the outflow of tritium. The evoked overflow was blocked by tetrodotoxin, not changed by alpha,beta-methylene-ATP (9 mumol/l) or prazosin, and enhanced by phentolamine, phenoxybenzamine and yohimbine. We conclude that, in the branch of the mesenteric artery examined, both noradrenaline and ATP or a closely related compound transmit information from sympathetic neurones to smooth muscle. An alpha-adrenoceptor antagonist can reduce neurogenic vasoconstriction by blockade of post-junctional alpha-(probably alpha 1) receptors, reserpine by selective depletion of noradrenaline, and alpha,beta-methylene-ATP by desensitization of the post-junctional ATP (probably P2) receptor mechanism. Noradrenaline and ATP appear to be released from the same neurone. In addition, prejunctional alpha 2-adrenergic autoinhibition of transmitter release operates in the artery. alp

Effects of tyramine on noradrenaline outflow and electrical responses induced by field stimulation in the perfused rabbit ear artery

In the perfused rabbit ear artery the basal outflows of noradrenaline (NA) and 3,4-dihydroxyphenylglycol (DOPEG) were less than 1 ng g-1 and 1-2 ng g-1 wet weight of tissue respectively. Field stimulation increased outflows of NA and DOPEG in a frequency-dependent manner, and they reached the maximum value at frequencies over 5 Hz. Tyramine (1 X 10(-6) -1 X 10(-4) M) increased basal outflow of NA and DOPEG, in a dose-dependent manner. This effect was not blocked by tetrodotoxin (TTX, 3 X 10(-7) M), but was prevented by pretreatment with 6-hydroxydopamine (6-OHDA). Tyramine increased the field stimulation-induced outflow of NA but not that of DOPEG in a dose-dependent manner. Cocaine (1 X 10(-5) M) reduced the increased outflow of NA induced by tyramine at rest and during field stimulation, without modifying DOPEG-outflow. Guanethidine (5 X 10(-6) M), increased outflows of NA and DOPEG at rest, and reduced the NA outflow induced by field stimulation. Pretreatment with guanethidine (5 X 10(-6) M) did not block the action of tyramine on NA and DOPEG basal outflows. Additional application of guanethidine during the presence of tyramine did reduce the outflow of NA induced by field stimulation, but did not modify the outflow of NA and DOPEG at rest. Tyramine at concentrations over 1 X 10(-5) M depolarized the smooth muscle membrane of the rabbit ear artery. After chemical denervation with 6-hydroxydopamine (6-OHDA) the depolarizing action of tyramine was reduced. Tyramine-induced depolarization was attenuated by prazosin (5 X 10(-6) M) or phentolamine (5 X 10(-6) M), but not by guanethidine (5 X 10(-6) M). In 6-OHDA-denervated tissues, tyramine-induced depolarization was attenuated by phentolamine but not by prazosin. Field stimulation evoked excitatory junction potential (e.j.p.), slow depolarization and spike potential in the rabbit ear artery. Tyramine reduced, while guanethidine blocked these electrical responses. Tyramine did not alter the facilitation process of e.j.ps. In tissues pretreated with guanethidine, tyramine evoked either no electrical response or a slow depolarization during field stimulation. The slow depolarization was blocked by prazosin. Tyramine reduced the NA content of tissues in a dose-dependent manner (by 31% at 10(-4) M). Guanethidine (5 X 10(-6) M) reduced the NA content by 20%. 10 We conclude that in the rabbit ear artery, tyramine depolarizes the smooth muscle membrane indirectly by releasing neuronal NA which acts on alpha-adrenoceptors, and directly by an action on the smooth muscle cells. Two NA compartments (guanethidine-sensitive and tyramine-sensitive NA) could be identified. Field stimulation releases the former with associated generation of ej.p. and slow depolarization whilst the release of the latter is not accompanied by ej.p. generation.

