Microsatellite mutation of type II transforming growth factor-beta receptor is rare in atherosclerotic plaques

A somatic mutation within a microsatellite polyA tract in the coding region of the type II transforming growth factor (TGF)-beta receptor gene was reported to occur in human atherosclerotic and restenotic lesions. This mutation occurs frequently in colorectal cancer with the replication error repair phenotype and results in loss of sensitivity to the growth inhibitory effects of TGF-beta in cells from the tumors. The mutation was proposed to account for the clonal expansion of vascular smooth muscle cells observed in atherosclerotic plaques, through loss of the growth inhibitory effect of TGF-beta. The frequency of the mutation and the extent of clonal expansion of the mutated cells have major implications for the mechanism of atherogenesis and therapeutic strategies. We analyzed a set of 22 coronary arterial and 9 aortic samples containing early to advanced atherosclerotic lesions for the mutation in the type II TGF-beta receptor polyA tract. Only 1 coronary arterial sample from an advanced lesion showed detectable amounts of the mutation, present at a low level (8% of the DNA sample). The data imply that the mutation occurs only at low frequency and is not a major mechanistic contributor to the development of atherosclerosis.

Tamoxifen effects on endothelial function and cardiovascular risk factors in men with advanced atherosclerosis

BACKGROUND

Tamoxifen and its analogues act as selective estrogen receptor modulators (SERMs) in women, with estrogen-like activities on some plasma cardiovascular risk factors (eg, lipoproteins). Effects of SERMs on men with coronary artery disease (CAD) have not been reported.

METHODS AND RESULTS

Thirty-one men with angiographically proven CAD were recruited; 16 were treated with tamoxifen (40 mg/d) for 56 days, and 15 were untreated. All the CAD patients were medicated with aspirin and an HMG-CoA reductase inhibitor for >/=6 weeks before entering the study. Ten men with angina-like symptoms but normal coronary arteries by angiography (NCA group) were also treated with tamoxifen. Blood samples were collected at days -7, 0, 7, 14, 21, 28, and 56 of treatment. Endothelium-dependent flow-mediated dilatation (ED-FMD) of the brachial artery was measured by high-resolution ultrasound at 5 visits. Tamoxifen caused an increase in %ED-FMD maximal at 28 days in the CAD group (2.1+/-0.3% to 7.5+/-0.7%; P<0.0001) and the NCA group (3.8+/-0.4% to 7.9+/-1.0%; P<0.0001), with no significant change in the untreated group. Tamoxifen also caused decreases in several plasma cardiovascular risk factors, including total cholesterol, triglycerides, lipoprotein(a), and fibrinogen. Except for the triglyceride response, these effects were similar to those reported for postmenopausal women treated with tamoxifen.

CONCLUSIONS

Tamoxifen substantially increased ED-FMD in men with CAD who were taking conventional medication. Together with the effects on risk factors, the data strongly support clinical evaluation of SERMs for the treatment of men with CAD.

Chronic cold exposure potentiates CRH-evoked increases in electrophysiologic activity of locus coeruleus neurons

BACKGROUND

Chronic stress exposure can produce sensitization of norepinephrine release in the forebrain in response to subsequent stressors. Furthermore, the increase in norepinephrine release in response to the stress-related peptide corticotropin-releasing hormone (CRH) is potentiated by prior chronic stress exposure. To explore possible mechanisms underlying these alterations in norepinephrine release, we examined the effect of chronic stress on the electrophysiologic activity of locus coeruleus (LC) neurons in response to centrally applied CRH.

METHODS

Single-unit recordings of LC neurons in halothane-anesthetized rats were used to compare the effect of intraventricular administration of CRH (0.3-3.0 microg) in control and previously cold-exposed (2 weeks at 5 degrees C) rats.

RESULTS

The CRH-evoked increase in LC neuron activity was enhanced following chronic cold exposure, without alteration in basal activity of LC neurons. The enhanced CRH-evoked activation was apparent at higher doses of CRH but not at lower ones, resulting in an increased slope of the dose-response curve for CRH in previously cold-exposed rats.

CONCLUSIONS

These data, in combination with previous data, suggest that the sensitivity of LC neurons to excitatory inputs is increased following chronic cold exposure. The altered functional capacity of LC neurons in rats after continuous cold exposure may represent an experimental model to examine the role of central noradrenergic neurons in anxiety and mood disorders.

