Yohimbine blockade of ionic channels in myocardial cells

Yohimbine Directly Induces Cardiotoxicity on Human-Induced Pluripotent Stem Cell-Derived Cardiomyocytes

Yohimbine is a highly selective and potent α₂-adrenoceptor antagonist, which is usually treated as an adjunction for impotence, as well for weight loss and natural bodybuilding aids. However, it was recently reported that Yohimbine causes myocardial injury and controversial results were reported in the setting of cardiac diseases. Here, we used human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) as a model system to explore electrophysiologic characterization after exposure to Yohimbine. HiPSC-CMs were differentiated by employment of inhibitory Wnt compounds. For analysis of electrophysiological properties, conventional whole-cell patch-clamp recording was used. Specifically, spontaneous action potentials, pacemaker currents (I_f), sodium (Na⁺) channel (I_Na), and calcium (Ca⁺⁺) channel currents (I_Ca) were assessed in hiPSC-CMs after exposure to Yohimbine. HiPSC-CMs expressed sarcomeric-α-actinin and MLC2V proteins, as well as exhibited ventricular-like spontaneous action potential waveform. Yohimbine inhibited frequency of hiPSC-CMs spontaneous action potentials and significantly prolonged action potential duration in a dose-dependent manner. In addition, rest potential, threshold potential, amplitude, and maximal diastolic potential were decreased, whereas APD₅₀/APD₉₀ was prolonged. Yohimbine inhibited the amplitude of I_Na in low doses (IC₅₀ = 14.2 μM, n = 5) and inhibited I_Ca in high doses (IC₅₀ = 139.7 μM, n = 5). Whereas Yohimbine did not affect the activation curves, treatment resulted in left shifts in inactivation curves of both Na⁺ and Ca⁺⁺ channels. Here, we show that Yohimbine induces direct cardiotoxic effects on spontaneous action potentials of I_Na and I_Ca in hiPSC-CMs. Importantly, these effects were not mediated by α₂-adrenoceptor signaling. Our results strongly suggest that Yohimbine directly and negatively affects electrophysiological properties of human cardiomyocytes. These findings are highly relevant for potential application of Yohimbine in patients with atrioventricular conduction disorder.

Comparative effects of idazoxan, prazosin, and yohimbine on coronary ligation-induced arrhythmias in spontaneously hypertensive rats

We investigated whether certain drugs with alpha-adrenergic antagonist activity display anti-arrhythmic effects in hypertensive animals subjected to acute coronary artery ligation. The left anterior descending coronary artery (LAD) was ligated in open-chest pentobarbital-anesthetized spontaneously hypertensive rats (SHR); arrhythmias were subsequently recorded for 30 min. Drugs were administered intravenously, (i.v.) 5 min before ligation. The effects of yohimbine and idazoxan were compared with those of prazosin. Prazosin (100 mu g/kg) increased the occurrence of ventricular tachycardia (VT). In contrast, yohimbine 1.6 mg/kg decreased both the occurrence and the duration of VT and the occurrence and the duration of ventricular fibrillation, (VF). The results obtained with idazoxan 1 mg/kg were similar to those with yohimbine. The ECG alterations induced by coronary artery ligation in rats treated with yohimbine and idazoxan were more pronounced than in controls and in rats treated with prazosin, suggesting that the antiarrhythmic effects observed were not mediated by antiischemic activity. The protective effects against ligation-induced arrhythmias were preceded by a hypotensive effect and a decrease in the rate-pressure product in yohimbine-treated but not in idazoxan-treated animals. In rats treated with prazosin, more arrhythmic events were observed, although hemodynamics were similar to those in rats treated with yohimbine. Our results suggest that the yohimbine-induced antiarrhythmic action is not due to an alteration of conduction or repolarization rates. In this model, yohimbine and idazoxan appear to protect against ligation-induced arrhythmias. These data suggest that drugs with alpha-adrenergic properties might influence the nervous drive to the heart in SHR with cardiac ischemia. However, further investigations are needed to ascertain whether the alpha-adrenoceptor blockade participates in this effect.

