3H-NA Release Research Articles

Mechanism of nicotine-evoked release of 3H-noradrenaline in human cerebral cortex slices.

1. The mechanism of stimulation of noradrenaline (NA) release by nicotine (NIC) was investigated in human cerebral cortex slices preloaded with 3H-noradrenaline. 2 NIC (10-1000 micro M) increased 3H-NA release in a concentration-dependent manner. 3. NIC (100 micro M)-evoked 3H-NA release was largely dependent on external Ca2+, and was attenuated by omega-conotoxin GVIA (0.1 micro M) but not by nitrendipine (1 micro M). 4. Tetrodotoxin (1 micro M) and nisoxetine (0.1 micro M) attenuated the NIC (100 micro M)-evoked release of 3H-NA. 5. Mecamylamine (10 micro M), dihydro-beta-erythroidine (10 micro M) and d-tubocurarine (30 micro M), but not alpha-bungarotoxin (alpha-BTX, 0.1 micro M), attenuated the NIC (100 micro M)-evoked release of 3H-NA. 6. NIC (100 micro M)-evoked release of 3H-NA was not affected by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 30 micro M) and D(-)-2-amino-5-phosphonopentanoic acid (D-AP5, 100 micro M), but attenuated by MK-801 (10 micro M). MK-801 (0.1-1000 micro M) displaced the specific binding of 3H-nisoxetine with K(i) values of 91.2 micro M. NIC (100, 300 and 1000 micro M) did not induce 3H-D-aspartate release in human cerebral cortex slices. 7. NIC (100 micro M)-evoked release of 3H-NA was attenuated by 7-nitroindazole (10 micro M), N(G)-nitro-L-arginine methyl ester HCl (L-NAME, 30 micro M), N(G)-monomethyl-L-arginine acetate (L-NMMA, 300 micro M). [(3)H]-NA release induced by NIC (100 micro M) was attenuated by methylene blue (3 micro M) and 1H-[1,2,4]oxadiazole[4,3-alpha]quinoxalin-1-one (ODQ, 10 micro M), and enhanced by zaprinast (30 micro M). 8. In conclusion, NIC stimulates the release of 3H-NA through activation of alpha-BTX-insensitive nicotinic acetylcholine receptors in the human cerebral cortex slices and this action of NIC is associated with modulation of the NO/cGMP pathway.

MgATP-dependent and MgATP-independent [3H]noradrenaline release from perforated synaptosomes both use N-ethylmaleimide-sensitive fusion protein.

In streptolysin-O (SLO)-perforated rat brain cortical synaptosomes, Ca2+-induced [3H]noradrenaline (3H-NA) release began with a phase lasting about 1 min that did not depend on MgATP. Subsequent release became increasingly MgATP-dependent. The first phase involved release from previously "primed" synaptic vesicles. MgATP-dependent release, on the other hand, was release from unprimed vesicles that needed to be primed by ATP hydrolysis before they could be fused with the presynaptic membrane. Vesicle depriming was detected by observing that the initial release decreased when the synaptosomes were perforated and incubated for 2 min in the absence of MgATP before increasing Ca2+ to promote release. One millimolar N-ethylmaleimide (NEM) inhibited both MgATP-dependent and MgATP-independent release at all times of incubation (0.5-5 min), and inhibition by NEM was partially reversed at short (0.5 min) and longer (5 min) times by adding intact N-ethylmaleimide sensitive fusion protein (NSF) to the perforated synaptosomes. Polyclonal antibodies against the N-terminal domain of NSF produced dose-dependent inhibition of Ca2+-induced 3H-NA release. This inhibition occurred in both early and late release phases and was highly significant at early times if the perforated synaptosomes were preincubated for 2 min with anti-NSF. These results indicate participation of NSF both after vesicular fusion, probably for separation of SNARE proteins in v/t-SNARE complexes before endocytosis, and, surprisingly, after docking, possibly to maintain vesicles in a primed state and reverse depriming during regulated secretion.

