Effects of Ca2+ channel antagonists and ryanodine on H1-receptor mediated electromechanical response to histamine in guinea-pig left atria

Summary Effects of organic Ca²⁺ channel antagonists, Ni²⁺ and ryanodine on the electrophysiological and positive inotropic responses to histamine were examined in isolated guinea-pig left atria. Histamine increased force of contraction, prolonged action potential duration (APD) and hyperpolarized the membrane in a concentration-dependent manner. Histamine at a concentration of 1 mol/l produced a dual-component positive inotropic response composed of an initial increasing phase (initial component) and a second an late developing, greater positive inotropic phase (second component), whereas causing monophasic changes in APD and resting potential. The electrophysiological and dual-component positive inotropic effects induced by histamine were antagonized by chlorpheniramine (1 mol/l) but not by cimetidine (10 mol/l), indicating that both effects are exclusively mediated by H₁-receptors. The positive inotropic response to 1 mol/l histamine was changed by the pretreatment with nifedipine (1 mol/l) and nisoldipine (1 mol/l). In the presence of these dihydropyridines, the second component was almost completely abolished, while the initial component was hardly affected. On the other hand, verapamil (3 mol/l) and diltiazem (10 mol/l) failed to modify the multiphasic inotropic response to histamine. None of the Ca²⁺ channel antagonists affected the histamine-induced APD prolongation. In the presence of Ni²⁺ at a concentration of 0.3 mmol/l, at which it produced no negative inotropic action, the second component of the positive inotropic effect of histamine was specifically suppressed whereas the histamine-induced APD prolongation was unaffected. Preferential attenuation of the second component was also observed in the presence of 30 nmol/l ryanodine. However, the electrophysiological alterations caused by histamine remained unchanged. These results suggest that in guinea-pig left atria the H₁-receptor-mediated prolongation of APD seems unlikely to be due to enhancement of the slow inward Ca²⁺ current. Conversely, the increased Ca²⁺ influx as a result of the APD prolongation may contribute to the second component of the positive inotropic effect of histamine. Dihydropyridine Ca²⁺ channel antagonists, Ni²⁺ and ryanodine are all capable of inhibiting the second component, but do so possibly via different mechanisms, implying the complicated mechanisms underlying the H₁-receptor-mediated positive inotropic effect.Send offprint requests to Y. Hattori     

Chronic β1-adrenoceptor blockade sensitises the H1 and H2 receptor systems in human atrium: role of cyclic nucleotides

We have reported that chronic treatment of patients with ₁-adrenoceptor blockers sensitises isolated atrial preparations to adrenaline, noradrenaline and 5-HT. We have now examined the effect of chronic treatment with -adrenoceptor blockers on responses to histamine of human right atrial appendages. We compared the effects of histamine on contractile force, cyclic AMP and cyclic GMP levels as well as cyclic AMP-dependent protein kinase (PKA) activity and explored the arrhythmogenic effects of histamine in preparations obtained from patients chronically treated or not treated with -adrenoceptor blockers.Histamine increased contractile force in paced preparations; the effects were blocked by the H₂ receptor antagonist famotidine (0.1–30 mol/1). The maximum inotropic response to histamine was doubled and the inotropic potency of histamine 0.4 log units greater in atria from -adrenoceptor blocker-treated compared to non -adrenoceptor blocker-treated patients. Histamine elicited frequency-dependent arrhythmias that were blocked by famotidine (30 mol/1) but not by mepyramine (1 mol/1). The incidence of arrhythmias was higher in atria from -adrenoceptor blocker-treated compared to untreated patients. Histamine increased both cyclic AMP and cyclic GMP levels, as well as PKA activity, significantly more in atria from -adrenoceptor blocker-treated compared to those from untreated patients. Mepyramine 1 mol/l prevented the histamine-evoked increase in cyclic GMP levels, reduced the inotropic hyperresponsiveness and abolished the hyperresponsiveness in cyclic AMP levels and PKA activity observed in patients chronically treated with blockers. Sodium nitroprusside 10 mol/l caused smaller increases of cyclic GMP levels than histamine and restored the contracile force depressed by mepyramine to its original level in atria from -adrenoceptor blocker-treated patients.The evidence is consistent with sensitisation of both the histamine H₁ and histamine H₂ receptor systems by chronic ₁-adrenoceptor blockade. H₁ receptor-mediated increases in cyclic GMP, enhanced through an as yet unknown mechanism by chronic ₁-adrenoceptor blockade, may inhibit phosphodiesterase 3 activity, thereby causing enhanced histamine-evoked increases in cyclic AMP levels and PKA activity, and accounting partially for the increased inotropic responses to histamine through H₂ receptors.     

