Facilitatory effect of nicotine on adrenergic neuroeffector transmission in the isolated ear artery of the rabbit

The effects of nicotine were studied on perfusion pressure and vasoconstrictor responses to sympathetic nerve stimulation in the isolated ear artery of the rabbit. Infusions of nicotine (50 μM) produced a transient increase in perfusion pressure and potentiated responses to nerve stimulation; these effects of nicotine were unaffected by atropine (0–3 μM) and abolished or significantly reduced respectively by hexamethonium (300 μM) or mecamylamine (1 μM). In experiments with ear arteries previously labelled with [³ H] noradrenaline an infusion of nicotine (50 μM) produced a transient increase in tritium efflux and the potentiation of responses to nerve stimulation in the presence of nicotine was accompanied by a statistically significant increase in stimulation-induced tritium efflux; these effects of nicotine were abolished by hexamethonium (300 μM) or mecamylamine (1 μM).     

Effects of S9977 on adrenergic neurotransmission

1. Experiments were designed to determine whether or not the putative promnesic drug S9977 (1,3,7-trimethyl 8-[3-(4-diethylaminocarbonyl-1-piperazinyl) 1-propyl]-3,7-dihydro (1H)2,6-purinedione hydrochloride) affects peripheral adrenergic neurotransmission.2. Rings of canine saphenous veins (without endothelium) were suspended for isometric tension recording in conventional organ chambers filled with modified Krebs-Ringer bicarbonate solution. The adrenergic nerve endings were activated with electrical impulses (9 V, 2 msec, 0.25–8 Hz).3. At 10⁻⁵ M, S9977 significantly reduced the contraction to 0.25, 0.5 and 1 Hz. The compound did not affect the response to higher stimulation frequencies or to exogenous noradrenaline. The inhibitory effect of S9977 was prevented by methiothepin, and not affected by atropine or 8-phenyltheophylline.4. Helical strips of canine saphenous veins were incubated with [³H]noradrenaline and suspended for superfusion and isometric tension recording. Under basal conditions, S9977 (10⁻⁴ M) augmented, the total ³H-overflow which was due mainly to an augmented overflow of [³H]deoxyphenylglycol (DOPEG); the extraneuronal metabolites 3,4-dihydromandelic acid (DOMA) and 3-methoxy-4-hydroxymandelic acid (VMA) were reduced.5. During electrical stimulation of the adrenergic nerves, S9977 (10⁻⁴ M) augmented the total ³H-overflow but reduced the contractile response; the evoked overflow of [³H]noradrenaline was not significantly affected.6. These experiments suggest that S9977 the displacement of noradrenaline from the adrenergic varicosities; most of the displaced transmitter is metabolized by intraneuronal monoamine oxidase before reaching the junctional cleft. In addition, S9977 exerts an inhibitory effect on the extraneuronal metabolism of catecholamines. S9977 does not inhibit the exocytotic release of the adrenergic neurotransmitter.     

Release of [3H]noradrenaline induced by 5-hydroxy-tryptamine from cat pial arteries

Pial arteries of cats were used to analyse the effects of 5-hydroxytryptamine (5-HT) on the release of [³H]noradrenaline. To achieve this the vessels were preincubated with [³H]noradrenaline and the effect of different concentrations of 5-HT (10^?6, 10^?5, 10^?4 M) on the release of tritium was studied. 5-HT elicited release of radioactivity in a dose-dependent manner. Removal of both superior cervical sympathetic ganglia 15 days before the experiment or pretreatment of the animals with reserpine (3 mg kg^?1, total dose) produced a significant decrease in the outflow of tritium induced by 5-HT. In these arteries, the amount of radioactivity retained at the end of the experiment was much diminished. Cocaine (10^?6 M) caused a significant decrease in the tritium efflux induced by 5-HT (10^?5 M). These results show that 5-HT has an indirect adrenergic effect in the pial arteries of the cat only at high doses of 5-HT, and confirm that sympathetic innervation of these vessels mainly comes from the superior cervical ganglia.     

