Atrial Strain Imaging after Repair of Tetralogy of Fallot: A Systematic Review

We performed a systematic review of the literature on assessment of right atrial (RA) and left atrial (LA) deformation by myocardial strain imaging in patients with repaired tetralogy of Fallot. Ten studies with a total of 536 adolescent and adult patients were included, of which 5 evaluated RA deformation, 2 studied LA deformation and 3 assessed deformation of both atria. Seven studies used speckle tracking echocardiography, 2 employed tissue Doppler imaging and 1 applied cardiac magnetic resonance feature tracking. Main findings were (i) reduced regional and/or global RA and LA strain and strain rate consistent with reduced conduit, reservoir and contractile function of the two atria in patients; (ii) associations between RA and LA deformation indices suggestive of atrial–atrial interaction; and (iii) relationships between RA deformation and indices of right ventricular systolic and diastolic function. The lack of data on prognostic value of atrial strain is an important knowledge gap.     

Cellular mechanisms of impaired adrenergic responsiveness in neonatal dogs.

The myocardial responsiveness of conscious, instrumental dogs to exogenously administered isoproterenol and norepinephrine was investigated in neonatal, 6-wk-old, and adult animals. Comparable base-line values for peak left ventricular derivative of pressure with respect to time were observed in all age categories. However, when compared with adult responses, the sympathomimetic amine-induced increases in neonatal left ventricular dP/dt were significantly blunted at each concentration of adrenergic agonist examined, whereas the 6-wk-old puppies displayed an intermediate inotropic response. To investigate the cellular mechanisms of this blunted neonatal response, we correlated physiologic and biochemical measurements of the myocardial responses to catecholamines in each age category. When compared with adult myocardial membrane preparations, neonatal cardiac membranes were characterized in vitro by an increased density of beta-adrenergic binding sites, comparable affinity for adrenergic agonists and antagonists, and an enhanced coupling of adenylate cyclase activation to receptor occupancy. Simultaneous changes in either the serum catecholamine concentration or the membrane content of other intrinsic proteins failed to account for the observed neonatal increase in beta-adrenergic receptor density. These findings are most consistent with a compensatory mechanism of the cardiac cell membrane, whereby an inherent depression in the adrenergic responsiveness of the immature myocardium appears to induce the increase in receptor density and activation of adenylate cyclase.     

Differential regulation of right and left ventricular beta-adrenergic receptors in newborn lambs with experimental cyanotic heart disease.

To determine whether chronic hypoxemia secondary to an intracardiac right-to-left shunt alters regulation of the myocardial beta-adrenergic receptor/adenylate cyclase system, we produced chronic hypoxemia in nine newborn lambs by creating right ventricular outflow obstruction and an atrial septal defect. Oxygen saturation was reduced to 65-74% for 2 wk. Eight lambs served as normoxemic controls. beta-receptor density (Bmax) and ligand affinity (KD) were determined with the radio-ligand [125I]iodocyanopindolol and adenylate cyclase activity determined during stimulation with isoproterenol, sodium fluoride (NaF), and forskolin. During chronic hypoxemia, Bmax decreased 45% (hypoxemic, 180.6 +/- 31.5 vs. control, 330.5 +/- 60.1 fmol/mg) in the left ventricle (exposed to hypoxemia alone) but was unchanged in the right ventricle (exposed to hypoxemia and pressure overload). KD was not different from control in either ventricle. Left ventricular isoproterenol-stimulated adenylate cyclase activity was decreased by 39% (30.0 +/- 4.3% increase vs. 44.1 +/- 9.5% increase) whereas right ventricular adenylate cyclase activity was unchanged. Stimulation of adenylate cyclase with NaF or forskolin was not different from control in either ventricle. Circulating epinephrine was increased fourfold whereas circulating and myocardial norepinephrine were unchanged. These data demonstrate a down-regulation of the left ventricular beta-adrenergic receptor/adenylate cyclase system during chronic hypoxemia secondary to an intracardiac right-to-left shunt.     

