Functional beta(1)- and beta(2)-adrenoceptors in the left and right atrium of pre-hypertensive rats

There is a small increase in the functional beta(2)-adrenoceptor response on the spontaneously hypertensive rat (SHR) left atrium in the early stages of hypertension. In the present study, the functional beta(1)- and beta(2)-adrenoceptors of the left and right atrium in SHR pre-hypertension and age-matched (5-week-old) Wistar Kyoto (WKY) rats were characterized. Contractility methods with isoprenaline, T-0509 (a selective beta(1)-adrenoceptor agonist) and procaterol (a selective beta(2)-adrenoceptor agonist) were used. At 5 weeks, the SHRs were pre-hypertensive. Isoprenaline was more potent on the left atrium of 5-week-old SHRs than WKY rats. Bisoprolol, a selective beta(1)-adrenoceptor antagonist, was more potent against isoprenaline and T-0509 on the SHR than WKY rat left atrium. ICI 118,551, a selective beta(2)-adrenoceptor antagonist, was more potent against procaterol and T-0509 on the SHR than WKY rat left atrium. The results with bisoprolol and ICI 118,551 suggest that there are more functional beta(1)- and beta(2)-adrenoceptors on the left atrium of 5-week-old SHRs than WKY rats. Isoprenaline, T-0509 and procaterol were equipotent on the right atrium of 5-week-old WKY rats and SHRs. Bisoprolol was more potent against isoprenaline, T-0509 and procaterol on the SHR than WKY rat right atrium. ICI 118,551 was more potent against T-0509, but not isoprenaline and procaterol, on the SHR than WKY rat left atrium. This suggests there are more functional beta(1)-adrenoceptors, and probably more functional beta(2)-adrenoceptors, on the right atrium of 5-week-old SHRs than WKY rats. These functional differences in beta(1)-and beta(2)-adrenoceptor-mediated responses of the left and right atria of pre-hypertensive SHRs cannot be caused by hypertension, and may be associated with the onset of hypertension.

Abolition of (-)-CGP 12177-evoked cardiostimulation in double beta(1)/beta(2)-adrenoceptor knockout mice. Obligatory role of beta(1)-adrenoceptors for putative beta 4-adrenoceptor pharmacology

Some beta (1)- and beta (2)-adrenoceptor-blocking agents, such as (-)-CGP 12177, cause cardiostimulant effects at concentrations considerably higher than those that antagonise the effects of catecholamines. The cardiostimulant effects of these non-conventional partial agonists are relatively resistant to blockade by (-)-propranolol and have been proposed to be mediated through putative beta (4)-adrenoceptors or through atypical states of either beta (1)- or beta (2)-adrenoceptors. We investigated the effects of (-)-CGP 12177 on sinoatrial rate and left atrial contractile force as well as the ventricular binding of (-)-[H-3]CGP 12177 in tissues from wild-type, beta (2)-adrenoceptor knockout and beta (1)/beta (2)-adrenoceptor double knockout mice. The cardiostimulant effects of (-)-CGP 12177 were present in wildtype and beta (2)-adrenoceptor knockout mice but were absent in beta (1)/beta (2)-adrenoceptor double knockout mice. Thus, the presence of beta (1)-adrenoceptors is obligatory for the cardiostimulant effects of (-)-CGP 12177. It appears therefore that an atypical state of the beta (1)-adrenoceptor contributes to the mediation of the cardiostimulant effects induced by non-conventional partial agonists. Ventricular beta (1)- and beta (2)-adrenoceptors, labelled in wild-type with a K(D)similar to0.5 nmol/l (similar to 16 fmol/mg protein), were absent in beta (1)/beta (2)-adrenoceptor double knockout mice. However, a high density binding site (similar to 154-391 fmol/mg protein) that did not saturate completely (K(D)similar to 80-200 nM) was labelled by (-)-[H-3]CGP 12177 in the three groups of mice, being distinct from beta (1)- and beta (2)-adrenoceptors, as well as from the site mediating the agonist effects of(-)-CGP 12177.