Prolonged treatment with (+/-) propranolol induces supersensitivity to (L)noradrenaline in mesenteric arteries in the rat

In the isolated perfused mesenteric arteries of the rat, neither (+/-) propranolol (0.1 microM) nor (+/-)isoproterenol (0.05 microM) modified the overflow of DL-[3H]noradrenaline (DL-[3H]NA) induced by sympathetic nerve stimulation at either 5 or 10 Hz. The blockade of alpha presynaptic receptors with phentolamine (4.7 microM) increased the 3H-transmitter overflow at 5 and 10 Hz. (+/-)Propranolol (0.1 microM) failed to modify this effect. Vasoconstrictor responses to exogenous NA or sympathetic nerve stimulation were not modified by (+/-)propranolol (0.1 microM). Prolonged treatment with (+/-)propranolol (7 mg/kg) for 15 days potentiates responses to both exogenous NA and sympathetic nerve stimulation; however, the fractional release per pulse of DL-[3H]NA was not modified at either 5 or 10 Hz. These results provide no evidence to support the hypothesis that the release of NA is regulated by presynaptic beta-adrenoreceptors in the mesenteric arteries of the rat. The enhancement of vascular responses after prolonged treatment with propranolol could be caused by postsynaptic supersensitivity.     

Caffeine enhances sympathetic purinergic and noradrenergic transmission in the guinea-pig isolated vas deferens

Intracellular recording techniques were used to monitor the resting membrane potential of smooth muscle cells and the excitatory junction potentials (EJPs) evoked by stimulation of the hypogastric nerve. Stimulation with trains of 15 pulses at 1 Hz or 0.33 Hz evoked individual EJPs which increased in amplitude from the first pulse and reached a plateau after 6–8 pulses. Stimulation at 1 Hz resulted in EJPs facilitating to a plateau level of approximately 25 mV, whereas with stimulation at 0.33 Hz the EJPs only facilitated to a plateau level of about 12 mV. With stimulation at 1 Hz, caffeine (3 mM and 10 mM), increased the amplitude of the first few EJPs in each train and decreased the extent of facilitation and reduced the amplitude of fully facilitated EJPs. In comparison, the amplitude of all EJPs evoked by stimulation at 0.33 Hz was increased by caffeine (3 mM and 10 mM). With 0.33 Hz stimulation, facilitation of the first few EJPs was observed in the presence of 3 mM caffeine but not in the presence of 10 mM caffeine. In the presence of the ₂-adrenoceptor antagonist idazoxan, caffeine (3 mM and 10 mM) still enhanced the amplitude of EJPs early in trains of stimulation but there was no depression of EJPs later in the trains. Similarly, in reserpine-treated vasa deferentia, caffeine (3 mM) enhanced EJPs early in the train of stimulation at 1 Hz and there was no depression of EJPs at the end of the train. In addition to electrophysiological experiments, the effect of caffeine (0.1–30 mM) on the resting and stimulation-induced (S-I) efflux of radioactivity was investigated in guinea-pig isolated vasa deferentia previously incubated with [³H] -noradrenaline. Caffeine (10 mM) did not affect the resting efflux of [³H]-noradrenaline but significantly enhanced the S-I efflux by 150–160%. The present findings suggest that caffeine enhances sympathetic purinergic and noradrenergic transmission at the sympathetic neuroeffector junction in the guinea-pig vas deferens. Moreover, the increased release of transmitter noradrenaline can modulate purinergic transmission by activation of ₂-adrenoceptors located at sympathetic neuroeffector sites.     