Amphetamine-induced dopamine release in human ventral striatum correlates with euphoria

BACKGROUND

Studies in experimental animals have implicated the mesolimbic dopaminergic projections into the ventral striatum in the neural processes underlying behavioral reinforcement and motivated behavior; however, understanding the relationship between subjective emotional experience and ventral striatal dopamine (DA) release has awaited human studies. Using positron emission tomography (PET), we correlated the change in endogenous dopamine concentrations following dextroamphetamine (AMPH) administration with the associated hedonic response in human subjects and compared the strength of this correlation across striatal subregions.

METHODS

We obtained PET measures of [(11)C]raclopride specific binding to DA D2/D3 receptors before and after AMPH injection (0.3 mg/kg IV) in seven healthy subjects. The change in [(11)C]raclopride binding potential (DeltaBP) induced by AMPH pretreatment and the correlation between DeltaBP and the euphoric response to AMPH were compared between the anteroventral striatum (AVS; comprised of accumbens area, ventromedial caudate, and anteroventral putamen) and the dorsal caudate (DCA) using an MRI-based region of interest analysis of the PET data.

RESULTS

The mean DeltaBP was greater in the AVS than in the DCA (p <.05). The AMPH-induced changes in euphoria analog scale scores correlated inversely with DeltaBP in the AVS (r = -.95; p <.001), but not in the DCA (r =.30, ns). Post hoc assessments showed that changes in tension-anxiety ratings correlated positively with DeltaBP in the AVS (r =.80; p [uncorrected] <.05) and that similar relationships may exist between DeltaBP and emotion ratings in the ventral putamen (as were found in the AVS).

CONCLUSIONS

The preferential sensitivity of the ventral striatum to the DA releasing effects of AMPH previously demonstrated in experimental animals extends to humans. The magnitude of ventral striatal DA release correlates positively with the hedonic response to AMPH.

Dopamine-mediated regulation of striatal neuronal and network interactions

The dopaminergic system exerts dynamic modulation of glutamatergic afferent drive that is dependent on the temporal pattern of the dopaminergic input and the subtypes of striatal neurons affected. The differences in feedforward inhibition between striatal neurons comprising the direct and indirect output pathway confer distinct response-pattern differences in their respective targets,supporting brief bursts of activity in Type-I neurons but attenuating repetitive activity in Type-II cells. This temporal patterning is further modulated by NO-mediated signaling, and by tonic and phasic dopamine-mediated stimulation, which exerts preferential actions on indirect and direct output neurons, respectively. As a result,the striatal network is forced into state-dependent patterns of activity that differentially regulate muscle tone and voluntary motor activity via distinct output projections from the striatum.

The tonic/phasic model of dopamine system regulation and its implications for understanding alcohol and psychostimulant craving

All drugs of abuse have been shown to act either directly or indirectly by increasing dopamine neurotransmission within the limbic system. Thus, alcohol has been shown to increase dopamine transmission primarily by activating dopamine cell spike activity, whereas psychostimulants increase dopamine transmission by inhibiting the removal of dopamine from the synaptic space after its release. The spike-dependent release of dopamine that is modulated by drugs of abuse to lead to their rewarding actions has been termed the phasic dopamine response. In contrast, with repeated drug administration, dopamine will also accumulate in the extracellular space of the nucleus accumbens in concentrations too low to stimulate postsynaptic receptors, but of sufficient magnitude to activate dopamine release-inhibiting autoreceptors. In addition, the level of extracellular dopamine is proposed to be under the regulatory influence of cortico-accumbens afferents. This steady-state level of extrasynaptic dopamine has been termed the tonic dopamine response. In this paper it is proposed that several of the aspects of drug addiction, withdrawal and craving associated with the continued use of these drugs can be explained on the basis of their effects on tonic versus phasic dopamine system function. Thus, the increase in tonic dopamine levels that occurs with repeated drug administration would serve to oppose phasic dopamine release via stimulation of dopamine terminal autoreceptors, causing the subject to increase drug administration to restore the phasic response. Moreover, after withdrawal from the drugs, exposure to priming doses of drug or to drug-related stimuli are proposed to increase tonic dopamine levels, again triggering drug-seeking behavior in order to restore balance between the tonic and phasic dopamine systems. Therefore, one consequence of continued drug use is that these parameters of dopamine system function that normally serve to keep the system stable will enter into a new steady-state homeostasis, from which the system is particularly susceptible to destabilizing influences that may precipitate relapse.