Carvedilol blocks beta(2)- more than beta(1)-adrenoceptors in human heart

Objective: To understand the basis of the effectiveness of carvedilol in heart failure by determining its specific properties at human heart and beta(2)-adrenoceptors. Methods: The positive inotropic effects of noradrenaline (in the presence of the beta(2)-selective antagonist ICI118551) and adrenaline (in the presence of the beta(1)-selective antagonist CGP20712), mediated through beta(1)- and beta(2)-adrenoceptors, respectively, were investigated in atrial and ventricular trabeculae. The patch-clamp technique was used to investigate effects of noradrenaline and adrenaline on L-type Ca2+ current in human atrial myocytes. Results: Carvedilol was a 13-fold more potent competitive antagonist of the effects of adrenaline at 1 2-adrenoceptors (-logK(B) = 10.13 +/- 0.08) than of noradrenaline at beta(1)-adrenoceptors (-logK(B) = 9.02 +/- 0.07) in human right atrium. Chronic carvedilol treatment of patients with non-terminal heart failure reduced the inotropic sensitivity of atrial trabeculae to noradrenaline and adrenaline 5.6-fold and 91.2-fold, respectively, compared to beta(1)-blocker-treated patients, consistent with persistent preferential blockade of beta(2)-adrenoceptors. In terminal heart failure carvedilol treatment reduced 1.8-fold and 25.1-fold the sensitivity of right ventricular trabeculae to noradrenaline and adrenaline, respectively, but metoprolol treatment did not reduce the sensitivity to the catecholamines. Increases of current (I-Ca,I-L) produced by noradrenaline and adrenaline were not different in atrial myocytes obtained from non-terminal heart failure patients treated with metoprolol or carvedilol, consistent with dissociation of both beta-blockers from the receptors. Conclusions: Carvedilol blocks human cardiac beta(2)-adrenoceptors more than beta(1)-adrenoceptors, thereby conceivably contributing to the beneficial effects in heart failure. The persistent blockade of beta-adrenoceptors is attributed to accumulation of carvedilol in cardiac tissue. (c) 2005 European Society of Cardiology. Published by Elsevier B.V. All rights reserved.

Augmented muscle vasodilatory responses in obese children with Glu27 beta(2)-adrenoceptor polymorphism

This study examined forearm vasodilatation during mental challenge and exercise in 72 obese children (OC; age = 10 +/- 0.1 years) homozygous with polymorphism in the allele 27 of the beta(2)-adrenoceptors: Gln27 (n = 61) and Glu27 (n = 11). Forearm blood flow was recorded during 3 min of each using the Stroop color-word test (MS) and handgrip isometric exercise. Baseline hemodynamic and vascular measurements were similar. During the MS, peak forearm vascular conductance was significantly greater in group Glu27 (Delta = 0.35 +/- 0.4 vs. 0.12 +/- 0.1 units, respectively, p = .042). Similar results were found during exercise (Delta = 0.64 +/- 0.1 vs. 0.13 +/- 0.1 units, respectively, p = .035). Glu27 OC increased muscle vasodilatory responsiveness upon the MS and exercise.

(-)-CGP 12177 causes cardiostimulation and binds to cardiac putative beta(4)-adrenoceptors in both wild-type and beta(3)-adrenoceptor knockout mice

