The peripheral, high affinity benzodiazepine binding site is not coupled to the cardiac Ca2+ channel

Targeting the Translocator Protein (18 kDa) in Cardiac Diseases: State of the Art and Future Opportunities

Mitochondria dysfunctions are typical hallmarks of cardiac disorders (CDs). The multiple tasks of this energy-producing organelle are well documented, but its pathophysiologic involvement in several manifestations of heart diseases, such as altered electromechanical coupling, excitability, and arrhythmias, is still under investigation. The human 18 kDa translocator protein (TSPO) is a protein located on the outer mitochondrial membrane whose expression is altered in different pathological conditions, including CDs, making it an attractive therapeutic and diagnostic target. Currently, only a few TSPO ligands are employed in CDs and cardiac imaging. In this Perspective, we report an overview of the emerging role of TSPO at the heart level, focusing on the recent literature concerning the development of TSPO ligands used for fighting and imaging heart-related disease conditions. Accordingly, targeting TSPO might represent a successful strategy to achieve novel therapeutic and diagnostic strategies to unravel the fundamental mechanisms and to provide solutions to still unanswered questions in CDs.

Beetles as Model Organisms in Physiological, Biomedical and Environmental Studies - A Review

Model organisms are often used in biological, medical and environmental research. Among insects, Drosophila melanogaster, Galleria mellonella, Apis mellifera, Bombyx mori, Periplaneta americana, and Locusta migratoria are often used. However, new model organisms still appear. In recent years, an increasing number of insect species has been suggested as model organisms in life sciences research due to their worldwide distribution and environmental significance, the possibility of extrapolating research studies to vertebrates and the relatively low cost of rearing. Beetles are the largest insect order, with their representative - Tribolium castaneum - being the first species with a completely sequenced genome, and seem to be emerging as new potential candidates for model organisms in various studies. Apart from T. castaneum, additional species representing various Coleoptera families, such as Nicrophorus vespilloides, Leptinotarsa decemlineata, Coccinella septempunctata, Poecilus cupreus, Tenebrio molitor and many others, have been used. They are increasingly often included in two major research aspects: biomedical and environmental studies. Biomedical studies focus mainly on unraveling mechanisms of basic life processes, such as feeding, neurotransmission or activity of the immune system, as well as on elucidating the mechanism of different diseases (neurodegenerative, cardiovascular, metabolic, or immunological) using beetles as models. Furthermore, pharmacological bioassays for testing novel biologically active substances in beetles have also been developed. It should be emphasized that beetles are a source of compounds with potential antimicrobial and anticancer activity. Environmental-based studies focus mainly on the development and testing of new potential pesticides of both chemical and natural origin. Additionally, beetles are used as food or for their valuable supplements. Different beetle families are also used as bioindicators. Another important research area using beetles as models is behavioral ecology studies, for instance, parental care. In this paper, we review the current knowledge regarding beetles as model organisms and their practical application in various fields of life science.

The mitochondrial translocator protein and arrhythmogenesis in ischemic heart disease

Mitochondrial dysfunction is a hallmark of multiple cardiovascular disorders, including ischemic heart disease. Although mitochondria are well recognized for their role in energy production and cell death, mechanisms by which they control excitation-contraction coupling, excitability, and arrhythmias are less clear. The translocator protein (TSPO) is an outer mitochondrial membrane protein that is expressed in multiple organ systems. The abundant expression of TSPO in macrophages has been leveraged to image the immune response of the heart to inflammatory processes. More recently, the recognition of TSPO as a regulator of energy-dissipating mitochondrial pathways has extended its utility from a diagnostic marker of inflammation to a therapeutic target influencing diverse pathophysiological processes. Here, we provide an overview of the emerging role of TSPO in ischemic heart disease. We highlight the importance of TSPO in the regenerative process of reactive oxygen species (ROS) induced ROS release through its effects on the inner membrane anion channel (IMAC) and the permeability transition pore (PTP). We discuss evidence implicating TSPO in arrhythmogenesis in the settings of acute ischemia-reperfusion injury and myocardial infarction.

Translocator protein (18 kDa): a promising therapeutic target and diagnostic tool for cardiovascular diseases

The translocator protein (18 kDa) (TSPO) is a five transmembrane domain protein in mitochondria, abundantly expressed in a variety of organs and tissues. TSPO contributes to a wide range of biological processes, including cholesterol transportation, mitochondrial membrane potential and respiratory chain regulation, apoptosis, and oxidative stress. Recent studies have demonstrated that TSPO might also be involved in the physiological regulation of cardiac chronotropy and inotropy. Accordingly, TSPO ligands play significant roles in protecting the cardiovascular systems under pathological conditions through cardiac electrical activity retention, intracellular calcium maintenance, mitochondrial energy provision, mitochondrial membrane potential equilibrium, and reactive oxygen species inhibition. This paper focuses on the physiological and pathological characteristics of TSPO in the cardiovascular systems and also summarizes the properties of TSPO ligands. TSPO represents a potential therapeutic target and diagnostic tool for cardiovascular diseases including arrhythmia, myocardial infarction, cardiac hypertrophy, atherosclerosis, myocarditis, and large vessel vasculitis.

