Beta-adrenergic receptors of the normal heart and in heart failure

The heart is often refereed to as an "beta-adrenergic organ" because beta-adrenergic agonists are powerful stimulants of cardiac contractility. Catecholamines acting through beta-adrenoceptors produce both positive inotropic and chronotropic effects in human heart. It is now generally accepted that in human heart both beta 1- and beta 2-adrenoceptors coexist. beta-Adrenergic transduction system consist of membrane-bound beta-receptors, the effector enzyme adenylyl cyclase and guanine nucleotide-binding transduction (G) proteins. Repeated long-lasting agonist stimulus evokes homologous or heterologous desensitization of transduction system. Chronic heart failure accompanies with decreased responsiveness to beta-adrenoceptor agonists and is thought to exacerbate the loss of cardiac contractility. Depending on the etiology of heart failure abnormalities of the beta-receptor-G protein-adenylyl cyclase system result from a reduced of beta 1-receptors, uncoupling of beta 1- or beta 2-receptors, alteration of G-protein function, or decreased catalytic subunit activity of adenylyl cyclase and enhanced expression of beta-adrenoceptor kinase. The model most widely used is that of circulating lymphocytes that contain a homogeneous population of beta 2-adrenoceptors. The biochemical and pharmacological properties of human lymphocyte beta 2-adrenoceptors are quite comparable to those of heart beta 2-receptors. The analysis of lymphocyte beta 2-adrenoceptor-adenylyl cyclase system can be used as a model for long-term regulation of human cardiac beta 1- and beta 2-adrenoceptors only if serial changes in response to administration of non-selective beta-adrenergic agonists or antagonists are being investigated. This review concentrates on beta-adrenoceptors in human healthy heart and in heart failure and also on lymphocyte beta 2-adrenoceptors and on the changes of these receptors properties under the influence of some cardiotropic drugs.     

Beta-receptor-mediated increase in venous return in humans

To assess the involvement of beta 1- and beta 2-receptors in the regulation of venous return in humans, changes in left ventricular end-diastolic (LVED) dimension were determined during beta-receptor stimulation either by exogenous catecholamines or by increased endogenous sympathetic activity after hydralazine, after placebo and during nonselective versus beta 1-selective blockade. Taking changes in heart rate and LV emptying into account, the three beta-agonists (isoproterenol, terbutaline, and epinephrine) as well as hydralazine increased venous return as inferred from LVED dimension. After hydralazine, nonselective and beta 1-selective blockade were equally effective in blunting the increases in venous return, in heart rate, and in positive inotropic response. Beta 1-Selective blockade did not affect the increase in heart rate caused by epinephrine and partially inhibited the positive inotropic effect and the increase in venous return. Nonselective blockade not only blocked the increase in venous return owing to epinephrine but actually led to a decrease, as evidenced by a decrease in LVED dimension despite the marked bradycardia and high afterload with this combination. The present findings in healthy humans indicate that stimulation of both beta 1- and beta 2-receptors increases venous return, heart rate, and myocardial contractility. Beta 1-Receptors appear to predominate in the response to neuronal sympathetic activity.     

Mammalian beta 1- and beta 2-adrenergic receptors. Immunological and structural comparisons