Some blockers of beta(1)- and beta(2)-adrenoceptors cause cardiostimulant effects through an atypical beta-adrenoceptor (putative beta(4)-adrenoceptor) that resembles the beta(3)-adrenoceptor. It is likely but not proven that the putative beta(4)-adrenoceptor is genetically distinct from the beta(3)-adrenoceptor. We therefore investigated whether or not the cardiac atypical beta-adrenoceptor could mediate agonist effects in mice lacking a functional beta(3)-adrenoceptor gene (beta(3)KO). (-)-CGP 12177, a beta(1)- and beta(2)-adrenoceptor blocker that causes agonist effects through both beta(3)-adrenoceptors and cardiac putative beta(4)-adrenoceptors, caused cardiostimulant effects that were not different in atria from wild-type (WT) mice and beta(3)KO mice. The effects of (-)-CGP 12177 were resistant to blockade by (-)-propranolol (200 nM) but were blocked by (-)-bupranolol (1 mu M) with an equilibrium dissociation constant of 15 nM in WT and 17 nM in beta(3)KO. (-)-[H-3]CGP 12177 labeled a similar density of the putative beta(4)-adrenoceptor in ventricular membranes from the hearts of both WT (B-max = 52 fmol/mg protein) and beta(3)KO (B-max = 53 fmol/mg protein) mice. The affinity of (-)-[H-3]CGP 12177 for the cardiac putative beta(4)-adrenoceptor was not different between WT (K-d = 46 nM) and beta(3)KO (K-d = 40 nM). These results provide definitive evidence that the cardiac putative beta(4)-adrenoceptor is distinct from the beta(3)-adrenoceptor.

Activation of beta(2)-adrenergic receptors hastens relaxation and mediates phosphorylation of phospholamban, troponin I, and C-protein in ventricular myocardium from patients with terminal heart failure

Background-Catecholamines hasten cardiac relaxation through beta-adrenergic receptors, presumably by phosphorylation of several proteins, but it is unknown which receptor subtypes are involved in human ventricle. We assessed the role of beta(1)- and beta(2)-adrenergic receptors in phosphorylating proteins implicated in ventricular relaxation. Methods and Results-Right ventricular trabeculae, obtained from freshly explanted hearts of patients with dilated cardiomyopathy (n=5) or ischemic cardiomyopathy (n=5), were paced at 60 bpm. After measurement of the contractile and relaxant effects of epinephrine (10 mu mol/L) or zinterol (10 mu mol/L), mediated through beta(2)-adrenergic receptors, and of norepinephrine (10 mu mol/L), mediated through beta(1)-adrenergic receptors, tissues were freeze clamped. We assessed phosphorylation of phospholamban, troponin I, and C-protein, as well as specific phosphorylation of phospholamban at serine 16 and threonine 17, Data did not differ between the 2 disease groups and were therefore pooled. Epinephrine, zinterol, and norepinephrine increased contractile force to approximately the same extent, hastened the onset of relaxation by 15+/-3%, 5+/-2%, and 20+/-3%, respectively, and reduced the time to half-relaxation by 26+/-3%, 21+/-3%, and 37+/-3%. These effects of epinephrine, zinterol, and norepinephrine were associated with phosphorylation (pmol phosphate/mg protein) of phospholamban 14+/-3, 12+/-4, and 12+/-3, troponin I 40+/-7, 33+/-7, and 31+/-6; and C-protein 7.2+/-1.9, 9.3 +/- 1.4, and 7.5 +/- 2.0. Phosphorylation of phospholamban occurred at both Ser16 and Thr17 residues through both beta(1)- and beta(2)-adrenergic receptors. Conclusions-Norepinephrine and epinephrine hasten human ventricular relaxation and promote phosphorylation of implicated proteins through both beta(1)- and beta(2)-adrenergic receptors, thereby potentially improving diastolic function.

Putative beta(4)-adrenoceptors in rat ventricle mediate increases in contractile force and cell Ca2+: comparison with atrial receptors and relationship to (-)-[H-3]-CGP 12177 binding