Functional and physical properties of chick atrial and ventricular GTP-binding proteins: relationship to muscarinic acetylcholine receptor-mediated responses

Activation of muscarinic acetylcholine receptors in chick atria, but not ventricles, causes an increase in K+ permeability. Because of suggestions that this difference in muscarinic receptor-mediated physiological responses may be due to changes in the guanine nucleotide regulatory proteins (termed Go and Gi) associated with the receptor, we compared the functional and biochemical properties of these proteins in atria and ventricles from 8-day chick embryos. The affinity of agonist for the muscarinic receptor in either the absence or presence of guanine nucleotides was the same in membranes from atria and ventricles; similar concentrations of guanyl-5'-yl imidodiphosphate were required to regulate agonist binding in both tissues (EC50 of 2.7 X 10(-8) and 2.0 X 10(-8) M for atria and ventricles, respectively). Forskolin-stimulated adenylate cyclase activity in atria and ventricles was equally sensitive to inhibition by guanyl-5'-yl imidodiphosphate. In addition, the muscarinic agonist carbachol inhibited adenylate cyclase in ventricles with an IC50 similar to that observed in atria, although the magnitude of inhibition was slightly less in ventricles. The physical properties of the alpha subunits of the guanine nucleotide regulatory proteins were examined after covalent modification by islet activating protein. Two [32P]ADP-ribosylated polypeptides were detected by one dimensional gel electrophoresis, with molecular weights equal to those reported for Go and Gi from other tissues. Similar amounts of both proteins were found in atrial and ventricular membranes. Peptide mapping demonstrated that although the 39 and 42 kD proteins had nonidentical peptide maps, atrial and ventricular peptide maps were identical.(ABSTRACT TRUNCATED AT 250 WORDS)     

Positive inotropic responses mediated by endothelin ET(A) and ET(B) receptors in human myocardial trabeculae.

The aim of the present study was to determine possible inotropic effects mediated by endothelin ET(A) and ET(B) receptors in human myocardial trabeculae from the right atrium and the left ventricle. Isolated trabeculae from human hearts were paced at 1.0 Hz in tissue baths, and changes in isometric contractile force upon exposure to agonist were studied. Endothelin-1 (ET-1) and ET-3 had a strong positive inotropic effect in all trabeculae. ET-1 was significantly more potent than ET-3 in both atrial (P < 0.001) and ventricular (P < 0.05) trabeculae. Preincubation with the ET(A) receptor antagonist FR139317 (1 microM) decreased significantly (P < 0.005) the potency of ET- I in both atrial and ventricular trabeculae, without any significant changes in Emax (maximum effect obtained with an agonist). The ET(B) receptor agonist IRL 1620 had a positive inotropic effect only in some trabeculae, and the ET(B) receptor antagonist BQ 788 (1 microM) almost completely blocked this effect. These results suggest that both ET(A) and ET(B) receptors mediate positive inotropic responses at both the atrial and ventricular level in the human heart.     

组织多普勒成像技术评价阵发性房颤心房收缩和电-机械时间的变化

目的：采用组织多普勒成像技术(TDl)，观察阵发性房颤(PAF)患者左、右心房和房室环处心房收缩时间(A)和电—机械时间(P—A)的变化。方法：PAF组61例，正常组32例，取心尖四腔观，将TDI取样容积分别置于室间隔房室环、左室侧壁二尖瓣环和右室侧壁三尖瓣环处，分别记录各点的的运动频谱，测量不同部位P—A和A值。结果：与对照组比较，PAF组左、右心房径明显增大；各部位的P—A、A均显著延长；P—A、A与左、右心房径之间无明显相关，而与房颤持续时间、病史及年龄呈相关性，线性回归分析显示房颤持续时间是P—A的主要影响因素。结论：阵发性房颤患者心房收缩、电—机械时间显著延长，且与左房、右房扩大无关，而可能取决于房颤事件的持续时间。     

Chronic norepinephrine elicits desensitization by uncoupling the beta-receptor.

