The role of neuronal and extraneuronal systems in the metabolism of adrenaline and noradrenaline released from nerve terminals by electrical stimulation

The role of MAO-A and MAO-B in the metabolic degradation of noradrenaline in human arteries

1. Segments of human cystic, gastric and ileocolic arteries were obtained from patients undergoing surgery. 2. Segments of arterial tissues, the noradrenaline content of which ranged between 0.27 and 0.52 microg g(-1), were incubated with 0.1 micromol l(-1) [3H]-noradrenaline for 30 min and the accumulation of the amine as well as the formation of metabolites was measured. 3. In all the arteries, oxidative deamination predominated over O-methylation; the mean values of the deaminated and O-methylated metabolites formed for the three arteries were 247.6 and 82.4 pmol g(-1) tissue, respectively. Dihydroxymandelic acid (DOMA) was the most abundant metabolite. 4. Both clorgyline (a selective MAO-A inhibitor) and selegiline (a selective MAO-B inhibitor) reduced the formation of dihydroxyphenylglycol (DOPEG), DOMA and O-methylated-deaminated metabolites (OMDA), and increased that of normetanephrine (NMN). However, clorgyline depressed the formation of DOPEG more than that of DOMA, while selegiline depressed the formation of DOMA more than that of DOPEG. 5. In conclusion, three major differences distinguish the metabolism of noradrenaline by human arteries from that observed in other species: (1) the large predominance of deamination over O-methylation; (2) the extremely high formation of DOMA; and (3) the relative lack of selectivity of clorgyline and selegiline for MAO-A and B, respectively. Since the arterial vessels used were collected from patients older than 60 years, the morphological changes depending on age may explain the increase in DOMA formation.

Prazosin-induced blockade of extraneuronal uptake facilitates dopaminergic modulation of muscle twitches in rat vas deferens

Preliminary findings in our laboratory have shown that prazosin augmented the inhibitory effects of dopamine on the electrically-evoked muscle twitches in rat vas deferens. In this study, we opted to investigate the underlying mechanism and whether a prazosin-induced blockade of extraneuronal uptake process may be involved. Cumulative additions of dopamine (1.8 x 10(-7)-4.4 x 10(-5) M) elicited slight (< 30%) but dose-related inhibition of electrically-evoked (0.05 Hz, 1 ms duration and supramaximal voltage) muscle twitches of the vas deferens. Pretreatment with cocaine (10 microM), prazosin (50 nM) or oestradiol (10 microM) produced comparable potentiation of the inhibitory responses of dopamine; the pD2 values to dopamine amounted to 4.47 +/- 0.20, 4.72 +/- 0.21 and 4.56 +/- 0.19, respectively. A lower concentration of prazosin (5 nM) failed to alter dopaminergic responses. Further potentiation of dopamine responses was demonstrated in tissues preincubated with a combination of cocaine plus prazosin (50 nM), or cocaine plus oestradiol (pD2, 5.40 +/- 0.11 and 5.42 +/- 0.05, respectively). However, a mixture of all three drugs failed to elicit any further increase in dopamine responses, a finding that may suggest an extraneuronal uptake blocking activity for prazosin. Inhibition of muscle twitches evoked by bromocriptine, a dopaminoceptor agonist which is not a substrate for extraneuronal uptake, was not affected by prazosin (50 nM) pretreatment. The findings presented in this study emphasize the role of dopamine in modulating noradrenergic neurotransmission in rat vas deferens. More importantly, the results suggest that prazosin may act to block the extraneuronal uptake at noradrenergic sites, an effect that may account for its capability to facilitate dopaminergic modulation of noradrenergic neurotransmission.

Accumulation and release of adrenaline, and the modulation by adrenaline of noradrenaline release from rabbit blood vessels in vitro