Comparison of the effects of caffeine and procaine on noradrenergic transmission in the guinea-pig mesenteric artery

The effects of caffeine and procaine on noradrenergic transmission in the guinea-pig mesenteric artery were investigated by recording electrical responses of smooth muscle cells and by measuring the outflow of noradrenaline (NA) and 3,4-dihydroxyphenylglycol (DOPEG) induced by perivascular nerve stimulation. Caffeine possessed dual actions on the membrane, i.e., at low concentrations (2.5 X 10(-4)-5 X 10(-4)M), it hyperpolarized the membrane and decreased the membrane resistance and at high concentrations (over 2.5 X 10(-3)M) it depolarized the membrane and increased the membrane resistance. Procaine (over 10(-4)M) consistently depolarized the membrane and increased the membrane resistance. The amplitude of the excitatory junction potential (e.j.p.) produced by perivascular nerve stimulation was increased by low concentrations of procaine (2.5 X 10(-5)-10(-4)M) or high concentrations (10(-3)-5 X 10(-3)M) of caffeine and was decreased by low concentrations of caffeine (2.5 X 10(-5)-10(-4)M) or high concentrations of procaine (5 X 10(-4)-10(-3)M). Higher concentrations of caffeine (over 5 X 10(-3)M) induced a spike potential on the e.j.p., while higher concentrations of procaine (over 2.5 X 10(-3)M) inhibited the generation of e.j.ps. Facilitation of e.j.ps produced by repetitive stimulation of perivascular nerves remained unchanged by caffeine, while it was enhanced by procaine at any given concentration (caffeine 2.5 X 10(-4)-10(-3)M; procaine 10(-4)-10(-3)M). The membrane depolarization produced by exogenously applied NA (10(-5)M) was not blocked by pretreatment with procaine. Conduction velocity of perivascular nerve excitation remained unchanged by application of caffeine (up to 5 X 10(-3)M), and was reduced by application of procaine (over 2.5 X 10(-4)M). Outflow of NA during perivascular nerve stimulation remained unchanged by caffeine (10(-4)-3 X 10(-3)M), while it was enhanced by procaine (over 2.5 X 10(-4)M). The outflow of DOPEG was slightly reduced by caffeine (10(-3)-5 X 10(-3)M) and by lower concentrations of procaine (10(-4)-2.5 X 10(-4)M) but was not altered by higher concentrations of procaine (10(-3)-5 X 10(-3)M). It is concluded that in the guinea-pig mesenteric artery, high concentrations of caffeine (over 10(-3)M) increased the e.j.p. amplitude which might be due to an increase in membrane resistance of the smooth muscle cells. No marked effect of caffeine was observed on transmitter release from the nerve terminals. Procaine (over 2.5 X 10(-4)M) increased transmitter release from perivascular nerves and blocked the re-uptake mechanism of released NA. The mechanisms underlying the decrease in ej.p. amplitude by procaine remain to be determined.

Electrical responses of smooth muscle cells of the rabbit ear artery to adenosine triphosphate

Ionophoretic application of ATP to smooth muscle cells of the rabbit ear artery produced rapid depolarization of the membrane and, in the case of large doses of ATP, spike potentials or slow oscillatory potentials. The ATP response desensitized rapidly, and required over 70 s for recovery. When the intervals between repetitive application of ATP were shorter than 70 s, the amplitudes of the ATP responses successively decreased. Ejection of ATP with increasing intensities of current (10-15% of the first) was required to produce successively increasing amplitudes of ATP responses. Repetitive stimulation of perivascular nerves (at intervals of less than 10 s) evoked increasing amplitude of excitatory junction potentials (e.j.p.s). Quinidine (over 5 X 10(-5) M) inhibited and theophylline (over 5 X 10(-4) M) enhanced the ATP response, with associated depolarization or hyperpolarization of the membrane, respectively. Cocaine (over 10(-6) M) depolarized the membrane and enhanced the ATP response. Phentolamine reduced the amplitude of the ATP response with no change in the membrane potential, only when the concentration was extremely high (over 10(-4) M). These all therefore appear to represent non-specific interactions with the effects of ATP. Bath application of ATP depolarized the membrane dose dependently and, at concentrations over 5 X 10(-7) M, produced spike potentials. The amplitude of electrotonic potentials decreased during the ATP-induced depolarization, thereby suggesting an increase in ionic conductance of the membrane. ADP depolarized the membrane, the effect being weaker than that of ATP. Both AMP and adenosine hyperpolarized the membrane. The results provide evidence that in the rabbit ear artery, the e.j.p. could be mimicked by ATP. ATP can however only account for the fast e.j.p. if it is released in increasing amounts with successive nerve discharges. Reported blocking agents for ATP receptors did not block the response to ATP in this tissue.