Effect of ω‐Conotoxin GVIA on Noradrenaline Release from Postganglionic Sympathetic Neurones in Rabbit Aorta

Abstract: The aim of the present work was to examine the effect of the selective N‐type calcium blocking agent ω‐conotoxin GVIA on stimulation‐evoked release of noradrenaline from sympathetic nerves in rabbit isolated aorta with regard to stimulation frequency, extracellular Ca²⁺ concentration, and transmitter uptake. Rings of rabbit isolated aorta were preloaded with (‐)‐³H‐noradrenaline and the fractional ³H‐overflow evoked by electrical‐field stimulation was determined by liquid scintillation spectrometry. ω‐Conotoxin GVIA (3×10^?10– 3×10^?8 M) did not alter the spontaneous ³H‐outflow. ω‐Conotoxin GVIA (3×10^?10– 3×10^?8 M) caused a slowly developing reduction of stimulation‐evoked ³H‐overflow at 1 and 30 Hz. The Emax for the ω‐conotoxin‐induced inhibition was less (70%) at 30 Hz than that (96%) seen at 1 Hz. Short‐term incubation with ω‐conotoxin GVIA caused a subsequent steady‐state inhibition. The inhibitory action of ω‐conotoxin GVIA (3×10^?10– 3×10^?9 M) was inversely related to the extracellular Ca²⁺ concentration (6.5×10^?4– 2.7×10^?3 M). Cocaine (3×10^?5 M) plus corticosterone (4×10^?5 M), neuronal and extraneuronal uptake inhibitors, respectively, did not alter the inhibitory effect of ω‐conotoxin GVIA (3×10^?9 M) on ³H‐overflow evoked by stimulation at a frequency of either 1 or 30 Hz. It is concluded that ω‐conotoxin GVIA acts on prejunctional N‐type calcium channels to inhibit stimulation‐evoked noradrenaline release from sympathetic neurone terminals in rabbit aorta. At a high frequency, another subtype calcium channel may possibly be involved. The action of ω‐conotoxin GVIA is independent of neuronal and extraneuronal uptake mechanisms for noradrenaline, but dependent on the amount of Ca²⁺ to be transported across the neurilemma from the extracellular space into the neurone.     

Prejunctional effects of cromakalim,nicorandil and pinacidil on noradrenergic transmission in rat isolated mesenteric artery

1 The present study investigated the effects of cromakalim, nicorandil and pinacidil on resting and stimulation-induced (S-I) effluxes of radioactivity from rat isolated mesenteric artery preparations in which the noradrenergic transmitter stores had been radiolabelled with [³H]-noradrenaline. The efflux of radioactivity evoked by field stimulation of peri-arterial sympathetic nerves (pulses at 2 Hz frequency in trains of 60 s duration) was taken as an index of transmitter noradrenaline release. 2 Cromakalim (1–100 μm ) and nicroandil (1–1000 μm ) produced minor effects on resting and S-I effluxes of radioactivity, but these did not exhibit concentration-dependency. 3 Pinacidil (1–1000 μm ) produced concentration-dependent increases, in both resting and S-I effluxes of radioactivity. With 1000 μm pinacidil, resting and S-I effluxes were increased to approximately 348% and 358% of their respective control values. 4 The effects of pinacidil on resting and S-I effluxes were unaltered when the neuronal amine transport system was inhibited by desipramine (1 μm ). 5 Inhibition of monoamine oxidase with pargyline (100μm ) treatment markedly reduced the enhancement of resting efflux by 1000 μm pinacidil but did not alter its effect on S-I efflux. It is proposed that the enhanced resting efflux produced by pinacidil without pargyline treatment consists of deaminated [³H]-noradrenaline metabolites formed from [³H]-noradrenaline displaced from transmitter storage vesicles by pinacidil. 6 The enhancement of S-I efflux by pinacidil does not appear to involve disruption of α₂-adrenoceptor auto-inhibition of transmitter release since equi-effective concentrations of phentolamine (1 μm ) and pinacidil (1000 μm ) produced additive effects on S-I efflux, whereas increasing the concentration of phentolamine from 1 to 2m produced no further increases in S-I efflux. 7 In conclusion this, study has provided no evidence of a prejunctional inhibitory effect of the potassium channel openers cromakalim, nicorandil and pinacidil on transmitter noradrenaline release. However, the findings with pinacidil suggest that, in high concentrations, pinacidil displaces noradrenaline from transmitter stores, such that deaminated noradrenaline metabolites are released from the nerve terminals. Furthermore, pinacidil enhances S-I transmitter noradrenaline release, possibly by blocking neuronal potassium channels.     