The accumulation of (-)-3H-adrenaline (3H-A) by rabbit isolated aorta was studied. In all experiments, monoamine oxidase and catechol-O-methyltransferase were inhibited by treatment with pargyline and 3',4'-dihydroxy-2-methyl-propiophenone, respectively. The relationship between the accumulation of 3H derived from 3H-A and the duration of incubation was linear. The 3H-accumulation after 3 h incubation was 22.5 ml/g. In reserpine-treated tissue, the 3H-accumulation levelled off after 30 min and was 8.5 ml/g after 3 h. The concentration of 3H-A or (-)-3H-noradrenaline (3H-NA) and the 3H-accumulation (ml/g) were inversely related. At 10(-8) M, the 1-hour accumulation of 3H derived from 3H-A and 3H-NA was 7.8 and 15.2 ml/g, respectively. With increasing concentrations the accumulation values approached each other. The accumulation of 3H derived from 3H-A by reserpine-treated tissue also showed an inverse relationship with concentration. The accumulation of 3H derived from 3H-A was dependent on the bath temperature. Storage of tissue (0-5 days in salt solution without equilibration with 95% O2/5% CO2; 4 degrees C) did not affect the accumulation of 3H derived from 3H-A. Thereafter (7-14 days), the accumulation decreased. The inhibitory potency (IC50; -log M) of desipramine, cocaine, propranolol, isoprenaline, and normetanephrine on accumulation of 3H derived from 3H-A was found to be 8.26; 6.50; 5.48; 4.88, and 4.02, respectively. The maximal degree of inhibition was almost the same for these drugs, while that of clonidine and corticosterone was 50 and 20%, respectively. In the presence of desipramine, either clonidine, corticosterone or isoprenaline reduces the accumulation of 3H derived from 3H-A. Ouabain and iodoacetic acid, but not sodium cyanide and 2,4-dinitrophenol, reduced the accumulation of 3H derived from 3H-A. Anoxia (95% N2/5% CO2; 37 degrees C; 1-24 h) did not alter the accumulation of 3H derived from 3H-A. Glucose deprivation alone or combined with anoxia markedly reduced the 3H-accumulation. The release of 3H-A from rabbit isolated aorta was studied. This release was compared with that of 3H-NA. The stimulation-evoked 3H-overflow from aorta preloaded with 3H-A decreased with repeated stimulation. In contrast, prestimulation enhanced subsequent stimulation-evoked 3H-overflows. For both 3H-amines, the 3H-overflow increased concomitantly to the same degree with the number of pulses. The time course of 3H-overflows with either 3H-A or 3H-NA was compared.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of hypoxia on noradrenaline release and neuronal reuptake in isolated rabbit thoracic aortic strips

To clarify the effects of hypoxia on stimulus-evoked noradrenaline release and on neuronal reuptake of the released noradrenaline, we examined the effects of hypoxia on contraction responses of rabbit thoracic aortic strips to transmural electrical stimulation and on the stimulation-evoked overflow of total [3H] and [3H]noradrenaline from the strips prelabelled with [3H]noradrenaline. This was done in the presence or absence of an inhibitor of neuronal uptake (cocaine). In a medium equilibrated with a gas mixture of 95% O2/5% CO2 (control), cocaine doubled the stimulation-evoked overflow of total [3H] and [3H]noradrenaline; there was a concomitant increase (130%) in contractions to electrical stimulation. At 0% O2 (95% N2/5% CO2, hypoxia), cocaine had no significant effects on either the stimulation-evoked overflow of total [3H] and [3H]noradrenaline or contractions. In the absence of the drug, hypoxia decreased the stimulation-evoked overflow of total [3H] and [3H]noradrenaline to 47% and 43%, respectively, of the control values, whereas these values were 31% and 28%, respectively, after exposure to cocaine. The inhibition by hypoxia of contraction responses to electrical stimulation was greater in the presence of cocaine than in its absence. These results show that hypoxia inhibits both noradrenaline release evoked by a given stimulus and neuronal uptake.

Spontaneous and ouabain-induced efflux of catecholamines and dihydroxyphenylglycol in two canine blood vessels