1 We identified putative beta(4)-adrenoceptors by radioligand binding, measured increases in ventricular contractile force by (-)-CGP 12177 and (+/-)-cyanopindolol and demonstrated increased Ca2+ transients by (-)-CGP 12177 in rat cardiomyocytes. 2 (-)-[H-3]-CGP 12177 labelled 13-22 fmol mg(-1) protein ventricular beta(1), beta(2)-adrenoceptors (pK(D) similar to 9.0) and 50-90 fmol mg(-1) protein putative beta(4)-adrenoceptors (pK(D) similar to 7.3). The affinity values (PKi) for (beta(1),beta(2)-) and putative beta(4)-adrenoceptors, estimated from binding inhibition, were (-)-propranolol 8.4, 5.7; (-)-bupranolol 9.7, 5.8; (+/-)-cyanopindolol 10.0,7.4. 3 In left ventricular papillary muscle, in the presence of 30 mu M 3-isobutyl-1-methylxanthine, (-)CGP 12177 and (+/-)-cyanopindolol caused positive inotropic effects, (pEC(50) (-)-CGP 12177, 7.6; (+/-)-cyanopindolol, 7.0) which were antagonized by (-)-bupranolol (pK(B) 6.7-7.0) and (-)-CGP 20712A (pK(B) 6.3-6.6). The cardiostimulant effects of(-)-CGP 12177 in papillary muscle, left and right atrium were antagonized by (+/-)-cyanopindolol (pK(i), 7.0-7.4). 4 (-)-CGP 12177 (1 mu M) in the presence of 200 nM (-)-propranolol increased Ca2+ transient amplitude by 56% in atrial myocytes, but only caused a marginal increase in ventricular myocytes. In the presence of 1 mu M 3-isobutyl-1-methylxanthine and 200 nM (-)-propranolol, 1 mu M (-)-CGP 12177 caused a 73% increase in Ca2+ transient amplitude in ventricular myocytes. (-)-CGP 12177 elicited arrhythmic transients in some atrial and ventricular myocytes. 5 Probably by preventing cyclic AMP hydrolysis, 3-isobutyl-1-methylxanthine facilitates the inotropic function of ventricular putative beta(4)-adrenoceptors. suggesting coupling to G(s) protein-adenylyl cyclase. The receptor-mediated increases in contractile force are related to increases of Ca2+ in atrial and ventricular myocytes. The agreement of binding affinities of agonists with cardiostimulant potencies is consistent with mediation through putative beta(4)-adrenoceptors labelled with (-)-[H-3]-CGP 12177.

Both beta(2)- and beta(1)-adrenergic receptors mediate hastened relaxation and phosphorylation of phospholamban and troponin I in ventricular myocardium of fallot infants, consistent with selective coupling of beta(2)-adrenergic receptors to G(s)-protein

Background-In adult human heart, both beta(1)- and beta(2)-adrenergic receptors mediate hastening of relaxation; however, it is unknown whether this also occurs in infant heart. We compared the effects of stimulation of beta(1)- and beta(2)-adrenergic receptors on relaxation and phosphorylation of phospholamban and troponin I in ventricle obtained from infants with tetralogy of Fallot. Methods and Results-Myocardium dissected from the right ventricular outflow tract of 27 infants (age range 2-1/2 to 35 months) with tetralogy of Fallot was set up to contract 60 times per minute. Selective stimulation of beta(1)-adrenergic receptors with (-)-norepinephrine (NE) and beta(2)-adrenergic receptors with (-)-epinephrine (EPI) evoked phosphorylation of phospholamban (at serine-16 and threonine-17) and troponin I and caused concentration-dependent increases in contractile force (-log EC50 [mol/L] NE 5.5+/-0.1, n=12; -EPI 5.6+/-0.1, n=13 patients), hastening of the time to reach peak force (-log EC50 [mol/L] NE 5.8+/--0.2; EPI 5.8+/-0.2) and 50% relaxation (-log EC50 [mol/L] NE 5.7+/-0.2: EPI 5.8+/-0.1), Ventricular membranes from Fallot infants, labeled with (-)-[I-125]-cyanopindolol, revealed a greater percentage of beta(1)- (71%) than beta(2)-adrenergic receptors (29%). Binding of (-)-epinephrine to beta(2)-receptors underwent greater GTP shifts than binding of (-)-norepinephrine to beta(1)-receptors. Conclusions-Despite their low density, beta(2)-adrenergic receptors are nearly as effective as beta(1)-adrenergic receptors of infant Fallot ventricle in enhancing contraction, relaxation, and phosphorylation of phospholamban and troponin I, consistent with selective coupling to G(s)-protein.

