Agonist-like Action Research Articles

Central inhibitory effects of water extract of Acori graminei rhizoma in mice

The present study evaluated in mice the central inhibitory effects of a water extract of shichangpu (Acori graminei rhizoma (AGR), the dry rhizome of Acorus gramineus Soland. (Araceae)). AGR (0.5–5.0 g/kg) dose-dependently decreased the locomotor activity and increased the pentobarbital-induced sleeping time, but had no significant effect on the treadmill performance. AGR also dose-dependently inhibited the intensity of apomorphine-induced stereotypic behavior. At the highest dose (5.0 g/kg), AGR had a weak anticonvulsant effect on the pentylenetetrazol-induced seizures. Receptor binding assays showed that AGR competed with [ 3H]SCH-23390 and [ 3H]YM-09151-2 for specific binding to striatal dopamine D 1 and D 2 receptors with K i values of 5.6 and 4.2 mg/ml, respectively. AGR also competed with [ 3H]muscimol for specific binding to the γ-aminobutyric acid (GABA) binding site of cortex GABA A receptors with a K i value of 0.31 mg/ml. It also increased the specific binding of [ 3H]flunitrazepam to the benzodiazepine binding site of the GABA A receptors, suggesting a GABA agonist-like action. These results suggested that the central inhibitory effects of AGR were probably effected through an action on the central dopamine receptors and GABA A receptors. The principle of AGR acting at these ligand binding sites was not α-asarone, one of the important principles of AGR, since that α-asarone (10 −6–10 −4 M) had no significant interactions with these binding sites.

Discovery and synthesis of [6-hydroxy-3-[4-[2-(1-piperidinyl)ethoxy]phenoxy]-2-(4-hydroxyphenyl)]b enzo[b]thiophene: a novel, highly potent, selective estrogen receptor modulator.

Raloxifene,[2-(4-hydroxyphenyl)-6-hydroxybenzo[b]thien-3-yl] [4-[2-(1-piperidinyl)ethoxy]phenyl]methanone hydrochloride (2), is representative of a class of compounds known as selective estrogen receptor modulators (SERMs) that possess estrogen agonist-like actions on bone tissues and serum lipids while displaying potent estrogen antagonist properties in the breast and uterus. As part of ongoing SAR studies with raloxifene, we found that replacement of the carbonyl group with oxygen ([6-hydroxy-3-[4-[2-(1-piperidinyl)ethoxy]phenoxy]-2-(4-hydroxyphenyl)]b enzo[b]thiophene hydrochloride, 4c) resulted in a substantial (10-fold) increase in estrogen antagonist potency relative to raloxifene in an in vitro estrogen dependent cell proliferation assay (IC50 = 0.05 nM) in which human breast cancer cells (MCF-7) were utilized. In vivo, 4c potently inhibited the uterine proliferative response to exogenous estrogen in immature rats following both sc and oral dosing (ED50 of 0.006 and 0.25 mg/kg, respectively). In ovariectomized aged rats, 4c produced a significant maximal decrease (45%) in total cholesterol at 1.0 mg/kg (p.o.) and showed a protective effect on bone relative to controls with maximal efficacy at 1.0 mg/kg (p.o.). These data identify 4c as a novel SERM with greater potency to antagonize estrogen in uterine tissue and in human mammary cancer cells compared to raloxifene, tamoxifen or ICI-182,780.

A model of ???antipredator??? defense in Swiss-Webster mice

A mouse defense test battery (MDTB) has been designed to assess defensive reactions of Swiss-Webster mice to situations associated with nonpainful threat. When compared to mice approached by a leather glove, animals confronted with an anesthetized or a conscious rat displayed potentiated flight responses and defensive threat/attack reactions, while risk assessment performances were generally similar in all three conditions. Furthermore, escape attempt responses following removal of the stimulus were higher in the conscious rat condition compared to the two other groups. Taken together, these results suggest that flight reactions and defensive threat/attack responses are specific to the rat, and thus indicate that the MDTB may relate to 'antipredator' defense. In mice confronted with an anaesthetized rat, administration of the benzodiazepine (BZ) receptor full agonist chlordiazepoxide (5-25mg/kg, i.p., 30min) and the BZ partial agonist Ro 19-8022 (0.5-2mg/kg, i.p., 30min) altered one of two risk assessment measures and inhibited defensive attack behaviors, but failed to counter the post-predator increase in escape attempts. In addition, Ro 19-8022 also strongly reduced flight responses. The overall behavioral profile suggests a fear/anxiety-reducing action of both drugs. By contrast, administration of the BZ inverse agonist Ro 19-4603 (0.025-0.1mg/kg, i.p., 30min) reliably released these defensive responses. Interestingly, the BZ antagonist flumazenil (5-20mg/kg, i.p., 30min) manifested differential intrinsic activity depending upon the level of threat. Thus, in a weakly threatening situation, the drug potentiated flight reactions, indicating an inverse agonist-like action, decreased defensive biting in a highly threatening situation, indicating an agonist activity. These findings demonstrated that BZ ligands differently modulated 'antipredator' defense in Swiss-Webster mice, depending upon their intrinsic (positive or negative) efficacy, but also depending upon the defense strategy required by the threat.