The spontaneous efflux of endogenous noradrenaline, dopamine, dihydroxyphenylglycol (DOPEG) from adrenergic nerve endings of 2 canine blood vessels (the mesenteric artery and the saphenous vein) were studied during 8 successive incubation periods of 15 min each. Extraneuronal uptake and O-methylation were minimized by the presence of adequate concentrations of tropolone and hydrocortisone. Both vessels had an efflux characterized by a decline in the 3 catechols, which was most marked for noradrenaline; the mesenteric artery lost larger amounts than the saphenous vein. Ouabain caused a large increase in the efflux of noradrenaline and dopamine and a reduction of DOPEG efflux. Cocaine had only a modest effect, more evident in the case of the mesenteric artery, increasing noradrenaline and reducing DOPEG effluxes. The combination of ouabain and cocaine had no additive effects, and the effects of ouabain were even reduced (on some parameters) by cocaine. Accordingly, the noradrenaline:DOPEG ratio was markedly increased by ouabain, but not by cocaine; cocaine significantly reduced the effects of ouabain. The ratio dopamine:noradrenaline was decreased by cocaine and by ouabain. Comparison of tissue content and efflux allowed us to conclude that apparently no significant de novo synthesis of noradrenaline occurred during the incubation period. We conclude that a fast and early component of spontaneous efflux is due to loss from the neurons and that its greater magnitude in the mesenteric artery may be due to differences in neuronal [Na+] and/or to differences in neuronal membrane adenosine triphosphatase activity. The results also suggest that neuronal reuptake plays only a minor role in the handling of spontaneously released noradrenaline.

Evidence that 6-hydroxydopamine is an inhibitor of catechol-O-methyltransferase in intact tissue

The effect of in-vitro sympathetic denervation on the O-methylation of 2-hydroxyoestradiol was examined using the rabbit thoracic aorta. Aortic segments were exposed to 6-hydroxydopamine (400 mg litre-1, 10 min), incubated in 2-hydroxyoestradiol (5 microM, 60 min), and the 2-methoxyoestradiol formed was quantified using HPLC with electrochemical detection. Pre-exposure to 6-hydroxydopamine reduced O-methylation by 90% in intact aortic strips. However, O-methylation was also reduced (81%) in rabbit aorta that had been surgically denervated before exposure to 6-hydroxydopamine. Furthermore, the O-methylation of 2-hydroxyoestradiol (20 microM, 15 min) by partially purified soluble catechol-O-methyltransferase was inhibited by 6-hydroxydopamine (400 mg litre-1) by 95% and 51% in the absence and presence of antioxidant, respectively. These results suggest that the inhibition of catechol-O-methyltransferase by 6-hydroxydopamine reported for the purified enzyme, applies to the intact tissue and that the inhibition is dependent on oxidation of the 6-hydroxydopamine. Subsequent experiments using dialysis techniques demonstrated that the inhibition of soluble catechol-O-methyltransferase by 6-hydroxydopamine was irreversible in part, but the degree of irreversibility was influenced by pH and by the extent of in-vitro oxidation of 6-hydroxydopamine.

Influence of MAO A and MAO B on the inactivation of noradrenaline in the saphenous vein of the dog

Specific inhibitors of MAO A (clorgyline 0.1 mumol/l) and of MAO B [(-)deprenyl 10 mumol/l] were used in dog saphenous vein strips in order to study the relative influence of the two types of MAO on: 1. Termination of contractile response to exogenous noradrenaline (NA); 2. Metabolism and accumulation of exogenous (3H)-NA; 3. Metabolism of 3H-NA released by electrical stimulation. To study the termination of contractile response to exogenous NA, the oil immersion technique was used to determine the time for half-relaxation. The experiments were performed on strips with or without treatment with cocaine 10 mumol/l (to inhibit neuronal uptake) plus U-0521 100 mumol/l (to inhibit catechol-O-methyl transferase) before and after exposure to MAO inhibitors. Clorgyline, but not (-)deprenyl enhanced significantly the time for half-relaxation of the strips, whether cocaine was present or not. To study the metabolism and accumulation of exogenous 3H-NA, the strips were incubated (with or without preincubation with cocaine) with 3H-NA 0.23 mumol/l and 2.3 mumol/l in the presence and in the absence of MAO inhibitors. The formation of deaminated metabolites was significantly reduced by clorgyline, but not by deprenyl. To study the metabolism of 3H-NA released by electrical stimulation, the strips were incubated with 3H-NA 1.4 mumol/l. In the presence of cocaine and U-0521, field stimulation was applied during two periods of 5 min (10 Hz, 100 V, 2 ms), in the absence or presence of MAO inhibitors. Under these experimental conditions clorgyline, but not deprenyl, abolished DOPEG formation, without affecting the other metabolites.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural and functional alterations caused at the extraneuronal level by sympathetic denervation of blood vessels