Effects of (-)-RO363 at human atrial beta-adrenoceptor subtypes, the human cloned beta(3)-adrenoceptor and rodent intestinal beta(3)-adrenoceptors

1 Chronic treatment of patients with beta-blockers causes atrial inotropic hyperresponsiveness through beta(2)-adrenoceptors, 5-HT4 receptors and H-2-receptors but apparently not through beta(1)-adrenoceptors despite data claiming an increased beta(1)-adrenoceptor density from homogenate binding studies. We have addressed the question of beta(1)-adrenoceptor sensitivity by determining the inotropic potency and intrinsic activity of the beta(1)-adrenoceptor selective partial agonist (-)-RO363 and by carrying out both homogenate binding and quantitative beta-adrenoceptor autoradiography in atria obtained from patients treated or not treated with beta-blockers. In the course of the experiments it became apparent that (-)-RO363 also may cause agonistic effects through the third atrial beta-adrenoceptor. To assess whether (-)-RO363 also caused agonistic effects through beta(3)-adrenoceptors we studied its relaxant effects in rat colon and guinea-pig ileum, as well as receptor binding and adenylyl cyclase stimulation of chinese hamster ovary (CHO) cells expressing human beta(3)-adrenoceptors. 2 beta-Adrenoceptors were labelled with (-)-[I-125]-cyanopindolol. The density of both beta(1)- and beta(2)-adrenoceptors was unchanged in the 2 groups, as assessed with both quantitative receptor autoradiography and homogenate binding. The affinities of (-)-RO363 for beta(1)-adrenoceptors (pK(i) = 8.0-7.7) and beta(2)-adrenoceptors (pK(i) = 6.1-5.8) were not significantly different in the two groups. 3 (-)-RO363 increased atrial force with a pEC(50) of 8.2 (beta-blocker treated) and 8.0 (non-beta-blocker treated) and intrinsic activity with respect to (-)-isoprenaline of 0.80 (beta-blocker treated) and 0.54 (non-beta-blocker treated) (P<0.001) and with respect to Ca2+ (7 mM) of 0.65 (beta-blocker treated) and 0.45 (non-beta-blocker treated) (P<0.01). The effects of (-)-RO363 were resistant to antagonism by the beta(2)-adrenoceptor antagonist, ICI 118,551 (50 nM). The effects of 0.3-10 nM (-)-RO363 were antagonized by 3-10 nM of the beta(1)-adrenoceptor selective antagonist CGP 20712A. The effects of 20-1000 nM (-)-RO363 were partially resistant to antagonism by 30-300 nM CGP 20712A. 4 (-)-RO363 relaxed the rat colon, partially precontracted by 30 mM KCl, with an intrinsic activity of 0.97 compared to (-)-isoprenaline. The concentration-effect curve to (-)-RO363 revealed 2 components, one antagonized by (-)-propranolol (200 nM) with pEC(50)=8.5 and fraction 0.66, the other resistant to (-)-propranolol (200 nM) with pEC(50)=5.6 and fraction 0.34 of maximal relaxation. 5 (-)-RO363 relaxed the longitudinal muscle of guinea-pig ileum, precontracted by 0.5 mu M histamine, with intrinsic activity of 1.0 compared to (-)-isoprenaline and through 2 components, one antagonized by (-)-propranolol (200 nM) with pEC(50)=8.7 and fraction 0.67, the other resistant to (-)-propranolol with pEC(50)=4.9 and fraction 0.33 of maximal relaxation. 6 (-)-RO363 stimulated the adenylyl cyclase of CHO cells expressing human beta(3)-adrenoceptors with pEC(50)=5.5 and intrinsic activity 0.74 with respect to (-)-isoprenaline (pEC(50)=5.9). (-)-RO363 competed for binding with [I-125]cyanopindolol at human beta(3)-adrenoceptors transfected into CHO cells with pK(i)=4.5. (-)-Isoprenaline (pk(i)=5.2) and (-)-CGP 12177A (pK(i)=6.1) also competed for binding at human beta(2)-adrenoceptors. 7 We conclude that under conditions used in this study, (-)-RO363 is a potent partial agonist for human beta(1)- and beta(3)-adrenoceptors and appears also to activate the third human atrial beta-adrenoceptor. (-)-RO363 relaxes mammalian gut through both beta(1)- and beta(3)-adrenoceptors. (-)-RO363, used as a beta(1)-adrenoceptor selective tool, confirms previous findings with (-)-noradrenaline that beta(1)-adrenoceptor mediated atrial effects are only slightly enhanced by chronic treatment of patients with beta-blockers. Chronic treatment with beta(1)-adrenoceptor-selective blockers does not significantly increase the density of human atrial beta(1)- and beta(2)-adrenoceptors.