Striatal nitric oxide signaling regulates the neuronal activity of midbrain dopamine neurons in vivo

A major component of the cortical regulation of the nigrostriatal dopamine (DA) system is known to occur via activation of striatal efferent systems projecting to the substantia nigra. The potential intermediary role of striatal nitric oxide synthase (NOS)-containing interneurons in modulating the efferent regulation of DA neuron activity was examined using single-unit recordings of DA neurons performed concurrently with striatal microdialysis in anesthetized rats. The response of DA neurons recorded in the substantia nigra to intrastriatal artificial cerebrospinal fluid (ACSF) or drug infusion was examined in terms of mean firing rate, percent of spikes fired in bursts, cells/track, and response to electrical stimulation of the orbital prefrontal cortex (oPFC) and striatum. Intrastriatal infusion of NOS substrate concurrently with intermittent periods of striatal and cortical stimulation increased the mean DA cell population firing rate as compared with ACSF controls. This effect was reproduced via intrastriatal infusion of a NO generator. Infusion of either a NOS inhibitor or NO chelator via reverse microdialysis did not affect basal firing rate but increased the percentage of DA neurons responding to striatal stimulation with an initial inhibition followed by a rebound excitation (IE response) from 40 to 74%. NO scavenger infusion also markedly decreased the stimulation intensity required to elicit an IE response to electrical stimulation of the striatum. In single neurons in which the effects of electrical stimulation were observed before and after drug delivery, NO antagonist infusion was observed to decrease the onset latency and extend the duration of the initial inhibitory phase induced by either oPFC or striatal stimulation. This is the first report showing that striatal NO tone regulates the basal activity and responsiveness of DA neurons to cortical and striatal inputs. These studies also indicate that striatal NO signaling may play an important role in the integration of information transmitted to basal ganglia output centers via corticostriatal and striatal efferent pathways.

Amphetamine withdrawal alters bistable states and cellular coupling in rat prefrontal cortex and nucleus accumbens neurons recorded in vivo

Repeated amphetamine administration is known to produce changes in corticoaccumbens function that persist beyond termination of drug administration. We have found previously that long-term alteration in dopamine systems leads to changes in gap junction communication, expressed as dye coupling, between striatal neurons. In this study, the cellular bases of amphetamine-induced changes were examined using in vivo intracellular recordings and dye injection in ventral prefrontal-accumbens system neurons of control and amphetamine-treated rats. Rats that had been withdrawn from repeated amphetamine displayed a significant increase in the incidence of dye coupling in the prefrontal cortex and nucleus accumbens, which persisted for up to 28 d after withdrawal. The increased coupling was limited to projection neurons in both prefrontal cortical and accumbens brain regions, as identified by their axonal trajectory or the absence of interneuron-selective immunocytochemical markers. These changes occurred with no substantial loss of tyrosine hydroxylase-immunoreactive terminals in these cortical and accumbens regions, ruling out dopamine degeneration as a precipitating factor. Previous studies showed that nitric oxide plays a role in the regulation of coupling; however, amphetamine-withdrawn rats had fewer numbers of neurons and processes that stained for nitric oxide synthase immunoreactivity. In amphetamine-treated rats, a higher proportion of cortical cells fired in bursts, and a larger proportion of accumbens and prefrontal cortical neurons exhibited bistable membrane oscillations. By increasing corticoaccumbens transmission, amphetamine withdrawal may lead to neuronal synchronization via gap junctions. Furthermore, this adaptation to amphetamine treatment persists long after the drug is withdrawn.

Estimation of systolic and diastolic free intracellular Ca2+ by titration of Ca2+ buffering in the ferret heart