Effect of urapidil on the action potentials in the guinea-pig ventricular myocardium

The electrophysiological effects of urapidil, a new alpha 1-adrenoceptor antagonist, were assessed in the reserpinized guinea-pig ventricular myocardium. Urapidil suppressed the maximal rate of rise (Vmax) of steady-state action potentials elicited by the fast responses at high concentrations independently of blockade of myocardial alpha-adrenoceptors, but not the Vmax of Ca(2+)-dependent slow action potentials of partially depolarized muscles in concentrations tested (up to 1.1 mM). Urapidil at high concentrations prolonged the action potential durations of the fast and slow responses in a manner similar to the quinidine-like antiarrhythmic drugs. These results suggest that the inhibitory effect of urapidil on the slow inward Ca2+ current and the Na+ current is in practice negligible.

Effects of midaglizole, a new hypoglycaemic drug on the electrophysiological properties of guinea-pig papillary muscle

1. The electrophysiological effects of midaglizole, a new oral hypoglycaemic agent which is an alpha 2-adrenoceptor antagonist, were assessed in the reserpinized ventricular myocardium of the guinea-pig. 2. Midaglizole suppressed the maximal rate of rise (Vmax) and the amplitude of action potentials (APA) of the fast and the slow responses in a concentration-dependent manner without influencing the resting potentials. 3. Voltage-dependency of the Vmax block of the action potentials by midaglizole (10 microM) did not appear in solutions containing various concentrations of KCl. 4. It is concluded that midaglizole has inhibitory effects on the fast Na+ and slow Ca2+ currents of the membrane independent of the blockade of myocardial alpha-adrenoceptors, and that these effects may be significant in some clinical uses of the drug.

Characterization of endogenous noradrenaline release from intact and epithelium-denuded rat isolated trachea

1. Overflow of endogenous noradrenaline (NA) from the in vitro incubated rat trachea evoked by two periods of electrical field stimulation (S1, S2 at 3 or 15 Hz) or by high potassium (60 mM) was determined by high performance liquid chromatography (h.p.l.c.) with electrochemical detection. 2. In the presence of the neuronal uptake inhibitor desipramine, the alpha 2-adrenoceptor antagonist, yohimbine, enhanced the overflow of NA evoked by stimulation at 3 Hz by about 100% suggesting the presence of presynaptic inhibitory autoreceptors on the sympathetic nerves innervating the trachea. 3. When desipramine and yohimbine were present throughout the experiments, the overflow of NA evoked by the second period of electrical stimulation (S2) was significantly smaller than that evoked by the first (S1). This decline of overflow was prevented when the NA precursor, tyrosine, was additionally present throughout the experiments. 4. After removal of the epithelium, the tissue content of NA was reduced by about 30%, suggesting that part of the NA may be present and released within the epithelium. However, the overflow of NA evoked by stimulation at 3 Hz or 15 Hz was reduced by 70-80%, indicating that the epithelium may additionally exert a permissive role on the release of NA within the airways, possibly by suppressing inhibitory factors. 5. Stimulation by high potassium (60 mM for 10 min) caused a large overflow of NA (about 45% of the tissue NA), both from epithelium-free and epithelium-denuded tracheae. Thus the 'endogenous inhibition' of NA release after removal of the epithelium is surmountable when a high potassium stimulus is applied.