The goal of this study was to determine the mechanism of beta-adrenergic receptor desensitization after chronic elevation of circulating NE levels. Osmotic minipumps containing either NE or saline were implanted subcutaneously in dogs for 3-4 wk. Physiologic desensitization to isoproterenol was confirmed in conscious dogs, i.e., left ventricular dP/dt increased in response to isoproterenol (0.4 micrograms/kg per min) by 5,625 +/- 731 mmHg/s in control dogs with saline pumps, and significantly less, P less than 0.01, by 2,093 +/- 263 mmHg/s in dogs with NE pumps. Myocardial beta-adrenergic receptor density as determined with 125I-cyanopindolol binding was 49% higher (p less than 0.05) in the NE pump group. However, beta-adrenergic receptor agonist binding with isoproterenol demonstrated a significant shift into the low affinity state for the animals with NE pumps. Basal, GTP plus isoproterenol, 5'-guanylylimidodiphosphate, sodium fluoride, and forskolin-stimulated adenylate cyclase activity in the NE pump group were significantly depressed (P less than 0.05) by amounts ranging from 20 to 40%. The functional activity of the guanine nucleotide binding protein Gs was also reduced (P less than 0.05) in animals with NE pumps. Thus, the process of desensitization in response to chronic elevation of NE levels in intact, normal dogs does not involve a decrease in beta-adrenergic receptor density. Rather, it is characterized by reduced adenylate cyclase activation and uncoupling of the beta-adrenergic receptor in association with decreased activity of the GTP-coupling protein Gs.     

Effects of acute ischemia in the dog on myocardial blood flow, beta receptors, and adenylate cyclase activity with and without chronic beta blockade.

We ligated the left anterior descending coronary artery for 1 or 2 h in 31 purebred beagles. We did not detect any changes in beta-adrenergic receptor density or affinity when normal and ischemic zones were compared, either in the subendocardium or in the subepicardium. In the ischemic zones, there was a significant decline in all measures of adenylate cyclase activity, including activity mediated by the beta-adrenergic receptor. By contrast, after chronic beta-adrenergic blockade (1.5 mg/kg propranolol i.v. twice daily for 7 d), there was an increase in adenylate cyclase activity stimulated by (-)-isoproterenol relative to adenylate cyclase activity stimulated by guanyl-5'imidodiphosphate (GppNHp) in both normal and ischemic tissue, suggesting that one effect of chronic beta blockade may be to enhance coupling between the stimulatory guanine nucleotide regulatory protein (Gs) and the beta-adrenergic receptor, despite a reduction in the number or function of Gs units. Chronic beta blockade also led to up regulation of beta-adrenergic receptor density in subepicardial regions. After 20 min of reperfusion following 2 h of ischemia, adenylate cyclase activity tended to return to control levels, particularly in the subepicardium, where (-)-isoproterenol-stimulated adenylate cyclase activity was not different from normal myocardium. We conclude that chronic beta-adrenergic blockade may have beneficial effects during prolonged episodes of myocardial ischemia by preserving signal transduction mediated by the beta-adrenergic receptor.     

Concanavalin A amplifies both beta-adrenergic and muscarinic cholinergic receptor-adenylate cyclase-linked pathways in cardiac myocytes.

Concanavalin A (Con A) is a tetrameric plant lectin that disrupts plasma membrane-cytoskeletal interactions and alters plasma membrane fluidity. We used Con A as a probe to explore beta-adrenergic and muscarinic cholinergic receptor-mediated regulation of cAMP in intact neonatal rat ventricular myocytes. Preincubation with Con A, 0.5 micrograms/ml, attenuated 1 microM (-)-norepinephrine (NE)-induced downregulation of beta-adrenergic receptors and resulted in a 50% augmentation of cAMP accumulation stimulated by 1 microM NE. Con A also augmented forskolin (1-10 microM)-stimulated cAMP accumulation by an average of 37% (P less than 0.05); however, Con A preincubation had no effect on basal or cholera toxin-stimulated cAMP content. The muscarinic cholinergic agonist carbachol (1-100 microM) decreased 1 microM NE-stimulated cAMP generation by an average of 32% (n = 7, P less than 0.05); preincubation with Con A further enhanced the inhibitory effect of carbachol by 18% (n = 7, P less than 0.05). Carbachol (1 microM) for 2 h decreased muscarinic cholinergic receptor density in whole cells by 33%; preincubation with Con A prevented this receptor downregulation. Con A pretreatment did not affect (-)-isoproterenol- or forskolin-stimulated adenylate cyclase activity in cell homogenates, suggesting that an intact cytoarchitecture is necessary for Con A to augment cAMP formation. We conclude that Con A, through its modulation of beta-adrenergic and muscarinic cholinergic receptor signaling, amplifies both stimulatory and inhibitory adenylate cyclase-linked pathways in intact neonatal ventricular myocytes. These data suggest the possibility that plasma membrane-cytoskeletal interaction is an important regulator of transmembrane signaling because interference with this interaction results in alterations in cAMP accumulation mediated by both beta-adrenergic- and muscarinic cholinergic-adenylate cyclase pathways.     