Beta 1- and beta 2-adrenergic receptors, pharmacologically distinct proteins, have been reported to be structurally dissimilar. In the present study three techniques were employed to compare the nature of mammalian beta 1- and beta 2-adrenergic receptors. Antibodies against each of the receptor subtypes were raised separately. Polyclonal antisera against beta 1-receptors of rat fat cells were raised in mice, and antisera against beta 2-receptors of guinea pig lung were raised in rabbits. Receptors purified from rat fat cells (beta 1-), S49 mouse lymphoma cells (beta 2-), and rat liver (beta 2-) were probed with these antisera. Each anti-receptor antisera demonstrated the ability to immunoprecipitate purified receptors of both beta 1- and beta 2- subtypes. The mobility of beta-receptors subjected to polyacrylamide gel electrophoresis was probed using antireceptor antibodies and nitrocellulose blots of the gels. Fat cell beta 1-adrenergic receptors display Mr = 67,000 under reducing conditions and Mr = 54,000 under nonreducing conditions, as previously reported (Moxham, C. P., and Malbon, C. C. (1985) Biochemistry 24, 6072-6077). Both beta 1- and beta 2-receptors displayed this same shift in electrophoretic mobility observed in the presence as compared to the absence of disulfide bridge-reducing agents, as detected both by autoradiography of the radiolabeled receptors and by immunoblotting of native receptors. Finally, isoelectric focusing of purified radioiodinated beta 1- and beta 2-adrenergic receptors revealed identical isoelectric points. These data are the first to provide analyses of immunological, structural, and biochemical features of beta 1- and beta 2-subtypes in tandem and underscore the structural similarities that exist between these pharmacologically distinct receptors.     

Regulation of diamine oxidase expression by beta 2-adrenoceptors in normal and hypertrophic rat kidney

The administration of preferential adrenergic receptor antagonists to uninephrectomized rats revealed the beta 2-adrenergic mediation in diamine oxidase activity increase that occurs in the remaining kidney undergoing compensatory hypertrophy. In fact, beta 1, beta 2- or beta 2, but not alpha 1-, alpha 2-, or beta 1-receptor-blocking agents prevented this enzyme enhancement. Further studies with adrenoceptor agonists, such as epinephrine (alpha 1, alpha 2, beta 1, beta 2), isoproterenol (beta 1, beta 2) or terbutaline (beta 2) showed that also in normal rat kidney diamine oxidase activity is under the control of catecholamine-beta 2-receptors through a mechanism that involves new synthesis of mRNA and protein. Theophylline, an inhibitor of phosphodiesterase, or forskolin, an activator of adenyl cyclase, increased diamine oxidase activity as does epinephrine or nephrectomy. Thus, catecholamine-triggered beta 2-receptors coupled to adenyl cyclase are involved in the regulation of diamine oxidase activity in normal and hypertrophic rat kidney.     

Adrenergic Receptors in Aging and Alzheimer''s Disease: Increased β2-Receptors in Prefrontal Cortex and Hippocampus

Loss of pigmented noradrenergic locus ceruleus neurons occurs in Alzheimer's disease (AD) and, to a lesser extent, in aging. We studied beta-adrenergic receptors and their subtypes, beta 1 and beta 2, by the specific binding of 125I-pindolol to particulate membrane preparations from prefrontal cortex, hippocampus, putamen, and cerebellum and to sections from frontal cortex by in vitro autoradiography. In prefrontal cortex from controls, numbers of total beta- and beta 2-adrenoceptors did not significantly correlate with age, but number of beta 1-adrenoceptors showed a weak but significant negative correlation. Binding in tissue particulate preparations to total beta-receptors did not reveal significant differences in samples from prefrontal cortex between AD subjects and age-matched controls. However, beta 1-adrenoceptors were decreased and beta 2-adrenoceptors were increased in number by approximately 30-50% in AD subjects. Thus, the relative ratio of beta 1-/beta 2-receptors was decreased in AD. Binding by in vitro receptor autoradiography performed in a subset of samples of frontal cortex also showed beta 2-adrenoceptors, and less consistently total beta- and beta 1-receptors, to be increased significantly in number in cortical laminae II, III, IV, and V of tissue sections from AD subjects. In these subjects, number of locus ceruleus cells and norepinephrine concentrations in putamen and frontal cortex were markedly reduced compared with values in controls. In the hippocampus, total beta- and both beta 2- and beta 1-adrenoceptors were increased in number in AD. In contrast, in the putamen, where beta 1-receptors predominate, total beta- and beta 1-receptors were significantly decreased in number with no consistent change in content of beta 2-receptors in AD. There were no significant changes in the cerebellum. Specific pindolol binding was not affected by interval between death and sampling of tissue at autopsy. Our results indicate selective changes in number of beta-receptors in AD. These changes in the cortex and hippocampus suggest receptor upregulation in response to noradrenergic deafferentation from the locus ceruleus or may simply reflect glial proliferation in AD.     