Muscarinic toxins from the venom of Dendroaspis snakes with agonist-like actions

The venom of some Dendroaspis snakes contains small proteins (7500 mol. wt) that inhibit the binding of radiolabelled muscarinic antagonist to brain synaptomal membranes. There were no peptides described among muscarinic ligands until Adem et al. ( Biochim. biophys. Acta 968, 340–345, 1988) reported that muscarinic toxins (MTxs), MTx1 and 2 were able to inhibit 3H-QNB binding to rat brain membranes. Since MTxs inhibit around half of specific binding of 3H-quinuclidinyl benzilate ( 3H-QNB) and 3 H- N-methyl-scopolamine ( 3H-NMS), which do not discriminate between subtypes of muscarinic receptors, it has been proposed that MTxs might selectively bind to some subtype. MTx1 and 2 from Dendroaspis angusticeps almost completely inhibit the binding of 3H-pirenzepine ( 3H-PZ), a preferential M 1 muscarinic receptor subtype ligand to cerebral cortex synaptosomal membranes. A much higher concentration was needed to inhibit partially 3H-PZ binding to atrial muscarinic receptors. These results support the hypothesis that MTx1 and 2 may be m 1 selective muscarinic ligands. Similar activities have been found in Dendroaspis polylepis and D. viridis venoms, but with lower affinities. The K i obtained from inhibition curves of the binding of 3H-PZ showed that MTx1 has higher affinity for the putative M 1 muscarinic receptor subtype, followed by MTx2. DpMTx has lower affinity, while DvMTx seems to have the lowest affinity. All these peptides are devoid of anticholinesterase activity. Dendrotoxin and fasciculin from D. angusticeps venom do not inhibit the binding of muscarinic radioligands to cerebral cortex membranes. The injection of MTxs into dorsal hippocampus of rats immediately after training in an inhibitory avoidance task improves memory consolidation, as does oxotremorine. This improvement is antagonized by the joint administration of the muscarinic antagonist scopolamine. These results suggest that MTxs behave as a muscarinic agonist, at least in this case. Therefore, the muscarinic toxins from elapid snakes may be useful tools since they are proteins that might be selective muscarinic ligands, that seem to be agonists in the case of MTxs from D. angusticeps venom.

Muscarinic toxins from the venom of Dendroaspis snakes with agonist-like actions: D. Jerusalinsky, 1I. Izquierdo2 and C. Cervenansky3 ( 1Inst. Cell Biol., Fac. Med., Univ. of Buenos Aires, 1121 Buenos Aires, Argentina; 2Dept Biochem., Inst. Biosci., Federal Univ. of Rio Grande do Sul, Porto Alegre, Brazil; and 3Neurochem. Div., Inst. Biol. Invest., 11600 Montevideo, Uruguay)

Muscarinic toxins from the venom of Dendroaspis snakes with agonist-like actions: D. Jerusalinsky, 1I. Izquierdo2 and C. Cervenansky3 ( 1Inst. Cell Biol., Fac. Med., Univ. of Buenos Aires, 1121 Buenos Aires, Argentina; 2Dept Biochem., Inst. Biosci., Federal Univ. of Rio Grande do Sul, Porto Alegre, Brazil; and 3Neurochem. Div., Inst. Biol. Invest., 11600 Montevideo, Uruguay)

A peptide muscarinic toxin from the Green Mamba venom shows agonist-like action in an inhibitory avoidance learning task

A peptide, muscarinic toxin 2 (MTX2), isolated from Dendroaspis angusticeps venom was previously shown to displace the specific binding of [ 3H]pirenzepine, a muscarinic M 1 receptor ligand, from rat brain synaptosomal membranes. We have tested MTX2 for muscarinic agonist or antagonist actions in an inhibitory avoidance task in rats. Infusion of the muscarinic receptor antagonist scopolamine into the hippocampus of rats immediately after the training period produced amnesia, whereas the muscarinic agonist oxotremorine increased retention. When MTX2 was injected into the hippocampus of rats after the inhibitory avoidance task, it caused memory facilitation, which could be suppressed by the concomitant infusion of scopolamine. Hence, in this test, MTX2 showed muscarinic receptor agonist-like actions, which are probably mediated by the M 1 subtype of muscarinic acetylcholine receptors.