Canine cardiac muscarinic receptors, G proteins, and adenylate cyclase after long-term morphine.

Short-term morphine stimulates vagal bradycardia. This led us to propose the hypothesis that chronically administered morphine would down-regulate myocardial muscarinic receptor systems. Dogs received morphine continuously for 2 weeks through an s.c. catheter, and cellular aspects of parasympathetic control of the heart were examined. Contrary to expectations, morphine increased muscarinic receptor density in the right atrium and left ventricle by 17 and 34%, respectively, with no change in the apparent affinity of the receptor (K(D)). Morphine also increased the expression of the G protein G(ialpha) by 115 and 233%, respectively, in right atrial and left ventricular sarcolemmal membranes. Morphine increased ventricular and atrial G(salpha) to a much lesser degree (49 and 25%). Morphine failed to alter basal or maximally stimulated (forskolin plus MnCl(2)) adenylate cyclase activity. The maximum cyclase activation by isoproterenol and the maximum inhibition by carbachol were similarly unaltered by morphine. Morphine reduced the ventricular but not atrial norepinephrine. Both long- and short-term morphine lowered tissue epinephrine content, suggesting that short-term morphine reduces extraneuronal uptake. Potential systemic and cellular models for myocardial adaptation to morphine are proposed, including sequential sympathetic and parasympathetic compensations.     

Beta-adrenergic neuroeffector abnormalities in the failing human heart are produced by local rather than systemic mechanisms.

In order to investigate the general cause of beta-adrenergic receptor neuroeffector abnormalities in the failing human heart, we measured ventricular myocardial adrenergic receptors, adrenergic neurotransmitters, and beta-adrenergic receptor-effector responses in nonfailing and failing hearts taken from nonfailing organ donors, subjects with endstage biventricular failure due to idiopathic dilated cardiomyopathy (IDC), and subjects with primary pulmonary hypertension (PPH) who exhibited isolated right ventricular failure. Relative to nonfailing PPH left ventricles, failing PPH right ventricles exhibited (a) markedly decreased beta 1-adrenergic receptor density, (b) marked depletion of tissue norepinephrine and neuropeptide Y, (c) decreased adenylate cyclase stimulation in response to the beta agonists isoproterenol and zinterol, and (d) decreased adenylate cyclase stimulation in response to Gpp(NH)p and forskolin. These abnormalities were directionally similar to, but generally more pronounced than, corresponding findings in failing IDC right ventricles, whereas values for these parameters in nonfailing left ventricles of PPH subjects were similar to values in the nonfailing left ventricles of organ donors. Additionally, relative to paired nonfailing PPH left ventricles and nonfailing right ventricles from organ donors, failing right ventricles from PPH subjects exhibited decreased adenylate cyclase stimulation by MnCl2. These data indicate that: (a) Adrenergic neuroeffector abnormalities present in the failing human heart are due to local mechanisms; systemic processes do not produce beta-adrenergic neuroeffector abnormalities. (b) Pressure-overloaded failing right ventricles of PPH subjects exhibit decreased activity of the catalytic subunit of adenylate cyclase, an abnormality not previously described in the failing human heart.     

Loss of high affinity cardiac beta adrenergic receptors in dogs with heart failure.

We studied the alterations in myocardial beta-adrenergic receptor-adenylate cyclase activity and muscarinic receptor density in a canine model of left ventricular (LV) failure. LV failure was characterized by a doubling of LV weight/body weight ratio (3.3 +/- 0.1 to 6.9 +/- 0.4 g/kg) and an elevation of LV end-diastolic pressure, 32 +/- 4.5 mmHg, compared with 7.7 +/- 0.6 mmHg in normal dogs. Despite a 44% increase in receptor density as measured by antagonist binding studies with [3H]dihydroalprenolol, there was a twofold decrease in receptor affinity, i.e., an increase in the dissociation constant (Kd) (5.6 +/- 0.7 to 12 +/- 1.6 nM) in heart failure. Agonist displacement of [3H]dihydroalprenolol binding with isoproterenol in the presence and absence of 5'-guanylylimidodiphosphate [Gpp(NH)p] demonstrated a striking loss of high affinity binding sites in heart failure (51 +/- 16 to 11 +/- 5%). Beta-Adrenergic receptor-mediated stimulation of adenylate cyclase and maximal stimulation with Gpp(NH)p or sodium fluoride was reduced in heart failure. There was a concomitant marked, P less than 0.01, reduction in muscarinic receptor density (242 +/- 19 vs. 111 +/- 20 fmol/mg). Thus, while muscarinic receptor density fell, beta-adrenergic receptor density actually increased in LV failure. However, a larger portion of the beta-adrenergic receptors are not functionally coupled to the GTP-stimulatory protein (Ns), as evidenced by a decrease in the fraction of receptors that bind agonist with high affinity.     