Action of agonists and antagonists on adrenergic receptors in isolated porcine coronary arteries

The adrenergic receptors of porcine coronary arteries were investigated in helically cut strips of small (less than or equal to 0.5 mm outer-diameter (od), medium (0.8-1.2 mm od), large (1.5-2.5 mm od), and very large (greater than 4 mm od) coronary arteries. Both the beta1 agonist dobutamine and the beta2 agonist terbutaline relaxed coronary arteries partially contracted by 25 mM of KCl. Dobutamine contracted small coronary arteries at 10(-5) M concentrations, then relaxed them at 10(-4) M. The beta1-adrenoceptor antagonist metoprolol contracted coronary arteries relaxed by either dobutamine or terbutaline, but the beta2 antagonist H35/25 did so only in high and probably nonselective concentrations. Alpha1-adrenoreceptor stimulating concentrations of phenylephrine did not contract any of the arteries. Metoprolol and high concentrations of H35/25 further contracted large coronary arteries partially contracted by 25 mM potassium. These contractions were blocked by verapamil and papaverine but not by atropine, phentolamine, yohimbine, mepyramine, or methysergide. This seems to indicate that beta-adrenergic receptors in porcine coronary arteries are beta1-receptors, or closely resemble beta1-receptors. They differ from many other beta1-receptors, however, in that they are stimulated by terbutaline. Alpha1 adrenoreceptors seem not to be present in these porcine coronary arteries to a significant extent. Metoprolol and high concentrations of H35/25 have a direct contractile effect in large porcine coronary artery that is not mediated by alpha-adrenergic, muscarinic, histaminergic, or serotonergic receptors but requires verapamil-sensitive calcium.     

Endothelial CB1-receptors limit infarct size through NO formation in rat isolated hearts

The aim of the present study was to document the presence of cannabinoid receptors in the rat heart, and to assess the cardioprotective properties of CB(1)- and CB(2)-receptor agonists. Rat isolated hearts were exposed to low-flow ischemia and reperfusion, with selective cannabinoid agonists administered prior to and during the ischemic period. In some hearts, RT-PCR, Western blots, and immunohistological techniques were used to identify and localize both cannabinoid-receptor subtypes. The effect of cannabinoids on infarct size was evaluated in additional hearts using TTC staining. Protein and mRNA for both CB(1)- and CB(2)-receptors were found in rat heart extracts. CB(1)-receptors were localized almost exclusively on arterial and capillary endothelial cells in intact hearts, whereas CB(2)-receptors appeared on cardiomyocytes and endothelial cells of larger arteries. Both the CB(1)-agonist, ACEA (50 nM), and the CB(2)-agonist, JWH015 (50 nM), reduced infarct size. However, only the cardioprotective effect of the CB(1)-agonist was blocked by the NO-synthase inhibitor, N(G)-nitro-L-arginine (30 microM). In conclusion, CB(1)-receptors are present mainly on endothelial cells in the rat heart, and exert their effect through production of NO. In contrast, CB(2)-receptors present on cardiomyocytes exert a cardioprotective effect independent of this endothelial factor.     

beta(1)-Receptors increase cAMP and induce abnormal Ca(i) cycling in the German shepherd sudden death model

We studied the role of beta-adrenergic receptor subtype signaling to cAMP and calcium in the genesis of catecholamine-dependent arrhythmias in German shepherd dogs that develop lethal arrhythmias at ~5 mo of age. There were three major findings in this study: 1) isoproterenol induces similar increases in cAMP in afflicted and control dogs exclusively through beta(1)-receptors (not beta(2)), 2) cells from afflicted dogs display prolonged relaxation kinetics at long cycle lengths and large frequent spontaneous calcium oscillations (and aftercontractions) with little increase in calcium transient amplitude in response to beta(1)-receptor agonists, and 3) beta(2)-receptor agonists induce a similar marked increases in calcium transient and twitch amplitude, with only rare spontaneous calcium oscillations in afflicted and control cells. These results indicate that catecholamines provide inotropic support to canine cardiomyocytes through distinct beta(1)- and beta(2)-receptor pathways with differing requirements for cAMP. The propensity to develop arrhythmias is not induced by beta(2)-receptors (or a rise in calcium alone), but rather occurs in the context of beta(1)-receptor activation of the cAMP-dependent pathway.     