A new GABA-A receptor subtype coupled with Ca++/Cl- synporter modulates aminergic release from rat brain neuron terminals.

The aim of the present study was to give a better characterization of GABA receptors that modulate aminergic release. GABA or muscimol (15 microM) increased basal noradrenaline (3H-NA) release but reduced the following K+-evoked 3H-NA release in the synaptosomes from rat cerebellar cortex. Bicuculline and picrotoxin counteracted these two effects. The same GABA modulation resulted to operate also on dopaminergic and serotoninergic neuron terminals. The increased basal noradrenaline release resulted to be both calcium and chloride dependent and associated with an increased entry of 45Ca++ into the synaptosomes. We therefore advance the hypothesis of an involvement of a Cl-/Ca++ synporter system coupled to the receptor. Baclofen also reduced the K+-evoked 3H-NA release, but did not increase basal 3H-NA release; moreover, the interaction of baclofen G with GABA-B receptors resulted to be associated with the inhibition of 45Ca++ entry into synaptosomes. GABA-B receptors resulted to be present also on serotoninergic but not on dopaminergic neuron terminals. The GABA-C receptor agonist cis-4-aminocrotonic acid (CACA) did not influence either basal or K+-evoked 3H-NA release. These results point to a new type of GABA functional role through a different A-family receptor subtype, coupled with calcium influx in aminergic neuron terminals, modulating aminergic release.

Interaction between glutamate and GABA on 3H-noradrenaline release from rat hypothalamus

Glutamate has been shown to stimulate noradrenaline (NA) release from hypothalamic nerve terminals. In the present study, we evaluated the possible interaction between the excitatory amino acid glutamate and gamma-aminobutyric acid (GABA), an inhibitory transmitter, on noradrenaline (NA) release from mediobasal hypothalamus (MBH) of adult male rats. Hypothalamic slices loaded in vitro with 3H-NA were superfused and exposed to glutamate, N-methyl- d-aspartic acid (NMDA), or kainate (KA). We found that 3H-NA release evoked by the excitatory amino acids glutamate and NMDA was dramatically decreased by GABA. The facilitatory effects of NMDA and KA were prevented concentration-dependently by the GABA B receptor antagonist 2-hydroxy saclofen which restored the NMDA effect. In addition, beclofen blocked K +-induced 3H-NA release. Activation of GABA A receptors by muscimol and THIP was ineffective. In conclusion, glutamate and GABA, through GABA B receptors, may interact to modulate NA release from the rat mediobasal hypothalamus.

Prejunctional alpha-2 adrenoceptors and K + channel blockers in rat vas deferens

A parallel increase in systemic plasma levels of neuropeptide Y (NPY)-like immunoreactivity (LI) and noradrenaline (NA) was found during thoracotomy and surgery involving cardiopulmonary bypass in man. Thus, plasma levels of NPY-LI increased from 29 ± 4 pmol/l before anaesthesia to 59 ± 10 after thoracotomy and to 87 ± 8 pmol/l upon cardiopulmonary bypass. The corresponding NA levels increased from 1.3 ± 0.1 nmol/l before anaesthesia to 3.0 ± 0.6 and 4.2 ± 5 nmol/l after thoracotomy and cardiopulmonary bypass, respectively. A significant correlation was found between plasma levels of NPY-LI and NA during the operation but not between NPY-LI and adrenaline. The NPY-LI in human plasma was found to be similar to synthetic porcine NPY on reversed phase high performance liquid chromatography. Human submandibular arteries contained high levels of NPY-LI (24 ± 3 pmol/g). In in vitro experiments on isolated human submandibular arteries, NPY in low concentrations (1000 pmol/l) was found to potentiate the contractile effects of NA or transmural nerve stimulation and to exert vasoconstrictor activity per se in higher concentrations. The calcium-entry antagonist nifedipine abolished both the NPY-induced contractions and the enhancement of NA-evoked contractions. NPY depressed the nerve stimulation-evoked 3H-NA release from human submandibular arteries via a prejunctional mechanism which was resistant to nifedipine. NPY contracted human mesenteric veins and renal arteries, but not mesenteric arteries. In conclusion, NPY seems to be co-released with NA upon sympathetic activation in man. Furthermore, NPY exerts both pre- and postjunctional effects on sympathetic control of human blood vessels.