Studies of the role of membrane lipid order in the effects of delta 9-tetrahydrocannabinol on adenylate cyclase activation in heart

The studies in this report were carried out to investigate the effects of delta 9-tetrahydrocannabinol (delta 9-THC) on cardiac membrane adenylate cyclase activity and to determine the role of changes in membrane lipid order in these effects. delta 9-THC and its psychoactive metabolite, 11-OH-delta 9-THC, increased isoproterenol (ISO) stimulation of adenylate cyclase in rat cardiac ventricular membranes. Cannabidiol, cannabinol and (+)-delta 9-THC were all without effect, indicating that this effect of delta 9-THC is stereoselective and specific for cannabinoids with psychoactive potency. delta 9-THC also increased glucagon stimulation of adenylate cyclase. The enhancement of both ISO and glucagon-stimulated adenylate cyclase was due to an increase in the Vmax of these agonists with no significant change in Kact. delta 9-THC did not affect basal adenylate cyclase activity or the activation of the enzyme by forskolin, guanine nucleotides or fluoride ion. Those cannabinoids which increased ISO-stimulated adenylate cyclase activity also decreased the break temperature of the Arrhenius plot; evidence that the effects of delta 9-THC involve changes in membrane phospholipid order. The effects of the cannabinoids on cardiac membrane phospholipid order were investigated directly using diphenylhexatriene fluorescence polarization. delta 9-THC and 11-OH-delta 9-THC alone decreased the break temperature of the diphenylhexatriene temperature profile, i.e., decreased the temperature of the lipid phase separation. This effect of delta 9-THC was stereoselective.(ABSTRACT TRUNCATED AT 250 WORDS)     

Decreased stimulatory guanosine triphosphate binding protein in dogs with pressure-overload left ventricular failure.

Alterations in the level and function of the stimulatory guanyl nucleotide binding protein (Gs) from the cardiac sarcolemma were examined in a canine model of heart failure. The present study is based on our previous investigations that demonstrated both a loss of beta-adrenergic agonist high-affinity binding sites and a decreased adenylate cyclase activity in sarcolemma from failing hearts. Using cholera toxin and [32P]NAD, we labeled the alpha subunit of Gs (Gs alpha) and found a 59% reduction in the level of this protein. Further, a 50% reduction in Gs activity was noted in a reconstitution assay utilizing membranes from the mouse S49 lymphoma cell line cyc-, which is deficient in Gs. These data suggest that, in this model of pressure-overload left ventricular failure, the acquired defect in the beta-adrenergic receptor/adenylate cyclase system involves a deficiency in the coupling protein Gs. Such an abnormality may explain the decreased adrenergic responsiveness of the failing left ventricle.     

Ectopic beta-adrenergic receptor binding sites. possible molecular basis of aberrant catecholamine responsiveness of an adrenocortical tumor adenylate cyclase.