Translocator protein (Tspo) gene promoter-driven green fluorescent protein synthesis in transgenic mice: an in vivo model to study Tspo transcription

Translocator protein (TSPO), previously known as the peripheral-type benzodiazepine receptor, is a ubiquitous drug- and cholesterol-binding protein primarily found in the outer mitochondrial membrane as part of a mitochondrial cholesterol transport complex. TSPO is present at higher levels in steroid-synthesizing and rapidly proliferating tissues and its biological role has been mainly linked to mitochondrial function, steroidogenesis and cell proliferation/apoptosis. Aberrant TSPO levels have been linked to multiple diseases, including cancer, endocrine disorders, brain injury, neurodegeneration, ischemia-reperfusion injury and inflammatory diseases. Investigation of the functions of this protein in vitro and in vivo have been mainly carried out using high-affinity drug ligands, such as isoquinoline carboxamides and benzodiazepines and more recently, gene silencing methods. To establish a model to study the regulation of Tspo transcription in vivo, we generated a transgenic mouse model expressing green fluorescent protein (GFP) from Aequorea coerulescens under control of the Tspo promoter region (Tspo-AcGFP). The expression profiles of Tspo-AcGFP, endogenous TSPO and Tspo mRNA were found to be well-correlated. Tspo-AcGFP synthesis in the transgenic mice was seen in almost every tissue examined and as with TSPO in wild-type mice, Tspo-AcGFP was highly expressed in steroidogenic cells of the endocrine and reproductive systems, epithelial cells of the digestive system, skeletal muscle and other organs. In summary, this transgenic Tspo-AcGFP mouse model recapitulates endogenous Tspo expression patterns and could be a useful, tractable tool for monitoring the transcriptional regulation and function of Tspo in live animal experiments.

The translocator protein (peripheral benzodiazepine receptor) mediates rat-selective activation of the mitochondrial permeability transition by norbormide

We have investigated the mechanism of rat-selective induction of the mitochondrial permeability transition (PT) by norbormide (NRB). We show that the inducing effect of NRB on the PT (i) is inhibited by the selective ligands of the 18kDa outer membrane (OMM) translocator protein (TSPO, formerly peripheral benzodiazepine receptor) protoporphyrin IX, N,N-dihexyl-2-(4-fluorophenyl)indole-3-acetamide and 7-chloro-5-(4-chlorophenyl)-1,3-dihydro-1-methyl-2H-1,4-benzodiazepin-2-one; and (ii) is lost in digitonin mitoplasts, which lack an intact OMM. In mitoplasts the PT can still be induced by the NRB cationic derivative OL14, which contrary to NRB is also effective in intact mitochondria from mouse and guinea pig. We conclude that selective NRB transport into rat mitochondria occurs via TSPO in the OMM, which allows its translocation to PT-regulating sites in the inner membrane. Thus, species-specificity of NRB toward the rat PT depends on subtle differences in the structure of TSPO or of TSPO-associated proteins affecting its substrate specificity.

Roles of mitochondrial benzodiazepine receptor in the heart

Mitochondrial benzodiazepine receptor (mBzR) is a type of peripheral benzodiazepine receptor that is located in the outer membrane of mitochondria. It is an 18-kDa protein that can form a multimeric complex with voltage-dependent anion channel (32 kDa) and adenine nucleotide translocator (30 kDa). mBzR is found in various species and abundantly distributed in peripheral tissues, including the cardiovascular system. The mitochondria are well known as the site of energy production, and the heart is the organ that highly requires this energy supply. In the past decades, it has been shown that mBzR plays a critical role in regulating mitochondrial and heart functions. A growing body of evidence demonstrates that mBzR is associated with regulation of mitochondrial respiration, mitochondrial membrane potential, apoptosis, and reactive oxygen species production. Moreover, mBzR has been suggested to play a role in alteration of physiological effects in the heart such as contractility and heart rate. mBzR is involved in the pathologic condition such as ischemia/reperfusion injury, responses to stress, and changes in electrophysiological properties and arrhythmogenesis. In this review, evidence of the roles of mBzR in the heart under both physiological and pathologic conditions is presented. Clinical studies regarding the use of pharmacologic intervention involving mBzR in the heart are also discussed as a possible target for the treatment of electrical and mechanical dysfunction in the heart.

Effects of 4'-chlorodiazepam on cellular excitation-contraction coupling and ischaemia-reperfusion injury in rabbit heart

AIMS

Recent evidence indicates that the activity of energy-dissipating ion channels in the mitochondria can influence the susceptibility of the heart to ischaemia-reperfusion injury. In this study, we describe the effects of 4'-chlorodiazepam (4-ClDzp), a well-known ligand of the mitochondrial benzodiazepine receptor, on the physiology of both isolated cardiomyocytes and intact hearts.

METHODS AND RESULTS

We used current- and voltage-clamp methods to determine the effects of 4-ClDzp on excitation-contraction coupling in isolated rabbit heart cells. At the level of the whole heart, we subjected rabbit hearts to ischaemia/reperfusion in order to determine how 4-ClDzp influenced the susceptibility to arrhythmias and contractile dysfunction. In isolated rabbit cardiomyocytes, 4-ClDzp evoked a significant reduction in the cardiac action potential that was associated with a decrease in calcium currents and peak intracellular calcium transients. In intact perfused normoxic rabbit hearts, 4-ClDzp mediated a dose-dependent negative inotropic response, consistent with the observation that 4-ClDzp was reducing calcium influx. Hearts that underwent 30 min of global ischaemia and 30 min of reperfusion were protected against reperfusion arrhythmias and post-ischaemic contractile impairment when 4-ClDzp (24 microM) was administered throughout the protocol or as a single bolus dose given at the onset of reperfusion. In contrast, hearts treated with cyclosporin-A, a classical blocker of the mitochondrial permeability transition pore, were not protected against reperfusion arrhythmias.

CONCLUSION

The findings indicate that the effects of 4-ClDzp on both mitochondrial and sarcolemmal ion channels contribute to protection against post-ischaemic cardiac dysfunction. Of clinical relevance, the compound is effective when given upon reperfusion, unlike other pre-conditioning agents.