The effects of labetalol (AH 5158) on metabolism of 3H(-)-noradrenaline released from the cat spleen by nerve stimulation

The competitive α and β adrenoceptor antagonist labetalol, in concentrations up to 10^?4 M, produced a dose dependent increase in overflow of ³H and [³H]noradrenaline in the isolated blood perfused cat spleen following stimulation of the splenic nerves at a frequency of 10 Hz. Labetalol had no effect on the pattern of overflow of label following stimulation. In experiments in which the metabolism of [³H]noradrenaline released on nerve stimulation was examined, labetalol produced a concentration dependent increase in the percentage of [³H]noradrenaline and a decrease in the percentage of [³H]DOPEG in the venous blood following nerve stimulation. Production of [³H]COMT metabolites and [³H]DOMA was not affected. It is suggested that in the isolated blood perfused cat spleen labetalol produces the elevation of overflow and effects on noradrenaline metabolism by inhibition of neuronal uptake of noradrenaline. The drug has no detectable effects on the enzymes MAO or COMT or on extraneuronal uptake.     

Fate of [3H]amezinium in sympathetically innervated rabbit tissues

Perfused rabbit hearts accumulated intra-aortically infused [³H]amezinium. At concentrations of 1 or 10 nM, the accumulation proceeded at a constant rate for at least 60 min. After 60 min, tissue medium ratios were between 6 (1 μM) and approx. 100 (1 or 10 nM [³H]amezinium). Cocaine or pretreatment with 6-hydroxydopamine abolished, and pretreatment with reserpine reduced the accumulation of [³H]amezinium (1nM). Kinetic analysis yielded a K_m value of 0.9 μM and a V_max of 1.2 nmole g^?1 min^?1. When hearts had been labelled with 1 or 10 nM [³H]amezinium, the fractional rate of the subsequent efflux was very low (0.001 min^?1). It was greatly increased when the animals had been pretreated with reserpine. Electrical stimulation of the sympathetic nerves released [³H] amezinium from pre-labelled rabbit pulmonary artery strips. The electrically evoked overflow was abolished by tetrodotoxin or omission of Ca²⁺; it was enhanced by cocaine, desipramine and yohimbine and decreased by clonidine. The results show that amezinium, at least at low concentrations, is selectively taken up into postganglionic sympathetic neurones, that it is partly sequestered in the vesicles, and that it is released by action potentials. Amezinium is a structurally novel substrate of both the noradrenaline transport mechanism of the axolemma and the transport mechanism of the noradrenaline-storing synaptic vesicles.     

Prejunctional Muscarinic Receptor Modulation of Noradrenaline Release from Sympathetic Neurones in Rabbit Aorta *

