Determination of beta-adrenoceptor subtype on rat isolated ventricular myocytes by use of highly selective beta-antagonists

[Radioligand Method of Assessment of β-Adrenoceptor's Activity on Human T-Lymphocytes]

We proposed a new method of evaluation of beta-receptor's activity on the surface of human T-lymphocytes based on the radioligand method. Optimal conditions for evaluation of specific binding to β2-adrenoceptors of 0.5 fmol ligand per 1 million cells using [125I]-cyanopindolol were found. The possibility of using of β2-adrenoceptor's activity assessment in clinical settings was demonstrated on human T-lymphocyte.

Paroxetine-mediated GRK2 inhibition reverses cardiac dysfunction and remodeling after myocardial infarction

Heart failure (HF) is a disease of epidemic proportion and is associated with exceedingly high health care costs. G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptor (GPCR) kinase 2 (GRK2), which is up-regulated in the failing human heart, appears to play a critical role in HF progression in part because enhanced GRK2 activity promotes dysfunctional adrenergic signaling and myocyte death. Recently, we found that the selective serotonin reuptake inhibitor (SSRI) paroxetine could inhibit GRK2 with selectivity over other GRKs. Wild-type mice were treated for 4 weeks with paroxetine starting at 2 weeks after myocardial infarction (MI). These mice were compared with mice treated with fluoxetine, which does not inhibit GRK2, to control for the SSRI effects of paroxetine. All mice exhibited similar left ventricular (LV) dysfunction before treatment; however, although the control and fluoxetine groups had continued degradation of function, the paroxetine group had considerably improved LV function and structure, and several hallmarks of HF were either inhibited or reversed. Use of genetically engineered mice indicated that paroxetine was working through GRK2 inhibition. The beneficial effects of paroxetine were markedly greater than those of β-blocker therapy, a current standard of care in human HF. These data demonstrate that paroxetine-mediated inhibition of GRK2 improves cardiac function after MI and represents a potential repurposing of this drug, as well as a starting point for innovative small-molecule GRK2 inhibitor development.

Role of β-adrenergic receptor regulation of TNF-α and insulin signaling in retinal Muller cells

PURPOSE

The goal of this study was to determine the relationship of TNF-α and the downregulation of insulin receptor signaling in retinal Müller cells cultured under hyperglycemic conditions and the role of β-adrenergic receptors in regulating these responses.

METHODS

Retinal Müller cells were cultured in normal (5 mM) or high (25 mM) glucose until 80% confluent and then were reduced to 2% serum for 18 to 24 hours. The cells were then treated with 10 μM salmeterol followed by Western blot analysis or ELISA. For TNF-α inhibitory studies, the cells were treated with 5 ng/mL of TNF-α for 30 minutes or by a 30-minute pretreatment with TNF-α followed by salmeterol for 6 hours. In the TNF-α short hairpin (sh)RNA experiments, the cells were cultured until 90% confluent, followed by transfection with TNF-α shRNA for 18 hours.

RESULTS

TNF-α-only treatments of Müller cells resulted in significant decreases of tyrosine phosphorylation of the insulin receptor and Akt in high-glucose conditions. Salmeterol (10 μM), a β-2-adrenergic receptor agonist, significantly increased phosphorylation of both insulin receptor and Akt. TNF-α shRNA significantly decreased phosphorylation of IRS-1(Ser307), which was further decreased after salmeterol+TNF-α shRNA. Both TNF-α shRNA and salmeterol significantly reduced death of the retinal Müller cells.

CONCLUSIONS

These studies demonstrate that β-adrenergic receptor agonists in vitro can restore the loss of insulin receptor activity noted in diabetes. By decreasing the levels of TNF-α and decreasing the phosphorylation of IRS-1(Ser307) while increasing tyrosine phosphorylation of insulin receptor, these results suggest a possible mechanism by which restoration of β-adrenergic receptor signaling may protect the retina against diabetes-induced damage.