Cardiovascular characterization of pyrrolo[2,1-d][1,5]benzothiazepine derivatives binding selectively to the peripheral-type benzodiazepine receptor (PBR): from dual PBR affinity and calcium antagonist activity to novel and selective calcium entry blockers

The synthesis and cardiovascular characterization of a series of novel pyrrolo[2,1-d][1,5]-benzothiazepine derivatives (54-68) are described. Selective peripheral-type benzodiazepine receptor (PBR) ligands, such as PK 11195 and Ro 5-4864, have recently been found to possess low but significant inhibitory activity of L-type calcium channels, and this property is implicated in the cardiovascular effects observed with these compounds. In functional studies both PK 11195 (1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxa mide) and Ro 5-4864 (4'-chlorodiazepam) did not display selectivity between cardiac and vascular tissue. Therefore, several 7-(acyloxy)-6-arylpyrrolo[2,1-d][1,5]benzothiazepines, potent and selective peripheral-type benzodiazepine receptor ligands recently developed by us (3, 7-20), were subjected to calcium channel receptor binding assay. Some of these compounds showed an unexpected potency in displacing the binding of [3H]nitrendipine from L-type calcium channels, much higher than that reported for PK 11195 and Ro 5-4864 and equal to or higher than that of reference calcium antagonists such as verapamil and (+)-cis-diltiazem. Specifically, in rat cortex homogenate, our prototypic PBR ligand 7-acetoxy-6-(p-methoxyphenyl)pyrrolo[2,1-d][1,5]benzothiazepine (3) showed an IC50 equal to 0.13 nM for inhibition of [3H]nitrendipine binding. Furthermore, in functional studies this compound displayed a clear-cut selectivity for cardiac over vascular tissue. Comparison of calcium antagonist activity on guinea pig aorta strips with the negative inotropic activity, determined by using isolated guinea pig left atria, revealed that 3 displayed higher selectivity than the reference (+)-cis-diltiazem. Thus, the pyrrolobenzothiazepine 3 might represent a new tool for characterizing the relationship between the PBR and cardiac function. Furthermore, we have also investigated the structural dependence of binding to PBR and L-type calcium channels, and this study allowed us to identify a new class of potent calcium channel blockers selective for cardiac over vascular tissue, with no affinity for PBR. A number of structure-activity relationship trends have been identified, and a possible explanation is advanced in order to account for the observed differences in selectivity. Three structural features, namely, (i) the saturation of the C(6)-C(7) double bond, with a consequent higher molecular flexibility, (ii) the presence of a substituent in the benzofused ring, and (iii) a basic side chain at C-10 of the pyrrolobenzothiazepine ring system, were found to be responsible for potent L-type calcium channel antagonism and clear-cut selectivity for cardiac over vascular tissue. Among the synthesized compounds the pyrrolobenzothiazepine 62 was found to be the most promising selective calcium channel blocker. Additionally, the molecular structure determination of the key intermediate 48 by X-ray diffraction, molecular modeling, and NMR analysis is reported.

Direct relaxant effects of intravenous anesthetics on airway smooth muscle

Ketamine, at concentrations achieved with the usual clinical doses, has a direct relaxant effect on airway smooth muscle (ASM). This study investigates the dose-dependent direct relaxation effects of midazolam and propofol on both proximal and distal ASM compared with ketamine. The proximal and distal airways were dissected from eight mongrel dogs and cut into 2-mm rings. The rings were attached to pressure transducers and equilibrated in a Krebs-Ringer bicarbonate bath kept at 37 degrees C, pH 7.4, CO2 37 mm Hg, and PaO2 > 100 mm Hg. Optimal length was determined, a dose-response curve to acetylcholine was established, and the 50% effective dose (ED50) of acetylcholine was calculated. Ketamine, midazolam, or propofol were given in random order to each ring preconstricted with ED50 of acetylcholine in cumulative log incremental doses from 10(-6) to 10(-4) M. Relaxation response was the tension during anesthetic equilibrium, expressed as a percentage of the tension from ED50 of acetylcholine. The drug vehicles were tested for their effects on the ASM. No bronchorelaxation was seen with any of the intravenous anesthetics at 10(-6) M. Ketamine 10(-5) M produced at 17.9% +/- 2.1% relaxation in the distal ASM but had no effect on the proximal ASM. Neither propofol nor midazolam affected the ASM at 10(-5) M. The distal ASM was significantly (P < 0.005) more sensitive to 10(-4) M of all three drugs compared with the proximal ASM. In the proximal ASM, 10(-4) M of ketamine, midazolam and propofol reduced ASM tension by 14.9% +/- 4.4%, 19.0% +/-8.8%, and 14.7% +/- 5.5%, respectively, versus 36.4% +/- 3.2%, 58.6% +/- 6.1%, and 64.4% +/- 9.0% in the distal ASM. The drug vehicles had no effect on the ASM. We conclude that ketamine, midazolam, and propofol have direct relaxant effects on ASM. All three intravenous anesthetics have a greater direct relaxant effect on distal ASM than on proximal ASM. Only ketamine showed significant direct bronchorelaxing effects at concentrations that are likely to be achieved with the usual clinical dosing patterns.

Hormonal modulation of benzodiazepines' actions on rat isolated uterus

Effects of various benzodiazepines were investigated in ovariectomized rat isolated uterus which had been chronically pre-treated with different female sex hormones: oestrogen, progesterone and oestrogen + progesterone. Uteri obtained from all groups developed a spontaneous, rhythmic activity. The spontaneous activity observed in control uterus was either inhibited in a concentration-dependent manner by diazepam, 4'-chlorodiazepam, clonazepam or 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxam ide (PK 11195), or was abolished in [Ca2+]o-free solution. Diazepam, 4'-chlorodiazepam, clonazepam and PK 11195 all caused a concentration-dependent relaxation of the [K+]o-pre-contracted uterus with the relative order of potency: PK 11195 > 4'-chlorodiazepam > diazepam > clonazepam. Administration of [Ca2+]o (1 microM to 10 mM) caused a concentration-dependent contraction of uterus, bathed in [Ca2+]o-free physiological salt solution obtained from different pre-treatment groups. Incubation with different concentrations (microM) of diazepam, 4'-chlorodiazepam, clonazepam and PK 11195 caused a decrease in response to [Ca2+]o-induced contraction in all groups of rat uteri. These results indicate that micromolar benzodiazepine binding sites exist in rat uterus. Diazepam, 4'-chlorodiazepam, clonazepam and PK 11195 caused relaxation of pre-contracted rat uterus and this effect may involve the inhibition of influx of [Ca2+]o and the relaxing effects of different benzodiazepines observed in this study can be modulated by pre-treatment with different female hormones.