Effects of (+)- and (±)-sotalol on repolarizing outward currents and pacemaker current in sheep cardiac Purkinje fibres

Summary This study was aimed to differentiate the action of (+)- and (±)-sotalol (10–1000 mol/l) on membrane currents which are active during the repolarization of cardiac action potentials Effects where studied in shortened sheep cardiac Purkinje fibres with the two-microelectrode voltage-clamp technique Action potentials were activated at a frequency of 0.25 Hz and membrane currents at 0.03 Hz or 0.05 Hz in most experiments.Out of the currents investigated the transient outward current (i_to) reacted most sensitively to (+)- and (±)-sotalol. I_to-amplitude was decreased on the average to 77% of reference at 10 mol/l and to 53% at 1000 mol/l (+)- or (±)-sotalol. The maximally available ito-current was decreased but the voltage-dependent control of inactivation was left nearly unchanged. The initial inwardly rectifying current (i_Ki), which propels the last repolarization phase of the action potential and controls resting potential to a large extent was reduced on the average to 93% of reference at 10 mol/l and to 62% at 1000 mol/l (+)- or (±)-sotalol. Time-dependent (delayed) outward current (i_K) was on the average not affected by (+)- or (±)-sotalol up to 100 mol/l and was decreased to 84% of reference current under the influence of 1000 mol/l. An initial outward current, which is activated at positive membrane potentials (i_inst) was not clearly affected by (+)- or (±)-sotalol at concentrations up to 1000 mol/l Pacemaker current (i_f) was not influenced by the drugs up to 100 mol/l. Only at 1000 mol/l was the amount of available i_f-current decreased to 79% of reference. (The potential-dependent control of activation was not affected) Time constants of time-dependent currents i_to, i_K and i_f did not change in concentrations up to 1000 mol/l of the drug.Action potential duration increased at (+)- or (±)-sotalol concentrations 10 mol/l and maximal prolongation was achieved at concentrations of 100–300 mol/l Resting potential remained nearly unchanged at these concentrations, but the membranes depolarized at 1000 mol/l. According to our data action potential prolongation in sheep Purkinje fibres under the influence of (+)- and (±)-sotalol correlates to the drug-induced block to i_to-current and inwardly rectifying i_K1-current.Supported by the Deutsche Forschungsgemeinschaft SFB 242, C 1 Send offprint requests to U. Borchard at the above address     

Presynaptic dopamine DA2-receptors in rabbit jejunal arteries

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

Inhibition of noradrenaline release from the sympathetic nerves of the human saphenous vein by presynaptic histamine H3 receptors

Summary The human saphenous vein was used to examine whether presynaptic histamine receptors can modulate noradrenaline release and, if so, to determine their pharmacological characteristics. Strips of this blood vessel were incubated with [³H]noradrenaline and subsequently superfused with physiological salt solution containing desipramine and corticosterone. Electrically (2 Hz) evoked ³H overflow was inhibited by histamine and the H₃ receptor agonist R-(–)--methylhistamine. Histamine-induced inhibition of electrically evoked tritium overflow was not affected by ₂-adrenoceptor blockade by rauwolscine. S-(+)--methylhistamine (up to 10 mol/l) as well as the histamine H₁ and H₂ receptor agonists 2-(2-thiazolyl)ethylamine (up to 3 mol/l) and dimaprit (up to 30 mol/l), respectively, were ineffective. The selective histamine H₃ receptor antagonist thioperamide abolished the inhibitory effect of histamine. The histamine H₂ and H₁ receptor antagonists ranitidine and pheniramine, respectively, did not affect the histamine-induced inhibition of evoked tritium overflow. The present results are compatible with the suggestion that the sympathetic nerves of the human saphenous vein are endowed with inhibitory presynaptic histamine receptors of the H₃ class. Send offprint requests to M. Gothert at the above address     

Blockade of γ-aminobutyric acid-receptors of the B-subtype inhibits the dopamine-induced enhancement of the release of cholecystokinin-like immunoreactivity from slices of rat dorsal caudatoputamen