Spectroscopic Ca(2+)-indicators are thought to report values of free intracellular Ca(2+) concentration ([Ca(2+)](i)) that may differ from unperturbed values because they add to the buffering capacity of the tissue. To check this for the heart we have synthesized a new (19)F-labelled NMR Ca(2+) indicator, 1, 2-bis-[2-bis(carboxymethyl)amino-4,5-difluorophenoxy]ethane ('4, 5FBAPTA'), with a low affinity (K(d) 2950 nM). The new indicator and four previously described (19)F-NMR Ca(2+) indicators 1,2-bis-[2-(1 - carboxyethyl)(carboxymethyl)amino - 5 - fluorophenoxy]ethane ('DiMe-5FBAPTA'), 1, 2-bis-[2-(1-carboxyethyl)(carboxymethyl)amino-4-fluorophenoxy]ethane ('DiMe-4FBAPTA'), 1, 2-bis-[2-bis(carboxymethyl)amino-5-fluorophenoxy]ethane ('5FBAPTA') and 1, 2-bis-[2-bis(carboxymethyl)amino-5-fluoro-4-methylphenoxy]ethane ('MFBAPTA'), with dissociation constants for Ca(2+) ranging from 46 to 537 nM, have been used to measure [Ca(2+)](i), over the range from less than 100 nM to more than 3 microM, in Langendorff-perfused ferret hearts (30 degrees C, pH 7.4, paced at 1.0 Hz) by (19)F-NMR spectroscopy. Loading hearts with indicators resulted in buffering of the Ca(2+) transient. The measured end-diastolic and peak-systolic [Ca(2+)](i) were both positively correlated with indicator K(d). The positive correlations between indicator K(d) and the measured end-diastolic and peak-systolic [Ca(2+)](i) were used to estimate the unperturbed end-diastolic and peak-systolic [Ca(2+)](i) by extrapolation to K(d)=0 (diastolic) and to K(d)=infinity (systolic) respectively. The extrapolated values in the intact beating heart were 161 nM for end-diastolic [Ca(2+)](i) and 2650 nM for peak-systolic [Ca(2+)](i), which agree well with values determined from single cells and muscle strips.

Ca2+ buffering in the heart: Ca2+ binding to and activation of cardiac myofibrils

The measurement of cardiac Ca(2+) transients using spectroscopic Ca(2+) indicators is significantly affected by the buffering properties of the indicators. The aim of the present study was to construct a model of cardiac Ca(2+) buffering that satisfied the kinetic constraints imposed by the maximum attainable rates of cardiac contraction and relaxation on the Ca(2+) dissociation rate constants and which would account for the observed effects of (19)F-NMR indicators on the cardiac Ca(2+) transient in the Langendorff-perfused ferret heart. It is generally assumed that the Ca(2+) dependency of myofibril activation in cardiac myocytes is mediated by a single Ca(2+)-binding site on troponin C. A model based on 1:1 Ca(2+) binding to the myofilaments, however, was unable to reproduce our experimental data, but a model in which we assumed ATP-dependent co-operative Ca(2+) binding to the myofilaments was able to reproduce these data. This model was used to calculate the concentration and dissociation constant of the ATP-independent myofilament Ca(2+) binding, giving 58 and 2.0 microM respectively. In addition to reproducing our experimental data on the concentration of free Ca(2+) ions in the cytoplasm ([Ca(2+)](i)), the resulting Ca(2+) and ATP affinities given by fitting of the model also provided good predictions of the Ca(2+) dependence of the myofibrillar ATPase activity measured under in vitro conditions. Solutions to the model also indicate that the Ca(2+) mobilized during each beat remains unchanged in the presence of the additional buffering load from Ca(2+) indicators. The new model was used to estimate the extent of perturbation of the Ca(2+) transient caused by different concentrations of indicators. As little as 10 microM of a Ca(2+) indicator with a dissociation constant of 200 nM will cause a 20% reduction in peak-systolic [Ca(2+)](i) and 30 microM will cause approx. 50% reduction in the peak-systolic [Ca(2+)](i) in a heart paced at 1.0 Hz.

Gating of information flow within the limbic system and the pathophysiology of schizophrenia

Although first thought of as a dopaminergic disorder, there is little direct evidence to support a primary pathology in the dopamine system as the etiological factor in schizophrenia. In contrast, evidence is amassing in support of a cortical disturbance in this disorder; one consequence of which is a disruption in the cortical regulation of subcortical dopamine systems. Our studies show that the hippocampus plays a major role in this interaction, in that, along with the dopamine system, it provides a gating influence over information flow from the prefrontal cortex at the level of the nucleus accumbens. Moreover, chemically-induced disruption of the development of the hippocampus and entorhinal cortex were found to lead to pathophysiological changes in these interactions in the limbic system of adult rats. Therefore, schizophrenia is proposed to be a developmentally-related disorder, in which disruption of the hippocampal influence over the limbic system during ontogeny results in a pathological alteration of corticoaccumbens interactions in the adult organism.