PK 11195 antagonizes the positive inotropic response of the isolated rat heart to diazepam but not the negative inotropic response

The influence of the ligand PK 11195 (1-(2-chlorophenyl)-N-methyl-N-(1- methylpropyl)-3-isoquinolinecarboxamide), antagonist of the peripheral-type benzodiazepine receptor, on the inotropic response of the perfused rat heart to diazepam (7-chloro-5-phenyl-methyl- 1,3-dihydro-2H-1,4-benzodiazepin-2-one) was studied. Diazepam induced a positive inotropic response which was preceded by a transient negative inotropic response. Concentrations of 10(-7) M PK 11195 were ineffective, whereas concentrations of 10(-6) and 10(-5) M PK 11195 reduced the positive inotropic response significantly. At 5 x 10(-5) M PK 11195 the response was completely abolished. The negative inotropic response was not changed by either concentration of PK 11195 used. It is concluded that the positive inotropic response of the isolated rat heart to diazepam may well be mediated through peripheral-type benzodiazepine receptors; the negative inotropic response must be related to other (more complex) mechanisms.

Pharmacological relevance of peripheral type benzodiazepine receptors on motor nerve and skeletal muscle

1. Effects of agonists and antagonists of peripheral and central benzodiazepine receptors (pBZR and cBZR) on neuromuscular transmission were studied in mouse isolated phrenic nerve-diaphragm preparations. 2. Ro5-4864, a pBZR agonist, had no effect on the neuromuscular transmission but increased muscle contractility and antagonized the tetanic fade induced by neostigmine. 3. Ro5-4864 inhibited the regenerative tonic endplate depolarization caused by repetitive stimulation in the presence of neostigmine without affecting the amplitude and decay time of miniature and evoked single endplate potentials. 4. All the effects of Ro5-4864 were shared by PK11195, a pBZR antagonist, but not by clonazepam and flumazenil, a cBZR agonist and antagonist, respectively. 5. It is suggested that peripheral type benzodiazepine receptors modulate presynaptic function and muscle contraction.

Effects of Ca2+ channel antagonists and benzodiazepine receptor ligands in normal and skinned rat urinary bladder

The effects of Ca2+ channel antagonists and benzodiazepine receptor ligands against concentration-dependent contractions of rat urinary bladder induced by CaCl2 (0.1-50 mM, in K(+)-depolarized tissues), KCl (1-100 mM) and acetylcholine (0.1 microM to 1 mM) were studied. Nifedipine (0.001-0.1 microM), verapamil (0.01-1 microM), diltiazem (0.01-1 microM), cinnarizine (1-100 microM), and trifluoperazine (1-100 microM) each produced a concentration-related inhibition of the log concentration-effect curve for CaCl2. The rank order of potencies of these antagonists, measured as the IC50 against Ca2+ (25 mM)-induced contraction of depolarized bladder, was nifedipine (0.01 microM) > diltiazem (0.36 microM) approximately verapamil (0.41 microM) > or = cinnarizine (2.57 microM) > trifluoperazine (17.4 microM). These antagonists depressed KCl-induced contractions with an effectiveness and potency similar to that displayed against CaCl2-induced contractions. Nifedipine, verapamil, and diltiazem but not cinnarizine and trifluoperazine had a preferential inhibitory effect on the contractions elicited by KCl when compared to those elicited by acetylcholine. Ro 5-4864, diazepam, midazolam and the non-benzodiazepine PK 11195, each at 1-100 microM, depressed CaCl2- and KCl-induced contractions (IC50 values in the micromolar range). Benzodiazepines and PK 11195, all at 100 microM, markedly depressed acetylcholine-induced contractions. Flumazenil was scarcely effective. Cinnarizine (100 microM) and trifluoperazine (100 microM), but not the other Ca2+ channel antagonists and benzodiazepine receptor ligands tested, depressed Ca2+ (20 microM)-evoked contractions of skinned bladder. It is concluded that the action of nifedipine, verapamil, and diltiazem is restricted to the plasmalemma whereas cinnarizine and trifluoperazine also act on the intracellular contractile apparatus.(ABSTRACT TRUNCATED AT 250 WORDS)

Ro 5-4864 has a negative inotropic effect on human atrial muscle strips that is not antagonized by PK 11195

The effects of Ro 5-4864, a peripheral benzodiazepine receptor agonist (9 x 10(-6) M and 9 x 10(-5) M) and of PK 11195, a peripheral benzodiazepine receptor antagonist (3 x 10(-6) M and 3 x 10(-5) M), alone or together, on contraction parameters of human atrial muscle strips were studied. Atrial muscle strips were obtained from patients undergoing cardiac surgery. The strips were suspended in Krebs-Henseleit solution at 36.8 +/- 0.2 degrees C, connected to an isometric force transducer and then stimulated at 15 V above threshold and paced at 1.5 Hz. Ro 5-4864 at its higher concentration, alone or mixed with PK 11195 at both concentrations, depressed the contraction amplitude to 80% of the control value (P < 0.05). In conclusion, Ro 5-4864 had a small but significant depressant effect on the contraction amplitude of human atrial strips. Surprisingly, this effect was not reversed by the peripheral benzodiazepine receptor antagonist PK 11195.