The prejunctional muscarinic modulation of stimulation‐evoked release of ³H‐noradrenaline from sympathetic neurones in rabbit aorta was examined. The role of transmitter uptake, α‐adrenoceptor blockade, stimulation frequency and endothelium on the modulation was investigated. Rings of aorta were incubated with (‐)‐³H‐noradrenaline and subsequently subjected to electrical‐field stimulation. Fractional ³H‐overflow was determined by liquid scintillation counting. Acetylcholine (10^?8–3×10^?6 M) added cumulatively, reduced the stimulation‐evoked ³H‐overflow up to 80%. The effect of acetylcholine was the same in intact and endothelium‐free aorta. The inhibitory effect of acetylcholine was inversely related to the frequency of stimulation (1–10 Hz). The maximal inhibition (%) was 80 (1 Hz), 53 (3 Hz) and 14 (10 Hz). The inhibitory effect of acetylcholine (10^?6 M) and carbachol (10^?5 M) reached a maximum 15 min. after addition and then remained almost constant. Cocaine (3×10^?5 M) did not alter the effect of acetylcholine. Desipramine (10^?6 M) and corticosterone (4×10^?5 M) attenuated the inhibition seen with low concentrations (10^?8–10^?7 M) of acetylcholine. The acetylcholine‐induced inhibition was antagonized by desipramine. Cocaine plus corticosterone attenuated the inhibition seen with high concentrations (10^?6–3×10^?6 M) of acetylcholine. Rauwolscine (10^?6 M) enhanced the maximal inhibitory effect of acetylcholine. We conclude that the inhibitory effect of acetylcholine on ³H‐overflow from rabbit aorta preloaded with ³H‐noradrenaline is (1) inversely related to stimulation frequency; (2) independent of endothelium; (3) unaffected by neuronal and extraneuronal transmitter uptake; (4) that cocaine is not a prejunctional muscarinic antagonist; (5) that cocaine, but not desipramine, is suited as a neuronal uptake inhibitor in studies of prejunctional muscarinic receptor subtypes; and (6) and that there is an inverse interaction between prejunctional α₂‐adrenoceptors and muscarinic receptors.     

The indirect sympathomimetic activity of etilefrine — a comparison with tyramine and ephedrine using [3H]noradrenaline

The indirect sympathomimetic activity of etilefrine has been examined using the ventral caudal artery of the rat. This vessel has a rich sympathetic innervation and lends itself to studies on [³H]noradrenaline efflux from these sites. Etilefrine possessed significant indirect activity on the artery and this action, although less than that of tyramine, was equivalent to that caused by ephedrine. Pretreatment of the vessels with a mixture of iproniazid, doca, cocaine and U0521 (3′,4′-dihydroxy-2-methyl propiophenone) significantly enhanced[³H]-noradrenaline efflux from the artery.     

On the reduced retention of extravesicular noradrenaline in rabbit atria produced by potassium

The effects of KCl on the retention of extravesicular (?)-[³H]noradrenaline were investigated. Atria, from reserpine-pretreated rabbits, were exposed to tropolone and pargyline before incubation with (?)-[³H]noradrenaline. After efflux for 50–60 min, exposure of tissues to media containing 65 mM KCl resulted in increased efflux of (?)-[³H]noradrenaline. This effect was not Ca²⁺-dependent and was not altered by oxymetazoline, phentolamine, indomethacin or methacholine. The KCl-induced reduction in retention of (?)-[³H]noradrenaline was, however, inhibited by cocaine and desipramine, but not by lidocaine. Efflux was not increased by RbCl and CsCl. It is suggested that KCl may have accelerated the efflux of extravesicular (?)-[³H]noradrenaline by increasing a cocaine- and desipramine-sensitive carrier-mediated efflux process.     

Functional activity of the noradrenergic innervation of large cerebral arteries.

1 The role of the sympathetic innervation of cerebral arteries remains controversial. Therefore, the functional activity of the adrenergic innervation of the rabbit basilar artery was characterized and compared to that of a peripheral artery, the ear artery. 2 Both the ear artery and basilar artery have similar endogenous noradrenaline (NA) contents but accumulation of [3H]-NA was considerably greater in the basilar artery. 3 Studies of tritium efflux after loading with [3H]-NA demonstrated a considerable non-neuronal component since neither guanethidine nor tetrodotoxin completely blocked tritium efflux during nerve stimulation. Pretreatment with blockers of uptake2 did not eliminate this problem. 4 Comparison of methods for estimating the functional activity of adrenergic nerves showed that, for the vessels studied, NA content and [3H]-NA accumulation gave markedly different answers. Fractional release of [3H]-NA did not correspond to fractional release of endogenous NA. 5 Adrenergic nerves innervating cerebral arteries are shown to have a high activity relative to a peripheral artery. While cerebrovascular sympathetic innervation may not play an important role in normal circumstances, its influence may be seen in pathological conditions.     