Cytoskeletal role in protection of the failing heart by β-adrenergic blockade

Formation of a dense microtubule network that impedes cardiac contraction and intracellular transport occurs in severe pressure overload hypertrophy. This process is highly dynamic, since microtubule depolymerization causes striking improvement in contractile function. A molecular etiology for this cytoskeletal alteration has been defined in terms of type 1 and type 2A phosphatase-dependent site-specific dephosphorylation of the predominant myocardial microtubule-associated protein (MAP)4, which then decorates and stabilizes microtubules. This persistent phosphatase activation is dependent upon ongoing upstream activity of p21-activated kinase-1, or Pak1. Because cardiac β-adrenergic activity is markedly and continuously increased in decompensated hypertrophy, and because β-adrenergic activation of cardiac Pak1 and phosphatases has been demonstrated, we asked here whether the highly maladaptive cardiac microtubule phenotype seen in pathological hypertrophy is based on β-adrenergic overdrive and thus could be reversed by β-adrenergic blockade. The data in this study, which were designed to answer this question, show that such is the case; that is, β(1)- (but not β(2)-) adrenergic input activates this pathway, which consists of Pak1 activation, increased phosphatase activity, MAP4 dephosphorylation, and thus the stabilization of a dense microtubule network. These data were gathered in a feline model of severe right ventricular (RV) pressure overload hypertrophy in response to tight pulmonary artery banding (PAB) in which a stable, twofold increase in RV mass is reached by 2 wk after pressure overloading. After 2 wk of hypertrophy induction, these PAB cats during the following 2 wk either had no further treatment or had β-adrenergic blockade. The pathological microtubule phenotype and the severe RV cellular contractile dysfunction otherwise seen in this model of RV hypertrophy (PAB No Treatment) was reversed in the treated (PAB β-Blockade) cats. Thus these data provide both a specific etiology and a specific remedy for the abnormal microtubule network found in some forms of pathological cardiac hypertrophy.

Increased spontaneous activity and reduced inotropic response to catecholamines in ventricular myocytes from footshock-stressed rats

Exposure to stressors has been shown to change atrial responsiveness to catecholamines, but it is not clear yet how it affects the ventricular myocardium, which plays a major role in the catecholamine-stimulated increase in cardiac output. Adult male rats were submitted to restraint (RST) or footshock (FS) sessions for 3 days. Reactivity to agonists of the beta-adrenergic pathway was analyzed in left ventricular myocytes isolated from stressed and control rats (CTR). Whereas no significant changes were detected after RST, enhancement of catecholamine-induced spontaneous activity, accompanied by decrease in inotropic maximal response, was observed in myocytes from FS rats. Changes were reversed by beta(1)-, but not by alpha(1)-or beta(2)-adrenoceptor (AR) blockade. Similar alterations were seen in response to forskolin. However, responsiveness to 3-isobutyl-1-methylxanthine and CaCl(2) was comparable in control and FS groups. A significant negative correlation was observed between the maximally stimulated spontaneous activity rate and contraction amplitude. Results indicate that: (a) enhanced automatism during adrenergic stimulation of myocytes from FS rats is mediated by beta(1)-ARs and seems to involve post-receptor mechanisms, probably decreased cAMP degradation; (b) the exaggerated spontaneous activity, which may contribute to generation of catecholaminergic arrhythmias, might limit the development of the inotropic response.

Inotropy and L-type Ca2+ current, activated by beta1- and beta2-adrenoceptors, are differently controlled by phosphodiesterases 3 and 4 in rat heart

BACKGROUND AND PURPOSE

beta(1)- and beta(2)-adrenoceptors coexist in rat heart but beta(2)-adrenoceptor-mediated inotropic effects are hardly detectable, possibly due to phosphodiesterase (PDE) activity. We investigated the influence of the PDE3 inhibitor cilostamide (300 nmol x L(-1)) and the PDE4 inhibitor rolipram (1 micromol x L(-1)) on the effects of (-)-catecholamines.