The molecular basis for the aberrant catecholamine responsiveness of the adenylate cyclase of adrenocortical carcinoma 494 was explored. The adenylate cyclase of this corticosteroid-producing, transplanted, adrenal cancer of the rat was stimulated not only by adrenocorticotropic hormone and fluoride, but also by the beta-adrenergic agonist, isoproterenol. The adenylate cyclase of normal adrenal tissue was unresponsive to isoproterenol. Direct binding studies with the specific high affinity B-adrenergic ligand, (-)[3H]dihydroalprenolol, demonstrated the pressure of 0.094 pmol of specific binding sites per milligram of tumor membrane protein. By contrast, normal adrenal membranes contained too few binding sites to accurately measure and study using these techniques. The tumor binding sites had high affinity for (-)[3H] dihydroalprenolol with an equilibrium dissociation constant of 2.1 nM. Adrenergic agonists competed for the binding sites in an order of potency, [(-) isoproterenol greater than (-) epinephrine (-) norepinephrine], paralleling their order of potency as beta-adrenergic agonists. The beta-adrenergic antagonist, (-) propranolol, competed for binding, causing half-mzximal inhibition of specific binding at a concentration of 6 nM. The alpha-adrenergic antagonist, phentolamine, and several catecholamine metabolites and precursors did not effectively compete for the binding sites at high concentrations. Binding was stereospecific, the (+) stereoisomers of beta-adrenergic agonists and antagonists requiring 40- to 300-fold higher concentrations than the corresponding (-) stereoisomers to half maximally inhibit (-) [3H] dihydroalprenolol binding. These results indicate that adrenocortical carcinoma 494 membranes contain beta-adrenergic receptor-binding sites which are not normally present in membranes of adrenal tissue. These ectopic beta-adrenergic receptors presumably confer on the neoplastic tissue the catecholamine sensitivity of its adenylate cyclase.     

Impaired cardiac muscarinic receptor function in dogs with heart failure.

Prior physiological studies have suggested that parasympathetic control is altered in heart failure. The goal of our studies was to investigate the influence of heart failure on the muscarinic receptor, and its coupling to adenylate cyclase. Ligand binding studies using [3H]quinuclidinyl benzilate and enriched left ventricular (LV) sarcolemma, demonstrated that muscarinic receptor density in heart failure declined 36% from a control of 5.6 +/- 0.6 pmol/mg, with no change in antagonist affinity. However, agonist competition studies with both carbachol and oxotremorine showed that it was a loss of high affinity agonist binding sites in the sarcolemma from failing LV that accounted for this difference. The functional efficacy of the muscarinic receptor was also examined. When 1 microM methacholine was added to 0.1 mM GTP and 0.1 mM isoproterenol, adenylate cyclase stimulated activity was inhibited by 15% in normal LV but only 5% in LV sarcolemma from animals with heart failure even when the reduced adenylate cyclase in these heart failure animals was taken into account. Even at 100-fold greater concentrations of methacholine, significantly less inhibition of adenylate cyclase activity was observed in LV failure as compared with normal LV sarcolemma. Levels of the GTP-inhibitory protein known to couple the muscarinic receptor to adenylate cyclase, as measured with pertussis toxin labeling, were not depressed in LV failure. Thus, the inhibitory pathway regulating LV adenylate cyclase activity is defective in heart failure. The decrease in muscarinic receptor density, and in particular the specific loss of the high affinity agonist binding component of this receptor population, appears to be the major factor underlying this abnormality.     

Adenylate cyclase and beta adrenergic receptor development in the mouse heart

We have demonstrated previously a postnatal peak for the beta adrenergic receptor in the heart and detected the appearance of a beta adrenergic receptor before an (-)-isoproterenol inducible increase in heart rate. The present study examined 1) agonist displaceable [3H] dihydroalprenolol (DHA) binding in the neonatal and adult mouse heart and 2) adenylate cyclase in fetal, neonatal and adult mouse heart. 3[H]DHA binding displaceable by (-)-isoproterenol gave a similar Ki from 1 day neonate through adult. Similar to the result found for antagonist displacement binding, there was a dramatic increase in the agonist displaceable [3H] DHA binding postnatally. The maximum was achieved in 2 weeks and then gradually declined to adult level. Cyclase activity (basal, (-)-isoproterenol- and NaF- stimulated) paralleled beta adrenergic receptor increases before birth. However, no early postnatal peak was present. In the 13 day fetal mouse heart, there is no (-)-isoproterenol increase in heart rate, but beta adrenergic receptor (13 +/- 4% of adult) and (-)-isoproterenol-stimulated adenylate cyclase activity (15 +/- 5% of adult) are present. It is concluded that 1) no significant difference exists between the agonist and antagonist displaceable [3H] DHA binding during development, 2) adenylate cyclase activity increases significantly during the last third of pregnancy in parallel with the beta adrenergic receptor, 3) both the beta adrenergic receptor and adenylate cyclase activity can be detected before the heart rate responses and 4) total adenylate cyclase activity does not increase in parallel with the early postnatal beta adrenergic receptor peak.     