Summary When slices of rat dorsal caudatoputamen (= neostriatum) are incubated in vitro, Choecystokinin-like immunoreactivity (CCK-LI) is released upon addition of veratridine (3.75 mol/l). This release is affected by dopamine and by -aminobutyric acid (GABA)-receptor agonists. Dopamine enhances the release by stimulating dopamine D₂-receptors and decreases it via D₁-receptors. GABA_A-receptor agonists enhance the veratridine-induced release of CCK-LI, while GABA_B-receptor agonists decrease it. In the present investigation, it was examined whether GABA-receptors are involved in the effect which dopamine exerts via D₂-receptors. The GABA_A-receptor antagonist bicuculline (10 mol/l)and the blocker of the GABA_A-receptor ionophore picrotoxin (1 mol/l) did not affect the dopamine (0.1 mol/1)-induced increase in the release of CCK-LI. However, the GABA_A-receptor agonist muscimol (1 mol/l) not only enhanced the release of CCK-LI, but also prevented a further enhancement by dopamine (0.1 mol/l). This effect of muscimol was blocked by bicuculline (10 mol/l). In the presence of -amino-n-valeric acid (0.1 mmol/l), which has been described to block GABA_B-receptors, dopamine no longer enhanced the veratridine-induced release of CCK-LI. -Amino-n-valeric acid also inhibited the pronounced enhancement of the release of CCK-LI caused by dopamine (0.1 mol/l) and 1 mol/l in the presence of the preferential D₁-receptor antagonist SCH 23390. The effect of -amino-n-valeric acid persisted in the presence of bicuculline (10 mol/l and 100 mol/l). (+)-Baclofen, a partial agonist at GABA_B-receptors, and the stereoisomer (–)-baclofen, a full agonist, also prevented the effect of dopamine on the veratridine-induced release of CCK-LI. The effects of both drugs may be due to desensitization of GABA_B-receptors, which has been described to develop quite rapidly. It is concluded that -amino-n-valeric acid blocks GABA_B-receptors and in this way prevents the enhancement of the veratridine-induced release of CCK-LI caused by dopamine via D₂-receptors. These data are interpreted as evidence that dopamine and GABA-neurons can directly or indirectly interact in the rat neostriatum. Send offprint requests to D. K. Meyer at the above address     

ATP and endogenous agonists inhibit evoked [3H]-noradrenaline release in rat iris via A1 and P2y-like purinoceptors

Summary Effects of ATP, adenosine and purinoceptor antagonists on field stimulation-evoked (3 Hz, 2 min) [³H]-noradrenaline overflow were investigated in the rat isolated iris.ATP and adenosine inhibited the evoked overflow of [³H]-noradrenaline. 1,3-Dipropyl-8-cyclopentylxanthine (DPCPX) shifted the concentration-response curve of ATP to the right in a concentration-dependent manner, but with a potency (–log K_B = 7.88) much lower than expected for an A1 adenosine receptor. In the continuous presence of DPCPX, the ATP-induced prejunctional inhibition was unaffected by suramin (100 mol/l) and DIDS (4,4-diisothiocyanatostilbene-2,2-disulfonic acid, 50 mol/l) but was antagonized by the P_2Y-receptor antagonist cibacron blue ( = reactive blue 2;30 and 100 mol/l, –log K_B = 4.7)and ,-methylene-ATP (10 mol/l). Whereas the evoked [³H]-noradrenaline overflow was unaffected by suramin and DIDS, cibacron blue and ,-methylene-ATP caused a small and transient increase. Cibacron blue at 30 mol/l failed to antagonize the inhibition of evoked [³H]-noradrenaline overflow that adenosine produced in the absence of DPCPX. Basal [³H]-noradrenaline overflow was enhanced by cibacron blue, not changed by ,-methylene-ATP and DIDS, and decreased by suramin.The results show that exogenous ATP inhibits sympathetic neurotransmission in the rat iris via A₁ and P_2Y-like purinoceptors. The latter have a low apparent affinity for cibacron blue and probably are blocked by ,-methylene-ATP. Under the present conditions, endogenous purines exert a tonic inhibition not only via A₁- but also via these P_2Y-receptors. Correspondence to: H. Fuder at the above address     