Increase in cardiac P2X1-and P2Y2-receptor mRNA levels in congestive heart failure.

We wanted to study the expression of P2-receptors at the mRNA-level in the heart and if it is affected by congestive heart failure (CHF). To quantify the P2 receptor mRNA-expression we used a competitive RT-PCR protocol which is based on an internal RNA standard. The P2 receptor mRNA-expression was quantified in hearts from CHF rats and compared to sham-operated rats. Furthermore, the presence of receptor mRNA was studied in the myocardium from patients with heart failure. In the sham operated rats the G-protein coupled P2Y-receptors were expressed at a higher level than the ligand gated ion-channel receptor (P2X1). Among the P2Y-receptors the P2Y6-receptor was most abundantly expressed (P2Y6 > P2Y1 > P2Y2 = P2Y4 > P2X1). A prominent change was seen for the P2X1- and P2Y2-receptor mRNA levels which were increased 2.7-fold and 4.7-fold respectively in the myocardium from the left ventricle of CHF-rats. In contrast, the P2Y1-, P2Y4- and P2Y6-receptor mRNA levels were not significantly altered in CHF rats. In human myocard the P2X1-, P2Y1-, P2Y2-, P2Y6- and P2Y11-receptors were detected by RT-PCR in both right and left atria and ventricles, while the P2Y4-receptor band was weak or absent. In conclusion, most of the studied P2-receptors were expressed in both rat and human hearts. Furthermore, the P2X1- and P2Y2-receptor mRNA were upregulated in CHF, suggesting a pathophysiological role for these receptors in the development of heart failure.     

beta-Adrenergic receptors in rabbit liver plasma membranes. Predominance of beta 2-receptors and mediation of adrenergic regulation of hepatic glycogenolysis

[3H]Dihydroalprenolol was used to study beta-adrenergic binding sites in plasma membranes isolated from rabbit liver. Specific binding was measured at 25 degrees C as the difference between total binding and binding in the presence of 2 microM dl-propranolol or 10 microM l-isoproterenol. Binding was saturable and stereoselective. The maximum number of binding sites (Bmax) was 434 +/- 41 fmol/mg of protein. The Kd for this binding as determined by Scatchard analysis was 1.39 +/- 0.09 nM. This value agreed well with the Kd value (1.27 +/- 0.12 nM) determined by kinetic analysis. The potency order for the displacement of bound [3H]dihydroalprenolol was isoproterenol greater than epinephrine greater than norepinephrine, indicative of beta 2-receptors. Use of beta 1- and beta 2-subtype-selective inhibitors also supported the interpretation that the binding characteristics are those of beta 2-receptors. Computer-aided analysis of this inhibition indicated that the beta-receptors in this membrane are predominantly, if not exclusively, of the beta 2-subtype. That these receptors are responsible for mediating catecholamine stimulation of hepatic glycogenolysis was deduced from the inhibition of agonist-stimulated glycogenolysis, in isolated hepatocytes, by beta-receptor subtype-selective antagonists. Thus, the hydrochloride of (t-butylamino-3-ol-2-propyl)oximino-9 fluorene, a beta-antagonist which has higher affinity at beta 2-sites than at beta 1-sites, was 3 orders of magnitude more potent in inhibiting isoproterenol-stimulated glycogenolysis than either atenolol or practolol, both of which are beta 1-selective antagonists. These results resemble the inhibition of [3H]dihydroalprenolol binding in plasma membranes. The glycogenolytic effects of catecholamines occurred with the potency order isoproterenol greater than epinephrine greater than norepinephrine. Thus, both by radioligand binding studies and by metabolic studies, the functional adrenergic receptor in the rabbit liver is shown to be of the beta 2-subtype.     