Differential effects of electrical stimulation, blockade of neuronal amine uptake and activation of alpha 2-adrenoceptors on the release of endogenous noradrenaline and 5-hydroxytryptamine from the isolated rat pineal gland

Isolated rat pineal glands were incubated in vitro and the release of endogenous noradrenaline or 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) was determined by HPLC with electrochemical detection. In the absence of test drugs, the spontaneous outflow of noradrenaline was about 10 fmol/10 min and electrical stimulation (5 Hz, 1500 pulses) evoked the release of about 70 fmol noradrenaline. Nomifensine enhanced the spontaneous outflow of noradrenaline about threefold and the electrically evoked release of noradrenaline about sixfold. In the presence of nomifensine, the alpha 2-adrenoceptor antagonist yohimbine markedly increased the electrically evoked release of noradrenaline, whereas the alpha 1-adrenoceptor antagonist prazosin had no effect. Clonidine inhibited the electrically evoked release of noradrenaline by about 65%, and this was antagonized by yohimbine in a competitive manner. In the absence of drugs, the initial spontaneous outflow of 5-HT was (compared with noradrenaline) very high 64 pmol/10 min). It declined by 80% within 1 h of incubation in vitro. The outflow of 5-HIAA amounted initially to 38 pmol 10 min and declined by 40% within 1 h of incubation. Addition of L-tryptophan (10 mumol/l) after 1 h of incubation in vitro largely enhanced the outflow of 5-HT and 5-HIAA within 30 min of incubation (about ten- and fourfold, respectively). When L-tryptophan was present from the onset of incubation the initial outflow of 5-HT and 5-HIAA was only slightly elevated, but the decline was largely attenuated. Neither omission of calcium nor addition of nomifensine, clonidine or yohimbine significantly affected the spontaneous outflow of 5-HT or 5-HIAA.(ABSTRACT TRUNCATED AT 250 WORDS)

Bunazosin induces use-dependent slowing of conduction in guinea-pig ventricular myocardium

The use-dependent effects of bunazosin on the maximal rate of rise (Vmax) of the action potential and conduction velocity were studied in isolated papillary muscles of guinea-pig. Standard microelectrode techniques were used to monitor the conduction and action potential of the muscles. In the presence of 30 microM bunazosin, the time constants for the start of the use-dependent inhibition of Vmax during a 0.2, 1, 2 and 3 Hz stimulation were (means +/- S.E.M. in s) 30.9 +/- 8.0, 15.0 +/- 1.6, 7.4 +/- 0.9 and 3.9 +/- 0.7 (n = 4) and those for conduction velocity were 17.3 +/- 2.3, 17.3 +/- 5.2, 6.5 +/- 0.9 and 3.4 +/- 0.2, respectively. These results showed that in the cardiac ventricular muscle of the guinea-pigs, bunazosin produces use-dependent changes in conduction velocity with onset kinetics comparable to those measured simultaneously using Vmax. The characteristics of the use-dependent inhibition of conduction velocity induced by bunazosin are similar to those found with slow kinetic drugs such as disopyramide rather than with fast ones.

Antiarrhythmic and antinecrotic effects of yohimbine stereoisomers in rats during coronary occlusion and reperfusion

In pentobarbital-anesthetized rats the left coronary artery was ligated for 5 or 30 min and then opened for reperfusion of the ischemic myocardial area. Twelve min prior to the coronary occlusion yohimbine stereoisomers, namely corynanthine and rauwolscine, or saline solution were given intravenously. In the saline controls both ischemia and reperfusion provoked severe tachyarrhythmias with ventricular fibrillation in 37.5% or 54.6%, respectively. Using corynanthine and rauwolscine a highly significant antiarrhythmic effect was observed. Corynanthine completely prevented ventricular fibrillation and delayed significantly the development of myocardial necrosis. The factors likely underlying the antiarrhythmic and antinecrotic effects are discussed.

Inhibition of aftercontractions and phasic calcium release by yohimbine in ferret papillary muscle

Phasic release of calcium from the sarcoplasmic reticulum occurs in all mammalian cardiac preparations when the intracellular calcium concentration is sufficiently high. The phasic calcium release is often sufficient to trigger electrophysiological responses and aftercontractions. These can be detrimental to normal cardiac function. We induced phasic calcium release in ferret papillary muscles loaded with the calcium indicator aequorin. Development of phasic calcium release was associated with an increase in resting and peak [Ca2+]i. Inhibiting sodium channels with yohimbine reduced resting [Ca2+]i and prevented phasic calcium release. We propose a mechanism where by reduced [Na+]i, and the subsequent increased efflux of calcium via sodium/calcium exchange reduced [Ca2+]i.