Calcium induced modulation of peripheral-type benzodiazepine receptors in rat kidney membranes

The effects of Ca2+ ions on 3H-RO 5-4864 binding to the peripheral benzodiazepine receptor were examined. Preincubation of rat kidney membranes with Ca2+ at 37 degrees C produced a dose-dependent inhibition of 3H-RO 5-4864 binding. No inhibition was observed in membranes preincubated at 0 degrees C. The effect of Ca2+ was competitive in nature and was fully reversed by the addition of EGTA. At 1 mM, the maximal effect was achieved with CaCl2, whereas CoCl2 and CdCl2 had lesser effects. No other divalent cation salts examined decreased 3H-RO 5-4864 binding to rat kidney membranes. Collectively, these data demonstrate that the affinity of 3H-RO 5-4864 binding to rat kidney membranes is regulated by Ca2+ and suggest the presence of cation recognition binding sites coupled to the peripheral benzodiazepine receptor.

Peripheral-type mitochondrial binding sites for benzodiazepines in GH3 pituitary cells

Benzodiazepines (BZs) interact with two classes of high affinity binding sites, equilibrium dissociation constants in the nanomolar range, a neuronal or central-type and a non-neuronal or peripheral-type. The peripheral-type binding site has been shown to be present on the outer mitochondrial membrane and appears to be involved in regulation of cholesterol transport in steroid hormone-producing endocrine cells. In rat pituitary GH3 cells, BZs bind to receptors for thyrotropin-releasing hormone (TRH) and via interaction at a different site block Ca2+ influx through voltage-sensitive channels. These, however, are low affinity interactions occurring at micromolar BZ concentrations. Here, using [3H]Ro 5-4864, we report that GH3 cells also have high affinity peripheral-type BZ binding sites. Apparent equilibrium dissociation constants of 7.8 +/- 1.7 nM and 9.3 +/- 4.5 nM for [3H]Ro 5-4864 were measured with intact cells and isolated mitochondria, respectively. As predicted from studies of these sites in other cells, the order of potencies of BZs to displace [3H]Ro 5-4864 was Ro 5-4864 greater than diazepam (DZP) much greater than clonazepam (CIZP); chlordiazepoxide (CDE) did not affect binding. Nifedipine, a dihydropyridine antagonist of Ca2+ channels that has been shown to displace BZs from peripheral-type sites in other cells, was shown to be a competitive inhibitor of [3H]Ro 5-4864 binding with a half-effective concentration in the micromolar range. Ro 5-4864, however, had no effect on Ca2+ influx or efflux in mitochondria isolated from GH3 cells. Hence, GH3 cells exhibit mitochondrial, peripheral-type BZ binding sites but the role of these putative receptors in these neuroendocrine cells, which do not produce steroid hormones, is unclear.

Effect of benzodiazepine ligands on Ca2+ channel currents in Xenopus oocytes injected with rat heart RNA

Voltage-dependent Ca2+ channel (VDCC) currents have been measured in Xenopus oocytes injected with heart RNA purified from 7 day old rats, using voltage clamp technique. The currents were evoked by depolarizing voltage steps, using Ba2+ as charge carrier. Electrophysiological analysis of the current revealed two components: a slow, L-type, dihydropyridine (DHP)-sensitive current and a transient, DHP-insensitive, current. The benzodiazepine (BZ) ligands diazepam and Ro5-4864 decreased the current with micromolar affinity, with potency order of Ro5-4864 greater than diazepam greater than clonazepam. The central antagonist Ro15-1788 did not interfere with the effect of these drugs, thus excluding the possible involvement of the "central type" receptor. The slow current that was increased about 3 fold in the presence of 0.5 microM Bay K 8644, was less potentiated when previously treated with Ro5-4864 or diazepam. Current-voltage relation of the peak inward current and the steady state activation curve showed a small shift towards negative potentials in the presence of 50 microM diazepam. It is concluded that the benzodiazepine ligands block DHP-sensitive voltage dependent Ca2+ channels with a very marginal effect on the transient, DHP-insensitive current. Also it is emphasized that Xenopus oocytes can serve as a useful model system to study the pharmacology of these important drugs on cardiac Ca2+ channels.

Inhibition of Ca-channels by diazepam compared with that by nicardipine in pheochromocytoma PC12 cells

The effects of diazepam on voltage-gated Ca channels were studied in PC12 pheochromocytoma cells using whole-cell voltage-clamp techniques. An inward current activated by a depolarizing voltage step to +10 mV from a holding potential of -60 mV in 10.8 mM Ba was larger than that activated in 10.8 mM Ca. The Ba current was completely blocked by a low concentration of Cd (30 microM) and was also sensitive to nicardipine (100 nM to 10 microM). Diazepam (1-100 microM) inhibited the Ba current in a concentration-dependent manner. Neither diazepam nor nicardipine affected the current-voltage relationship or the dependence on holding potentials of the Ba current. Both slightly accelerated the inactivation time course of the Ba current. When diazepam was applied to the cells in combination with nicardipine, the observed inhibition agreed with a value predicted assuming independent blockade by diazepam and by nicardipine. These results suggest that diazepam inhibits Ca channels in a manner similar to nicardipine, but that the binding sites for diazepam are different from those for nicardipine.

Differential interactions between benzodiazepines and the dihydropyridines, nitrendipine and Bay K 8644

The effects of the dihydropyridine calcium antagonist, nitrendipine and the calcium channel activator, Bay K 8644, have been compared on the anaesthetic, ataxic and anticonvulsant effects of benzodiazepines. Possible interactions between the peripheral benzodiazepine receptor antagonist, PK11195, and the classical benzodiazepines were also examined. Nitrendipine considerably potentiated the anaesthetic effects of benzodiazepines and increased their ataxic effects but had no effect on the anticonvulsant actions. Clonazepam did not produce anaesthesia, at doses up to 1 g kg-1 or when given with nitrendipine. When given alone, nitrendipine did not cause general anaesthesia. Nitrendipine did not appear to alter the metabolism of midazolam. The calcium channel activator, Bay K 8644, reduced the anaesthetic potency of midazolam and, when given alone, produced ataxia. It did not significantly alter central concentrations of midazolam. The "peripheral" benzodiazepine antagonist, PK11195, did not affect the ataxic or anaesthetic actions of benzodiazepines. These results suggest that dihydropyridine-sensitive calcium channels may be more important to the general anaesthetic than to the anticonvulsant actions of benzodiazepines. The "peripheral" benzodiazepine site did not appear to play a role in either of these properties.