alpha 1- and beta 2-adrenergic receptor expression in the Madin-Darby canine kidney epithelial cell line

The Madin-Darby canine kidney (MDCK) cell line, derived from distal tubule/collecting duct, expresses differentiated properties of renal tubule epithelium in culture. We studied the expression of adrenergic receptors in MDCK to examine the role of catecholamines in the regulation of renal function. Radioligand-binding studies demonstrated, on the basis of receptor affinities of subtype-selective adrenergic agonists and antagonists, that MDCK cells have both alpha 1- and beta 2- adrenergic receptors. To determine whether these receptor types were expressed by the same cell, we developed a number of clonal MDCK cell lines. The clonal lines had stable but unique morphologies reflecting heterogeneity in the parent cell line. Some clones expressed only beta 2-adrenergic receptors and were nonmotile, whereas others expressed both alpha 1- and beta 2-receptors and demonstrated motility on the culture substrate at low cell densities. In one clone, alpha- and beta- receptor expression was stable for more than 50 passages. Catecholamine agonists increased phosphatidylinositol turnover by activating alpha- adrenergic receptors and cellular cyclic adenosine monophosphate accumulation by activating beta-adrenergic receptors. Guanine nucleotide decreased the affinity of isoproterenol for the beta 2- receptor but did not alter the affinity of epinephrine for the alpha 1- receptor. These results show that alpha 1- and beta 2-receptors can be expressed by a single renal tubular cell and that the two receptors behave as distinct entities in terms of cellular response and receptor regulation. Heterogeneity of adrenergic receptor expression in MDCK clones may reflect properties of different types of renal tubule cells.     

Dopamine increases L-type calcium current more in newborn than adult rabbit cardiomyocytes via D1 and beta2 receptors

Dopamine is used to treat heart failure, particularly after cardiac surgery in infants, but the mechanisms of action are unclear. We investigated differences in the effect of dopamine on L-type calcium current (I(Ca)) between newborn (NB, 1-4 days) and adult (AD, 3-4 mo) rabbit ventricular myocytes. Myocytes were enzymatically dissociated from NB and AD rabbit hearts. I(Ca) was recorded by using the whole cell patch-clamp technique. mRNA levels of cardiac dopamine receptor type 1 (D1), type 2 (D2), and beta-adrenergic receptors (beta-ARs) were measured by real-time RT-PCR. Dopamine (100 microM) increased I(Ca) more in NB (E(max) 87 +/- 10%) than in AD ventricular cells (E(max) 21 +/- 3%). Further investigation of this difference showed that mRNA levels of the D1 receptor were significantly higher in NB, and, with beta-AR blockade, dopamine increased I(Ca) more in NB than AD cells. Additionally, SKF-38393 (selective D1 receptor agonist) significantly increased I(Ca) by 55 +/- 4% in NB (P < 0.05, n = 4) and by 11 +/- 1% in AD (P < 0.05, n = 6). Dopamine in the presence of SCH-23390 (D1 receptor antagonist) increased I(Ca) in NB cells by 67 +/- 5% and by 22 +/- 2% in AD cells, suggesting a role for beta-AR stimulation. Selective blockade of beta(1)- or beta(2)-receptors (with block of D1 receptors) showed that the beta-AR action of dopamine in the NB was largely mediated via beta(2)-AR activation. Dopamine produces a larger increase in I(Ca) in NB cardiomyocytes compared with ADs. The mechanism of action is not only through beta(2)-ARs but also due to higher expression of cardiac D1 receptor in NB.     