EXPERIMENTAL APPROACH

Cardiostimulation evoked by (-)-noradrenaline (ICI118551 present) and (-)-adrenaline (CGP20712A present) through beta(1)- and beta(2)-adrenoceptors, respectively, was compared on sinoatrial beating rate, left atrial and ventricular contractile force in isolated tissues from Wistar rats. L-type Ca(2+)-current (I(Ca-L)) was assessed with whole-cell patch clamp.

KEY RESULTS

Rolipram caused sinoatrial tachycardia. Cilostamide and rolipram did not enhance chronotropic potencies of (-)-noradrenaline and (-)-adrenaline. Rolipram but not cilostamide potentiated atrial and ventricular inotropic effects of (-)-noradrenaline. Cilostamide potentiated the ventricular effects of (-)-adrenaline but not of (-)-noradrenaline. Concurrent cilostamide + rolipram uncovered left atrial effects of (-)-adrenaline. Both rolipram and cilostamide augmented the (-)-noradrenaline (1 micromol x L(-1)) evoked increase in I(Ca-L). (-)-Adrenaline (10 micromol x L(-1)) increased I(Ca-L) only in the presence of cilostamide but not rolipram.

CONCLUSIONS AND IMPLICATIONS

PDE4 blunts the beta(1)-adrenoceptor-mediated inotropic effects. PDE4 reduces basal sinoatrial rate in a compartment distinct from compartments controlled by beta(1)- and beta(2)-adrenoceptors. PDE3 and PDE4 jointly prevent left atrial beta(2)-adrenoceptor-mediated inotropy. Both PDE3 and PDE4 reduce I(Ca-L) responses through beta(1)-adrenoceptors but the PDE3 component is unrelated to inotropy. PDE3 blunts both ventricular inotropic and I(Ca-L) responses through beta(2)-adrenoceptors.

Relaxations to beta-adrenoceptor subtype selective agonists in wild-type and NOS-3-KO mouse mesenteric arteries

We have investigated the role of nitric oxide (NO) in relaxations to beta-adrenoceptor agonists in mesenteric artery from wild-type (WT) and NO synthase-3 knockout (NOS-3-KO) mice. Isoprenaline, formoterol and BRL 37344 ((R(),R())-(+/-)-4-[2-[(2-(3-chlorophenyl)-2-hydroxyethyl)amino]propyl]phenoxyacetic acid) were chosen as non-selective and beta(2)- and beta(3)-adrenoceptor agonists, respectively. Atenolol, ICI 118,551 ((+/-)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol hydrochloride) and SR59230A (1-(2-ethylphenoxy)-3-[[(1S)-1,2,3,4-tetrahydro-1-naphthalenyl]amino]-(2S)-2-propanol hydrochloride) were chosen as selective beta(1)-, beta(2)- and beta(3)-adrenoceptor antagonists, respectively. Experiments employing isoprenaline were carried out in the presence of prazosin (0.1 microM). Isoprenaline produced relaxations with a potency of 5.68+/-0.36 (-log M, n=6) in WT mice. Relaxations to isoprenaline were blocked by atenolol (10 microM) and were absent in vessels from NOS-3-KO animals. Formoterol produced relaxations with two components. ICI 118,551 (1 microM) abolished relaxations to low concentrations of formoterol (0.1-10 microM), but failed to affect relaxations to formoterol (100 microM). In NOS-3-KO mice only the highest concentration of formoterol (100 microM) produced relaxations: the relaxation was resistant to all of the beta-adrenoceptor antagonists employed. BRL 37344 (5.75+/-0.28, n=9) was approximately equipotent with isoprenaline but produced a smaller degree of relaxation, in WT mice. SR59230A (1 microM) abolished relaxations to BRL 37344 in WT mice. In NOS-3-KO mice, BRL 37344 produced concentration-dependent relaxations which were abolished by SR59230A. It is concluded that the predominant beta-adrenoceptor mediating relaxations in mouse mesenteric artery is beta(1), and relaxations involve NOS-3. In addition, beta(3)-adrenoceptors mediate smaller relaxations at least partly independent of NOS-3, and beta(2)-adrenoceptors may mediate smaller relaxations dependent on NOS-3.