Atrial natriuretic factor modifies noradrenaline release in a sodium-free medium.

1. The effects of atrial natriuretic factor (ANF) on 3H-noradrenaline (3H-NA) release evoked by a sodium-free medium (SFM) were studied. The experiments were carried out in rat hypothalamic slices incubated in vitro. 2. ANF (1, 10 and 100 nM) decreased NA release evoked by the omission of sodium in a concentration-dependent way. When calcium was omitted from a SFM, NA output was partially diminished. However, if ANF was added to the SFM/calcium free medium NA secretion showed no modifications. 3. Present results suggest that, in rat hypothalamus, NA release evoked by Na+ omission is divided into two fractions: one independent of and the other dependent on extracellular calcium. In addition, ANF modifies NA release evoked by SFM dependent on extracellular calcium.

Release of 3H-noradrenaline by excitatory amino acids from rat mediobasal hypothalamus and the influence of aging

The present study was designed to analyze the effects of glutamate (GLU) and its agonists on the release of noradrenaline (NA) from the mediobasal region of rat hypothalamus (MBH). Slices from hypothalamus were loaded in vitro with 3H-NA and thereafter exposed to GLU and the glutamate agonists N- methyl- d-aspartic acid (NMDA) and kainate (KA), in superfusion chambers. GLU evoked a significant 3H-NA release in a concentration-dependent manner. The EC 50 was 35 mM. 6-Cyano-7-nitro-quinoxaline-2,3-dione (CNQX), a non-NMDA selective antagonist, and amino-7-phosphonoheptanoic acid (AP 7), a NMDA selective antagonist, both decreased the GLU-evoked response to about 50% of its value. NMDA, superfused in Mg 2+-free Krebs-Ringer, exhibited a greater potency than GLU with an EC 50 = 124 μM. KA was also able to evoke 3H-NA release, although overall responses to KA were lower than those of NMDA. The maximal response to KA was a 36% increase of release at a concentration of 200 μM. The effect of KA was blunted by CNQX. NMDA-induced 3H-NA release was progressively altered with age. In old rats (16–18 months) and middle-aged rats (10 months), responses to 200 μM NMDA were decreased respect to young (4 months) male rats. These results show that NMDA and KA receptors mediate the excitatory effects of GLU on NA release from nerve terminals in the MBH and suggest that GLU, in association with NA, participates in the complex mechanisms that regulate neuroendocrine functions. In addition, the age-related changes in NA responsiveness point to a GLU contribution on the maintenance of the reproductive status.

Age-related changes in rat brain monoamines release: peculiarity of dopamine release.

Our aim has been to investigate the ability of the rat brain to retain its level of neurotransmitter release over life. We have investigated the neurotransmitter release from the rat brain synaptosomes prelabeled with 3H-DA, 3H-NA, or 3H-5HT, and perfused with Krebs-Ringer medium alone (basal release) or containing a high K+, calcium ionophore, tyramine or amphetamine (evoked release). Brain areas have been dissected of animals 45 days and 4, 6, and 11 months old. The results have shown a gradual reduction of the 3H-NA release evoked by a high K+ from 45 days to 6 months, which is stabilized until 11 months of age. The reduction rate has been relatively different from the brain areas investigated (36% for the frontal cortex and 26% for the hippocampus and cerebellar cortex). A similar reduction has been seen with 3H-5HT released from synaptosomes of the frontal cortex, hippocampus, and striatum. Surprisingly, the 3H-DA release that was evoked by high K+ was greater in rats 11 months old than in younger rats; this effect has been seen in synaptosomes from the caudate and the frontal cortex. The calcium ionophore A23187 has shown a releasing picture similar to a high K+. When we analyzed a nonexocitotic, but probably carrier mediated, release (evoked by tyramine or amphetamine), there was reduced release of all of the above neurotransmitters from 45 days to 11 months of age. We presume that there have been adaptive changes in neurotransmitter evoked release due to changes in Ca++ utilization, as inferred from the results from calcium ionophore experiments and carrier performance.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of neuronal nicotinic acetylcholine receptors (nAchRs) in human brain