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

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

Inhibitory adenosine A1-receptors on rat locus coeruleus neurones

Summary Intracellular recordings were performed in ₁-pontine slice preparation of the rat brain containing the locus coeruleus (LC). Adenosine (100, 300 mol/l) and its structural analogues, namely (–)-N⁶-(R-phenyliso-propyl)-adenosine (R-PIA; 3 – 30 mol/l) and S-PIA (10, 30 mol/l), as well as 5-N-ethylcarboxamido-adenosine (NECA; 3–30 mol/l) inhibited the firing rate of spontaneous action potentials and produced hyperpolarization; their rank order of potency was RPIA - NECA > S-PIA > adenosine. When applied by superfusion, all agonists strongly desensitized the LC cells; the hyperpolarization never surmounted 6 mV. Upon pressure ejection of adenosine 10 mmol/l from ₁- micropipette positioned close to an LC neurone, the membrane potential was raised by 14 mV and the apparent input resistance decreased by 20%. When the membrane potential was hyperpolarized by current injection to ₁- similar extent as adenosine did, the fall in input resistance was only 7%. The adenosine uptake inhibitor S-(p-nitrobenzyl)-6-thioguanosine (NBTG) 30 mol/l decreased the frequency of action potentials alone; on simultaneous bath-application with adenosine 300 mol/l it potentiated the hyperpolarization caused by the purine derivative. 8-Cyclopentyl-1,3-dipropylxanthine (CPDPX) 0.1 mol/l had no effect on its own, but it antagonized both R-PIA 30 mol/l and NBTG 30 mol/l. A higher concentration of CPDPX (1 mol/l) facilitated the spontaneous firing. In conclusion, both exogenous and endogenous adenosine activates somatic and/or dendritic A₁-receptors of LC neurones leading to an enhancement of potassium conductance and thereby to ₁- decreased firing rate and ₁- hyperpolarization. Send offprint requests to P. Illes at the above address     

Sodium dependent 3H-noradrenaline release from rat neocortical slices in the absence of extracellular calcium presynaptic modulation by μ-opioid receptor and adenylate cyclase activation

Summary In Ca²⁺-free EGTA (1 mmol/l)-containing medium veratrine (3 mol/l) and ouabain (100 mol/l) strongly enhanced the efflux of ³H-noradrenaline from superfused rat brain neocortical slices prelabelled with the radioactive amine. In both cases ³H-noradrenaline release was prevented by tetrodotoxin (1 mol/l). These effects of veratrine and ouabain were virtually additive and independent of whether the noradrenaline uptake carrier was blocked with 1 mol/l desipramine or not. The adenylate cyclase activator forskolin (10 nmol/l–10 mol/l) strongly enhanced veratrine- and ouabain-induced ³H-noradrenaline release, without affecting spontaneous tritium efflux. The release induced by both stimuli was profoundly inhibited by the selective -opioid receptor agonist [d-Ala, MePhe⁴, Gly-ol⁵]enkaphalin (DAGO, 3 nmol/l–1 mol/l) in a concentration-dependent manner. The inhibitory effects of 1 mol/l DAGO were abolished by 1 mol/l naloxone. On the other hand, preincubation of the slices for 1 h with the -opioid receptor-selective irreversible ligand fentanyl isothiocyanate (1 pmol/l) did not change the inhibitory effects of DAGO.These data show that veratrine- and ouabain-induced ³H-noradrenaline release from central noradrenergic nerve terminals is facilitated by increasing intracellular cyclic AMP levels and reduced by activation of presynaptic -opioid receptors, indicating the involvement of exocytotic neurotransmitter release. The results provide further evidence for the hypothesis that under these conditions neurotransmitter release from central noradrenergic neurons is triggerred by a Na⁺-induced efflux of Ca²⁺ ions from intracellular stores.Abbreviations DAGO [d-Ala², McPhe⁴, Gly-ol⁵]enkephalin Send offprint requests to A. N. M. Schoffelmeer at the above address     

Blockade of α2-adrenoceptors increases opioid μ-receptor-mediated inhibition of the firing rate of rat locus coeruleus neurones

Summary In pontine slices of the rat brain, the frequency of spontaneous action potentials of locus coeruleus (LC) neurones was recorded extracellularly. Noradrenaline 0.1–100 mol/l, UK 14,304 0.01–100 nmol/l, [Met⁵]-enkephalin 1–10,000 nmol/l and [D-Ala², D-Leu⁵]enkephalin 0.1–1,000 nmol/l, all depressed the firing rate. Rauwolscine 1 mol/l antagonized the effects of both noradrenaline and UK 14,304, but potentiated the effects of [Met']enkephalin and [D-Ala², D-Leu⁵]enkephalin. Idazoxan 1 mol/l acted in a similar manner. Prazosin 1 mol/l did not change the effects of either noradrenaline or [Met⁵]enkephalin. Naloxone 0.1 mol/l antagonized both [Met']enkephalin and [D-Ala², D-Leu⁵]enkephalin, but failed to alter the effects of either noradrenaline or UK 14,304. Rauwolscine, idazoxan and prazosin, all 1 mol/l, as well as naloxone 0.1 mol/l, did not influence the firing rate when given alone. Desipramine 1 mol/l inhibited the discharge of action potentials in a rauwolscine-antagonizable manner. Noradrenaline 10 mol/l produced the same depression of firing, both in the presence of noradrenaline 1 mol/l and [Met⁵]enkephalin 0.03 mol/l. Likewise, the effect of [Met⁵]enkephalin 0.3 mol/l was the same, irrespective of whether it was added to a medium containing [Met⁵]enkephalin 0.03 mol/l or noradrenaline 1 mol/l. The spontaneous activity of LC neurones is inhibited by somatic ₂-adrenoceptors and opioid -receptors. We suggest that the two receptors interact with each other at a site located between themselves and not in the subsequent common signal transduction system.Send offprint requests to: P. Illes at the above address     