Somatostatin inhibits the release of noradrenaline induced by electrical stimulation of the rat mesenteric artery.

Somatostatin, a peptide with antisecretory and antiproliferative effects, coexists with noradrenaline in sympathetic neurons. Octreotide, a stable somatostatin analogue, prevents hypertension and cardiovascular structural changes induced by prolonged infusion of DPSPX (1,3-dipropyl-8-sulfophenylxanthine, a non-selective adenosine receptor antagonist) in rats. In the present work we investigated the effect of somatostatin and its analogue octreotide on the release of [(3)H]noradrenaline from sympathetic nerves in the rat mesenteric artery. Rat mesenteric arteries were incubated for 60 min with [(3)H]noradrenaline (0.2 microm), mounted in perifusion chambers, washed out for 90 min and electrically stimulated (2 Hz, 5 min, 50 mA). Radioactivity was measured in the tissue and in the perifusion fluid before, during and after stimulation. Both somatostatin and octreotide inhibited tritium release evoked by electrical stimulation of in vitro preparations of rat mesenteric arteries preloaded with [(3)H]noradrenaline. The maximal effects produced by octreotide and somatostatin were a 56 and 70% inhibition of noradrenaline release, respectively. For somatostatin an EC(50)=0. 18 n m (0.01 n m-2.2 n m;n =16) was calculated. When used alone, the somatostatin receptor antagonist, cyclo(7-aminoheptanoyl-Phe- d -Trp-Lys-Thr[BZL]) (CYCAM; 1 microm), had no effect on noradrenaline release induced by electrical stimulation. However, it was able to significantly antagonize the inhibitory effects of octreotide and somatostatin. These results are compatible with a negative modulatory role of somatostatin on sympathetic neurotransmission.     

Attenuation of vasoconstriction by endogenous nitric oxide in rat caudal artery.

1. The effects of NG-nitro-L-arginine (L-NNA), NG-nitro-L-arginine methyl ester (L-NAME), haemoglobin and methylene blue have been examined on vascular reactivity in the rat isolated caudal artery. The effects of L-NNA and sodium nitroprusside were also investigated on the stimulation-induced (S-I) efflux of noradrenaline in the rat caudal artery. 2. L-NNA (10 microM) and L-NAME (10 microM) significantly attenuated the vasodilator responses to acetylcholine (1 nM-1 microM), but had no effect on vasodilator responses to papaverine (1-100 microM). 3. Vasoconstrictor responses to sympathetic nerve stimulation (3 Hz, 10 s), noradrenaline (0.01-1 microM), methoxamine (1-10 microM), 5-hydroxytryptamine (0.01-0.3 microM), phenylephrine (0.1-10 microM), endothelin-1 (10 nM) and KCl (40 mM) were significantly enhanced by 10 microM L-NNA. L-NAME (10 microM) caused a significant enhancement of vasoconstrictor responses to noradrenaline and sympathetic nerve stimulation in endothelium-intact, but not in endothelium-denuded tissues. 4. Haemoglobin and methylene blue (both 10 microM) enhanced the vasoconstrictor responses to sympathetic nerve stimulation and noradrenaline. The enhancements were absent in endothelium-denuded arterial segments. 5. In endothelium-denuded arterial segments precontracted with phenylephrine, the vasodilator responses to the nitric oxide donor, sodium nitroprusside (0.1-300 nM) were decreased by increasing the level of precontraction. 6. L-NNA (10 microM) had no effect on the S-I efflux of radioactivity from arteries in which transmitter stores had been labelled with [3H]-noradrenaline.(ABSTRACT TRUNCATED AT 250 WORDS)     