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.

Isomeric yohimbine alkaloids block calcium-activated K+ channels in medullary thick ascending limb cells of rabbit kidney

The alpha2-adrenergic antagonist yohimbine (YOH) and the closely related isomers corynanthine (COR) and rauwolscine (RAU) caused brief interruptions in current characteristic of a fast blocker Ca2+-activated K+ channels in cultured medullary thick ascending limb (MTAL) cells. The apparent dissociation constants (Kapp) for COR, YOH, and RAU, respectively, at the intracellular face of the channel in the presence of 200 mM K+ are 45 +/- 1, 98 +/- 2, and 310 +/- 33 microM. The Kapp for COR on the extracellular side also in the presence of 200 mM K+ was much greater at 1.6 +/- 0.17 mM. Increasing K+ on the same side as the blocker relieves the blocking reaction. The Kapp for the alkaloids varies with K+ in a manner quantitatively consistent with K+ and the alkaloids competing for a common binding site. Finally, blocking by the charged form of these alkaloids is voltage dependent with changes in Kapp of 86 +/- 7 and 94 +/- 6 microM per e-fold change in voltage for blockers applied either from the inside or outside. The alkaloids block at an electrical distance similar to tetraethylammonium, suggesting that the site within the channel pore of these molecules may be similar.

Antiarrhythmic effects of alpha-adrenoceptor antagonists in guinea pig ventricular myocardium

Antiarrhythmic effects of alpha-adrenoceptor antagonists were assessed in the reserpinized guinea pig ventricular myocardium. Both bunazosin (1 to 3 x 10(-7) M), a new alpha 1-adrenoceptor antagonist, and yohimbine (1 to 3 x 10(-7) M), another adrenoceptor antagonist, suppressed the transient depolarization and triggered activity induced by a train of rapid stimuli in the solution containing low potassium ion (K+), high calcium ion (Ca2+) and strophanthidin (1 to 5 x 10(-7) M). Bunazosin (3 x 10(-6) M) abolished the facilitatory effect of hypoxia on beta-adrenoceptor mediated abnormal automaticity. To clarify the mechanisms underlying the antiarrhythmic properties of alpha-adrenoceptor antagonists, their electrophysiologic effects on the fast and slow action potentials were investigated. Alpha-adrenoceptor antagonists (bunazosin, yohimbine and phentolamine) suppressed the slow response in a dose-related manner. The voltage-dependent block and use-dependent block of the maximal rate of rise (Vmax) of action potentials by bunazosin (10(-5) to 10(-4) M) and yohimbine (10(-6) to 10(-5) M) were studied. The analysis of the onset and recovery kinetics from the use-dependent block of drugs showed that both bunazosin and yohimbine act as slow kinetic drugs. It is concluded that alpha-adrenoceptor antagonists seem to have an antiarrhythmic effect through the inhibition of fast sodium ion (Na+) and slow Ca2+ currents of the cell membrane independently of blockade of myocardial alpha-adrenoceptors.

Excitatory and inhibitory cholinergic effects of yohimbine on isolated guinea-pig small intestine

The interaction of yohimbine with the cholinergic intestinal system was investigated in the isolated guinea-pig ileum using a wide range of drug concentrations from 3 x 10(-13) to 2 x 10(-4) g/ml. Low concentrations of yohimbine (3 x 10(-13) to 3 x 10(-11) g/ml) caused dose-dependent contractions of the ileal longitudinal muscle, which were potentiated by eserine 1 x 10(-8) g/ml and prevented by tetrodotoxin 1 x 10(-6) g/ml or by atropine 1 x 10(-12) g/ml; methysergide and diphenydramine were ineffective up to 3 x 10(-7) g/ml dose. Submaximal stimulatory responses evoked by twitch stimulation or by acetylcholine were significantly potentiated by the same concentrations of yohimbine (3 x 10(-13) to 3 x 10(-11) g/ml) and blocked by atropine 1 x 10(-12) g/ml. By contrast, high concentrations of yohimbine (1 x 10(-6) to 2 x 10(-4) g/ml) displayed dose-dependent inhibitory effects on cholinergic responses. The stimulant effect of yohimbine seems to be indirect and mediated by the increase in the release of acetylcholine, while the inhibitory action may be due to a molecular interaction with the muscarinic receptors allowing non-specific receptor blockade.