In this study 3H-noradrenaline (NA) release from rat neocortex slices evoked by electrical field-stimulation (1 Hz, 12 mA, 2 msec) was compared with that induced by K+-depolarization (13–30 mM K+) under similar experimental conditions, with a particular emphasis on the role of external Ca2+ and the releasable transmitter pool(s). Not only 3H-NA release evoked by electrical stimulation but also that induced by 13 mM K+ was almost completely blocked by 0.3 μM tetrodotoxin (TTX). Release induced by 20 mM K+ appeared to be less sensitive to TTX. Thus, under relatively mild stimulation conditions, the activation of sodium channels appears to be involved in 3H-NA release elicited by both stimuli.The electrically evoked 3H-NA release increased sigmoidally with the external Ca2+-concentration up to 1.2 mM. In contrast, 3H-NA release induced by 13–20 mM K+ reached a maximal value at 0.6–0.9 mM Ca2+ and gradually decreased at higher Ca2+-concentrations. The Ca2+-antagonist D-600 (1–30 μM) did not inhibit electrically evoked release, while K+-induced 3H-NA release was dose-dependently reduced. Upon repetitive K+-depolarization a strong depression of 3H-NA release could be demonstrated, while this phenomenon did not occur with repeated electrical stimulation. Moreover, a previous K+-induced (partial) depletion of 3H-NA stores did not affect the release evoked by electrical pulses and vice versa. Taken together these data are compatible with a much stronger activation of Ca2+-channels and a larger vesicle mobilizing capacity in case of electrical stimulation at physiological frequencies compared to sustained depolarization with moderate K+-concentrations.

Presynaptic inhibition of noradrenaline release by neuropeptide Y in selective rat brain regions in vitro is mediated by A Y 2 receptor

Adenosine and adenosine analogues inhibited electrically evoked 3H-noradrenaline (3H-NA) release from slices of the rat hippocampus in vitro in a dose -dependent manner in the concentration range 0.01–100 M. L-phenylisopropyladenosine (L-PIA) was more potent than 5′-N-carboxamidoadenosine (NECA), which was more potent than adenosine. The adenosine uptake blocker dipyridamole (3 M) enhanced the effect of exogenous adenosine, and had a slight inhibitory effect per se. The effect of L-PIA on NA release was competitively antagonized by 8-phenyltheopylline; pA2=7.1. Enprophylline (300 M), theophylline (300 M) and 8-phenyltheophylline (1–10 M) enhanced the evoked 3H-NA release per se, while no such enhancement was seen with the non-xanthine phosphodiesterase inhibitor ZK 62.711 (Rolipram) (30 M).It is concluded that adenosine, at physiologically relevant concentrations, inhibits electrically evoked NA release from terminals in the central nervous system. Alkylxanthines increase evoked NA release from hippocampal terminals, wich probably not related to cyclic AMP but may partly involve inhibition of endogenous adenosine acting as a modulator of transmitter release in the hippocampal slice preparation.

Presynaptic alpha 2-adrenoceptors mediating inhibition of noradrenaline and 5-hydroxytryptamine release in rat cerebral cortex: further characterization as different alpha 2-adrenoceptor subtypes.