Influence of diazepam, alpidem, zolpidem and zopiclone, on the response to adenosine of the guinea pig isolated trachea

It has been reported that dipyridamole and some benzodiazepines potentiate the responses to adenosine in peripheral organs and in particular in the guinea pig isolated atria or trachea by inhibition of adenosine uptake and/or metabolism. In this study, we have examined the sensitization of guinea pig isolated trachea to relaxant responses to adenosine produced by dipyridamole, diazepam and 3 compounds chemically unrelated to benzodiazepines but which display selective agonistic activity towards the central (zolpidem and zopiclone) or peripheral (alpidem) type benzodiazpine receptors. In preparations under spontaneous tone and in the absence of adenosine, dipyridamole (10(-5) M) and diazepam (10(-5)-10(-4) M), alpidem (3 x 10(-6) M-10(-5) M) and zopiclone (10(-6)-10(-4) M) induced a relaxation of the airway smooth muscle. In addition, dazepam (10(-4) M) attenuated the phasic response to histamine (10(-5) M). Dipyridamole (10(-5) M) and diazepam (10(-4) M) respectively produced a 56.2 and 32.4-fold potentiation of adenosine relaxant effects. Alpidem (10(-6)-10(-5) M), zolpidem (10(-6)-10(-4) M) and zopiclone (10(-6)-10(-4) M) were without any significant effect on the adenosine concentration-response curves. Moreover, alpidem, zolpidem, and zopiclone did not modify the 2-chloroadenosine dose-response curves nor the diazepam induced sensitization of adenosine-induced relaxation. In conclusion, adenosine sensitization of the guinea pig isolated trachea caused by diazepam might involve a peripheral benzodiazpine receptor subtype coupled to a nucleoside transporter system which is different from those recognized by compounds derived from the imidazopyridine series.

Platelet peripheral benzodiazepine receptors in repeated stress

[3H]PK 11195 binding to platelet membranes and plasma stress hormones were studied in soldiers at the beginning of a parachute training course, following 6 days of preparatory exercises, and after the fourth actual parachute jump. A slight reduction (15%; NS) in the number of peripheral benzodiazepine receptors (PBR) was detected at the end of the exercise period, prior to the first jump. Reduced (26%; P less than 0.05) density of PBR was observed immediately after the repeated actual jumps. Equilibrium dissociation constants were not affected by the stressful situation. Plasma cortisol and prolactin levels remained unaltered during the entire study period.

Release of Ca2+ from heart and kidney mitochondria by peripheral-type benzodiazepine receptor ligands

1. The effect of the benzodiazepines Ro5-4864, AHN 086 and clonazepam on the release of Ca2+ from rat heart and kidney mitochondria was studied. 2. The peripheral-type benzodiazepines Ro5-4864 and AHN 086 induced Ca2+ release which was blocked by Mg2+ whereas the central-type benzodiazepine clonazepam was ineffective. 3. An associated collapse of membrane potential and swelling were also induced by AHN 086 in the presence of Ca2+. 4. However, no oxidation of pyridine nucleotides or increased rate or respiration were observed. 5. Release of Sr2+ was induced by AHN 086 in the absence of inorganic phosphate but not in its presence. 6. These data are discussed in the context of the current hypotheses on the mechanism of mitochondrial Ca2+ release.

Nitrendipine decreases benzodiazepine withdrawal seizures but not the development of benzodiazepine tolerance or withdrawal signs

1. The effects of the calcium channel blocking agent, nitrendipine, were studied on seizures in mice produced during withdrawal from chronic benzodiazepine treatment and on the development of tolerance to benzodiazepines. 2. Nitrendipine produced a dose-dependent decrease in seizure incidence, when seizures were produced by the partial inverse agonist FG7142 during withdrawal from seven days treatment with flurazepam. 3. Nitrendipine did not raise the seizure thresholds in naïve mice to the full inverse agonist methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), or to the gamma-aminobutyric acid (GABA) antagonist, bicuculline. 4. When given concurrently with flurazepam for seven days, nitrendipine did not affect the incidence of seizures during flurazepam withdrawal. 5. When given concurrently with the benzodiazepines, nitrendipine did not prevent the development of tolerance to midazolam general anaesthesia or tolerance to the ataxic actions of flurazepam or midazolam. 6. Chronic treatment with flurazepam for seven days did not affect the Kd or Bmax of [3H]-nimodipine binding in mouse whole brain or cerebral cortex. 7. These results with benzodiazepines are partially in contrast with those for ethanol, where nitrendipine not only decreased ethanol withdrawal seizures when given acutely, but also prevented the development of tolerance and withdrawal signs when given concurrently with ethanol. However, they do confirm the selectivity of nitrendipine for withdrawal-induced seizures.

Identification and distribution of peripheral benzodiazepine binding sites in male rat genital tract

In the present study we identified and characterized the distribution of high-affinity peripheral benzodiazepine binding sites (PBzS) in male rat vas deferens (whole, and prostatic and epididymal portions), prostate, seminal vesicles, and Cowper's glands. [3H]PK 11195, an isoquinoline carboxamide derivative, was used as a radioligand specific for PBzS. Scatchard analysis of saturation curves of [3H]PK 11195 binding in the whole vas deferens, the prostatic and epididymal portions of the vas deferens, the prostate, the seminal vesicles, and Cowper's glands yielded mean maximal numbers of binding sites of 1211 +/- 158, 1012 +/- 311, 1451 +/- 156, 1805 +/- 86, 865 +/- 51, and 2251 +/- 135 fmol/mg protein, respectively. The equilibrium dissociation constant values ranged between 1 and 3 mM in all the above tissues. The ability of various drugs to displace the specific binding of [3H]PK 11195 from PBzS in Cowper's gland membranes was also tested. The inhibition constants for Ro 5-4864, diazepam, and PK 11195 were 28, 330, and 4 nM, respectively, whereas clonazepam, Ro 15-1788, and testosterone were inefficient in displacing [3H]PK 11195. The presence of high densities of PBzS in the male genital tract suggests a functional role in these hormone-dependent organs.