Regulation of gastric acid secretion by histamine H3 receptors in the dog: an investigation into the site of action

The involvement of histamine H₃ receptors in the regulation of gastric acid secretion was investigated in the conscious dog with gastric fistula, by the use of the selective agonist (R)-methylhistamine and the selective antagonist thioperamide. (R)-methylhistamine (0.3–1.2 mol/kg/h) induced a dose-related inhibition of the acid secretion induced by pentagastrin and by bombesin, maximum inhibition not exceeding 60–65%. The inhibitory effect of the H₃ agonist (0.6 mol/kg/h) was inhibitited by thioperamide (0.1 mol/kg/h), suggesting that the effect was entirely mediated by H₃ receptors. Thioperamide was also able to enhance the acid response to submaximal doses of pentagastrin and bombesin. The acid secretion induced by histamine was not modified by (R)a-methylhistamine (0.3–1.2 mol/kg/h) but it was significantly enhanced by thioperamide (0.1 mol/kg/h). Neither (R)-methylhistamine nor thioperamide significantly modified the increase in plasma gastrin levels induced by bombesin. In conclusion these data demonstrate that histamine H₃ receptors may represent an effective mechanism for the negative control of stimulated gastric acid secretion in the dog; however, since the inhibition was mainly evident against stimuli which involve the release of histamine, a location of H₃ receptors in paracrine cells of the gastric mucosa rather than in gastrin producing cells or parietal cells seems more likely. Correspondence to: G. Bertaccini at the above address     

In vivo evidence for a concordant response of terminal and dendritic dopamine release during intranigral infusion of drugs

Summary In the present study we have administered prototypical drugs of 5 different receptors (D-2, GABA-A, GABA-B, NMDA, kainate) to the substantia nigra by infusion via a microdialysis probe, whereas the release of dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) were recorded both in the substantia nigra and (by a second microdialysis probe) in the ipsilateral striatum. Infusion of the specific D-2 receptor agonist 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin ((-)-N0437) into the nigra induced a decrease in the release of dopamine in the nigra (after 1 mol/l) as well as in the ipsilateral striatum (after 10 mol/l). During these infusions extracellular DOPAC decresed in the nigra and increased in the striatum. Infusion of the D-2 specific receptor antagonist (-)-sulpiride into the nigra induced an increase in the release of dopamine in the nigra (after 1 mol/l) as well as in the ipsilateral striatum (10 mol/l). During these infusions a slight increase of extracellular DOPAC was noticed in the nigra. Infusion of the GABA-A receptor antagonist bicuculline into the nigra (50 mol/l) induced an increase in the release of dopamine and DOPAC both in the nigra and ipsilateral striatum. Infusion of the GABA-B receptor agonist d,I-baclofen into the nigra (10 mol/l) induced a decrease of in the release of dopamine in the nigra as well as in the ipsilateral striatum, whereas extracellular DOPAC decreased in the nigra and increased in the striatum. Infusion of the glutamate receptor agonist N-methyl-D-aspartate (NMDA) into the nigra induced an increase in the release of dopamine and DOPAC in the nigra (after 300 mol/l) as well as in the ipsilateral striatum (after 1 mmol/l). Infusion of the glutamate receptor agonist kainate into the nigra (10 mol/l) induced an increase in the release of dopamine and DOPAC in the nigra as well as in the ipsilateral striatum. The results showed that nigral as well as striatal dopamine release respond in a concordant way to intranigral infusion of drugs.Correspondence to B. H. C. Westerink at the above address     

The sympathetic axons innervating the sinus node of the rabbit possess presynaptic opioid к- but not μ- or δ-receptors