Alpha 2-receptor control over release of noradrenaline in rat thalamus

Rat brain thalamic tissue was examined for the presence of alpha 2-receptor control over noradrenaline (NA) utilization. Systemically administered clonidine (0.1 mg/kg i.p.) decreased NA turnover, yohimbine (10 mg/kg i.p.) increased NA turnover, and prazosin (0.5 mg/kg i.p.) had no effect. In vitro, yohimbine increased K+-stimulated overflow of endogenous NA in a dose-dependent fashion. It appears that locus coeruleus neurons which project to thalamus exhibit the same type of alpha 2-receptor control as those terminating in other forebrain regions.

Effect of yohimbine on action potentials recorded from isolated canine ventricular myocytes

Yohimbine is used extensively as an alpha 2-adrenoceptor antagonist. This drug also inhibits sodium channels in the squid axon. In this study we investigated the electrophysiological effects of yohimbine in isolated canine ventricular myocytes. Yohimbine produced a reversible and concentration-dependent decrease in the maximum upstroke velocity and a slight increase in the action potential duration. The maximum upstroke velocity was reduced by over 90% by 10(-4) M yohimbine. It did not effect the resting or plateau potentials in control or isoproterenol-treated cells. Delayed after-depolarizations were also inhibited. These data suggest that yohimbine can produce a local anesthetic effect which is primarily due to inhibition of sodium channels.

Enhancement by norepinephrine of automaticity in sheep cardiac Purkinje fibers exposed to hypoxic glucose-free Tyrode's solution: a role for alpha-adrenoceptors?

A period of drive in the presence of norepinephrine (NE) may be followed by the induction or acceleration of spontaneous activity. Experiments were carried out in sheep cardiac Purkinje fibers to determine whether the effects of NE on automaticity were modified during superfusion with hypoxic glucose-free Tyrode's solution and to assess the possible contribution of alpha-adrenergic influences on automaticity under these conditions. The following results were obtained: Low concentrations of NE (10(-7) and 3 X 10(-7)M) were able to induce automaticity after a period of drive in normal oxygenated (97% O2, 3% CO2) Tyrode's solution. Superfusion with hypoxic (97% N2, 3% CO2) glucose-free Tyrode's solution enhanced NE-induced automaticity. Practolol, in concentrations able to block the effects of NE in normal oxygenated solution, did not counteract the effects of NE in hypoxic glucose-free solution. Yohimbine, but not prazosin, antagonized the effects of NE in hypoxic glucose-free solution. At the same concentration, yohimbine did not affect transmembrane potentials or automaticity induced by isoproterenol. It is concluded that alpha-adrenergic responsiveness appears to be enhanced during superfusion in vitro with hypoxic glucose-free solution, and that alpha-adrenoceptors belonging to the alpha 2-subtype in sheep cardiac Purkinje fibers might influence abnormal automaticity, possibly through an effect on oscillatory potentials.