Peripheral type benzodiazepine receptors and response to adenosine on the guinea-pig isolated trachea

It has been reported that dipyridamole, an adenosine uptake inhibitor, and some benzodiazepines potentiate the responses to adenosine in peripheral organs and in particular, on guinea-pig isolated atria. In this paper, we have examined the potentiation of responses to adenosine produced by dipyridamole, diazepam and four compounds with selective agonistic activity towards the central (clonazepam) or peripheral (Ro5-4864) type benzodiazepine receptors or antagonistic activity towards the central (flumazenil) or peripheral (PK 11195) benzodiazepine receptors in guinea-pig trachea in vitro. In preparations under basal tone and in the absence of adenosine, dipyridamole (10(-5) M) and benzodiazepines (10(-4) M) with the exception of flumazenil induced a relaxation of the airway smooth muscle. In addition, diazepam (10(-4) M) attenuated the phasic response to histamine (10(-5) M). Dipyridamole, and the benzodiazepine agonists diazepam, Ro5-4864 and clonazepam (10(-5) to 10(-4) M) produced potentiation of the tracheal response to adenosine, the rank order of potency being dipyridamole (pKi = 7.77 +/- 0.12, n = 8) greater than Ro5-4864 (pKi = 5.43 +/- 0.18, n = 6) greater than or equal to diazepam greater than clonazepam (pKi = 4.84 +/- 0.11, n = 6). The two benzodiazepine receptor antagonists, flumazenil and PK 11195, gave a significant but small potentiation to adenosine only at 10(-4) M. In the presence of dipyridamole (10(-5) M), diazepam (10(-4) M) did not cause any further potentiation to adenosine. Additionally, the potentiation produced by diazepam was not antagonised by flumazenil, whereas it was potently antagonised by PK 11195. Similarly, PK 11195 potently inhibited the adenosine potentiation produced by Ro5-4864.(ABSTRACT TRUNCATED AT 250 WORDS)

Relaxation of rat vascular muscle by peripheral benzodiazepine modulators

Effects of peripheral benzodiazepine receptor modulating drugs, Ro 5-4864 and PK 11195, on tension induced by K+ and the calcium agonist SDZ 202 791 (S isomer), were studied in rat caudal arteries. A significant reduction of tonic phase tension occurred with 30 nM PK 11195 or 3 microM Ro 5-4864, but decreases of the initial (first 3 min), phasic contraction were detected only at the highest concentrations of Ro 5-4864 and PK 11195. Protoporphyrin IX, the putative endogenous ligand of the peripheral benzodiazepine receptor, (at 10-100 nM) markedly increased the effectiveness of Ro 5-4864 and PK 11195 in reducing phasic contraction. Intracellular calcium localization and distribution in fura-2 loaded single vascular cells were quantitated using a high sensitivity, two-stage microchannel plate, photon-counting (PMI-VIM) camera. Peripheral benzodiazepines reduced intracellular calcium release from centrally located calcium pools, and this decrease of calcium release was potentiated by protoporphyrin IX. The decrease in intracellular calcium activity, which was more pronounced in the central regions where sarcoplasmic reticular elements are numerous, was probably the major mechanism of these vasodilator properties. Measurements of soluble guanylate cyclase activity also supported the intracellular Ca2+ release mechanism. Under conditions where protoporphyrin IX did not significantly stimulate guanylate cyclase, Ro 5-4864 alone or more effectively in combination with protoporphyrin IX stimulated cGMP production and caused relaxation. Guanylate cyclase forms a possible target for these benzodiazepine modulators, a hypothesis that merits further investigation.

The effect of diazepam on ventricular automaticity induced by a local injury. Evidence of involvement of "peripheral type" benzodiazepine receptors

The effect of diazepam on ectopic cardiac automaticity has been examined in rats. We also investigated whether "central" or "peripheral type" benzodiazepine receptors are involved, as well as the role of calcium, on the possible effect of diazepam, by studying the interaction of this drug with either GABA, picrotoxin, RO 15-1788, PK 11195, diltiazem or Bay K 8644. A local injury of the rat isolated right ventricle produced a sustained abnormal rhythm which was completely abolished by diazepam (30-50 microM). This effect was not modified by the presence of either GABA (100 microM) picrotoxin (2 microM) or RO 15-1788 (5 microM) but it was reduced by the antagonist of "peripheral type" benzodiazepine receptors PK 11195 (0.1 microM). On the other hand the calcium channel blocker diltiazem (5 microM) and the calcium channel activator Bay K 8644 (3 nM), respectively, potentiated and reduced the effect of diazepam. It is concluded that diazepam effectively reduces ectopic cardiac automaticity in the rat. The "central type" benzodiazepine receptors are not involved in this effect, but it seems to be, at least, partially mediated by "peripheral type" receptors and is a calcium-dependent phenomenon.

