Structural requirements of 5-hydroxytryptamine-moduline analogues to interact with the 5-hydroxytryptamine1B receptor.

5-Hydroxytryptamine-moduline is an endogenous cerebral tetrapeptide that regulates the activity of 5-hydroxytryptamine1B receptors. Direct binding of 5-[3H]hydroxytryptamine-moduline on rat brain homogenate evidenced the existence of two interacting sites for the peptide, very likely corresponding to different conformations of the 5-hydroxytryptamine1B receptor: The peptide first binds to a low-affinity state of the receptor (pIC50 = 7.68+/-0.14) and then induces (or stabilizes) a high-affinity complex (pIC50 = 11.62+/-0.18). This work focuses on the ability of 5-hydroxytryptamine-moduline analogues to recognize the high- and low-affinity sites for 5-hydroxytryptamine-moduline. The results obtained show that the two conformers of the 5-hydroxytryptamine1B receptor have similar but not identical binding pockets for 5-hydroxytryptamine-moduline. These two sites proved to be stereoselective and selective for tetrapeptides and favored the binding of peptides with hydrophobic amino acids in positions 1 and 4, serine in position 2, and a short amino acid in position 3. However, the serine in position 2 seems to be more important for the interaction of the peptide with the low-affinity site than the high-affinity one, which only needs a short hydrophobic amino acid in position 2.

Endoproteolytic activity in mammalian brain membranes cleaves 5-hydroxytryptamine-moduline into dipeptides.

This work was intended to determine which enzymatic activities from crude synaptosomal mammalian brain membranes could qualify for the status of 5-hydroxytryptamine-moduline (5-HT-moduline, LSAL, Leu-Ser-Ala-Leu) inactivating enzymes. An enzymatic assay for 5-HT-moduline metabolism was developed using [3H]5-HT-moduline measurement and high performance liquid chromatography (HPLC) technique to identify and quantify 5-HT-moduline metabolites. 5-HT-moduline metabolism displayed all characteristics of metalloprotease activity: sensitivity to divalent ion chelators, reactivation by Zn2+ ions and a pH optimum in the 7-8 range. Bestatin, an aminopeptidase inhibitor, allowed the identification of two enzymatic activities responsible for this metabolism: a bestatin-sensitive aminopeptidase and an endoprotease cleaving 5-HT-moduline into LS (Leu-Ser) and AL (Ala-Leu) dipeptides. This latter enzyme was shown to have a Km of 37.1 +/- 3.6 microM and a Vmax of 5.5 micromol min(-1) l(-1) per mg of protein. Moreover, this enzyme was insensitive to peptidyl dipeptidase A (angiotensin converting enzyme, EC 3.4.15.1), endothelin converting enzyme and neutral endopeptidase (neprylisin, EC 3.4.24.11) inhibitors and displayed some specificity among 5-HT-moduline-analogues and in particular recognized only tetrapeptides. These results, together with the isolation of the LS and AL metabolites [Rousselle, J.C., Massot, O., Delepierre, M., Zifa, E., Rousseau, B., Fillion, G., 1996. Isolation and characterization of an endogenous peptide from rat brain interacting specifically with the serotonergic 1B receptor subtypes. J. Biol. Chem. 271, 726-735] during the purification process of 5-HT-moduline are strong arguments for the physiological implication of this endoprotease in 5-HT-moduline metabolism.

Growth inhibition of human in vitro and mouse in vitro and in vivo mammary tumor models by retinoids in comparison with tamoxifen and the RU-486 anti-progestagen.

