Experimental issues in assessment of G protein function in cardiac disease

βARK1 Inhibition Improves Survival in a Mouse Model of Heart Failure Induced by Myocardial Infarction

Heart failure (HF) is characterized by abnormalities in beta-adrenergic receptor (betaAR) signaling, including an increase in betaAR kinase 1 (betaARK1) levels and activity. Gene therapy using a peptide inhibitor of betaARK1 (betaARKct) in infarcted rabbit hearts has improved compromised cardiac function. To determine whether betaARK1 inhibition improves survival in a mouse model of HF induced by myocardial infarction (MI), we studied wild-type (WT) and transgenic (TG) mice overexpressing betaARKct following MI. There was no difference in infarct size. Survival of WT mice with MI was 25% at 26 weeks. In contrast, 92% of betaARKct TG mice with MI survived (P = 0.01). betaARKct TG mice with MI at 8 weeks showed significantly higher fractional shortening compared with WT mice with MI (25.1 +/- 2.7% versus 14.2 +/- 1.0%; P < 0.05). Moreover, the biochemical betaAR abnormalities in WT mice with MI were prevented in betaARKct TG mice with MI. In conclusion, betaARK1 inhibition results in a marked increase in survival and improved cardiac function in a mouse model of HF induced by MI.

Attenuation of changes in G(i)-proteins and adenylyl cyclase in heart failure by an ACE inhibitor, imidapril

Cardiac dysfunction in animals with congestive heart failure due to myocardial infarction (MI) is known to be associated with a wide variety of defects in receptor and post-receptor mechanisms. Since the heart function have been shown to be improved by treatment with different angiotensin converting enzyme (ACE) inhibitors, we examined the effects of imidapril, an ACE inhibitor, on changes in post-receptor mechanisms involving adenylyl cyclase (AC) and G proteins in the failing heart. Heart failure in rats was induced by occluding the coronary artery and 3 weeks later the animals were treated daily with 1 mg/kg (orally) imidapril for 5 weeks. The animals were assessed for their left ventricular function and crude membranes were isolated from the viable left ventricle and examined for AC activities as well as G-protein activities and expression. Animals with heart failure exhibited depressions in ventricular function and AC activities in the absence or presence of forskolin, NaF and Gpp(NH)p. The AC activity in the presence of pertussis toxin was increased whereas that in the presence of cholera toxin was decreased in the failing heart. Protein contents and mRNA levels for G(i)-proteins were increased whereas those for G(s)-proteins were unaltered in the infarcted heart. All these changes due to MI were prevented by imidapril treatment. The results indicate that the depressed cardiac function in the failing heart may partly be due to the direct effects of changes in AC and G(i) proteins.

G protein and adenylate cyclase complex-mediated signal transduction in the rat heart during sepsis

Changes in the protein level of various subunits of GTP-binding protein and the activity of adenylate cyclase in the rat heart during different phases of sepsis were studied. Sepsis was induced by cecal ligation and puncture (CLP). Experiments were divided into three groups: control, early sepsis, and late sepsis. Early and late sepsis refers to those animals sacrificed at 9 and 18 h, respectively, after CLP. The protein levels of various subunits of GTP-binding protein were determined by Western blot analysis. The activity of adenylate cyclase was measured based on the rate of formation of cAMP from [alpha-32P]ATP. The results show that protein levels of G alphas and G beta remained stable during the early and the late phases of sepsis. The protein levels of G alpha i-2 and G alpha i-3 remained relatively unaltered during the early phase of sepsis, but they were increased by 46.5% (P < 0.05) and 61.3% (P < 0.01), respectively, during the late phase of sepsis. The basal adenylate cyclase activity remained unchanged during the early phase while it was decreased by 25.7% (P < 0.05) during the late phase of sepsis. The isoproterenol-stimulated adenylate cyclase activity was unchanged during early sepsis while it was decreased by 44.6% (P < 0.01) during late sepsis. These data demonstrate that during the late hypodynamic phase of sepsis, myocardial G alpha i-2 and G alpha i-3 protein levels were increased and the increases were coupled with a reduction in adenylate cyclase activity. Because GTP-binding proteins mediate sympathetic control of cardiac function, the present findings may have a pathophysiological significance in contributing to the understanding of the pathogenesis of cardiac dysfunction during the late stage of sepsis.