Modulation of basolateral amygdala neuronal firing and afferent drive by dopamine receptor activation in vivo

The basolateral amygdala (BLA) is implicated in responding to affective stimuli. Dopamine (DA) is released in the BLA during numerous conditions; however, the neurophysiological effects of DA in the BLA have not been examined in depth. In this study, the effects of DA receptor manipulation on spontaneous and afferent-driven neuronal firing were examined using in vivo extracellular single-unit recordings in parallel with systemic and iontophoretic drug application, and stimulation of the substantia nigra/ventral tegmental area in the rat. The effects of DA receptor activation in the BLA were found to depend on the characteristics of the BLA neuron examined, causing an increase in the firing rate of putative interneurons and a decrease in the firing of identified projection neurons. Additionally, DA receptor activation attenuated short-latency spikes evoked by electrical stimulation of prefrontal cortical and mediodorsal thalamic inputs to the BLA while potentiating the responses evoked by electrical stimulation of sensory association cortex. DA receptor activation can thus attenuate BLA projection neuron firing via two mechanisms: (1) by a direct inhibition, and (2) by indirect actions mediated via activation of BLA interneurons. This is hypothesized to lead to a global filtration of weaker inputs. Moreover, DA potentiates sensory inputs and attenuates medial prefrontal cortex inputs to the BLA. Conditions in which DA is released in the BLA, such as during the presentation of an affective stimulus, will lead to a potentiation of the strongest sensory input and a dampening of cortical inhibition over the BLA, thus augmenting the response to affective sensory stimuli.

The regulation of forebrain dopamine transmission: relevance to the pathophysiology and psychopathology of schizophrenia

Since the discovery that the therapeutic efficacy of antipsychotic drugs was significantly correlated to their ability to block dopamine D2 receptors, abnormal dopamine transmission in the forebrain has been postulated to underlie psychosis in schizophrenia. In the past 15 years, an impressive amount of clinical and basic research aimed at the study of schizophrenia has indicated that prefrontal and temporal cortical abnormalities may be more important in the etiology of many of the symptoms of schizophrenia, including psychosis. However, the cortical systems that appear to have structural and/or metabolic abnormalities in schizophrenia patients potently regulate forebrain dopamine transmission through a number of mechanisms. In turn, dopamine modulates excitatory transmission mediated by frontal and temporal cortical projections to the basal ganglia and other regions. The present review summarizes the multiple interactions between forebrain DA systems and frontal and temporal corticostriatal transmission. It then examines the role of these interactions in normal behaviors and the psychopathology of schizophrenia.

Alterations in electrophysiological activity and dye coupling of striatal spiny and aspiny neurons in dopamine-denervated rat striatum recorded in vivo

We recently reported that pharmacological manipulations of the dopamine system can produce more than a 4-fold increase in dye coupling between dopaminoceptive neurons in the adult rat striatal complex. During in vivo intracellular recordings, striatal neurons in control rats and in rats that had been treated with 6-hydroxydopamine were injected with either Lucifer yellow or Neurobiotin. Only rats that exhibited severe loss (i.e., larger than approximately 95%) of striatal dopamine terminals displayed a significant increase in the incidence of dye coupling between neurons in adult striatum. Moreover, this increased coupling was present only between neurons of the same morphological cell class, i.e., among clusters of spiny neurons or between aspiny neurons. Combining intracellular labeling of spiny neurons with parvalbumin immunocytochemistry demonstrated that coupling did not occur between anatomically adjacent neurons that comprised immunocytochemically and morphologically distinct cell classes. Therefore, gap junction conductance as reflected by dye coupling appears to undergo upregulation as a consequence of compromises in nigrostriatal and mesolimbic dopamine transmission.

Modulation of cell firing in the nucleus accumbens

Pennartz et al. have proposed that functions of the nucleus accumbens (NA) are subserved by the activity of ensembles of neurons rather than by an overall neuronal activation. Indeed, the NA is a site of convergence for a large number of inputs from limbic structures that may modulate the flow of prefrontal cortical information and contribute to defining such ensembles, as exemplified in the ability of hippocampal input to gate cortical throughput in the nucleus accumbens. NA neurons exhibit a bistable membrane potential, characterized by a very negative resting membrane potential (down state), periodically interrupted by plateau depolarizations (up state), during which the cells may fire in response to cortical inputs. A dynamic ensemble can be the result of a distributed set of neurons in their up state, determined by the moment-to-moment changes in the spatial distribution of hippocampal inputs responsible for transitions to the up state. Ensembles may change as an adaptation to the contextual information provided by the hippocampal input. Furthermore, for dynamic ensembles to be functionally relevant, the model calls for near synchronous transitions to the up state in a group of neurons. This can be accomplished by the cell-to-cell transfer of information via gap junctions, a mechanism that can allow for a transfer of slow electrical signals, including "up" events between coupled cells. Furthermore, gap junction permeability is tightly modulated by a number of factors, including levels of dopamine and nitric oxide, and cortical inputs, allowing for fine-tuning of this synchronization of up events. The continuous selection of such dynamic ensembles in the NA may be disputed in schizophrenia, resulting in an inappropriate level of activity of thalamocortical systems.