Retinoids constitute a very promising class of agents for the chemoprevention or treatment of breast cancer. These retinoids exert their biological activity through two distinct classes of retinoic acid (RA) receptors (R), the RAR isotypes (alpha, beta, and gamma) and the three RXR isotypes (alpha, beta, and gamma) and their numerous isoforms which bind as RXR/RAR heterodimers to the polymorphic cis-acting response elements of RA target genes. With respect to these numerous receptor sub-types, the retinoid-induced effects at the biological level include marked modifications with respect to both cell proliferation and cell death (apoptosis), and also in the induction of differentiation processes. The present study aims to characterize the effect which four retinoids (TTNPB, 9-cis-RA, LGD 1069, 4-HPR) with distinct RAR/RXR binding properties induced on various in vitro and in vivo mouse and human breast cancer models. The experiments with the retinoids were carried out in comparison with the anti-estrogen tamoxifen and the anti-progestagen RU-486 compounds. The results show that the 6 compounds under study were markedly more efficient in terms of growth inhibition in the human T-47D cell line when maintained under anchorage-independent culture conditions than when maintained under anchorage-dependent ones. While RU-486 exhibited a weak statistically significant (p < 0.05) influence on the growth of the T-47D stem cells, tamoxifen had a marked inhibitory influence on the growth of these cells. Of the four retinoids, 4-HPR was the least effective since the lowest doses tested (1 and 0.1 nM) exhibited no statistically (p > 0.05) significant influence on the growth of the stem cells. The most efficient retinoid was TTNPB. It was only at the highest dose (10 microM) that tamoxifen and RU-486 showed a weak inhibitory influence on the growth of the T-47D non-stem cells while all 4 retinoids exerted a significant inhibitory influence on the growth of these non-stem cells, with 4-HPR being the most efficient (P < 0.001) at the highest dose, but ineffective (P > 0.05) at the lowest. Tamoxifen and TTNPB were tested in vivo on hormone-sensitive (HS) and hormone-insensitive (HI) strains of the MXT murine mammary carcinoma. While TTNPB appeared to be equally efficient in terms of growth inhibition in both MXT-HS and MXT-HI models, tamoxifen had only a marginal inhibitory influence on the growth of the MXT-HI strain but did inhibit growth in the case of the MXT-HS one. TTNPB was markedly more efficient than tamoxifen in terms of both inhibiting the cell proliferation level (measured by means of computer-assisted microscopy applied to Feulgen-stained nuclei, a method which enables the percentage of cells in the S phase of the cell cycle to be determined) and triggering cell death (measured by means of the determination of the transglutaminase activity) in both the MXT-HI and MXT-HS models. The very significant TTNPB-induced inhibition of the macroscopic MXT-HS growth rate relates to the triggering of cell death (apoptosis) rather than to an inhibition of cell proliferation. All these results clearly indicate that retinoids are very efficient agents against breast cancer, at least as efficient as tamoxifen.

Synthesis and biodistribution of [5-131I]iodotropapride: a potential D2 dopamine receptor imaging agent.

[5-131I]Iodotropapride is a benzamidic compound which displays high affinity and selectivity for dopaminergic receptors. It was prepared from the corresponding brominated compound by a nucleophilic substitution with [131I]iodine (t1/2 = 8.02 days, E gamma = 364 keV) based on the use of Cu(I) as catalyst and high specific activity of [131I]NaI. After i.v. injection in rats the tracer crosses the blood-brain barrier (0.42 +/- 0.06% of injected dose in the total brain) and demonstrates a high affinity binding to the striatum. The striatum-to-cerebellum ratio increases with time and reaches values of 9 and 22 at 30 and 120 min after injection, respectively. This specific uptake in the striatum is saturable and can be blocked by pretreatment with different D2 antagonists. When labeled with 123I (t1/2 = 13 h, E1 = 159 keV), the corresponding [123I]iodotropapride may be useful for the investigation of the D2 dopamine receptors in humans with single photon emission computer tomography (SPECT).

Effects of tilting in the anaesthetized and mechanically ventilated rat: a model of orthostatic hypotension.

The main of the present study was to determine the experimental conditions for an animal model of orthostatic hypotension based on tilting in rats. Blood pressure, heart rate and plasma catecholamine levels were measured before and after tilting in conscious and anaesthetized rats either breathing spontaneously or mechanically ventilated. In conscious rats, tilting did not alter arterial blood pressure, but increased heart rate and plasma catecholamine levels. In anaesthetized rats, tilting induced a drop in arterial blood pressure without any modification in plasma catecholamine concentrations. Mechanical ventilation significantly increased the postural hypotension and the percentage of rats showing a decrease in heart rate. Tyramine infusion increased plasma noradrenaline levels, and reduced or counteracted the postural hypotension and bradycardia. The results show that anaesthetized mechanically ventilated rats might constitute an appropriate model of orthostatic hypotension.

Action of heptaminol hydrochloride on contractile properties in frog isolated twitch muscle fibre.

1. Heptaminol stopped or delayed the progressive decline in tension which characterizes the phenomenon of fatigue in frog isolated twitch muscle fibre. 2. Heptaminol had no action on the sodium, potassium and calcium voltage-dependent ionic conductances. 3. The hypothesis of an action via an internal alkalinization was tested by comparison with the action of NH4Cl. Both substances increased the tension. 4. The action of heptaminol was suppressed in sodium-free (TRIS) solution or in the presence of amiloride while the action of NH4Cl was always observed. 5. These results could be explained by a stimulation of the Na/H antiport by heptaminol.