Proposed antagonists at GABA B receptors that inhibit adenylyl cyclase in cerebellar granule cell cultures of rat

The effects of various proposed GABA B receptor antagonists on baclofen-mediated inhibition of adenylyl cyclase were studied in cultured cerebellar granule cells from rat. (±)-Baclofen maximally inhibited adenylyl cyclase by approximately 60% of the basal enzyme activity with an EC 50 value of 10 μM. 3-Aminopropane sulfonic acid (3-APS) and 5-aminovaleric acid (5-AVA) produced similar responsesto that seen with (±)-baclofen. Saclofen reversed the action of (±)-baclofen , 50 μM, with a half maximal inhibitory concentration (IC 50) of about 1.0 mM. The most effective antagonist in blocking the action of (±)-baclofen was 3-aminopropyl-diethoxy-methyl-phosphonic acid (CGP 35 348). In the presence of (±)-baclofen, 50 μM, the IC 50 for CGP 35 348 was 290 μM and its inhibitory constant (K A) was 180 μM. The agonist-like actions of 3-APS and 5-AVA were antagonized by CGP 35 348 suggesting that 3-APS and 5-AVA may act as weak agonists at the GABA B receptor that inhibits adenylyl cyclase. All antagonists tested, except the new compound CGP 35 348, have very low potencies at GABA B receptors that inhibit adenylyl cyclase, though these compounds have been quite effective at other GABA B receptor-mediated events. Thus, the GABA B receptor which inhibits adenylyl cyclase differs pharmacologically from other reported GABA B receptor/effector systems and supports the existence of multiple receptor subtypes.

Supraspinal administration of [D-Met 2]FMRFamide produces a naloxone-sensitive increase in heart rate in unrestrained spontaneously hypertensive rats

Intracerebroventricular (i.c.v.) administration of either Phe-Met-Arg-Phe-NH 2 (FMRFamide; molluscan cardioexcitatory neuropeptide; 3–30 μg) or the FMRFamide analog Phe-D-Met-Arg-Phe-NH 2 ([D-Met 2]FMRFamide; 15 μg) to conscious unrestrained spontaneously hypertensive rats (SHR) produced a relatively long lasting (> 1 h) increase in heart rate. The increase in heart rate produced by [D-Met 2]FMRFamide was attenuated by i.c.v. injection of the opiate antagonist naloxone (2 μg). These results extend to a second endpoint an apparent opioid agonist-like (naloxone-reversible) action of [D-Met 2]FMRFamide.

A monoclonal anti-idiotypic antibody to μ and δ opioid receptors

A mouse monoclonal, anti-idiotypic, anti-opioid receptor antibody (Ab2-AOR) has been generated from monoclonal anti-morphine antibodies (Ab1). Hybridoma culture supernatants were screened by a solid phase radioimmunoassay (RIA), based on their competition with radiolabelled morphine for Ab1. One of the Ab2s that gave a positive RIA also competed at rat brain opioid receptors with tritiated opioid ligands dihydromorphine (DHM), naloxone, etorphine, Tyr- d-Ala-Gly-Phe- d-Leu (DADLE), Tyr- d-Ala-Gly-NMe-Phe-Gly-ol (DAMGE) and Tyr- d-Pen-Gly-Phe- d-Pen (DPDPE). SDS-PAGE revealed Ab2-AOR to be highly purified after successive affinity and protein A-Sepharose chromatography. Ab2-AOR at concentrations of 10–100 nM competed with both μ- and δ-selective specific ligands for brain opioid receptors. Less than 13 μg/ml Ab2-AOR completely inhibited specific opioid radioligand binding to both soluble and membrane-bound opioid receptors. To demonstrate its anti-δ receptor activity further, a double-antibody ELISA procedure was developed that is based on the binding of Ab2-AOR to immobilized NG 108-15 cells (which contain only δ opioid receptors). Dose-dependent, opioid peptide- and opiate alkaloid-competitive binding of Ab2-AOR-containing ascites fluid to NG 108-15 cells was observed. A μ opioid agonist effect was demonstrated for Ab2-AOR, in that it decreased by 70% [ 3H]thymidine incorporation into DNA of fetal brain cell aggregates. This agonist-like action of Ab2-AOR was blocked by naltrexone. The antibody bound specifically to brain tissue sections and the presence of diprenorphine blocked this interaction. Hence, an Ab2 with μ and δ specificity has been characterized.