Distribution of mRNA and binding sites of adrenoceptors and muscarinic receptors in the rat heart

Since there exist some obscurities in the expression of mRNAs and their receptors in the heart, we have investigated the gene expression (mRNA levels) of adrenoceptors (alpha1A-, alpha1B-, beta1-, beta2-, beta3-) and muscarinic receptors (M2) and the density of receptor binding sites (alpha1A-, alpha1B-, beta1-, beta2-adrenoceptors, muscarinic receptors). Moreover, the heart regions consist of tissue rich in ganglion cells (that are of importance in heart neural circuits) and those virtually free of them (myocytes). Therefore, we have examined the differences in the distribution of mRNAs/receptor binding sites in the atrial samples of the heart rich in ganglion cells vs. those are virtually free of them. Binding sites and mRNAs of muscarinic receptors and alpha1B-adrenoceptors differ in their distribution in different heart regions. The mRNAs for beta1- and beta2-adrenoceptors were almost equally distributed herein, while the amount of beta-adrenoceptors significantly differs in the heart regions. The alpha1A- and beta3-adrenoceptors mRNAs were also found in all investigated heart regions, but at significantly lower level and have not shown region differences. This is a new finding, especially to beta3-adrenoceptors, as they were not regularly found in each heart regions. alpha1B-adrenoceptors have similar distribution of their mRNAs and binding sites in some heart parts. Thus, we can conclude that there are noticeable differences in the presence of receptors in heart regions that contain ganglion cells in comparison to those are virtually free of them.     

Differential cardioprotective/cardiotoxic effects mediated by beta-adrenergic receptor subtypes

Recent data suggest that beta-adrenergic receptor subtypes couple differentially to signaling pathways regulating cardiac function vs. cardiac remodeling. To dissect the roles of beta1- vs. beta2-receptors in the pathogenesis of cardiomyopathy, doxorubicin was administered to beta1, beta2, and beta1/beta2 knockout (-/-) and wild-type mice. Expression and activation of MAPKs were measured. Wild-type and beta1-/- mice showed no acute cardiovascular effects, whereas beta2-/- mice all died within 30 min. The additional deletion of the beta1-receptor (beta1/beta2-/-) totally rescued this toxicity. beta2-/- mice developed decreased contractile function, hypotension, QTc prolongation, and ST segment changes and a 20-fold increase in p38 MAPK activity not seen in the other genotypes. The MAPK inhibitor SB-203580 rescued beta2-/- mice from this acute toxicity. The enhanced toxicity in beta2-/- mice was also recapitulated in wild-type mice with the beta2-selective antagonist ICI-118,551, although the rescue effect of the beta1-deletion was not recapitulated using the beta1-selective antagonist metoprolol or the nonselective beta-antagonist propranolol. These data suggest that beta2-adrenergic receptors play a cardioprotective role in the pathogenesis of cardiomyopathy, whereas beta1-adrenergic receptors mediate at least some of the acute cardiotoxicity of anthracyclines. Differential activation of MAPK isoforms, previously shown in vitro to regulate beta-agonist as well as doxorubicin cardiotoxicity, appears to play a role in mediating the differential effects of these beta-adrenergic receptor subtypes in vivo.     

Myotoxic effects of clenbuterol in the rat heart and soleus muscle.

Myocyte-specific necrosis in the heart and soleus muscle of adult male Wistar rats was investigated in response to a single subcutaneous injection of the anabolic beta(2)-adrenergic receptor agonist clenbuterol. Necrosis was immunohistochemically detected by administration of a myosin antibody 1 h before the clenbuterol challenge and quantified by using image analysis. Clenbuterol-induced myocyte necrosis occurred against a background of zero damage in control muscles. In the heart, the clenbuterol-induced necrosis was not uniform, being more abundant in the left subendocardium and peaking 2.4 mm from the apex. After position (2.4 mm from the apex), dose (5 mg clenbuterol/kg), and sampling time (12 h) were optimized, maximum cardiomyocyte necrosis was found to be 1.0 +/- 0.2%. In response to the same parameters (i.e., 5 mg of clenbuterol and sampled at 12 h), skeletal myocyte necrosis was 4.4 +/- 0.8% in the soleus. These data show significant myocyte-specific necrosis in the heart and skeletal muscle of the rat. Such irreversible damage in the heart suggests that clenbuterol may be damaging to long-term health.     