The lateral saphenous vein of the dog and the rabbit ear artery were surgically denervated, by clamping the vessel or by removal of the superior cervical ganglion, respectively. Both procedures resulted in denervation of the vessels. The denervated, lateral saphenous vein was supersensitive to exogenous noradrenaline and inactivation of the amine (in oil immersion experiments) was slower in denervated vein strips than in control strips treated with cocaine. Incubation experiments with 3H-noradrenaline confirmed that denervated strips formed considerably fewer metabolites than control ones (in the absence or presence of cocaine) and that O-methylation of noradrenaline was reduced by about 50%. When the strips were incubated with 3H-isoprenaline, the denervated ones accumulated and metabolized isoprenaline to a lesser degree than control strips. Hydrocortisone did not reduce the accumulation of isoprenaline in the denervated vein and had only minor effects on O-methylation. The metabolism of noradrenaline and isoprenaline gradually recovered with time. In the ear artery, denervation was accompanied by a marked reduction in O-methylation, but not in accumulation, of isoprenaline. In both vessels there was a highly significant positive correlation between noradrenaline content and O-methylating capacity; in the saphenous vein accumulation of isoprenaline was also positively correlated to noradrenaline content. Morphological changes observed in the denervated vessels consisted essentially in dedifferentiation of smooth muscle cells (which attained larger dimensions, had an indented, large nucleus, augmented euchromatin and an increased amount of ribosomes), abundance of extracellular material and fibroblasts.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of uptake2 on the inhibition by unlabelled noradrenaline of the stimulation-evoked overflow of 3H-noradrenaline in rabbit aorta with regard to surface of amine entry

The release by electrical field stimulation of 3H-noradrenaline from the adrenergic nerve endings in the rabbit aorta was studied in a special double-chambered organ bath. Independently of the frequency (1-10 Hz) and the number of pulses used (300-3,000 pulses), only 10-20% of the stimulation-evoked overflow of tritium (total evoked overflow) left the aortic wall via the intimal surface (intimal evoked overflow). The corresponding percentage value for the basal efflux was twice that for the evoked overflow, thus indicating that part of the radioactivity in the basal efflux originated from an extraneuronal compartment. The radioactivity in the total evoked overflow (in response to stimulation at 10 Hz) originated from at least two compartments (compartment I and II) with half times for efflux of about 2 and 6 min, respectively. In the intimal evoked overflow, compartment II (but not compartment I) was involved. When uptake1 was inhibited, 70% of the radioactivity in the intimal evoked overflow (stimulation at 1 Hz) consisted of metabolites, while unchanged amine prevailed by far in the total and adventitial evoked overflow, respectively. Additional inhibition of uptake2 thus had a striking effect only on the composition of the radioactivity in the intimal evoked overflow. The intimal surface was exposed to unlabelled noradrenaline in order to inhibit the evoked overflow of tritium (stimulation at 1 Hz; uptake1 inhibited). When uptake2 was inhibited additionally, the dose-response curve for the inhibitory effect of noradrenaline was shifted to the left by a factor of 4.(ABSTRACT TRUNCATED AT 250 WORDS)

Stereospecific inhibition by ozolinone of stimulated chloride secretion in rabbit colon descendens

The effects of the diuretic drug ozolinone on electrogenic Cl- secretion by rabbit colonic mucosa were investigated in vitro. Electrical properties and unidirectional Cl- fluxes were measured in stripped preparations mounted in Ussing-type chambers. After abolition of electrogenic Na+ absorption by amiloride (10(-4) mol/l) on the mucosal side electrogenic Cl- secretion was induced by addition of PGE1 (10(-6) mol/l, serosal side) and theophylline (10(-2) mol/l, both sides). Under these conditions, the monitored short-circuit current (Isc) equals the amount of Cl- secreted as evidenced by determination of unidirectional Cl- fluxes. After establishing a stable Cl- secretion its sensitivity to the enantiomers of the diuretic was studied. Only levorotatory (-)-ozolinone, but not the dextrorotatory (+)form, inhibited Cl- secretion on serosal application. This effect was fully accounted for by a reduction in the serosal-to-mucosal Cl- fluxes (JClsm). It was readily reversible and concentration-dependent with a Ki value of 6 x 10(-4) mol/l, but absent when the drug was added to the mucosal side. The results are in agreement with the hypothesis that loop diuretics inhibit a coupled NaCl entry mechanism across the baso-lateral membrane into colonic epithelial cells. This mechanism is though to account for Cl- influx into the cells.