In a previous investigation it was suggested that the alpha 2-adrenoceptors regulating 3H-noradrenaline (3H-NA) release and the alpha 2-heteroreceptors regulating the release of 3H-5-hydroxytryptamine (3H-5-HT) from rat cerebrocortex synaptosomes represent different subtypes of the alpha 2-adrenoceptor in that (-)-mianserin potently blocked the receptors sited on 5-HT terminals but was ineffective at the autoreceptors (Raiteri et al. 1983). In this work a number of alpha 2-adrenoceptor antagonists were tested against NA as an inhibitor of the K+ (15 mmol/l)-evoked release of 3H-NA or 3H-5-HT (in presence of 1 mumol/l desipramine or citalopram, respectively) from superfused rat neocortex synaptosomes. The order of apparent affinity of the antagonists was: idazoxan greater than or equal to ORG 20769 (2-amino-4-(1-methyl-1,2,3,6-tetrahydropyridin-4-yl)-thiazole-5-ca rbonitrile (Z)-2-butenedioate (1:1) salt) greater than ORG 20350 (5-chloro-4-(1-butyl-1,2,5,6-tetrahydropyridin-3-yl)-thiazole-2- amine (Z)-2-butenedioate (1:1) salt) greater than or equal to ORG 20091 (5-chloro-4-(1-methyl-1,2,5,6-tetrahydropyridin-3-yl)-thiazole-2- amine (Z)-2-butenedioate (2:1) salt) at the alpha 2-autoreceptor and idazoxan greater than or equal to ORG 20769 greater than ORG 20091 much greater than ORG 20350 at the alpha 2-heteroreceptor. Prazosin (1 mumol/l) or AR-C 239 (1 mumol/l) (2-[2-[4-(o-methoxyphenyl)piperazine-1-yl]ethyl]-4,4-dimethyl- 1,3(2H,4H)-isoquinolinedione) were ineffective in both systems.(ABSTRACT TRUNCATED AT 250 WORDS)

Estradiol effect on indomethacin and prostaglandin E 2-modulation of adrenergic transmission in rabbit oviduct

The effect of prostaglandin E 2/PGE 2/ and indomethacin on 3H-noradrenaline (3H-NA) release- and on contractions-evoked by field electrical stimulation (FES) was studied in vitro in oviductal isthmus of mature rabbits (untreated and treated with estradiol). FES evoked guanethidine-sensitive contractions and calcium-dependent tritium overflow, which reflected 3H-NA overflow. Markeed and concentration-dependent decrease of FES-evoked contractions by PGE 2 (0.1–100 nM) was observed in both groups of animals. The inhibitory effect of PGE 2 was more pronounced in estradiol treated animals (RC 50 1.5 nM, n=9) than in untreated animals (IC 50 18 nM, n=6). Indomethacin, 1 μM, induced a remarkably pronounced increase of FES-evoked contractions in estradiol treated (by 57.3 ± 6.3%, n=8), in comparison with untreated rabbits (21.4 ± 3.8%, n=7). The amount of FES-evoked release of tritium was significantly higher in untreated than in estradiol treated rabbits. PGE 2 decreased and indomethacin increased tritium-evoked release. The effects of PGE 2 and indoemethacin on tritium-evoked release showed no estradiol dependence. The competitive results of PGE 2 and indomethacin on both evoked contraction and 3H-NA release suggest that endogenous prostaglandin E 2 takes part in modulation of adrenergic mediated contraction and that estradiol enhanced the prostaglandin effect.