Effect of maturation and aging on beta-adrenergic signal transduction in rat kidney and liver

The characteristics of the beta-adrenergic signal transduction system were analyzed in kidney and liver membrane preparations from neonatal (2-3 days), mature (2 months), and old (2 years) rats. When comparing kidneys from adult to neonatal rats, we found a higher beta-receptor density and a higher percentage of beta(1)-receptor subtype, lower immunoreactive G(salpha)-protein, a lower ratio between the high and low molecular weight splice variant of G(salpha), lower immunoreactive G(ialpha)-protein, and lower basal adenylate cyclase activity. When comparing livers from adult to neonatal rats, we found lower beta-receptor density and basal adenylate cyclase activity. Very few differences could be detected when comparing mature to old kidneys or livers. Stimulated adenosine 3',5'-cyclic monophosphate (cAMP) synthesis was tissue- and age-dependent. In liver, G-protein- and beta-receptor-stimulated cAMP synthesis mirrored basal adenylate cyclase activity and was highest in liver from neonatal animals. In contrast, cAMP synthesis was significantly more stimulated in kidneys from mature animals than from neonatal and senescent rats. We conclude that: (i) the stoichiometry of the components within the beta-receptor/G-protein/adenylate cyclase complex is not fixed but is both tissue- and age-dependent; (ii) adenylate cyclase enzyme activity is possibly but not necessarily the rate-limiting step in the beta-receptor-mediated synthesis of cAMP; and (iii) there is in vivo evidence for a preferential co-expression of the large splice variant of the G(s)-protein and beta(2)-receptor subtype. It is speculated that this could have important physiological consequences for the development of the kidney.

Properties of the ventricular adrenergic signal transduction system during ontogeny of spontaneous hypertension in rats

The purpose of this study was to characterize adrenergic receptors and associated G proteins in ventricles of spontaneously hypertensive rats (SHRs) at different stages of development. The beta- and alpha1-adrenoceptor densities and subtype distribution, and beta-adrenoceptor-G protein coupling were studied by radioligand binding, and levels of G(Salpha), G(ialpha), and G(q/11alpha) protein species were determined by Western blotting in SHRs and Wistar-Kyoto (WKY) control rats aged 3.5 weeks, 3 months, and 8 months. In 3.5-week-old SHRs, both the beta-adrenoceptor density and the percentage of agonist high-affinity binding sites were higher than in age-matched WKY rats. The beta1/beta2-subtype distribution, the alpha1-adrenoceptor density, and the alpha1B/alpha1A-subtype distribution were similar in rats of both strains at all ages. Although essentially no differences in G(salpha) levels between SHRs and WKY rats were detected, higher G(ialpha) and lower Gq/1alpha concentrations were found in 3.5-week-old SHRs. In 3-month-old SHRs, increased levels of Gq/11alpha proteins were observed. In 8-month-old SHRs, none of the parameters was different from those of controls. We conclude that the differences in properties of the adrenergic signal transduction system between SHRs and WKY rats are exclusively observable before and at the onset of the overt hypertension. Moreover, the hypertensive genotype apparently affects G proteins more readily than adrenoceptors.

Evidence of phenotypic alteration as a cause of systolic dysfunction in the failing heart

This article describes the phenotype of dilated cardiomyopathy in humans and describes parallel studies in experimental models that support or refute the relevance of these changes to the development of the heart failure phenotype.

Diminished responsiveness of Gs-coupled receptors in severely failing human hearts: no difference in dilated versus ischemic cardiomyopathy