Noradrenergic modulation of 4-aminopyridine-induced acetylcholine release from rat cerebral cortex

The noradrenergic influence on cortical acetylcholine (ACh) release was investigated by the cortical cup technique in urethane anaestetized rats treated with 4-aminopyridine (4-AP). The following results were obtained: 1) The increase in ACh release induced by 4-AP (3 mg/kg i.p.) was strongly potentiated by pretreatment with -methyl-p-tyrosine (alpha-MPT) which inhibits catecholamine biosynthesis or by N-(2-chloroethyl)-N-ethyl-bromobenzylamine (DSP4) bringing about a selective degeneration of noradrenergic fibres. Neither pretreatment enhanced the spontaneous ACh output. 2) Pretreatment with p-chlorophenylalanine (PCPA), an inhibitor of serotonin synthesis, did not modify 4-AP effect on ACh output. 3) The alpha blockers, yohimbine (1 mg/kg i.p.) and prazosin (4 mg/kg i.p.), did not enhance the 4-AP effect on ACh release but only delayed its onset. 4) Yohimbine (7 mg/kg i.p.) completely reversed 4-AP effect on ACh release which was significantly decreased. It is concluded therefore that pretreatments with alpha-MPT and DSP4 remove an inhibitory noradrenergic control on cortical ACh release. On the other hand, the alpha blockers might interfere with the ionic mechanisms underlaying the 4-AP effect thus, masking the removal of the noradrenergic control, due to an alpha blockade.

Subsensitivity of enteric cholinergic neurones to alpha 2-adrenoceptor agonists after chronic sympathetic denervation

The concentration-effect relationships of noradrenaline, dopamine and clonidine in inhibiting resting and stimulated acetylcholine output have been studied in intact and in sympathetically denervated preparations of guinea pig isolated distal colon. The order of potencies for the inhibition of resting acetylcholine release in intact preparations was clonidine greater than dopamine greater than noradrenaline while the order of intrinsic activities was noradrenaline greater than dopamine greater than clonidine. Sympathetic denervation was able to modify the potency of either clonidine, dopamine and noradrenaline. Noradrenaline was 6 times more potent in inhibiting resting acetylcholine release in denervated than in intact preparations, while clonidine and dopamine underwent a 18-fold and a 11-fold decrease in potency after denervation. The potency of clonidine relative to noradrenaline was 110 in intact preparations and only 1.2 in denervated organs. The intrinsic activities of noradrenaline, dopamine and clonidine were almost unchanged in denervated organs. A dose-dependent facilitatory effect of yohimbine on both the resting acetylcholine output and the peristaltic reflex could be observed in intact but not in sympathetically denervated preparations at concentrations ranging from 2.5 X 10(-8) M to 2.5 X 10(-7) M. Yohimbine was able to counteract the inhibitory effect of dopamine and to remove the inhibitory effect of periarterial nerve stimulation on both acetylcholine release and the peristaltic reflex. Our results are consistent with the existence of a tonic physiological modulation of enteric cholinergic neurones by postganglionic sympathetic fibres. The order of potencies of adrenoceptor agonists and the antagonism by yohimbine is consistent with such a modulation being entirely carried out through alpha 2-heteroceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition by yohimbine of the calcium-dependent evoked release of [3H]GABA in rat and mouse brain slices in vitro

Calcium-dependent release of [3H]GABA was elicited by electrical stimulation in slices of rat and mouse cerebral cortex or by potassium stimulation in the mouse brain-stem. The stimulation-evoked release of [3H]GABA was inhibited by yohimbine in a concentration-dependent manner. High concentrations of other alpha-adrenoceptor antagonists such as phentolamine, RS 21361, idazoxan, rauwolscine and corynanthine also inhibited [3H]GABA release. This effect was not observed with pseudoyohimbine or prazosin. [3H]GABA release was not affected by exposure to the alpha-adrenoceptor agonists clonidine, M7, noradrenaline or methoxamine. In addition, clonidine did not antagonize the yohimbine-induced inhibition of [3H]GABA release. The inhibitory effect of yohimbine did not result from an interaction with endogenously released noradrenaline since the inhibition was still observed in reserpine-pretreated animals. It is concluded that yohimbine and other alpha 2-adrenoceptor antagonists inhibit the stimulation-evoked release of [3H]GABA through a mechanism which appears to be independent of the blockade of alpha 2-adrenoceptors and which does not involve an interaction with endogenous noradrenaline. The present results indicate that yohimbine exerts non-specific actions on the release of [3H]GABA and that similar effects can be observed with other alpha-adrenoceptor blocking agents in high concentrations. Consequently, when studying the effects of yohimbine and other alpha 2-adrenoceptor antagonists on noradrenergic neurotransmission, the possibility of non-specific effects should be taken into consideration, particularly in the high concentrations range.