[Heptaminol chlorhydrate: new data]. / Le chlorhydrate d'heptaminol: nouvelles données.

Heptaminol hydrochloride is widely used for the treatment for orthostatic hypotension. It continues to elicit controversy as to its mode of action since Loubatières (1951) described its powerful inotropic action on an ischaemic preparation. Twenty five years later it has been suggested that this drug exerts its action by interfering with the release and uptake of catecholamines (Grobecker and Grobecker, 1976). The aim of this review was to report new experimental approaches and new data relative to the mode of action of heptaminol hydrochloride. In the rat, heptaminol hydrochloride prevented orthostatic hypotension, and increased the noradrenaline plasma concentration. In bovine chromaffin cells maintained in primary cultures, it was found to be a competitive inhibitor of noradrenaline uptake. This inhibition may partially account for its antihypotensive effect. The cardiotonic effect was studied using 31 P nuclear magnetic resonance spectroscopy and left ventricular pressure measurement in rat isolated hearts. The results suggest that the inotropic effect during moderate ischemia could be related to a restoration of internal pH possibly mediated by a stimulation of the Na+/H+ exchange. The satellite cells of adult skeletal muscles are myogenic cells involved in muscle regeneration. In culture, they differentiate into myotubes and thus mimic some aspects of the in vivo myogenic process. Heptaminol hydrochloride, which did not significantly alter the cloning efficiency or proliferation, increased the capacity of satellite cells to differentiate into myotubes.

Effect of tuamine, heptaminol and two analogues on uptake and release of catecholamines in cultured chromaffin cells.

The effects of tuamine (1-methylhexylamine), a sympathomimetic compound with hypertensive properties, heptaminol (6-amino-2-methyl-2-heptanol), an aliphatic amine with pressor properties, and two structural analogues of tuamine on high-affinity Na(+)-dependent noradrenaline uptake and on nicotine-evoked release were examined in bovine chromaffin cells maintained in primary culture for 3 to 6 days. Tuamine was found to be a potent competitive inhibitor of noradrenaline uptake with an effect similar to that of cocaine. Its inhibition constant (Ki) was 1.1 +/- 0.1 microM while Ki values of heptaminol, of 1-methylamino-5-pentanol oxalate and of 5-amino-2-methylhexanol oxalate, which were also found to be competitive inhibitors of noradrenaline uptake, were 60 +/- 2 microM, 260 +/- 28 microM and 48 +/- 76 microM, respectively. Tuamine, hepataminol and 5-amino-2-methyl-2-hexanol were also shown to be inhibitors of nicotine-induced release of catecholamines, with IC50 values of 26 +/- 2 microM, 650 +/- 11 microM and 500 +/- 10 microM, respectively. Tuamine and hepataminol did not inhibit noradrenaline release evoked by 59 mM K+, suggesting that it acts at a step prior to calcium entry. The pharmacological properties of heptaminol as an anti-hypotension agent may partially account for its inhibitory effect on catecholamine uptake and release.

Decrease in internal H+ and positive inotropic effect of heptaminol hydrochloride: a 31P n.m.r. spectroscopy study in rat isolated heart.

1. The cardiotonic effect of heptaminol hydrochloride (Hept-a-myl, Delalande) was studied using 31P-nuclear magnetic resonance (n.m.r.) spectroscopy and left ventricular pressure (LVP) measurements in rat isolated hearts. The possibility of this effect being mediated by an intracellular realkalinisation was tested. 2. Isolated hearts were perfused at 10 ml min-1 by the Langendorff method with Krebs-Henseleit solution at 37 degrees C and stimulated at 5 Hz. Mechanical activity was measured as variations of left ventricular pressure (LVP). 31P-n.m.r. spectra were recorded every 2 min. Changes in cardiac adenosine triphosphate (ATP), phosphocreatine (PCr) and inorganic phosphate (Pi) were followed and intracellular pH (pHi) was estimated from the chemical shift of Pi. 3. The effects of heptaminol were tested in different conditions: normoxia, moderate ischaemia, severe ischaemia, and moderate ischaemia in the presence of amiloride or guanidinium chloride as inhibitors of the Na-H exchange. 4. In normoxia, heptaminol induced a cyclic increase of systolic LVP, associated with an increase in Pi. No significant effect on pHi was observed. In changing from normoxia to moderate ischaemia, PCr and systolic LVP decreased; a mild intracellular acidification (pHi 6.96) was obtained. Heptaminol induced a restoration of pHi and increased LVP. In severe ischaemia, the realkalinization effect and the restoration of LVP induced by heptaminol were no longer observed. During moderate ischaemia, Na-H exchange inhibitors decreased pHi and LVP. Heptaminol applied in the presence of these inhibitors was unable to restore pHi and LVP. In severe ischaemia, the realkalinization effect and the restoration of LVP induced by heptaminol were no longer observed. During moderate ischaemia, Na-H exchange inhibitors decreased pHi and LVP. Heptaminol applied in the presence of these inhibitors was unable to restore pHi and LVP. 5. These results suggest that the positive inotropic effect of heptaminol during moderate ischaemia could be related to a restoration of internal pH, possibly mediated by a stimulation of the Na-H exchange.