In end-stage heart failure, cardiac beta-adrenoceptors are decreased and cardiac Gi protein is increased. We assessed beta-adrenoceptors, G proteins, and effects of several beta-adrenoceptor agonists, histamine, and 5-HT on adenylyl cyclase activity in right and left atria and left ventricles and on left ventricular contractility in six potential heart transplant donors (nonfailing hearts; NFHs) and in nine patients with end-stage dilated cardiomyopathy (DCM) and 11 patients with end-stage ischemic cardiomyopathy (ICM) to establish whether the functional responsiveness of all cardiac Gs-coupled receptors is reduced. Beta-adrenoceptors were reduced in all three tissues; in DCM, beta1-adrenoceptors were more markedly downregulated; in ICM, both beta1- and beta2-adrenoceptors were diminished. In all three tissues, isoprenaline-, terbutaline-, histamine- and 5-HT-induced adenylyl cyclase activation was reduced similarly in DCM and ICM. Moreover, in DCM and ICM, guanosine triphosphate (GTP)- (involving Gs and Gi) activated adenylyl cyclase was significantly diminished, whereas NaF-activated (involving only Gs) and Mn2+-activated (acting at the catalytic unit of the enzyme) adenylyl cyclase was unaltered. Left ventricular positive inotropic responses to beta1- (noradrenaline, dopamine, and dobutamine), beta2- (terbutaline), and beta1- and beta2-adrenoceptors (isoprenaline, adrenaline, and epinine), as well as H2-receptor (histamine) stimulation were significantly reduced. The extent of reduction was not different for each agonist in ICM and DCM. We conclude that in DCM and ICM, functional responsiveness of all cardiac Gs-coupled receptors is similarly reduced.

Uncoupling of G-protein coupled receptors in vivo: insights from transgenic mice

Heart failure is a problem of increasing importance in medicine. An important characteristic of heart failure is reduced agonist-stimulated adenylyl cyclase activity (receptor desensitization) due to both diminished receptor number (receptor down regulation) and impaired receptor function (receptor uncoupling). These changes in the beta-adrenergic receptor (beta-AR) system, may in part account for some of the abnormalities of contractile function in this disease. Myocardial contraction is closely regulated by G-protein coupled beta-adrenergic receptors through the action of the second messenger cAMP. The beta-AR receptors themselves are regulated by a set of specific kinases, termed the G-protein-coupled receptor kinases (GRKs). The study of this complex system in vivo has recently been advanced by the development of transgenic and gene targeted ("knockout") mouse models. Combining transgenic technology with sophisticated physiological measurements of cardiac hemodynamics is an extremely powerful strategy to study the regulation of myocardial contractility in the normal and failing heart.

Myocardial beta-adrenergic receptor signaling in vivo: insights from transgenic mice

Heart failure is a problem of increasing importance in cardiovascular medicine. An important characteristic of heart failure is reduced agonist-stimulated adenylyl cyclase activity (receptor desensitization) due to both diminished receptor number (receptor downregulation) and impaired receptor function (receptor uncoupling). These changes in the section-adrenergic receptor (section-AR) system may in part account for some of the abnormalities of contractile function in this disease. Myocardial contraction is closely regulated by G protein coupled beta-adrenergic receptors through the action of the second messenger cAMP. The beta-adrenergic receptors themselves are regulated by a set of specific kinases, termed the G-protein-coupled receptor kinases. The study of this complex system in vivo has recently been advanced by the development of transgenic and gene targeted ("knockout") mouse models. Combining transgenic technology with sophisticated physiological measurements of cardiac hemodynamics is an extremely powerful strategy to study the regulation of myocardial contractility in the normal and failing heart.

Signal transduction mechanisms in the ischemic and reperfused myocardium

The cellular mechanisms regulating myocardial dysfunction during ischemia and subsequent reperfusion are complex. As can be determined from this review, it is clear that signal transduction pathways are altered during these conditions, which may explain, in part, the pathophysiology of ischemia and reperfusion. With respect to beta-adrenoceptor signal transduction, adaptive changes during ischemia and reperfusion ensure that this critical pathway for the regulation of cardiac function remains intact. Additionally, although the relative contribution of alpha 1-adrenoceptors to the regulation of cardiac function is minimal in normal myocardium, these receptors clearly exacerbate conditions associated with the generation of arrhythmias during reperfusion. It is likely that this enhancement of arrhythmogenesis is related to the activation of NHE by a PKC-dependent mechanisms. The importance of non-receptor-mediated signal transduction as a mediator of ischemia and reperfusion injury has long been established with respect to products of membrane lipid breakdown. As discussed, recent evidence now suggests that other compounds formed during ischemia and reperfusion, such as reactive oxygen species and NO, are also linked to cellular second messenger systems. In conclusion, as signal transduction is critical for normal myocardial function, signal transduction pathways are of even more importance during ischemia and reperfusion. There is an increasing interest in the role of non-receptor-mediated signal transduction as a mediator of ischemia and reperfusion injury and it is hoped that these pathways may represent new levels for therapeutic intervention.