Modulation of dopaminergic terminal excitability by D 1 selective agents: Further characterization

We have previously shown that stimulation of striatal D 1 receptors affects dopaminergic nigrostriatal terminal excitability, which is thought to be an index of biophysical events resulting from the activation of receptors on the presynaptic membrane. The experiments presented here further examine the locus and bases of these D 1 effects in the rat. We now report that striatal administration of the D 1 receptor selective antagonist R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazapine-7-ol-HCl (SCH 23390) produces a paradoxical agonist-like decrease in dopaminergic terminal excitability. This effect is blocked by pretreatment with the dopamine synthesis inhibitor, alpha-methyl-paratyrosine, suggesting that the action of SCH 23390 is dependent upon endogenous dopamine. Further, haloperidol pretreatment also prevents the SCH 23390-induced decrease in terminal excitability, confirming that dopamine, acting through a dopamine receptor, is responsible for this agonist-like action. Striatal application of the active R-(+) enantiomer of the dopaminergic D 1-selective agonist 1-phenyl-2,3,4,5-tetrahydrol-(1H)-3-benzazepine-7, 8-diol-HCl (R-SKF 38393) decreases terminal excitability in the alphamethyl-paratyrosine pretreated animal, indicating that dopamine is not required for the agonist action. In an effort to ascertain the presynaptic or postsynaptic location of these actions, an extensive destruction of postsynaptic neurons in the neostriatum was produced by local administration of the neurotoxin, kainic acid. It was observed that the neurotoxin-induced neostriatal neuronal loss did not disrupt the action of R-SKF 38393 nor its reversal by SCH 23390. Finally, to exclude the possibility that R-SKF 38393 may act through a D 2 receptor, an additional group of animals was pretreated with the D 2-selective antagonist, (−)-sulpiride. The D 1-selective agonist still decreased excitability following this pretreatment, indicating that the agonist action is not mediated through D 2 receptors.

Benzodiazepines and alcohol

The frequency and quantity of alcohol consumption is a major consideration in patients who need treatment with benzodiazepines. Alcohol affects the GABA-benzodiazepine-chloride ionophore complex and has an agonist-like action. Thus, additive interactions should be expected from combining alcohol with benzodiazepines. Furthermore, alcohol has clinically meaningful anxiolytic efficacy, and many anxious patients may take advantage of that fact. Therefore, co-administration of alcohol and benzodiazepines is to be expected in an anxious patient receiving benzodiazepines who does not totally abstain from alcohol. This article reviews three clinically relevant issues concerning benzodiazepines and alcohol: (1) interactions of benzodiazepines with social drinking in patients taking benzodiazepines for indications unrelated to alcoholism; (2) use of benzodiazepines in treatment of alcohol withdrawal; and (3) use of benzodiazepines in patients with alcohol dependence.

Phencyclidine (PCP)-like inhibition of N-methyl-D-aspartate-evoked striatal acetylcholine release, H-TCP binding and synaptosomal dopamine uptake by metaphit, a proposed PCP receptor acylator

The phencyclidine (PCP) receptor acylator, metaphit, has been reported to act as a PCP antagonist. Recent electrophysiological and behavioral assessments of metaphit action have revealed, however, that this compound can also act as a PCP-like agonist. The present study examined the effects of metaphit on the inhibition of N-methyl-D-aspartate (NMDA)-induced 3H-acetylcholine (ACh) release, 3H-TCP binding and synaptosomal 3H-dopamine (DA) uptake in the rat striatum. Preincubation of striatal slices for 10 min in the presence of metaphit, followed by a prolonged washout, produced a concentration-dependent inhibition of the ACh release evoked by 300 μM NMDA. At high concentrations, preincubation with PCP also resulted in inhibition of this measure. However, this could be reduced by extending the washout period, a procedure which had no effect on the inhibition produced by metaphit. At 10 μM, metaphit resulted in a 53% reduction in NMDA-evoked ACh release while PCP had no effect under identical conditions. Preincubation of slices in 10 μM PCP and metaphit reduced the metaphit inhibition by 62%. The effects of PCP and metaphit, alone or in combination, on NMDA- induced ACh release were paralleled by a loss of 3H-TCP binding sites in striatal tissue incubated under identical conditions suggesting that metaphit exerts long-lasting agonist-like actions on PCP receptors coupled to NMDA receptors. Although these results do not explain the ability of metaphit to antagonize PCP effects in other assays, we did observe that preincubation of striatal synaptosomes with metaphit also resulted in an irreversible inhibition of 3H-DA uptake. These data are discussed in relation to the interaction of metaphit with PCP receptors in various systems.