Calcium blocking properties of piprofurol.

Piprofurol is a benzofuran chalcon derivative. It was studied under various experimental conditions which allow the recognition of calcium antagonistic activity. Piprofurol inhibited in a concentration-dependent manner the calcium-induced contractions in isolated potassium depolarized preparations of rat aorta (pA2: 9.29) and relaxed the K+-induced contraction of the dog coronary artery and the rabbit basilar artery (IC's 50: 2 10(-8) M; 3 10(-9) M). Piprofurol also inhibited noradrenaline-induced vascular smooth muscle contractions but the antagonism was clearly noncompetitive and the contractions induced were altered by concentrations two orders of magnitude higher than the concentration inhibiting calcium-induced contractions. Calcium antagonism was demonstrated in cardiac muscle: calcium mediated slowly rising action potentials were evoked in partially depolarized guinea-pig papillary muscle by electrical stimulation in the presence of isoprenaline. Piprofurol decreased the rate of rise of these slow action potentials. The inhibitory effect was reversed by an elevation of the calcium concentration in the bath fluid. Piprofurol exerts a negative inotropic effect (IC50: 5 10(-6) M) on guinea-pig papillary muscle. The ratio IC50 inotropic action/IC50 relaxant activity was 230, i.e. higher than that obtained with verapamil or diltiazem, and near that observed for cinnarizine. The pharmacological profile from in vivo dog experiments is in agreement with its in vitro properties: coronary sinus blood flow was increased and heart rate decreased. These effects suggested a potentially anti-ischaemic activity. This is confirmed in anaesthetized dogs, where piprofurol reduced the epicardial ST-segment elevation following coronary artery occlusion, and in isolated heart preparations, where it decreased the leakage of LDH during periods of anoxia and reoxygenation.

The chronotropic effect of adenosine and ATP in dogs. The antagonism by theophylline.

We analyzed the action of adenosine and adenosine triphosphate (ATP) on the chronotropism of the dog heart in situ. The compounds were administered either intravenously (i.v.) after bivagotomy and propranolol treatment, or intracoronary (i.c.) in the sinus node blood supply. Under these conditions the intervention of reflex reactions was eliminated, and a constant and dose-dependent negative chronotropic effect was obtained. From the dose-response curves the relative potencies of ATP and adenosine were calculated, and found to be similar for both routes of administration (potency ratio 0.77 (0.72-0.82) by i.v. administration, and 0.52 (0.36-0.78) by intracoronary administration). The effect was also mimicked by 2-chloroadenosine, (a long acting P1 agonist), and by 5'-adenylyl (beta, gamma-imido) diphosphonate, a non-hydrolysable ATP analog. The chronotropic effect of ATP and adenosine were not prevented by 1 mg/kg atropine intravenous. Theophylline, at 3 mg/kg i.v., shifted the dose-response curves of i.v. ATP and adenosine to the right, suggesting a competitive antagonism. At a dose of 6 mg/kg theophylline, the effects of i.c. adenosine were competitively blocked. Theophylline also antagonized, in a competitive manner, the hypotensive effect of the i.v. administration of both compounds. Our results suggest that the chronotropic effect of purine compounds in anaesthetized animals could be brought about by a specific purinergic theophylline-sensitive mechanism and not by direct vagal activation.

Cardiotoxicity of high doses of isoproterenol on cardiac haemodynamics and metabolism in SHR and WKY rats.