Beta-adrenergic receptors in failing human myocardium

In the human heart the beta-adrenergic receptor-G-protein-adenylyl cyclase system is the most powerful physiologic mechanism to acutely increase contractility and/or heart rate. In the failing human myocardium beta 1-adrenergic receptor number is decreased, and this is accompanied by a reduced beta 1-adrenergic receptor mediated positive inotropic effect. Cardiac beta 2-adrenergic receptor number may or may not decrease; however, beta 2- adrenergic receptor mediated positive inotropic effects are also reduced, possibly because the functional activity of myocardial Gi is increased, thereby inhibiting cyclic AMP formation. The aging human heart shows some similarities with the failing human heart: in both settings, of chronic heart failure and age, beta-adrenergic receptor mediated effects and all other cyclic AMP dependent effects are depressed and Gi-protein is increased.

Inotropic responses to isoproterenol in congestive heart failure subsequent to myocardial infarction in rats

It is well known that the positive inotropic effect of catecholamines is mediated through the activation of adrenergic receptors and the formation of cyclic adenosine monophosphate (AMP) in the cardiac cell. Although attenuated responses of failing hearts to catecholamines are commonly seen in patients and experimental animals with coronary artery disease, their mechanisms are poorly understood. To examine the status of beta-adrenergic receptors and postadrenergic receptor mechanisms in congestive heart failure due to myocardial infarction, the left coronary artery in rats was ligated for 4 and 8 weeks before studying the hemodynamic and biochemical changes due to isoproterenol, a beta-adrenoceptor agonist, or forskolin, a postreceptor agonist. Isolated perfused rat heart preparations were also used for studying changes in contractile function and cyclic AMP content. The hemodynamic actions and changes in the left ventricular cyclic AMP content in the experimental animals were depressed, not only in response to isoproterenol but also to forskolin. The density of beta-adrenoceptors was also decreased in the failing myocardium. In isolated perfused hearts from the 8-week experimental animals, isoproterenol-induced as well as forskolin-induced increases in both contractile force and cyclic AMP content in the left ventricle were depressed. These data suggest that defects in both beta-adrenergic receptor and postreceptor sites may be involved in attenuating the response of infarcted hearts to catecholamines.

Strategies for pharmacologic modulation of the heart failure phenotype

The end-stage heart failure phenotype is characterized by marked dyspnea on exertion, edema, and overwhelming fatigue, and by a high incidence of sudden death. Patients who display the end-stage phenotype have transitioned from a normal phenotype with myocardial damage at a cellular level. This transition appears to be mediated by events at both the cellular and molecular levels. Until recently, it was generally believed that this transition was irreversible. However, recent clinical trials have demonstrated that the phenotype can be changed with pharmacologic agents. These agents have been demonstrated to improve exercise capability, increase ventricular function, and improve symptoms. Important recent studies have shown that pharmacologic agents can substantially alter the high mortality rates associated with the end-stage heart failure phenotype. As we learn more about the molecular and cellular events that initiate and support the transition from cardiac compensation to decompensation, we will be able to improve our pharmacologic targeting and, we hope, be able to delay the development of the end-stage heart failure phenotype to an even greater degree.

Effect of theophylline on beta-adrenergic receptor density and cAMP content in bovine aortic smooth muscle cells