Asymmetric release of cyclic AMP from guinea-pig and rabbit gallbladder

The release of cyclic adenosine 3':5'-monophosphate (cAMP) from guinea-pig and rabbit gallbladder was investigated in vitro. Serosal addition of prostaglandin E1 (PGE1) to luminally perfused guinea-pig gallbladders caused a concentration-dependent efflux of cAMP to the mucosal side, the threshold concentration of PGE1 being 10(-7) M. The efflux of cAMP to the serosal side was 7-fold lower. A mucosal sidedness of cAMP release was also observed in stripped preparations of rabbit gallbladder mucosa mounted between two half chambers. No cAMP was found in the solutions bathing the serosal layers isolated from rabbit gallbladders. Fluid secretion was observed at 10(-7) M PGE1, an effect mimicked by serosal, but not mucosal application of cAMP (3.3 x 10(-3) M). This is taken to indicate that the basolateral membrane is more easily permeated by cAMP than the apical membrane, since cAMP is believed to exerts its physiological effects from inside the cell. It is concluded that preferential release of cAMP to the mucosal side is not due to a higher permeability of the brush border membrane but rather represents an as yet undefined transport process which may be of importance for the regulation of excessive intracellular cAMP levels.

Deamination of released 3H-noradrenaline in the canine saphenous vein

Experiments were designed to determine the effect of monoamine oxidase (MAO) inhibitors on the release and the metabolism of noradrenaline in the canine saphenous vein. Helical strips were incubated with 3H-noradrenaline and mounted for superfusion and measurement of the efflux of labelled transmitter and its metabolites; in certain experiments the tissue content of 3H-noradrenaline and its metabolites was also determined. The MAO-A inhibitor clorgyline, and the non-specific inhibitor pargyline, but not the MAO-B inhibitor deprenyl decreased the appearance of deaminated and O-methylated deaminated metabolites under basal conditions and during electrical stimulation. The MAO-A and the non-specific MAO inhibitor did not decrease the efflux of VMA to the same extent as that of the other deaminated metabolites. During superfusion with etidocaine, an agent causing increased leakage of stored transmitter, clorgyline abolished the appearance of DOPEG. Addition of semicarbazide in preparations treated with pargyline did not affect the efflux of deaminated and O-methylated deaminated metabolites. From the measurement of tissue VMA, it appeared that the efflux of VMA poorly reflects quick changes in the rate of its formation but that formation is abolished by pretreatment with pargyline. These experiments indicate that in the canine saphenous vein: (1) DOPEG is formed mainly in intraneuronal sites, while DOMA, MOPEG and VMA are formed extraneuronally; (2) VMA is retained in the tissue after its formation; and (3) the only subtype of MAO involved in the metabolism of 3H-noradrenaline released from adrenergic nerve endings can be classified as MAO-A.

A pre-junctional action of 5-hydroxytryptamine and methysergide on noradrenergic nerves in dog isolated saphenous vein

Electrical stimulation (2 Hz for 2 min) of dog isolated saphenous vein strips pre-incubated with tritiated noradrenaline increased the overflow of tritium of which about 80% was noradrenaline. 5-Hydroxytryptamine (5-HT; 1.0 x 10-9-1.0 x 10-7 mol litre-1) and methysergide (3.0 x 10-8-3.0 x 10-6 mol litre-1) inhibited the induced overflow of total tritium by a maximum of 78 +/- 4% and 47 +/- 7% respectively (mean +/- s.e. mean, n=6 for each). Methysergide was about 30 times less potent than 5-HT and the maximum inhibition obtained was less than with 5-HT. Both compounds inhibited electrically-induced contractions and overflow of tritiated noradrenaline. Their inhibitory actions on tritium overflow were little affected by phentolamine (1.0 x 10-6 mol litre-1) or cyproheptadine (1.0 x 10-6 mol litre-1), nor was the inhibitory effect of methysergide on electrically induced contractions antagonized by atropine, mepyramine, cimetidine or propranolol. The findings suggest that the prejunctional inhibitory effect of methysergide may be mediated via stimulation of a 5-HT receptor which, unlike the D-receptor, is not blocked by cyproheptadine. The possibility that the pre-junctional 5-HT receptor in the dog saphenous vein is the same as the post-junctional receptor in this preparation is discussed.