The translocation of Botulinum A neurotoxin by chromaffin cells is promoted in low ionic strength solution and is insensitive to trypsin

Botulinum A neurotoxin (BoNtx) produced a partial inhibition of carbachol induced 3H-noradrenaline ( 3H-NA) release from bovine adrenal chromaffin cells in monolayer culture. Each of the polysialogangliosides GD1a, GT1b and GD1b enhanced the block of exocytosis when they were applied prior to the toxin exposure. The monosialoganglioside GM1 was not effective. Chromaffin cells treated with neuraminidase lost their sensitivity to BoNtx. Application of gangliosides to these cells, however, restored their susceptibility to the toxin. Treatment of the cells with trypsin did not affect the BoNtx-blockade of 3H-NA-release. The potency of botulinum A toxin was increased in a solution of low ionic strength in which sodium chloride was replaced by sucrose. In agreement with the potency of botulinum A neurotoxin in blocking exocytosis under the various conditions, binding of 125I-botulinum A neurotoxin to chromaffin cells was enhanced in low ionic strength solution and by pretreatment of the cells with gangliosides. The binding was decreased by digestion of gangliosides with neuraminidase. It is concluded that botulinum A neurotoxin binds exclusively to polysialogangliosides which subsequently serve as carriers for the toxin. The low ionic strength may increase some physico-chemical interaction of the toxin with the polysialogangliosides.

The regulation of 3H-noradrenaline (3H-NA) release from rat myenteric plexus synaptosomes

The regulation of 3H-noradrenaline (3H-NA) release from rat myenteric plexus synaptosomes

Reduced inhibitory effects of clonidine and neuropeptide Y on 3H-noradrenaline release from synaptosomes of the medulla oblongata of the spontaneously hypertensive rat.

The release of 3H-noradrenaline (3H-NA) evoked by high-K+ (15 mM) was studied in synaptosomes isolated from the medulla oblongata of the normotensive Wistar-Kyoto and spontaneously hypertensive male rat (14 weeks old) using a superfusion apparatus. Based on concentration-response curves clonidine was shown to have a reduced ability to inhibit 3H-NA release in synaptosomes isolated from the spontaneously hypertensive rat (SHR) versus the normotensive rat. Furthermore, only a high concentration of NPY (100 nM) had the ability to enhance the inhibitory effects of clonidine on 3H-NA release in synaptosomes isolated from the medulla oblongata of the SH male rat, while 1 nM of NPY was effective in synaptosomes isolated from the medulla oblongata of the normotensive Wistar-Kyoto rat. These results may indicate a reduced presynaptic alpha 2-adrenoceptor and NPY receptor function to inhibit 3H-NA release from NA and/or adrenaline (A) nerve terminals in the medulla oblongata of the adult 14 weeks old SHR.

Effect of CGRP on the evoked 3H-NA release in the rabbit central ear artery

Effect of CGRP on the evoked 3H-NA release in the rabbit central ear artery

Neuropeptide Y increases the inhibitory effects of clonidine on potassium evoked 3H-noradrenaline but not 3H-5-hydroxytryptamine release from synaptosomes of the hypothalamus and the frontoparietal cortex of the male Sprague-Dawley rat.

The release of 3H-noradrenaline (3H-NA) and of 3H-5-hydroxytryptamine (3H-5-HT) evoked by high-K+ (15 mM) was studied in synaptosomes isolated from the hypothalamus and the frontoparietal cortex of the male Sprague-Dawley rat using a superfusion apparatus. Based on concentration-response curves obtained by analyzing the full-time course of the inhibitory effects of clonidine on 3H-NA and on 3H-5-HT release neuropeptide Y (NPY) (1 nM) was shown to significantly increase the ability of clonidine to inhibit 3H-NA release in synaptosomes isolated from the hypothalamus and from the frontoparietal cortex. NPY (1 nM) alone had no effect on K+-evoked 3H-NA release from these regions. In contrast, NPY (1 nM) did not modulate the inhibitory effects of clonidine on 3H-5-HT release in the above mentioned regions. These results indicate that NPY can increase the sensitivity of the alpha 2-autoreceptors belonging to hypothalamic NA and/or to adrenaline nerve terminals and to cortical NA nerve terminals, while the alpha 2-heteroreceptors inhibiting 3H-HT release in the same brain regions appear not to be regulated by high affinity NPY receptors. Thus, alpha 2-autoreceptors and alpha 2-heteroreceptors appear to be differentially controlled by high affinity NPY receptors at least with regard to regulation of 3H-NA and 3H-5-HT release, respectively.