Activation of vascular beta-adrenergic receptors prevents an increase in vascular permeability caused by free radicals or inflammatory peptides. Methylxanthines seem to have similar protective effects on vascular endothelium. In the present study we investigated the effect of theophylline on the beta-adrenergic receptor expression and cAMP concentrations in cultured endothelial and smooth muscle cells from bovine aorta. Comparable values for beta-receptor density and binding affinity were detected in both cell types. Isoproterenol induced significant downregulation of beta-receptors in endothelial (BAEC: -60.5%) and smooth muscle cells (BASMC: -52.5%; P < 0.01). Incubation of endothelial cells with theophylline (4 micrograms/ml and and 40 micrograms/ml) for 24 hours did not affect beta-receptor expression, whereas in smooth muscle cells the beta-receptor density was reduced for -31.5% and -28.7%, respectively. In endothelial cells a transient effect on cAMP concentrations was observed after stimulation with isoproterenol (1 microM), but no effect was found in theophylline treated endothelial cells. Stimulation of intact smooth muscle cells with isoproterenol and theophylline (4 micrograms/ml and 40 micrograms/ml) resulted in a significant increase of cAMP concentrations after 60 and 240 minutes. The present data suggest a novel, celltype specific effect of theophylline on the beta-adrenergic receptor expression in vascular smooth muscle cells in vitro.

Altered adenylyl cyclase activities and G-protein abnormalities in portal hypertensive rabbits

Portal hypertension (PHT) is characterized by splanchnic hyperemia due to a reduction in mesenteric vascular resistance. We hypothesized that alterations in the activity of a guanine-nucleotide regulatory protein (G-protein) might be partially responsible for the marked circulatory disturbances observed in PHT. We, therefore, determined alterations in adenylyl cyclase/cAMP system in prehepatic portal hypertensive rabbits and correlated these changes to the activity of a G-protein. Basal and G-protein-stimulated adenylyl cyclase activities were lower in the PHT superior mesenteric artery (22-26%) and thoracic aorta (31-46%) membranes, but higher (178-321%) in portal vein. The functional activity of Gi alpha proteins (pertussis toxin-catalyzed ADP-dependent ribosylation) increased in the PHT superior mesenteric artery and thoracic aorta, but decreased in portal vein. Immunodetection revealed an increase in the Gi alpha protein subunits (Gi alpha 1/Gi alpha 2 and Gi alpha 3/Go alpha) in PHT thoracic aorta, without any change in Gs alpha proteins; and a decrease in the amount of Gi alpha proteins in PHT portal vein. There was no change in the amount of Gs alpha/Gi alpha in the PHT superior mesenteric artery. We conclude the hemodynamic alterations of PHT are associated with intrinsic alterations in G-protein-enzyme effector systems. These alterations are vessels specific and suggest a possible unique global derangement underlying the vasculopathy of PHT.

Cardiac beta-adrenoceptors and adenylyl cyclase activity in human left ventricular hypertrophy due to pressure overload

The effect of left ventricular hypertrophy (LVH) due to chronic pressure overload on right atrial (RA) and left ventricular (LV) myocardial beta-adrenergic receptor (beta-AR) density and subtypes, adenylyl cyclase (AC) activity and ADP-pertussis toxin ribosylated proteins was investigated in humans with LVH due to aortic stenosis and in patients without LVH undergoing heart surgery for mitral stenosis or coronary artery disease taken as controls. Both groups presented normal systolic function or plasma catecholamine levels. In LVH and controls, beta-AR density was similar in RA (62 +/- 6 vs 77 +/- 12 fmol.mg-1 protein) and LV (39 +/- 7 vs 32 +/- 2 fmol.mg-1 protein). In LVH, beta 1-AR percentage was < than in controls in LV (35 +/- 11 vs 73 +/- 5%, P < 0.05) but not in RA (79 +/- 5 vs 73 +/- 8%). Basal AC activity in RA (19 +/- 4 vs 21 +/- 6 pmol.mg-1 protein) and LV (22 +/- 5 vs 27 +/- 3 pmol.mg-1 protein) was similar in LVH and in controls. Isoprenaline-induced stimulation of AC in RA was similar in LVH and in controls (51 +/- 18 vs 36 +/- 18%) but < in LV of LVH (7 +/- 6 vs 45 +/- 6%, P < 0.05). In the presence of ICI-118,551 (a beta 2-adrenoceptor antagonist), isoprenaline failed to induce any increase in cAMP in LVH. The quantification of ADP-pertussis toxin ribosylated proteins indicated a lower concentration of substrates in LV myocardial membranes from LVH. These data indicate that in LVH due to pressure overload, there is a down-regulation of beta 1-AR and an increase in beta 2-AR density. This is associated with alterations of the transmembrane signalling marked by a decreased capacity of isoprenaline to stimulate AC and an impaired expression of Gi proteins.