The phosphodiesterase 3 inhibitor cilostamide enhances inotropic responses to glucagon but not to dobutamine in rat ventricular myocardium

The effects of phosphodiesterase (PDE) inhibitors (1-3) on tissue cAMP concentrations and the inotropic responses to dobutamine and glucagon were investigated in electrically driven right ventricular strips of the rat heart. Dobutamine (0.3-100 microM) produced a concentration-dependent positive inotropic effect which was not affected by 50 nM (+/-)-1-(2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy)-3-((1-methylethyl)amino)-2-butanol hydrochloride (ICI 118551), a beta2-receptor antagonist, but was virtually abolished by 0.3 microM (+/-)-2-hydroxy-5-(2-((2-hydroxy-3-(4-(1-methyl-4-(trifluoromethyl)-1H-imidazol-2-l)phenoxy)propyl)amino)ethoxy)-benzamide methanesulfonate (CGP 20712A), a beta1-receptor antagonist. Glucagon (0.01-1 microM) also enhanced the contractility of the preparation in a concentration-dependent way. Selective inhibitors of PDE 1 8-methoxymethyl-3-isobutyl-1-methylxantine (MIMX, 1 muM), PDE 2 erythro-9-[2-hydroxy-3-nonyl]adenine (EHNA, 1 microM) and PDE 3 cilostamide (0.1 microM) did not affect basal contractility. Cilostamide increased the positive inotropic effects of glucagon but not those of dobutamine. MIMX and EHNA did not alter the effects of either dobutamine or glucagon. Dobutamine (3 microM), but not glucagon (0.1 microM), increased tissue levels of cAMP. 1 microM of MIMX or EHNA were devoid of effects and failed to alter the effects of dobutamine and glucagon on cAMP. Cilostamide (0.1 microM) did not increase the effects of dobutamine but caused glucagon to enhance cAMP. The pharmacological and biochemical data presented in this study can be explained quantitatively by a cell compartment model in which PDE 3 appears to be colocalized with the contractile machinery responsible for the effects of glucagon but not those of dobutamine. Neither PDE 1 nor PDE 2 appears to regulate the inotropic effects of dobutamine and glucagon in rat ventricular myocardium.

Density and binding characteristics of beta-adrenoceptors in the normal and failing equine myocardium

Beta-adrenoceptors are important regulators of cardiac function and their characteristics are known to change in human and canine diseased myocardium. This study aimed to determine the density and subtypes of beta-adrenoceptors in the normal and failing equine ventricular myocardium. Membrane preparations of the left papillary muscles were incubated with increasing concentrations of the nonselective beta-adrenoceptor antagonist [3H]-CGP12177. Saturable and reversible binding of [3H]-CGP12177 to myocardial membranes was demonstrated with Kd values (+/- s.d.) of 0.49 +/- 0.40 and 0.43 +/- 0.22 nmol/l and Bmax values of 93.4 +/- 20.5 and 110.0 +/- 21.2 and fmol/mg protein for normal (n = 19) and heart failure (n = 10) tissues, respectively. Heart failure had no significant effect on the density of ventricular beta-adrenoceptors. The cardiac beta-adrenoceptors were further characterised by studying displacement of [3H]-CGP12177 (0.6 nmol/l) with the beta1-selective antagonists CGP20712A and the beta2-selective antagonist ICI118.551. In normal ventricular muscle, CGP20712A was 26 times more potent than ICI118.551 (Ki values 30.4 +/- 24.8 and 814.1 +/- 485.2 nmol/l, respectively). In heart failure cases, CGP 20712A curves were monophasic with a Ki value of 45.6 +/- 39.7 nmol/l. ICI 118.551 curves were biphasic in 5 horses where 11-31% of the cardiac beta-adrenoceptors had a high affinity for ICI 118.551. These data suggest that the normal equine ventricular myocardium possesses predominately beta1-adrenoceptors, with no evidence for co-existence of a significant population of beta2-adrenoceptors. The density of beta-adrenoceptors did not appear to change in heart failure, but the appearance of receptors with a high affinity for ICI118.551 may suggest that, in some cases, heart failure increases the expression of beta2-adrenoceptors in equine ventricular myocardium. This study provides an insight into the role of the adrenergic system in heart disease in the horse. Further studies in this area are warranted.