Summary The postganglionic sympathic nerves of rabbit isolated hearts were stimulated with pulses delivered at 5 Hz and train durations of 1–5 s. Ethylketocyclazone 0.01–1 mol/l and fentanyl 1 and 10 mol/l but not morphine 1 and 10 mol/l, Met-enkephalin 1 and 4 mol/l or d-Ala², d-Leu⁵-enkephalin 0.5 and 5 mol/l diminished the stimulation-evoked increase in heart rate. The effect of ethylketocyclazocine 0.1 mol/l was antagonized by naloxone 1 and 10 mol/l. In contrast, the effect of fentanyl was not changed by naloxone 10 mol/l. Ethylketocyclazocine 0.03 and 1 mol/l did not reduce the tachycardia elicited by exogenous noradrenaline. The results suggest that, under in vitro conditions, only presynaptic opioid - but not - or -receptors inhibit the release of noradrenaline from the sympathetic neurones innervating the sinus node.     

Opposite modulation of ouabain cardiotoxicity by hexamethyleneamiloride and phenylephrine

Summary To see whether the Na/H antiporter plays a role in digitalis cardiotoxicity, we investigated the influence of modulators of Na/H exchange on the toxic effects of ouabain in isolated, paced (0.4 Hz) rat left atria. Ouabain (1 mmol/l) caused a transient positive inotropic effect followed by toxic events, including a complete loss of developed force and a gradual increase in resting force. In the presence of hexamethyleneamiloride (3 and 10 mo1/l), an inhibitor of Na/H exchange, ouabain (1 mmol/l) caused a sustained positive inotropic effect without toxicity. By contrast, phenylephrine (100 mol/ 1) an -adrenoceptor agonist reported to stimulate the antiporter, hastened the development of ouabain's toxicity. Neither ouabain, at a subtoxic concentration (650 ol/l), nor phenylephrine (100 mol/l) affected diastolic force, but in their combined presence, a substantial contracture developed and twitch contractions disappeared. Phenylephrine (30 or 100 mol/l) or adrenaline (30 mol/l), in the presence of a -adrenoceptor antagonist, increased the intracellular pH by up to 0.15 pH unit, as measured using ion-selective microelectrodes in quiescent preparations. This effect on pH₁ was prevented by hexamethyleneamiloride (10 mol/l). Consistent with phenylephrine's ability to stimulate Na⁺ influx via the Na/H antiporter, phenylephrine (100 mol/l) increased intracellular Na⁺ activity by about 3 mmol/l in ouabain (650 mol/l)-treated atria. These findings indicate that modulators of Na/H exchange affect the cardiotoxicity of digitalis glycosides and imply that the stimulation of myocardial -adrenoceptors may aggravate digitalis toxicity.This work was conducted in part under the auspices of the Association for US/French Biomedical Cooperation Send offprint requests to S. M. Vogel at the above address     

Characterization of an endothelial 5-hydroxytryptamine (5-HT) receptor mediating relaxation of the porcine coronary artery