Different genes specify hyporesponsiveness to seizures induced by caffeine and the benzodiazepine inverse agonist, DMCM

Two strains of inbred mice differed significantly in their susceptibility to tonic seizures induced by caffeine and the benzodiazepine inverse agonist, methyl 6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate (DMCM). The hyporesponsive strain, SWR, was not less susceptible to the convulsant action of other chemical convulsants, an observation which indicated that the response differences between the strains were pharmacologically specific. These observations and drug interaction studies suggested that caffeine-induced seizures might be mediated through an “inverse” agonist-like action of caffeine on benzodiazepine receptors associated with GABA receptor-benzodiazepine receptor-chloride ionophore complex. To determine whether the coincident alteration in susceptibility to DMCM and caffeine resulted from a single mutational change or was the result of two different genetic changes occurring coincidentally between these two strains of mice, progeny from conventional Mendelian crosses (F 1, F 2 and reciprocal backcrosses) were analyzed for the co-segregation of susceptibility to DMCM and caffeine. The inheritance of DMCM sensitivity was consistent with a single autosomal gene determinant in which the allele specifying increased responsiveness was dominant to the allele determining hyporesponsiveness. The frequent occurrence of recombinant phenotypes (e.g., caffeine hyporesponsive but DMCM sensitive mice) among F 2 and backcross established that different genetic determinants encode DMCM susceptibility and caffeine susceptibility in these two strains of mice. Thus, while these data establish a simply inherited difference in benzodiazepine responsiveness between the two mouse strains, they also indicate that this pair of strains is inappropriate for a genetic analysis aimed at probing the relationship between caffeine-induced seizures and the benzodiazepine receptor.

Serotonin decreases the duration of action potentials recorded from tetraethylammonium-treated bullfrog dorsal root ganglion cells

Neurotransmitter effects on calcium currents activated by sensory neuron action potentials have been previously studied in embryonic or neonatal dorsal root ganglion (DRG) cells in culture. In the present study we examined the effects of serotonin (5-HT) on the shape of action potentials recorded from fully differentiated primary afferent neurons in isolated DRG of adult bullfrogs. Intracellular recordings were obtained from cell bodies of type A and C neurons. Concentrations of 5-HT that had no effect on membrane potential or input resistance had little or no effect on action potential shape. Treatment with 5-20 mM tetraethylammonium ion (TEA) led to the appearance of a plateau phase on the falling limb of the spike. This plateau phase appears to result from calcium influx, as it was dramatically reduced in amplitude and duration by solutions containing low concentrations of calcium or the calcium channel blocker, manganese. In preparations treated with 7.5 mM TEA, low concentrations of 5-HT (10 nM-1 microM) produced a dose-dependent narrowing of the calcium-dependent plateau phase of the mixed sodium/calcium spike. A decrease in spike afterhyperpolarization was also noted. The decrease in spike duration was recorded from 74% of type A neurons and 57% of type C neurons, and was not secondary to a change in resting potential or input resistance. The 5-HT receptor antagonists methysergide and metergoline did not block the response to 5-HT. Instead, they exhibited weak agonist-like actions. Serotonin also reduced the rate of rise and peak amplitude of calcium spikes recorded in the presence of tetrodotoxin and TEA.(ABSTRACT TRUNCATED AT 250 WORDS)

Anti-morphine anti-idiotypic antibodies: Opiate receptor binding and isolated tissue responses

Anti-idiotypic antibodies which recognize the opiate receptor were generated in guinea pigs following immunization against purified rabbit anti-morphine antibodies. The anti-idiotypic antibodies produced a concentration-dependent inhibition of [ 3H]naloxone binding to opiate receptors in a membranous mouse brain preparation. Saturation analysis indicated that the antibodies produced a non-competitive inhibition of naloxone binding. The ability of the antibodies to interact with biological systems was investigated in in vitro systems. In both the isolated guinea pig ileal longitudinal muscle and mouse vas deferens, the antibodies produced a concentration-dependent, opiate agonist-like action. The anti-morphine anti-idiotypic antibodies appear to interact specifically with the opiate receptor and may serve as useful tools in characterization of this receptor system.