Regulation of the efficacy of adrenergic synaptic transmission by adrenergic autoreceptors in rat brain slices

A mathematical model was developed based on the putative functional role of adrenergic autoreceptors for quantifying processes of synaptic depression, facilitation, stabilization of PSP amplitude, and rise in this amplitude produced by increased stimulation rate. Synaptic depression and facilitation are brought about by activation of α- and β-adrenergic autoreceptors respectively. A "steady release zone" is found between curves of adrenergic autoreceptor activity and this is responsible for stabilizing noradrenaline release during a series of presynaptic impulses. Experimental findings on labeled noradrenaline (3H-NA) release from rat brain slices showed that adrenergic autoreceptors do in fact form a stable release zone responsible for autofacilitation and stabilization of3H-NA release produced by 5-fold consecutive K+ depolarizations of slices spaced 30 min apart. Lack of a stable release zone in curves showing adrenergic autoreceptor activity produces destabilization of3H-NA release. These curves (together with the stable release zone) shift in the direction of high noradrenaline concentrations when stimulus intensity is increased.

Noradrenaline releasing effect of an ouabain-like compound on pulmonary artery.

The effect of ouabain and an endogenous ouabain-like compound (OLC) was studied on (3H)noradrenaline [3H)NA) release from rabbit pulmonary artery. Similarly to ouabain, OLC enhanced the release of (3H)NA in response to electrical stimulation. These findings are the first evidence that OLC is able to act presynaptically and enhance the amount of (3H)NA release per impulse. It is suggested that this mechanism might be involved in the physiological regulation of blood pressure or in the genesis of hypertension.

Effect of isoprenaline on noradrenaline release from sympathetic neurones in rabbit isolated pulmonary artery

The aim of the present work was to demonstrate the presence of presynaptic beta-adrenoceptors in the rabbit isolated pulmonary artery by investigating the effect of isoprenaline on 3H-noradrenaline (3H-NA) release evoked by electrical field stimulation. (-)-Isoprenaline (10(-7)-10(-6) mol/l) had no effect on the 3H-overflow evoked by stimulation (3 Hz) of the pulmonary artery preloaded with 3H-NA. At 10(-5) mol/l, (-)-isoprenaline reduced the 3H-overflow by maximally 39%. (-)-Isoprenaline (10(-5) mol/l) caused an inhibition that remained almost constant with time. The same results were obtained with (-)-isoprenaline (10(-7)-3 X 10(-5) mol/l) in the presence of cocaine (3 X 10(-5) mol/l), corticosterone (4 X 10(-5) mol/l), and the catechol-O-methyltransferase inhibitor U-0521 (3',4'-dihydroxy-2-methylpropiophenone) (10(-4) mol/l). In the presence of cocaine plus corticosterone, (-)-isoprenaline (3 X 10(-10)-10(-7) mol/l) had no effect on the 3H-overflow evoked by stimulation at 1 Hz. At 10(-6) mol/l, (-)-isoprenaline slightly reduced the 3H-overflow. At 10 Hz, (-)-isoprenaline (10(-6)-3 X 10(-5) mol/l) decreased the 3H-overflow and had no effect at 10(-7) mol/l. In the presence of either rauwolscine (10(-6) mol/l), phentolamine (10(-6) mol/l) or the phosphodiesterase inhibitor ICI 63,197 (3 X 10(-5) mol/l), (-)-isoprenaline (10(-7)-10(-6) mol/l) did not enhance the stimulation-evoked 3H-overflow. (+/-)-Propranolol (10(-7)-10(-5) mol/l) did not alter the stimulation-evoked 3H-overflow.(ABSTRACT TRUNCATED AT 250 WORDS)