Properties of benzodiazepine binding sites in peripheral blood lymphocytes

Benzodiazepine (BDZ) binding sites were studied by using 3H-diazepam and 3H-Ro 5-4864 in intact lymphocytes from peripheral blood (PBL), in comparison to kidney and cerebellum. Experiments with 3H-diazepam performed at equilibrium and measuring kinetics revealed that BDZ binding sites are indeed present in rat PBL. The binding is saturable (Bmax 557 fmoles/10(6) cells), with high affinity (KD = 9.3 nM) and reversible. Specific binding sites are also observed by saturation experiments with 3H-Ro 5-4864 (Bmax 175 fmoles/10(6) cells, KD 2.2 nM). In addition, analysis of saturation isotherms obtained with 3H-diazepam indicates that BDZ binding sites are also present in human PBL. Scatchard plot of binding isotherms revealed an apparent single population of sites in all cases. The pharmacological characterization of BDZ binding sites in PBL, as compared with those of kidney and cerebellum, showed that these sites belong to the so-called "peripheral type."

High-affinity binding sites for PK 11195, but not for RO5-4864, in porcine aortic smooth muscle

Peripheral benzodiazepine (BZ) binding sites were characterized in porcine aortic smooth muscle membrane preparation. [3H]PK11195 bound with high affinity to the membranes (Kd = 8.6 + 0.9 nM), whereas [3H]Ro5-4864 bound slightly to the membranes. The Ki value of Ro5-4864 obtained from the inhibition of [3H]PK 11195 binding was 1200 + 200 nM, which was 480 times weaker than that obtained in rat kidney. Furthermore, the Ro5-4864 effect was temperature-insensitive. When [3H]PK 11195 binding was examined in porcine, human and rat platelets, Ro5-4864 inhibited the binding in porcine and human platelets one order of magnitude less potently than that in rat platelets. These results suggest that low affinity for Ro5-4864 in porcine aorta smooth muscle originates in porcine tissue, but not in smooth muscle.

Tissue specific regulation of "peripheral-type" benzodiazepine receptor density after chemical sympathectomy

The characteristics of [3H]Ro 5-4864 binding to "peripheral" benzodiazepine receptors (PBR) in the central nervous system and peripheral tissues were examined after chemical sympathectomy with 6-hydroxydopamine (6-OHDA). One week after the intracisternal administration of 6-OHDA, the number of [3H]Ro 5-4864 binding sites (Bmax) in the hypothalamus and striatum increased 41 and 50%, respectively, concurrent with significant reductions in catecholamine content. An increase (34%) in the Bmax of [3H]Ro 5-4864 to cardiac ventricle was observed one week after parenteral 6-OHDA administration. In contrast, the Bmax of [3H]Ro 5-4864 to pineal gland decreased 48% after 6-OHDA induced reduction in norepinephrine content. The Bmax values for [3H]Ro 5-4864 binding to other tissues (including lung, kidney, spleen, cerebral cortex, cerebellum, hippocampus and olfactory bulbs) were unaffected by 6-OHDA administration. The density of pineal, but not cardiac PBR was also reduced after reserpine treatment, an effect reversed by isoproterenol administration. These findings demonstrate that alterations in sympathetic input may regulate the density of PBR in both the central nervous system and periphery in a tissue specific fashion.

Benzodiazepine Ro 5-4864 increases coronary flow

Ro 5-4864 (chlorodiazepam) increased coronary flow in isolated retrograde perfused Langendorff rat heart preparations without affecting heart rate and left ventricular contractility (dP/dt). On the other hand Ro 5-4023 (clonazepam) produced very little effect. PK 11195 which has been shown to inhibit the binding of Ro 5-4864 to cardiac muscle did not antagonize this vasodilatory effect of Ro 5-4864 but increased coronary flow by itself. The data indicate a specific vasodilatory effect of certain benzodiazepines. The mechanism of action remains unknown.

Benzodiazepine interactions at neuronal and smooth muscle Ca2+ channels

Ro 5-4864 and PK 11195, ligands for the peripheral benzodiazepine binding site, blocked 45Ca2+ uptake into depolarized guinea-pig cortical synaptosomes. The fast phase of uptake was significantly more sensitive than the slow phase. The central benzodiazepine ligands, Ro 15-1788 and beta-CCE at 10(-4) and 5 X 10(-5) M respectively, were less effective and did not discriminate between fast and slow uptake phases. Ro 5-4864 and PK 11195 inhibited and potentiated respectively the binding of [3H]nitrendipine to synaptosomes by effects on the KD value. PK 11195 (10(-5) M), blocked K+ depolarization and Bay K 8644-induced tension responses in guinea-pig ileal longitudinal smooth muscle. Benzodiazepines do interact with voltage-dependent Ca2+ channels but the present data are insufficient to prove direct coupling of peripheral benzodiazepine sites with Ca2+ channels.

Affinity of various compounds for benzodiazepine binding sites in rat brain, heart and kidneys in vitro

Binding of several psychoactive, antiinflammatory, antihypertensive, and antiarrhythmic drugs to central and peripheral benzodiazepine (BZ) binding sites was studied in the brain, heart and kidneys of rats. Diazepam exhibited the highest affinity for all binding sites (Ki values at 0.01 microM level); another 1,4-BZ, oxazepam, had markedly lower affinity for peripheral binding sites (Ki 21-37 microM). Non-BZ compounds had low affinity for central BZ receptors; proquazone was the most potent (Ki 9.5 microM). The affinities of non-BZ compounds were higher for peripheral BZ binding sites. The Ki value for proquazone was approximately 0.1 microM; and many other antiinflammatory agents, and the vasodilators cyclandelate and nifedipine, produced Ki values in the micromolar level. beta-Blocking drugs, and several other antihypertensive and antiarrhythmic agents lacked affinity for both central and peripheral BZ binding sites. According to the results, the affinity for peripheral binding sites is independent of an affinity for central BZ receptors. Non-BZ compounds that bound to brain BZ receptors bound with equal affinity to both BZ1 and BZ2 subgroups of receptors. The compounds with affinity for peripheral BZ binding sites did not select between heart and kidneys, which suggests that these organs have similar binding sites. The role of the peripheral BZ binding sites has not yet been established. The findings of this study allow the selection of a more varied group of ligands to be used when investigating the physiological significance of these binding sites.