Summary The pharmacological properties of the endothelial 5-hydroxytryptamine (5-HT) receptors involved in relaxation of vascular smooth muscle were determined in rings of pig coronary artery contracted with 10 nmol/1 of the thromboxane A₂ receptor agonist 9,11-dideoxy-11,9-epoxy-methano-prostaglandin F₂ (U 46619).(1) In the presence of 10 mol/l ketanserin, relaxation was obtained with: 5-HT (apparent pD₂ value 7.00), 5-carboxamidotryptamine (5-CONH₂-T; 6.42), 5-aminotryptamine (5-NH₂-T; 5.96), 5-methoxytryptamine (5-OCH₃-T; 5.92), tryptamine, 7-trifluoromethyl-4(4-methyl-1-piperazinyl)-pyrrolo(1,2-a)quinoxaline maleate (CGS 12066 A) and 5-methoxy-3(1,2,3,6-tetrahydro-4-pyridinyl)-1H-indole succinate (RU 24969). The maximum relaxation obtainable with the agonists was about 40–60% of the U 46619-induced contraction and the concentration-response curves for 5-HT, 5-NH₂-T and 5-OCH₃-T were bell-shaped. The endothelium-dependence of this effect (i. e. the failure to relax the artery in endothelium-denuded preparations) was demonstrated for 5-HT, 5-CONH₂-T, RU 24969, CGS 12066A and tryptamine.(2) 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), 4-hydroxytryptamine, quipazine and yohimbine were ineffective in decreasing the tension of arteries with or without endothelium. Ipsapirone elicited full relaxation of U 46619-induced contraction, but this effect was not endothelium-dependent.(3) Metitepine (0.03-1 mol/l), 6-chloro-2-(1-piperazinyl)pyrazine (MK 212; 10 mol/l), methysergide (1 gmol/l) and cyanopindolol (0.1 mol/l) antagonized the relaxing effect of 5-HT in a non-surmountable manner, whereas metergoline (0.1 mol/l), quipazine (10 mol/l), yohimbine (1 mol/l), propranolol (1 mol/l) and (3-tropanyl)-1H-indole-3-carboxylic acid ester (ICS 205-930; 0.1 mol/l) did not. However, spiroxatrine (0.1 mol/l) and mesulergine (10 mol/l) enhanced the 5-HT-induced relaxation. The endothelium-dependent relaxation induced by 5-CONH₂-T was also inhibited by metitepine 1 gmol/l.(4) The 5-HT-induced relaxation was probably mediated by release of an endothelium-derived relaxing factor (EDRF). Gossypol, an inhibitor of EDRF, virtually abolished the 5-HT-induced relaxation while indometacin, an inhibitor of cyclooxygenase and accordingly of PGI₂ formation, did not.In conclusion, the failure of ketanserin and ICS 205–930 to counteract the relaxant effect of 5-HT receptor agonists excludes the involvement of 5-HT₂ and 5-HT₃ receptors, respectively, in the endothelium-dependent relaxation of the porcine coronary artery. The rather high potency of 5-CONH₂-T and the ability of certain 5-HT receptor antagonists, such as metitepine, methysergide and cyanopindolol, to counteract the effect of 5-HT are compatible with a 5-HT₁ character of the endothelial receptor. However, on the basis of the present data, no final classification, in particular with respect to the known 5-HT₁ receptor subtypes, is possible. Classification is also hampered by the bell-shaped character of the concentration-response curves for 5-HT receptor agonists and by their property to produce only partial relaxation. Send offprint requests to M. Gothert at the above address     

Evidence for a vasoconstriction-mediating receptor for UTP,distinct from the P2 purinoceptor,in rabbit ear artery

Summary 1. The mechanism of uridine 5-triphosphate-(UTP-)induced vasoconstriction was studied in the rabbit ear artery. The arteries were incubated and perfused at a constant rate of flow. Vasoconstriction was measured as an increase in perfusion pressure. 2. Noradrenaline, adenosine 5'-triphosphate (ATP) and UTP caused concentration-dependent vasoconstriction. ATP and UTP were approximately equipotent. 3. The vasoconstrictor effect of UTP 300 mol/l was enhanced by a mixture of atropine, diphenhydramine and methysergide (1 mol/l each) and not affected by indometacin 10 mol/l. 4. Prazosin (0.01 –1 mol/l) and phentolamine (1–10 mol/l) reduced the vasoconstrictor effect of UTP 300 mol/l by up to 34%. Prazosin 1 mol/l failed to diminish the vasoconstrictor effect of UTP 300 mol/l after the sympathetic nerves had been destroyed with 6-hydroxydopamine. 5. , -Methylene-ATP (10–50 ol/l) elicited transient vasoconstriction. Subsequently, vasoconstrictor responses to ATP 100 or 300 pmol/1 were reduced by 88%, whereas responses to UTP 100 gmol/1 were enhanced, responses to UTP 300 mol/l decreased by only 32% and responses to UTP 1000 gmol/1 reduced by 74%. After in vitro-denervation with 6-hydroxydopamine or in the presence of phentolamine 1 mol/l throughout, a, -methylene-ATP (10–50 mol/l) reduced the vasoconstrictor effect of UTP 300 mol/l by 44% and 43%, respectively. 6. We suggest that, in the rabbit ear artery, the non-adrenergic and , -methylene-ATP-resistant vasoconstrictor response to UTP is mediated by a separate receptor mechanism, distinct from the P₂ purinoceptor. Send offprint requests to K. Starke     

A neurally mediated inotropic effect of veratridine and cevadine on isolated guinea-pig papillary muscle