Beta-adrenoceptors in cardiac disease

The human heart contains both beta 1 and beta 2-adrenoceptors; both mediate positive inotropic and chronotropic effects. In chronic heart failure, beta-adrenoceptor number is reduced, presumably, by down-regulation by endogenous noradrenaline which is elevated due to increased sympathetic activity. Since the human heart contains only a few spare receptors for beta-adrenoceptor-mediated positive inotropic effects and the amount of spare receptors declines in chronic heart failure, it is not surprising that the reduced beta-adrenoceptor number is accompanied by decreased contractile responses to beta-adrenoceptor agonists (including endogenous catecholamines), and the extent of decrease in maximal inotropic response is more pronounced as the disease becomes more advanced. Moreover, in chronic heart failure myocardial G(i)-protein, which inhibits cAMP formation, is increased, which might further contribute to the reduction in beta-adrenoceptor-mediated effects. It appears that, at present, the best therapy for severe heart failure is a successful heart transplant, since in the transplanted heart beta-adrenoceptor number and function seems to be normalized. Moreover, the data currently available do not suggest any development of super- or subsensitivity of postsynaptic cardiac beta-adrenoceptors in the transplanted human heart.

Cardiac beta-adrenoceptors, G-proteins and adenylate cyclase regulation during myocardial hypertrophy

The role of the beta-adrenoceptor-G-protein-adenylate cyclase system in the pathogenesis of cardiac hypertrophy was studied. We have used a minipig model of pressure-overload cardiac hypertrophy secondary to aortic banding. Four groups of five animals were used: minipigs made hypertrophic were evaluated 2 months (CH2 group) and 9 months (CH9 group) later and compared to controls (C2 and C9 groups, respectively). A decrease in beta-adrenergic receptor density and an increase in antagonist affinity were shown in left ventricular membranes of hypertrophied animals compared with controls. In both groups, CH2 and CH9, an increase in EC50 for isoproterenol-stimulated adenylate cyclase activity, an increase in forskolin-stimulated adenylate cyclase activity and a diminished inhibition by carbachol of isoproterenol-stimulated adenylate cyclase were observed. In contrast, fluoride-stimulated adenylate cyclase activity was markedly increased only in the end stage of hypertrophy. alpha s-cholera toxin-catalysed ADP-ribosylation is increased in early hypertrophy and then decreases with late hypertrophy and a similar pattern is observed with alpha o pertussis toxin-catalysed ADP-ribosylation, whereas alpha i-ADP-ribosylation remains unchanged. Tissue content of Gs-, Gi- and Go-proteins, as assessed by specific antibodies, was found unchanged in CH9 and CH2 groups when compared with that in C9 and C2 control groups, respectively. Modifications in Gs functional activity in later hypertrophic stages, expressed as alterations in cholera toxin ADP-ribosylation and adenylate cyclase fluoride responsiveness, may be important in the pathogenesis of decompensation from compensated hypertrophy to cardiac failure.

Percutaneous transvenous mitral commissurotomy using the Inoue balloon catheter

Since its inception in 1982, percutaneous transvenous mitral commissurotomy (PTMC) with the Inoue balloon catheter has gained increasingly wide use internationally. The procedure is technically successful in over 90% of patients, and the long-term durability of commissurotomy is excellent in those with pliable mitral valve leaflets and minimally deformed submitral apparatus. PTMC offers an alternative to patients previously not considered candidates for surgery, where no alternative had existed in the past. After transseptal puncture, PTMC using the Inoue balloon can be accomplished easily in the majority of patients. In comparison to double balloon mitral valvotomy, the postdilatation valve area is similar, the incidence of mitral regurgitation is not different, and the fluoroscopic and procedure time are markedly shorter. While patients with little valve deformity are excellent candidates for this procedure, and those not considered candidates for surgical therapy are also easily defined, selection of patients for balloon dilatation among those with significant valve deformity who are otherwise candidates for valve replacement therapy remains a challenging problem.