Selective attenuation by adenosine of arrhythmogenic action of isoproterenol on ventricular myocytes

We examined whether adenosine equally attenuated the stimulatory effects of isoproterenol on arrhythmic activity and twitch shortening of guinea pig isolated ventricular myocytes. Transmembrane voltages and whole cell currents were recorded with patch electrodes, and cell twitch shortening was measured using a video-motion detector. Isoproterenol increased the action potential duration at 50% repolarization (APD50), L-type Ca2+ current [ I Ca(L)], and cell twitch shortening and induced delayed afterdepolarizations (DAD), transient inward current ( I Ti), and aftercontractions. Adenosine attenuated the arrhythmogenic actions of isoproterenol more than it attenuated the effects of isoproterenol on APD50, I Ca(L), or twitch shortening. Adenosine (0.1–100 μmol/l) decreased the amplitude of DADs by 30 ± 6% to 92 ± 5% but attenuated isoproterenol-induced prolongation of the APD50 by only 14 ± 4% to 59 ± 4% and had no effect on the voltage of action potential plateau. Adenosine (30 μmol/l) inhibited I Ti by 91 ± 4% but decreased isoproterenol-stimulated I Ca(L) by only 30 ± 12%. Isoproterenol-induced aftercontractions were abolished by adenosine (10 μmol/l), whereas the amplitude of twitch shortening was not reduced. The effects of adenosine on twitch shortenings and aftercontractions were mimicked by the A1-adenosine receptor agonist CPA ( N 6-cyclopentyladenosine) and by ryanodine. In conclusion, adenosine antagonized the proarrhythmic effect of β-adrenergic stimulation on ventricular myocytes without reducing cell twitch shortening.

Renin-angiotensin system and sympathetic nervous system in cardiac pressure-overload hypertrophy

Angiotensin II and norepinephrine (NE) have been implicated in the neurohumoral response to pressure overload and the development of left ventricular hypertrophy. The purpose of this study was to determine the temporal sequence for activation of the renin-angiotensin and sympathetic nervous systems in the rat after 3-60 days of pressure overload induced by aortic constriction. Initially on pressure overload, there was transient activation of the systemic renin-angiotensin system coinciding with the appearance of left ventricular hypertrophy (day 3). At day 10, there was a marked increase in AT(1) receptor density in the left ventricle, increased plasma NE concentration, and elevated cardiac epinephrine content. Moreover, the inotropic response to isoproterenol was reduced in the isolated, perfused heart at 10 days of pressure overload. The affinity of the beta(2)-adrenergic receptor in the left ventricle was decreased at 60 days. Despite these alterations, there was no decline in resting left ventricular function, beta-adrenergic receptor density, or the relative distribution of beta(1)- and beta(2)-receptor sites in the left ventricle over 60 days of pressure overload. Thus activation of the renin-angiotensin system is an early response to pressure overload and may contribute to the initial development of cardiac hypertrophy and sympathetic activation in the compensated heart.