beta(2)-Agonists and cAMP inhibit protein degradation in isolated chick (Gallus domesticus) skeletal muscle

1. The role of beta(2)-agonist and of cAMP in chick skeletal muscle proteolytic pathways and protein synthesis was investigated using an in vitro preparation that maintains tissue glycogen stores and metabolic activity for several hours.2. In extensor digitorum longus (EDL) muscle total proteolysis decreased by 15 to 20% in the presence of equimolar concentrations of epinephrine, clenbuterol, a selective beta(2)-agonist, or dibutyryl-cAMP. Rates of protein synthesis were not altered by clenbuterol or dibutyryl-cAMP.3. The decrease in the rate of total protein degradation induced by 10(-5) M clenbuterol was paralleled by a 44% reduction in Ca2+-dependent proteolysis, which was prevented by 10(-5) M ICI 118.551, a selective beta(2)-antagonist.4. No change was observed in the activity of the lysosomal, ATP-dependent, and ATP-independent proteolytic systems. Ca2+-dependent proteolytic activity was also reduced by 58% in the presence of 10(-4) M dibutyryl-cAMP or isobutylmethylxanthine.5. The data suggest that catecholamines exert an inhibitory control of Ca2+-dependent proteolysis in chick skeletal muscle, probably mediated by beta(2)-adrenoceptors, with the participation of a cAMP-dependent pathway.

The beta 2-Adrenoceptor Agonist Formoterol Improves Structural and Functional Regenerative Capacity of Skeletal Muscles From Aged Rat at the Early Stages of Postinjury

Skeletal muscles from old rats fail to completely regenerate following injury. This study investigated whether pharmacological stimulation of beta 2-adrenoceptors in aged muscles following injury could improve their regenerative capacity, focusing on myofiber size recovery. Young and aged rats were treated with a subcutaneous injection of beta 2-adrenergic agonist formoterol (2 mu g/kg/d) up to 10 and 21 days after soleus muscle injury. Formoterol-treated muscles from old rats evaluated at 10 and 21 days postinjury showed reduced inflammation and connective tissue but a similar number of regenerating myofibers of greater caliber when compared with their injured controls. Formoterol minimized the decrease in tetanic force and increased protein synthesis and mammalian target of rapamycin phosphorylation in old muscles at 10 days postinjury. Our results suggest that formoterol improves structural and functional regenerative capacity of regenerating skeletal muscles from aged rats by increasing protein synthesis via mammalian target of rapamycin activation. Furthermore, formoterol may have therapeutic benefits in recovery following muscle damage in senescent individuals.

Cardiac implications for the use of beta(2)-adrenoceptor agonists for the management of muscle wasting

There are proposals for the implementation of beta(2)-adrenoceptor agonists for the management of muscle wasting diseases. The idea has been initiated by studies in animal models which show that beta(2)-adrenoceptor agonists cause hypertrophy of skeletal muscle. Their use in clinical practice will also need an understanding of possible effects of activation of human heart beta(2)-adrenoceptors. Consequences could include an increased probability of arrhythmias in susceptible patients.