The haemodynamic and metabolic effects on the heart due to high doses of isoproterenol were compared in spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats. In baseline conditions, the hypertrophied SHR heart displayed perfectly constant haemodynamics but had fewer energy reserves than the WKY heart. Isoproterenol (2 X 25 mg/kg, s.c.) caused high mortality, myocardial ischaemia, and heart failure in the SHR. These effects were accompanied by anaerobic metabolism. In the WKY rats, on the other hand, isoproterenol caused no changes in ST-segment elevation and no cardiac insufficiency; in addition, aerobic metabolism was maintained. A marked drop in coronary perfusion pressure and excessive accumulation of calcium in the myocardium account, in part, for the effects seen in the SHR. The results indicate that isoproterenol-induced heart failure in the SHR might be a useful model for selecting compounds designed to treat this disease.

Effects of carocainide (770207) on canine cardiac automaticity.

The benzofuran derivative carocainide is a new class I antiarrhythmic drug which has been shown to prevent and stop experimental ventricular arrhythmias. The effects of this drug on different types of cardiac automaticity were investigated by experimental models using "in situ" dogs' hearts. Complete A-V block was produced by local formaldehyde injection into the A-V junction. Carocainide at doses of 2.5, 5 and 7.5 mg/kg body weight i.v. produced the following effects: a significant and dose-related slowing of the idioventricular rate; a non-significant change of the sinus rate at 2.5 and 5 mg/kg b.w.; and a significant decrease of the total number of triggered ventricular beats induced by ventricular pacing during slow norepinephrine infusion. Our results suggest that carocainide could be a safe and effective drug for the treatment of ventricular arrhythmias.

Electrophysiological effects of carocainide (770207) on the dog heart.

The electrophysiological effects of carocainide (770207) were assessed in "in vivo" and "in vitro" experiments on dogs' hearts. Intravenous administration of 5 mg/kg body weight of carocainide significantly lengthened the following: A-H interval, the shortest atrial pacing cycle length maintaining 1:1 atrioventricular (A-V) nodal conduction, H-V interval, left bundle-ventricle (LB-V) interval, the shortest His bundle pacing cycle length maintaining 1:1 His-Purkinje conduction, QRS duration, right atrial and right ventricular effective refractory periods. In Purkinje fiber-papillary muscle junction preparations driven at a constant cycle length of 1000 msec, perfusion of carocainide at the concentration of 1.25 10(-5) M and 2.5 10(-5) M, increased conduction time between Purkinje and muscle fibers and decreased the rate of depolarization of Purkinje fibers and papillary muscle. These results suggest that carocainide depresses conduction and lengthens refractory periods at atrial and ventricular levels and provide an explanation for the positive effect of this compound against experimental cardiac arrhythmias.

Electrophysiological effects of carocainide (770207) on canine ventricular muscle and Purkinje fibers.

Carocainide is a new benzofuran derivative showing antiarrhythmic properties in animal models. In isolated papillary muscles and Purkinje fibers it decreases the maximum rate of rise of the action potential and in Purkinje fibers it decreases the plateau amplitude and the duration of the action potential. Carocainide increases the ratio of the effective refractory period to the action potential duration at 90% of repolarization, shifts the membrane responsiveness curve towards more negative membrane potentials and slows down the recovery of the maximum rate of rise of the action potentials of the Purkinje fibers. Carocainide also increases the conduction time at the Purkinje papillary muscle junction.

Effects of MD770207 (carocainide) on experimental ventricular arrhythmias.

Administered by intravenous or oral route, carocainide, MD770207, abolished experimental ventricular arrhythmias in anaesthetized and conscious dogs. Slow rate intravenous infusion (0.5 mg/kg/min) studies in conscious coronary ligated dogs demonstrated a good arrhythmia conversion as well as a wide margin of safety; mean conversion to 95% of normal sinus rhythm was achieved at 10.5 +/- 2.1 mg/kg while first signs of reversible neuro and cardio-toxicity occurred at 37.8 +/- 8 mg/kg. The reference agents disopyramide and lidocaine showed less favourable safety margin. Carocainide reduced epicardial ST-segment elevation produced by short coronary artery occlusion and prevented the occurrence of ventricular fibrillation resulting from reperfusion after acute coronary artery ligation. At antiarrhythmic dose levels there were no undesirable effects noted on cardiovascular function. Given intravenously the compound increased the ventricular fibrillation threshold in spontaneously hypertensive rats.