Agonist-independent effects of muscarinic antagonists on Ca2+ and K+ currents in frog and rat cardiac cells

1. The whole-cell patch clamp and intracellular perfusion techniques were used for studying the effects of atropine and other muscarinic acetylcholine receptor (mAChR) antagonists on the L-type calcium currents (ICa) in frog and rat ventricular myocytes, and on the mAChR-activated K+ current (IK(ACh)) in frog atrial myocytes. 2. In frog ventricular myocytes, atropine (0.1 nM to 1 microM) reversed the inhibitory effect of acetylcholine (ACh, 1 nM) on ICa previously stimulated by isoprenaline (Iso, 2 microM), a beta-adrenergic agonist. However, in the concomitant presence of Iso, ACh and atropine, ICa was > 50% larger than in Iso alone. 3. The effects of atropine were then examined in the absence of mAChR agonists. After a preliminary stimulation of ICa with Iso (0.1 or 2 microM), atropine induced a dose-dependent stimulation of ICa. EC50 (i.e. the concentration of atropine at which the response was 50% of the maximum) and Emax (i.e. maximal stimulation of ICa expressed as percentage increase in ICa with respect to the level in Iso alone) were respectively 0.6 nM and 35%. The stimulatory effect of atropine on ICa was not voltage dependent. 4. Atropine (1 microM) had no effect on frog ICa (i) under basal conditions, (ii) upon stimulation of ICa by the dihydropyridine agonist (-)-Bay K 8644 (1 microM), or (iii) when ICa had been previously stimulated by intracellular perfusion with cyclic AMP (3 microM). However, atropine increased ICa after a stimulation by forskolin (0.3 microM). Therefore, an increased adenylyl cyclase activity was required for atropine to produce its stimulatory effect on ICa. 5. The order of potency of mAChR antagonists to reverse the inhibitory effect of ACh on Iso elevated ICa in frog ventricle was atropine > AF-DX 116 >> pirenzepine. In the absence of ACh, mAChR antagonists produced their stimulatory effect on Iso elevated ICa with the same order of potency. 6. Intracellular substitution of Gpp(NH)p (5'-guanylylimidiphosphate) for GTP (420 microM) induced a strong inhibition of frog ICa in the presence of Iso (2 microM). This effect was attributed earlier to the spontaneous and irreversible activation of the GTP-binding regulatory protein (G protein), Gi, responsible for adenylyl cyclase inhibition. Atropine (1 microM) slowed down by a factor of 2 the rate of ICa inhibition induced by Gpp(NH)p. 7. In frog atrial myocytes, intracellular perfusion with 1 mM Gpp(NH)p induces spontaneous activation of IK(ACh). This effect was attributed earlier to the spontaneous and irreversible activation of the G protein, GK.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of alpha-subunits of G proteins in failing human heart: a reappraisal utilizing quantitative polymerase chain reaction

A decrease in the density of the beta 1-adrenergic receptor and an increase in the functional activity of the G inhibitory protein Gi accompany human heart failure; however, the molecular and biochemical mechanisms responsible for these changes are unclear. We previously reported that the steady-state levels of the mRNAs encoding both alpha Gi-3 and alpha Gs were significantly increased in failing human heart. However, these results are not consistent with recent studies demonstrating that immunodetectable levels of alpha G proteins are not different in failing human hearts when compared with non-failing controls. In addition, analysis of the 5' flanking regions of alpha Gi and alpha Gs suggests that these two genes are unlikely to be co-regulated as their regulatory domains are quite different. Therefore, we hypothesized that the disparity between the measurements of alpha G protein gene expression and assessment of the actual levels of alpha G proteins might be due to technical limitations of the Northern blot technique utilized in previous studies for assessment of the mRNA levels; (i) cytoskeletal beta-actin mRNA was used as a standard for normalization; and (ii) only relative levels of alpha G mRNAs were measured. The recent application of the polymerase chain reaction to quantification of mRNA levels in small quantities of human heart provided the tool with which to test this hypothesis. When expressed in molecules of mRNA per microgram of total RNA, there were no differences in the levels of alpha Gi and alpha Gs mRNAs in failing human heart when compared with non-failing controls.(ABSTRACT TRUNCATED AT 250 WORDS)