Murine ventricular L-type Ca2+ current is enhanced by zinterol via beta(1)-adrenoceptors, and is reduced in TG4 mice overexpressing the human beta(2)-adrenoceptor

1 The functional coupling of B-2-adrenoceptors (beta (2)-ARs) to murine L-type Ca2+ current (I-Ca(L)) was investigated with two different approaches. The beta (2)-AR signalling cascade was activated either with the beta (2)-AR selective agonist zinterol (myocytes from wild-type mice), or by spontaneously active, unoccupied beta (2)-ARs (myocytes from TG4 mice with 435 fold overexpression of human beta (2)-ARs). Ca2+ and Ba2+ currents were recorded in the whole-cell and cell-attached configuration of the patch- clamp technique, respectively. 2 Zinterol (10 muM) significantly increased I-Ca(L) amplitude of wild-type myocytes by 19+/-5%, and this effect was markedly enhanced after inactivation of Gi-proteins with pertussis-toxin (PTX; 76+/-13% increase). However, the effect of zinterol was entirely mediated by the beta (1)-AR subtype, since it was blocked by the beta (1)-AR selective antagonist CGP 20712A (300 nM). The beta (2)-AR selective antagonist ICI 118,551 (50 nM) did not affect the response of I-Ca(L) to zinterol. 3 In myocytes with beta (2)-AR overexpression I-Ca(L) was not stimulated by the activated signalling cascade. On the contrary, I-Ca(L) was lower in TG4 myocytes and a significant reduction of single-channel activity was identified as a reason for the lower whole-cell I-Ca(L). The beta (2)-AR inverse agonist ICI 118,551 did not further decrease I-Ca(L). PTX-treatment increased current amplitude to values found in control myocytes. 4 In conclusion, there is no evidence for beta (2)-AR mediated increases of I-Ca(L) in wild-type mouse ventricular myocytes. Inactivation of Gi-proteins does not unmask beta (2)-AR responses to zinterol, but augments beta (1)-AR mediated increases of I-Ca(L). In the mouse model of beta (2)-AR overexpression I-Ca(L) is reduced due to tonic activation of Gi-proteins.

Both alpha 1 and alpha 2-adrenoceptors mediate the cardiovascular responses to noradrenaline microinjected into the bed nucleus of the stria terminal of rats

Background and purpose: We have previously shown that noradrenaline microinjected into the bed nucleus of stria terminalis (BST) elicited pressor and bradycardiac responses in unanaesthetized rats. In the present study, we investigated the subtype of adrenoceptors that mediates the cardiovascular response to noradrenaline microinjection into the BST. Experimental approach: Cardiovascular responses following noradrenaline microinjection into the BST of male Wistar rats were studied before and after BST pretreatment with different doses of the selective alpha(1)-adrenoceptor antagonist WB4101, the alpha(2)-adrenoceptor antagonist RX821002, the combination of WB4101 and RX821002, the non-selective beta-adrenoceptor antagonist propranolol, the selective beta(1)-adrenoceptor antagonist CGP20712 or the selective beta(2)-adrenoceptor antagonist ICI118,551. Key results: Noradrenaline microinjected into the BST of unanaesthetized rats caused pressor and bradycardiac responses. Pretreatment of the BST with different doses of either WB4101 or RX821002 only partially reduced the response to noradrenaline. However, the response to noradrenaline was blocked when WB4101 and RX821002 were combined. Pretreatment with this combination also shifted the resulting dose-effect curve to the left, clearly showing a potentiating effect of this antagonist combination. Pretreatment with different doses of either propranolol or CGP20712 increased the cardiovascular responses to noradrenaline microinjected into the BST. Pretreatment with ICI118,551 did not affect cardiovascular responses to noradrenaline. Conclusion and implications: The present results indicate that alpha(1) and alpha(2)-adrenoceptors mediate the cardiovascular responses to noradrenaline microinjected into the BST. In addition, they point to an inhibitory role played by the activation of local beta(1)-adrenoceptors in the cardiovascular response to noradrenaline microinjected into the BST.