G(i)-dependent suppression of beta(1)-adrenoceptor effects in ventricular myocytes from NE-treated guinea pigs

It has been suggested that there is a preferential coupling in heart muscle between the inhibitory G protein (G(i)) and the beta(2)-subtype of the beta-adrenergic receptor (beta-AR), since pertussis toxin (which inactivates G(i)) reveals latent beta(2)-ARs in rat and mouse myocytes. We have previously shown that guinea pigs treated with norepinephrine (NE) for 7 days have myocytes that are desensitized to beta-AR-agonist stimulation, and that pertussis toxin restores these responses. The purpose of the present investigation was to determine whether pertussis toxin specifically upregulated beta(2)-ARs in myocytes from NE-treated guinea pigs. The sole beta-AR subtype in control guinea pig myocytes was confirmed as beta(1)-AR by radioligand binding, single-cell autoradiography, and concentration-response curves to isoproterenol in contracting myocytes. In contrast, a minor pool of beta(2)-ARs was observed in rat myocytes by use of the same methods. NE treatment decreased the maximum isoproterenol response (relative to high Ca(2+)) from 0.89 +/- 0.06 to 0.58 +/- 0.08 (n = 7, P < 0.01) and the pD(2) (-log EC(50)) from 8.8 +/- 0.2 to 7.5 +/- 0.2 (n = 7, P < 0.01). Pertussis toxin treatment increased the isoproterenol-to-Ca(2+) ratio to 0.88 +/- 0.04 (n = 6, P < 0.05) and the pD(2) to 8.6 +/- 0.3 (P < 0.01). This was not mediated by increases in either number or function of beta(2)-ARs. G(i) is therefore able to modulate beta(1)-AR responses in guinea pig myocytes.

Diabetes-induced down-regulation of beta1-adrenoceptor mRNA expression in rat heart

The present study addressed the question of whether the number of myocardial beta-adrenoceptors in rats with 4- to 6-week streptozotocin-induced diabetes is regulated in a transcriptional or translational manner. Radioligand binding experiments with [3H]CGP 12177 {4-(3-t-butylamino-2-hydroxypropoxy)-[5,7-3H]benzimidazol-2-one} showed that the density of beta-adrenoceptors fell by 50% with no change in affinity in diabetic rat ventricular myocardium compared with age-matched control myocardium. The relative content of beta1-adrenoceptor mRNA in diabetic myocardium also was reduced from the control level by 57%, as determined by northern blot analysis. The reductions in myocardial beta-adrenoceptor number and beta1-adrenoceptor mRNA observed in diabetes were prevented by insulin therapy. These data indicate that the diminished density of myocardial beta-adrenoceptors in diabetes occurred, at least in part, at the mRNA level.

Inverse agonist activities of beta-adrenoceptor antagonists in rat myocardium

1. Negative inotropic effects of several beta-adrenoceptor (betaAR) antagonists on electrically-stimulated right atria, left atria, right ventricles and left ventricular papillary muscles from reserpine-treated rats were used as a measure of their inverse agonist activities. 2. Beta1AR antagonists acebutolol, atenolol and metoprolol, beta2AR antagonist ICI-181,551 and nonselective betaAR antagonists alprenolol, nadolol, propranolol and timolol produced negative inotropic effects, which were most marked on the right atria. 3. The nonselective betaAR antagonist pindolol did not exhibit inverse agonist activity but inhibited the negative inotropic activities of ICI-118,551, atenolol and propranolol. 4. The negative inotropic effects of lidocaine, nifedipine and pentobarbitone were similar on all the four myocardial preparations. 5. The positive inotropic efficacy of salbutamol on right and left atria but not on right ventricles and papillary muscles was comparable to that of isoprenaline. The antagonist activity of ICI-118,551 against isoprenaline was greater on right atria than on other cardiac regions. 6. Beta1AR proteins were expressed in all regions of the heart but of beta2AR were primarily localized in the right atrium. 7. It is concluded that beta2AR play a greater role in right atria than in other cardiac regions and almost all betaAR antagonists behave as inverse agonists.

Do beta 2-adrenergic receptors modulate Ca2+ in adult rat ventricular myocytes?