Modulation of noradrenaline release in human saphenous vein via presynaptic alpha 2-adrenoceptors

Strips of human saphenous veins were incubated with [3H]noradrenaline and subsequently superfused with physiological salt solution containing cocaine, corticosterone and propranolol. The electrically (6 Hz) evoked overflow of tritium (78% of which was accounted for by unmetabolized [3H]noradrenaline) was abolished by tetrodotoxin or omission of Ca2+ from the superfusion fluid. Unlabelled noradrenaline, alpha-methylnoradrenaline, B-HT 920 and clonidine inhibited the evoked overflow (maximum effect of clonidine lower than that of the other compounds) whereas methoxamine was ineffective. The alpha 2-adrenoceptor antagonists, BDF 6143 and rauwolscine, facilitated the evoked overflow but no effect was obtained with prazosin. Rauwolscine produced a shift to the right of the concentration-response curve of B-HT 920 for its inhibitory effect on evoked outflow and BDF 6143 caused a shift to the right of the corresponding curve of clonidine. It is concluded that the stimulation-evoked release of noradrenaline from the sympathetic nerve fibres of the human saphenous vein is modulated via presynaptic alpha 2-adrenoceptors.

A contribution, from a possible local anaesthetic action, to the effects of yohimbine on evoked noradrenaline overflow

Both yohimbine (0.1 to 10 microM) and phentolamine (10 microM) increased the tritium overflow evoked by electrical stimulation (2.5 Hz, 1 ms, 15 V, for 90 s every 20 min) of mouse isolated vas deferens previously incubated with (-)-[3H]noradrenaline. At their maximally effective concentrations, phentolamine (10 microM) produced an effect that was sustained over the 2 h of the experiment while the effect of yohimbine (6 microM) decreased by about 60% over the first 40 min and was then sustained at a lower level. At higher concentrations of yohimbine, the increase in evoked tritium overflow was less marked and at the highest concentration tested (30 microM) evoked overflow was reduced below the levels seen before exposure to the drug. It is concluded that at concentrations maximally effective in inhibiting the presynaptic alpha-adrenoceptor-mediated mechanism controlling transmitter release, the known local anaesthetic effect of yohimbine may contribute to the overall effect on evoked transmitter overflow.

Characterization of the receptor subtype involved in alpha-adrenoceptor-mediated modulation of serotonin release from rat brain cortex slices

Rat brain cortex slices preincubated with 3H-5-hydroxytryptamine and superfused with physiological salt solution were stimulated electrically at a frequency of 3 Hz. 1. The electrically evoked 3H-overflow was decreased by clonidine, noradrenaline and B-HT 920 in a concentration-dependent manner (negative logarithms of the IC30 values: 6.66, 6.55 and 4.40, respectively) 2. Phenylephrine 10-5 M (which increased basal 3H-efflux) and methoxamine 10-4 M decreased the impulse-evoked 3H-overflow by less than 25% whereas lower concentrations were ineffective. 3. Yohimbine produced a shift to the right of the concentration-response curves of noradrenaline (apparent pA2: 6.93) and clonidine (apparent pA2: 7.06) for their inhibitory effects on evoked 3H-overflow. Rauwolscine also shifted the concentration-response curve of noradrenaline to the right (apparent pA2: 7.29), whereas prazosin (10-6 and 3.2 x 10-6 M) was ineffective in this respect. These results suggest that the alpha-adrenoceptors on the serotoninergic nerve fibres belong to the alpha2-subtype.