Presynaptic muscarinic receptor subtypes involved in the inhibition of acetylcholine and noradrenaline release in bovine cerebral arteries

Summary Experiments were performed in bovine cerebral arteries preincubated with [³H]-choline or [³H]-noradrenaline to analyze the presynaptic muscarinic receptors involved in inhibition of acetylcholine and noradrenaline release induced by electrical stimulation (4 Hz, 200 mA, 0.3 ms, 1 min). For this purpose, the actions of several muscarinic receptor antagonists on the ³H overflow and on the carbacol-induced inhibition of this overflow were assessed. The evoked [³H]-acetylcholine release and [³H]-noradrenaline release were markedly reduced by the presence of tetrodotoxin, Ca²⁺-free medium, and the inhibitor of both choline transport and choline acetyltransferase, AF64A. Chemical sympathetic denervation with 6-hydroxydopamine (6-OHDA) decreased the uptake of[³H]-noradrenaline, and AF64A reduced mainly the uptake of [³H]-choline, but also of [³H]-noradrenaline. Carbachol reduced the evoked [³H]-noradrenaline and [³H]-acetylcholine release; the IC₅₀ values were 0.37 and 0.43 mol/l, respectively.Atropine and 4-DAMP, but not AF DX 116, methoctramine or pirenzepine, increased the evoked [³H]-acetylcholine release. However, these muscarinic antagonists failed to modify the evoked [³H]-noradrenaline release. Carbachol inhibited the release of both acetylcholine and noradrenaline. The inhibition was blocked by the antagonists. The rank orders of potency (based on plC₅₀ values) were, in the case of [³H]-acetylcholine release, atropine > 4-DAMP >AF-DX 116 >- pirenzepine >- methoctramine, and, in the case of [³H]-noradrenaline release, atropine > 4-DAMP > AF-DX 116 >- methoctramine >-pirenzepine. These results suggest (1) that the prosynaptic receptors that modulate endogenous acetylcholine release are likely of the M₃ subtype, whilst those involved on the effect of the exogenous agonist Carbachol are of M₂ subtype, and (2) that those which inhibit noradrenaline release are probably a mixture of M₂ and M₃ subtypes as well. The autoinhibition of the acetylcholine release was funtionally active under our experimental conditions, while noradrenaline release does not appear to be modulated by muscarinic receptors in physiological conditions.Send offprint requests to G. Balfagón at the above address     

Involvement of cAMP in modulation of noradrenaline release in the human pulmonary artery

Summary After incubation with³H-noradrenaline, strips of human pulmonary arteries from patients undergoing surgery for lung tumour were superfused with physiological salt solution containing cocaine and corticosterone. Forskolin, AH 21-132 (a cAMP phosphodiesterase inhibitor), 8-Br-cAMP and isoprenaline did not affect the basal tritium efflux from the strips, but produced a concentration-dependent faciliatation of the tritium over-flow evoked by transmural electrical stimulation (2 Hz). The facilitatory effect of isoprenaline was potentiated by forskolin which produced a shift to the left of the concentration-response curve of isoprenaline. It is concluded that cAMP plays a role in the modulation of noradrenaline release in the human pulmonary artery and that presynaptic-adrenoceptors appear to be coupled to an adenylate cyclase in the sympathetic nerve terminals.     

Presynaptic effects of scopolamine,oxotremorine, noradrenaline and morphine on [3H] acetylcholine release from the myenteric plexus at different stimulation frequencies and calcium concentrations