We examined the role of beta 2-adrenergic receptors (ARs) in modulating calcium homeostasis in rat ventricular myocytes. Zinterol (10 microM), an agonist with a 25-fold greater affinity for beta 2-ARs over beta 1-ARs, modestly enhanced L-type calcium current (ICa) magnitude by approximately 30% and modestly accelerated the rate of Ca2+ concentration ([Ca2+]) decline (approximately 35%) but had little effect on the magnitude of the [Ca2+] transient (a nonsignificant 6% increase). However, 1 microM of the highly selective beta 1-AR antagonist CGP-20712A completely blocked the ICa increase induced by 10 microM zinterol. Pretreatment of cells with pertussis toxin (PTX) did not alter ICa enhancement by 10 microM zinterol, although it did abolish the ability of acetylcholine to block the forskolin-induced enhancement of ICa. Zinterol (10 microM) approximately doubled adenosine 3',5'-cyclic monophosphate (cAMP) accumulation, although one-half of this increase was blocked by CGP-20712A. In contrast, 1 microM of the nonselective beta-agonist isoproterenol increased cAMP production 15-fold. Thus we found no evidence that activation of beta 2-ARs modulates calcium homeostasis in rat ventricular myocytes, even after treatment with PTX.

Alterations in beta-adrenoceptor mechanisms in the aging heart. Relationship with heart failure

In chronic heart failure substantial and characteristic changes occur in the function of the adrenergic nervous system. Studies in isolated left ventricular muscle and in single cardiomyocytes from experimental models of aging and, recently, from humans show an age-related reduced contractile response to beta-adrenoceptor stimulation. "beta-adrenoceptor desensitization" is thought to be a general and common mechanism to explain the age- and heart failure-related decrease in beta-adrenoceptor response. The aim of this review is to compare alterations in beta-adrenoceptor mechanisms in physiological cardiovascular aging and chronic heart failure. From an analysis of the overall data on the role of aging in beta-adrenoceptor regulation in human and animal hearts, it is possible to conclude that the reduced response to beta-agonists is common to all species and all cardiac tissues. Moreover, the age-related changes are limited to beta-adrenoceptor-G-protein (s)-adenylyl cyclase system abnormalities, while the type and level of abnormalities change with species and tissues. The modifications shown in the aging heart are not very different from some observed in heart failure. In particular, both in aged and failing hearts we may see that the decrease in beta-adrenoceptor responsiveness is related to changes in G-protein function.

Diltiazem derivatives modulate the dihydropyridine-binding to intact rat ventricular myocytes

To examine whether the modulation of the 1,4-dihydropyridine-binding by diltiazem derivatives, which has been shown in cardiac and skeletal muscle membranes, takes place in intact cardiac myocytes, effects of diltiazem on the specific binding of [3H](+)-PN200-110 to freshly isolated adult rat ventricular myocytes were investigated in normal Tyrode solution at 37 degrees C. Diltiazem consistently potentiated the [3H](+)-PN200-110-binding in a concentration-dependent manner, while DTZ323 (3-(acetyloxy)-5-[2-[[2- (3,4-dimethoxyphenyl)ethyl]-methylamino]ethyl]-2,3-dihydro-2(-4 methoxyphenyl)-1,5-benzothiazepin-4-(5H)-one), a potent diltiazem derivative, inhibited it in a non-competitive manner. In saturation studies, 100 microM decreased the Kd value of the 3[H](+)-PN200-110-binding (control, 0.102 +/- 0.008 vs. diltiazem, 0.074 +/- 0.004 (nM, n = 6), P < 0.05) without significant effect on Bmax (control, 65.7 +/- 6.4 vs. diltiazem, 76.7 +/- 4.4 (fmol/mg protein, n = 6). Moreover, membrane-impermeant quaternary diltiazem also potentiated the [3H](+)-PN200-110-binding in intact myocytes. These results suggest that diltiazem modulates the 1,4-dihydro-pyridine-binding even in intact cardiac myocytes, and the binding site of diltiazem is accessible from the extracellular side of the L-type Ca2+ channels.