The Utility of Pacemaker Evoked T Wave Amplitude for the Noninvasive Diagnosis of Cardiac Allograft Rejection

Previous work suggested that pacemaker evoked T wave amplitude (ETWA) may be a sensitive noninvasive marker of cardiac allograft rejection. A Topaz QT sensing rate responsive pacemaker (Vitatron Medical) was implanted at transplantation using epicardial ventricular leads in 45 recipients (35 males; median age 51 years, range 20-63). The median duration of follow-up was 129 days (range 4-327). The ETWA at a paced rate of 100 beats/min was measured daily during hospitalization and at each outpatient attendance (900 readings). Endomyocardial biopsies were at routine intervals or when otherwise clinically indicated (257 biopsies with concurrent ETWA data). There were 58 episodes of rejection > or = grade 3a in 28 patients. The biopsies were classed as either no rejection (grade < 3a) or rejection requiring treatment (grade > or = 3a). The median normalized ETWA was 100.8% (range 24.6-239.7) without rejection and 89.9% (17.0-189.7) with rejection (Mann-Whitney U Test: P = 0.028). The performance of ETWA monitoring as a diagnostic test for the individual recipient was evaluated with exponentially weighted moving average quality control charts. For the diagnosis of all rejection episodes, ETWA monitoring had a sensitivity of 55%, a specificity of 62%, a positive predictive value of 30%, and negative predictive value of 83%. It is concluded that although analysis of pooled data showed a significant reduction in normalized ETWA with biopsy proven rejection, ETWA monitoring requires further refinement to improve sensitivity before it can be considered a clinically useful technique for the non-invasive diagnosis of cardiac allograft rejection in individual recipients.

Binding across time: the selective gating of frontal and hippocampal systems modulating working memory and attentional states

Temporal binding via 40-Hz synchronization of neuronal discharges in sensory cortices has been hypothesized to be a necessary condition for the rapid selection of perceptually relevant information for further processing in working memory. Binocular rivalry experiments have shown that late stage visual processing associated with the recognition of a stimulus object is highly correlated with discharge rates in inferotemporal cortex. The hippocampus is the primary recipient of inferotemporal outputs and is known to be the substrate for the consolidation of working memories to long-term, episodic memories. The prefrontal cortex, on the other hand, is widely thought to mediate working memory processes, per se. This article reviews accumulated evidence for the role of a subcortical matrix in linking frontal and hippocampal systems to select and "stream" conscious episodes across time (hundreds of milliseconds to several seconds). "Streaming" is hypothesized to be mediated by the selective gating of reentrant flows of information between these cortical systems and the subcortical matrix. The physiological mechanism proposed for this temporally extended form of binding is synchronous oscillations in the slower EEG spectrum (< 8 Hz).

Blockade of putative beta4- and beta1-adrenoceptors by carvedilol in ferret myocardium

The putative beta4-adrenoceptor mediates positive inotropic effects, action potential shortening and arrhythmias in ferret ventricle. Here we compared the affinity of carvedilol at the putative beta4-adrenoceptor and beta1-adrenoceptor, activated by (+/-)-CGP 12177 and (-)-isoprenaline, respectively. In paced right ventricular preparations, carvedilol (0.01-10 micromol/l) was a simple competitive antagonist of the positive inotropic effects of (+/-)-CGP 12177 (slope of Schild-plot = 1.02, pK(B) = 6.8) and (-)-isoprenaline (slope of Schild-plot = 0.98, pK(B) = 8.1). Carvedilol also blocked putative beta4- and beta1-adrenoceptors of left ventricle, left atrium and sino-atrial pacemaker. Carvedilol therefore interacts with the putative beta4-adrenoceptor according to the law of mass action and may provide a lead in the development of putative beta4-adrenoceptor-selective antagonists.