Summary The inhibition by three modulators (oxotremorine, noradrenaline, morphine) of acetylcholine release from the myenteric plexus preincubated with [³H]choline was investigated at different stimulation frequencies and calcium concentrations. Moreover, [³H]acetylcholine release evoked by a low (0.1 Hz) or a high (10 Hz) stimulation rate was investigated at different calcium concentrations either in the absence or presence of scopolamine. A reduced calcium concentration (0.6 mmol/l) inhibited acetylcholine release more at 0.1 Hz (74% ± 3%) than at 10 Hz (44% ± 8%). Scopolamine enhanced the stimulated acetylcholine release at a calcium concentration of 1.8 mmol/l. At calcium concentrations higher than 1.8 mmol/l scopolamine failed to enhance transmitter release markedly. A reduction of the calcium concentration (< 1.8 mmol/l) significantly enhanced the effect of scopolamine, when acetylcholine release was evoked at 0.1 Hz. Oxotremorine (10 mol/l) completely suppressed acetylcholine release at 1 Hz (120 pulses). When 120 pulses were applied at 10 Hz the maximal effect was only a 64% inhibition and the concentration-response curve was significantly shifted to the right. However, after a reduction of both the train length or the calcium concentration oxotremorine produced a complete inhibition of acetylcholine release evoked at 10 Hz. In contrast to the effect of oxotremorine, the concentration-response curves for morphine and noradrenaline were similar at 1 Hz and 10 Hz. Following conclusions can be drawn: 1. The present findings fit into the concept that residual calcium accumulates in the nerve terminal during 10 Hz stimulation. 2. The results obtained with scopolamine and oxotremorine are consistent with the view that muscarine autoreceptor activation triggers a reduction of the intraneuronal availability of calcium for the stimulus-secretion coupling. 3. The presynaptic effect of morphine and partly that of noradrenaline might be mediated by a different mechanism, probably by a reduction of release sites. Send offprint requests to I. Wessler at the above address     

Selective inhibition by nifedipine of the purinergic component of neurogenic vasoconstriction in the dog mesenteric artery

The neurogenic contractions evoked by perivascular sympathetic nerve stimulation of dog mesenteric artery consist of purinergic and adrenergic components, and these components were selectively inhibited by alpha, beta-methylene ATP and prazosin, respectively. We examined the effects of Ca antagonists on both these components in dog mesenteric arteries. Nifedipine (10(-8)-10(-6) M) inhibited the purinergic and adrenergic contractions evoked by transmural electrical stimulation, and this inhibition was more evident for the purinergic component of the response. Nifedipine was also more potent to inhibit the contractile response to alpha, beta-methylene ATP than it was to inhibit the responses to noradrenaline. Verapamil and diltiazem also inhibited the purinergic and adrenergic responses induced by transmural electrical stimulation, alpha, beta-methylene ATP or noradrenaline, but the extend of the inhibition was less than that seen with nifedipine. These three Ca antagonists had little effect on the 3H efflux evoked by electrical transmural stimulation of arteries that had been preincubated with [3H]noradrenaline. These results show that nifedipine is a selective inhibitor of the purinergic component of contractions evoked by sympathetic nerve stimulation of blood vessels.     

The endothelium modulates adrenergic neurotransmission to canine pulmonary arteries and veins

We tested the postulate that endothelium-derived relaxing factor (EDRF) modulates adrenergic neuroeffector transmission in isolated canine pulmonary arteries and veins, using the technique of superfusion and measurement of the efflux of [2-14C]NE during transmural nerve stimulation at 1, 2, 4, 8, 16 and 32 Hz for 10 and 30 min. In endothelium-rubbed artery and vein the contractile responses to low frequency nerve stimulation were enhanced, when compared to those from endothelium rubbed blood vessels. Transmural nerve stimulation of endothelium competent arteries and veins for 10 min released less [2-14C]NE than denuded arteries and veins, especially at 1, 2 and 4 Hz, with smaller differences evident at higher frequencies (16 and 32 Hz) of stimulation. Superfusion of endothelium rubbed blood vessels with effluent from canine thoracic aorta decreased the release of [2-14C]NE during nerve stimulation. These findings suggest that the endothelium and EDRF can inhibit release of adrenergic neurotransmitter from canine pulmonary arteries and veins. The endothelium may act as an endogenous modulator of adrenergic neurotransmission to canine vascular smooth muscle.