Adenosine antagonists: Lack of effect on the inhibition of kindled seizures in rats by carbamazepine

This study investigates the ability of adenosine antagonists to reverse the anticonvulsant effects of carbamazepine on amygdala-kindled seizures in order to elucidate the possible physiological relevance of the potent effects of carbamazepine on adenosine receptors. At large but subconvulsant doses, neither caffeine nor theophylline altered the anticonvulsant potency of carbamazepine, even though caffeine by itself significantly increased the duration of the kindled afterdischarge. The adenosine agonist cyclohexyladenosine (CHA), administered intraperitoneally at a dose that produced sedation, had no effect on the kindled seizures. Although carbamazepine potently displaces the binding of several adenosine ligands in vitro, the present data do not suggest that the anticonvulsant effects of carbamazepine on amygdalakindled seizures are mediated by an adenosine agonist-like action.

A patch-clamp study of the partial agonist actions of tubocurarine on rat myotubes.

Single channels activated by (+)-tubocurarine (curare) were recorded from rat myotubes using the patch-clamp technique. The agonist-like action of curare does not result from a contaminant molecule, as the same effects were observed with curare purified by high-performance liquid chromatography. A curare-activated channel can adopt two levels of conductance: full (F) or partial (P). The F state has a slope conductance of 40 pS (identical to that of the acetylcholine (ACh)-activated channel) and the P state has a conductance of 13 pS. At low concentrations of agonist (ACh or curare), the distribution of channel open times is biphasic. The briefer channels may result from the binding of a single agonist molecule whereas the longer-lived channels probably occur following the binding of two agonist molecules. The mean open time of the F state decreases with increasing curare concentration. It is shown that band-width limitations are likely to account for only a very small part of this observed reduction. In contrast, the mean open time of the P state is independent of the concentration of curare. A simple interpretation is that the F state is susceptible to channel blockade by curare, whereas the P state is not. The P state preceded the F state almost as often as it followed the F state; it can also be observed separately from the F state. The fraction of events including a P state increases from about 4% in the presence of 1 microM-curare to 30% at 100 microM-curare. This fraction is also increased by hyperpolarization. When the curare concentration is increased, the F-state frequency first increases and then decreases at higher concentration. This frequency is also decreased by hyperpolarization. The decrease in F-state frequency is probably related to channel blockade by curare; it cannot be wholly accounted for by problems associated with limited time resolution. A synthetic analogue of curare, (+)- tubocurine dimethiodide presents an agonist activity similar to that of curare but with a faster closing rate for both F and P states. The various actions of curare are summarized in two possible models where the P state is interpreted as either a partially open channel or a channel which is partially blocked.

Electrophysiological and mechanical studies on the cardiac effects of a histamine H2 receptor antagonist, cimetidine, in the isolated guinea pig myocardium.

The effects of cimetidine, a new histamine H2 receptor antagonist, were studied electrophysiologically and mechanically in the isolated guinea pig myocardium. Cimetidine did not modify the electrical activity of the sinoatrial pacemaker cells at the concentrations up to 10-4 M, but depressed the pacemaker potential at the higher concentration, 3×10-4 M. Increase in the slope of the pacemaker potential produced by histamine was completely inhibited by cimetidine, indicating that the histamine receptor for the positive chronotropism is of H2-type. Cimetidine did not depress the maximum rate of rise of the atrial and ventricular action potentials ; thus, unlike most of the H1 receptor antagonists, cimetidine does not have quinidine like actions. The shape of the action potential was not modified significantly. Slow electrical and mechanical activities produced by histamine in the right ventricular myocardium whose fast Na+ channels were blocked by elevated external potassium were partially blocked by either cimetidine or diphenhydramine, supporting the view that both types of receptors are present in the right ventricle. Histamine-induced mechanical activity in the depolarized right atrium was inhibited by cimetidine, but that in the left atrium was not inhibited ; these results support the view that the histamine receptor in the left atrium is of H1-type and suggest that the histamine receptor for the positive inotropism in the right atrium is of H2-type. In addition, some results were shown suggesting the possibility that cimetidine might have some agonist-like action in the myocardium.