Beta adrenergic receptor subtypes in the atria: evidence for close coupling of beta-1 and beta-2 adrenergic receptors to adenylate cyclase

The relationship between occupancy of beta adrenergic receptors and stimulation of adenylate cyclase in dog atrial tissue was examined by studying the binding of [125I]iodopindolol and the activation of adenylate cyclase. Computer-assisted nonlinear regression analysis was used to analyze the inhibition of isoproterenol-stimulated adenylate cyclase activity by beta-1- or beta-2-selective antagonists. The Ki values for each subtype of receptor for the selective antagonists resulting from studies of the inhibition of adenylate cyclase activity were similar to those determined in studies of the inhibition of the binding of [125I]iodopindolol. To compare further the occupancy of beta-1 or beta-2 adrenergic receptors with the activation of adenylate cyclase mediated by each class of receptor, computer modeling of the stimulation of adenylate cyclase by the beta-1-selective agonist norepinephrine was carried out. The EC50 values of norepinephrine for each receptor subtype, as measured in studies of norepinephrine-stimulated adenylate cyclase activity, were similar to the Ki values for the inhibition by norepinephrine of the binding of [125I]iodopindolol to each receptor subtype. The data led to the conclusion that beta-1 adrenergic receptors make up about 70% of the total number of beta adrenergic receptors and mediate 70% of the increase in adenylate cyclase activity produced by isoproterenol. These results suggest that the relationship between occupancy of each class of receptor and activation of adenylate cyclase is linear and that, when agonist-stimulated adenylate cyclase activity is used as a functional response, neither spare beta-1 nor spare beta-2 adrenergic receptors exist in the atrium.(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of beta-adrenergic receptor linked to adenylate cyclase in a human cancer cell line (COLO 16).

1. A human cancer cell line (COLO 16) derived originally from an epidermal squamous cell carcinoma was found to possess adenylate cyclase responsiveness to beta-adrenergic agonists. 2. The adenylate cyclase response was characterized with respect to activation constants (KA) for various beta-adrenergic agonists and inhibition constants (Ki) for antagonists. 3. Intact cells responded with dose-dependent increases in production of cyclic adenosine 3':5'-monophosphate. 4. Properties of the beta-adrenergic receptor were evaluated by using the specific binding of [3H]propranolol to cell membranes. Specific binding was saturable, with KD 5.79 nmol/l and binding sites 0.68 pmol/mg of protein. 5. Competition for binding to cell membranes was shown by beta-adrenergic agonists and antagonists and was stereospecific. There was close agreement between the affinity of these various agents on adenylate cyclase and receptor binding. 6. It is likely that the beta-adrenergic receptor-linked adenylate cyclase in COLO 16 cells represents persistence in a cancer cell line of a receptor present normally in epidermal cells.     

Reduced beta-adrenergic receptor activation decreases G-protein expression and beta-adrenergic receptor kinase activity in porcine heart.

To determine whether beta-adrenergic receptor agonist activation influences guanosine 5'-triphosphate-binding protein (G-protein) expression and beta-adrenergic receptor kinase activity in the heart, we examined the effects of chronic beta 1-adrenergic receptor antagonist treatment (bisoprolol, 0.2 mg/kg per d i.v., 35 d) on components of the myocardial beta-adrenergic receptor-G-protein-adenylyl cyclase pathway in porcine myocardium. Three novel alterations in cardiac adrenergic signaling associated with chronic reduction in beta-adrenergic receptor agonist activation were found. First, there was coordinate downregulation of Gi alpha 2 and Gs alpha mRNA and protein expression in the left ventricle; reduced G-protein content was also found in the right atrium. Second, in the left ventricle, there was a twofold increase in beta-adrenergic receptor-dependent stimulation of adenylyl cyclase and a persistent high affinity state of the beta-adrenergic receptor. Finally, there was a reduction in left ventricular beta-adrenergic receptor kinase activity, suggesting a previously unrecognized association between the degree of adrenergic activation and myocardial beta-adrenergic receptor kinase expression. The heart appears to adapt in response to chronic beta-adrenergic receptor antagonist administration in a manner that would be expected to offset reduced agonist stimulation. The mechanisms for achieving this extend beyond beta-adrenergic receptor upregulation and include alterations in G-protein expression, beta-adrenergic receptor-Gs interaction, and myocardial beta-adrenergic receptor kinase activity.