Salivary secretion during selective β-adrenoreceptor stimulation and blockade in the parotid gland of red kangaroos,<Emphasis Type="Italic"> Macropus rufus</Emphasis>

Intracarotid infusions of noradrenaline (0.3 nmol.kg(-1) x min(-1)) stimulated salivary fluid secretion and caused increases in salivary concentrations of protein, potassium. magnesium. chloride and phosphate, and decreases in bicarbonate. These effects of intracarotid noradrenaline were not reduced by simultaneous intracarotid infusion of phentolamine (3.0 nmol.kg(-1) x min(-1)) but were significantly greater than the responses accompanying intravenous noradrenaline infusion. Concomitant administration of the beta-antagonist, CGP20712A, were much more effective in blocking the noradrenaline-induced changes in salivary composition than equimolar infusions of the beta2-antagonist, ICI118551, thereby confirming the presence of beta1-adrenoreceptors. Intracarotid infusion of salbutamol at 0.6 nmol x kg(-1) x min(-1) and 6.0 nmol x kg(-1) x min(-1) caused increasing but qualitatively similar changes in salivary composition to intracarotid noradrenaline but was less effective than noradrenaline in augmenting salivary protein release. Equimolar intravenous infusions of salbutamol and noradrenaline were equally potent in altering salivary electrolyte concentrations but salbutamol by this route had less effect on protein release and fluid secretion. Concurrent intravenous and intracarotid infusions of beta1-(CGP) and beta2-(ICI) antagonists with intracarotid salbutamol showed that the beta2-antagonist was more potent than the beta1-antagonist by the intracarotid route thereby demonstrating the presence of glandular beta2-receptors and eliminating the possibility that the response to salbutamol was due totally by reflex increases in general sympathetic tone triggered by lowered blood pressure. It was concluded that the kangaroo parotid has functional beta1- and beta2-adrenoreceptor subtypes in endpieces whereas the data provide little support for either adrenoreceptor subtype being present in the excurrent duct system.     

Reversible effects of isoproterenol-induced hypertrophy on in situ left ventricular function in rat hearts

The aim of the present study was to evaluate specifically left ventricular (LV) function in rat hearts as they transition from the normal to hypertrophic state and back to normal. Either isoproterenol (1.2 and 2.4 mg.kg(-1).day(-1) for 3 days; Iso group) or vehicle (saline 24 microl.day(-1) for 3 days; Sa group) was infused by subcutaneous implantation of an osmotic minipump. After verifying the development of cardiac hypertrophy, we recorded continuous LV pressure-volume (P-V) loops of in situ ejecting hypertrophied rat hearts. The curved LV end-systolic P-V relation (ESPVR) and systolic P-V area (PVA) were obtained from a series of LV P-V loops in the Sa and Iso groups 1 h or 2 days after the removal of the osmotic minipump. PVA at midrange LV volume (PVA(mLVV)) was taken as a good index for LV work capability (13, 15, 20, 21). However, in rat hearts during remodeling, whether PVA(mLVV) is a good index for LV work capability has not been determined yet. In the present study, in contrast to unchanged end-systolic pressure at midrange LV volume, PVA(mLVV) was significantly decreased by isoproterenol treatment relative to saline; however, these measurements were the same 2 days after pump removal. Simultaneous treatment with a beta(1)-blocker, metoprolol (24 mg.kg(-1).day(-1)), blocked the formation of cardiac hypertrophy and thus PVA(mLVV) did not decrease. The reversible changes in PVA(mLVV) reflect precisely the changes in LV work capability in isoproterenol-induced hypertrophied rat hearts mediated by beta(1)-receptors. These results indicate that the present approach may be an appropriate strategy for evaluating the effects of antihypertrophic and antifibrotic modalities.     