[Comparison of the effects of regional ischemia and of reperfusion on the hypertrophied heart of spontaneously hypertensive rats and the heart of Wistar-Kyoto rats with isolated perfused heart via the left atrium. Effects of lidocaine, propranolol and verapamil]. / Comparaison des effets de l'ischémie régionale et de la reperfusion sur le coeur hypertrophié du SHR et le coeur du WKY dans des conditions de coeur isolé perfusé par voie atriale gauche. Effets de la lidocaïne, du propranolol et du vérapamil.

1. A comparative study between the effects of ischemia and reperfusion on the hypertrophied heart of spontaneously hypertensive rat (SHR) and on the heart of Wistar-Kyoto (WKY) was carried out in working heart conditions. 2. The decrease of coronary flow induced by a coronary occlusion had an equal magnitude in the two groups. In the SHR group, the aortic output became null in 40% of the cases, whereas this effect was never seen in WKY group. 3. In reperfusion conditions, the recovery of haemodynamic values was slower in SHR group. The release of LDH and the rhythmic disturbances were also increased. 4. The perfusion of SHR hearts by lidocaine (4.3 10(-5) M), propranolol (7.7 10(-6) M) and verapamil (1.1 10(-7) M) decreased significantly the heart rate and the aortic output. 5. When these substances were added to the perfusion medium, the deleterious effects of reperfusion were greatly reduced. Aortic output recovered more quickly than his basal values and the enzymatic leakage was decreased. None of the hearts fibrillated after lidocaine and propranolol. Ventricular fibrillation continued in 14% of cases after verapamil compared to 80% of cases in the control group.

Pharmacokinetics and pharmacodynamics of the antiarrhythmic compound MD750819 in dogs with experimentally induced arrhythmias.

Pharmacokinetics and pharmacodynamics were studied in three dogs with interventricular coronary artery ligatures (ligature of Harris) and in three control animals. Weighted nonlinear analysis was used to fit equations describing two and three compartment open models to the experimental data, obtained after intravenous injection (5 mg/kg) of the drug. The three compartment model gave a reduction in the weighted sum of squared residuals and an improvement in the randomness of scatter of the experimental points about the theoretical curve. The postdistribution elimination half-life was longer, the area under the plasma elimination curve larger, and the total body plasma clearance and apparent volume of distribution was reduced in the animals with arrhythmias. The pharmacological response was assessed by recording the ECG and calculating the percentage of normal sinus rhythm/min. A combined pharmacokinetic-pharmacodynamic model was used to analyze data from individual animals. keO, a measure of the lag time of pharmacological response behind changes in plasma concentration, and Ce (50), a measure of the sensitivity of the cardiac site of action of the drug, were determined.

Comparison between the ventricular fibrillation thresholds of spontaneously hypertensive and normotensive rats--investigation of antidysrhythmic drugs.

Induction of ventricular fibrillation by intraventricular electric pulses is achieved with weaker currents in spontaneously hypertensive rats (SHR) than in normotensive rats of the Wistar-Kyoto (WKY) or Sprague-Dawley (SD) strains. The ventricular fibrillation threshold (VFT) is stable with time in SHR but not in WKY. Investigation of antidysrhythmic agents in SHR showed that most substances with membrane-stabilizing properties increase the VFT. There was no correlation between the elevation of VFT and the decrease in heart rate induced by the substances studied. Determination of the VFT in SHR may be useful for the screening of compounds with membrane-stabilizing properties.

Effects of 5-hydroxytryptamine on canine isolated coronary arteries.

The effects of 5-hydroxytryptamine (5-HT) were studied in vitro on proximal and distal portions of canine interventricular and circumflex coronary arterial strips. 5-HT produced concentration-related contractions in the proximal portion whether contracted previously with KCl or not. These responses were still present after either chemical sympathetic denervation or release of noradrenaline induced by K+-free salt solution. In contrast, the distal portions of coronary arteries did not respond to 5-HT. Concentration-response curves to 5-HT exhibited a classical hyperbolic shape with a calculated Hill-coefficient of approximately 1. Methysergide and phentolamine but not morphine shifted to the right and depressed the maximum of the dose-response curves to 5-HT. It is concluded that the contractions produced by 5-HT in the proximal portion of the interventricular and circumflex coronary arteries are not due to the release of endogenous noradrenaline. The vessels appear to possess separate receptors for 5-HT and noradrenaline and the 5-HT responses belong to neither the M nor the D type.