Summary The positive inotropic effect of veratridine and cevadine was investigated in the isolated, isometrically contracting guinea-pig papillary muscle. The increase of the force of the rested-state contraction, elicited after incubation of the resting muscle with veratridine or cevadine, served as a measure of the neurally mediated, sympathomimetic effect of these alkaloids. Concentrations exceeding 5 mol/l veratridine or 15 mol/l cevadine produced concentration-dependent increases of the force of the rested-state contraction. These concentrations are larger than those causing the maximum positive inotropic effect on muscles contracting continually at a rate of 1 Hz. The positive inotropic effect of 30 mol/l veratridine as manifested by the rested-state contraction was absent in the presence of 100 nmol/l tetrodotoxin and in muscles from reserpine-pretreated animals. It was significantly inhibited by 50 nmol/l (–)-propranolol, but not by the same concentration of (+)-propranolol. The effect of 30 or 60 mol/l cevadine was likewise absent in catecholamine-depleted preparations. It is concluded that the indirect inotropic effect of veratridine or cevadine, which is attributed to their noradrenaline-releasing effect on intracardiac nerves, requires higher concentrations than the direct positive inotropic effect, which is a consequence of the increased transsarcolemmal influx of Na ions into the myocardial cell.These results were communicated to the Deutsche Pharmakologische Gesellschaft (Honerjäger 1977)     

Cyclic AMP-dependent and cyclic AMP-independent actions of a novel cardiotonic agent,OPC-8212

Summary Possible cAMP-dependent and cAMP-independent mechanisms of action for the cardiac effects of OPC-8212, a novel piperazinyl-quinolinone derivative, were evaluated. OPC-8212 was tested for in vitro potency as an inhibitor of soluble bovine cardiac phosphodiesterases using a rapid isolation and assay method involving monoclonal antibodies that distinguish among isozymes. The drug was selective for a low-K _m, cGMP-inhibited phosphodiesterase (CGI-PDE) with an IC₅₀ (half-maximal inhibition concentration) of 7.4 mol/l when measured at a substrate level of 0.35 mol/l cAMP. Under the conditions used, sulfolane, the solvent for OPC-8212, did not affect CGI-PDE activity. In electrophysiological measurements, OPC-8212 prolonged the action potential duration in canine Purkinje strand preparations up to 148% (APD₉₀) at 10 mol/l. Concomitantly, OPC-8212 produced a 100% increase in developed force. Both prolongation of the action potential duration and the positive inotropic effect were readily reversed after exposure to tetrodotoxin, 3 mol/l. Using Na-selective microelectrodes, intracellular Na⁺ ion activity increased 225% upon exposure to 10 mol/l OPC-8212. OPC-8212 represents a novel type of positive inotropic agent, possessing both cAMP-dependent (selective PDE isozyme inhibition) and cAMP-independent (activation of intracellular Na⁺) mechanism of action. Send offprint requests to A. Schwartz at the above address     

The relaxant action of osthole isolated from Angelica pubescens in guinea-pig trachea

The effect of osthole, isolated from Angelica pubescens, on the contraction of guinea-pig trachea was studied. Osthole (25–100 mol/l), theophylline (10–1000 mol/l) and higher concentrations of nifedipine (0.1–100 mol/l) suppressed the contraction response curves of tracheal smooth muscle caused by carbachol, prostaglandin F₂ (PGF₂), U46619 (thromboxane A₂ analogue) and leukotriene C₄ (LTC₄) in a concentration-dependent manner. The contraction caused by high K⁺ (120 mmol/1) and cumulative concentrations of CaCl₂ (0.03–3 mmol/1) was also inhibited concentration-dependently by osthole (25–100 mol/l), theophyl line(10–1000 mol/l) and lower concentrations of nifedipine (0.01–0.1 mol/l). The relaxant actions of osthole were not affected by propranolol (1 mol/l), glibenclamide (10 mol/l) or removal of tracheal epithelium. Osthole (100 mol/l) was still effective in causing tracheal relaxation in the presence of nifedipine (1 mol/l). In Ca²⁺-free- and EGTA (0.2 mmol/1)-containing medium, the relaxing effect of osthole was more potent than in normal Krebs solution. Osthole (25 and 50 mol/l) caused 2.9 and 6.5, or 3.0 and 5.6 fold, respectively, increase in potency of forskolin or sodium nitroprusside in causing tracheal relaxation but did not affect that by cromakalim. Osthole (50 mol/l) enhanced the increase in tissue cAMP and cGMP levels induced by forskolin and sodium nitroprusside, respectively, and in higher concentrations (100 and 250 mol/l), itself increased markedly tissue cAMP and cGMP contents. Osthole (10–250 mol/l) inhibited the activity of cAMP and cGMP phosphodiesterases in a concentration-dependent manner. It is concluded that osthole exerts a nonspecific relaxant effect on the trachealis by inhibiting the cAMP and cGMP phosphodiesterases. Correspondence to: C. M. Teng at the above address