Adrenergic receptor density in brown adipose tissue of active and hibernating hamsters and ground squirrels

The ligand-binding characteristics (B(max) and K(D)) of alpha(1)- and beta(1)/beta(2)-adrenoceptors were investigated in membranes prepared from brown adipose tissue (BAT) of warm-acclimated, cold-acclimated, hibernating and arousing ground squirrels (Spermophillus undulatus) and hamsters (Mesocricetus auratus) by specific binding of [(3)H]prazosin and [(3)H]CGP-12177, respectively. The physiological state did not change the affinity for the adrenoceptors in the BAT of ground squirrels and hamsters. There was a significant decrease in alpha(1)-receptor density in arousing ground squirrels and a significant decrease in beta(1)/beta(2) density in hibernating ground squirrels. The level of alpha(1)-receptors was in all conditions higher than that of beta(1)/beta(2) receptors. The results indicate a possible change in balance of adrenoceptor density in the processes of cold acclimation, hibernation and arousal. The balance between the various adrenoceptor subtypes may be important for the final effect of catecholamines in BAT in different physiological states.     

Beta-blockade reduces tidal volume during heavy exercise in trained and untrained men

The effects of beta-blockade on tidal volume (VT), breath cycle timing, and respiratory drive were evaluated in 14 endurance-trained [maximum O2 uptake (VO2max) approximately 65 ml X kg-1 X min-1] and 14 untrained (VO2max approximately 50 ml X kg-1 X min-1) male subjects at 45, 60, and 75% of unblocked VO2max and at VO2max. Propranolol (PROP, 80 mg twice daily), atenolol (ATEN, 100 mg once a day) and placebo (PLAC) were administered in a randomized double-blind design. In both subject groups both drugs attenuated the increases in VT associated with increasing work rate. CO2 production (VCO2) was not changed by either drug during submaximal exercise but was reduced in both subject groups by both drugs during maximal exercise. The relationship between minute ventilation (VE) and VCO2 was unaltered by either drug in both subject groups due to increases in breathing frequency. In trained subjects VT was reduced during maximal exercise from 2.58 l/breath on PLAC to 2.21 l/breath on PROP and to 2.44 l/breath on ATEN. In untrained subjects VT at maximal exercise was reduced from 2.30 l/breath on PLAC to 1.99 on PROP and 2.12 on ATEN. These observations indicate that 1) since VE vs. VCO2 was not altered by beta-adrenergic blockade, the changes in VT and f did not result from a general blunting of the ventilatory response to exercise during beta-adrenergic blockade; and 2) blockade of beta 1- and beta 2-receptors with PROP caused larger reductions in VT compared with blockade of beta 1-receptors only (ATEN), suggesting that beta 2-mediated bronchodilation plays a role in the VT response to heavy exercise.     

Presynaptic localization of histamine H3-receptors in rat brain.

The localization of histamine H3-receptors in subcellular fractions from the rat brain was examined in a [3H] (R) alpha-methylhistamine binding assay and compared with those of histamine H1- and adrenaline alpha 1- and alpha 2-receptors. Major [3H](R) alpha-methylhistamine binding sites with increased specific activities ([3H]ligand binding vs. protein amount) were recovered from the P2 fraction by differential centrifugation. Minor [3H](R)alpha-methylhistamine binding sites with increased specific activities were also detected in the P3 fraction. Further subfractionation of the P2 fraction by discontinuous sucrose density gradient centrifugation showed major recoveries of [3H](R)alpha-methylhistamine binding in myelin (MYE) and synaptic plasma membrane (SPM) fractions. A further increase in specific activity was observed in the MYE fraction, but the SPM fraction showed no significant increase in specific activity. Adrenaline alpha 2-receptors, the pre-synaptic autoreceptors, in a [3H] yohimbine binding assay showed distribution patterns similar to histamine H3-receptors. On the other hand, post-synaptic histamine H1- and adrenaline alpha 1-receptors were closely localized and distributed mainly in the SPM fraction with increased specific activity. Only a negligible amount was recovered in the MYE fraction, unlike the histamine H3- and adrenaline alpha 2-receptors.