Biomedical subjects

D E Vatner

Publications and source records attributed to D E Vatner.

At least 37 records · Page 2Linked to original sources

PubMed1998

Overexpression of myocardial Gsalpha prevents full expression of catecholamine desensitization despite increased beta-adrenergic receptor kinase.

Inotropic and chronotropic responses to catecholamines in young adult transgenic mice overexpressing myocardial Gsalpha are enhanced. One might predict that over the life of the animal, this chronically enhanced beta-adrenergic receptor stimulation would result in homologous catecholamine desensitization. To test this hypothesis, old transgenic Gsalpha mice and age-matched controls were studied physiologically in terms of responsiveness of left ventricular function (ejection fraction) to isoproterenol in vivo and in vitro in terms of beta-adrenergic receptor signaling. Old transgenic mice still responded to isoproterenol with augmented (P < 0.05) left ventricular ejection fraction (+44+/-3%) compared with age-matched controls (+24+/-1%). Although total beta-adrenergic receptor density was reduced in the old transgenic mice, and G protein receptor kinase 2 (beta-adrenergic receptor kinase) levels were increased, the fraction of receptors binding agonist with high affinity as well as isoproterenol- and G protein-stimulated adenylyl cyclase activities were enhanced. Thus, classical catecholamine desensitization is not effective in attenuation of persistently enhanced responses to sympathetic stimulation in mice overexpressing myocardial Gsalpha. To support this conclusion further, experiments were performed with chronic isoproterenol, which elicited effective desensitization in wild-type controls, but failed to elicit desensitization in overexpressed Gsalpha mice. The results of this study suggest that the lack of protective desensitization mechanisms may be responsible in part for the dilated cardiomyopathy which develops with chronic sympathetic stress over the life of these animals.

Adenylyl Cyclases↗

PubMed1998

Depressed heart rate variability and arterial baroreflex in conscious transgenic mice with overexpression of cardiac Gsalpha.

Recently, we developed a transgenic mouse with cardiac-specific Gsalpha overexpression (TG mouse), which exhibits enhanced postsynaptic beta-adrenergic receptor signaling, ultimately developing a cardiomyopathy. The goal of the present study was to determine whether cardiac Gsalpha overexpression alters autonomic cardiovascular control, which could shed light on the mechanism responsible for the later development of cardiomyopathy. Mean arterial pressure was increased (P<.05) in conscious, chronically instrumented TG mice (123+/-1 mm Hg) compared with age-matched wild-type (WT) control mice (103+/-1 mm Hg). Respiratory frequency was increased (P<.05) in TG mice (269+/-26/min) compared with WT mice (210+/-20/min). By use of telemetric techniques, baseline heart rate (HR) was elevated (P<.05) in conscious, untethered TG mice (696+/-13 bpm) compared with WT mice (568+/-28 bpm). Intrinsic HR, after propranolol and atropine or after ganglionic blockade with hexamethonium, was not different between TG and WT mice. Both the normal minute-to-minute and circadian variations of HR observed in WT mice were markedly blunted in TG mice. HR variability was assessed by the time-domain and frequency-domain methods. At baseline, time-domain analysis indices were reduced (P<.05) in TG mice compared with WT mice. Although the low frequency (LF) component was higher (P<.05) than the high frequency (HF) component in WT mice, the LF component was less (P<.05) than the HF component in TG mice. In addition, arterial baroreflex regulation of HR was markedly blunted in TG mice in response to both nitroglycerin-induced hypotension and phenylephrine-induced hypertension. The reduced LF/HF ratio in TG mice was surprising in view of enhanced beta-adrenergic signaling and may be due to reduced neural tone secondary to the elevated arterial pressure or alterations in arterial baroreflex control. Dobutamine infusion in WT mice also resulted in depressed HR variability. The combination of elevated baseline HR, arterial pressure, and respiratory frequency suggests that enhanced beta-adrenergic signaling in TG mice results in reduced HR variability, in terms of both minute-to-minute variability and the lack of circadian variations in HR. The lack of normal HR variability in general and the failure of HR to decline, even during sleep, may actually be critical mechanisms contributing to the ultimate development of cardiomyopathy in these animals.

Adrenergic beta-Agonists↗

PubMed1998

Physiological and biochemical adrenergic regulation of the stunned myocardium.

A dual approach was employed to study beta-adrenergic receptor signal transduction in post ischemic (stunned) myocardium, examining physiological interventions in awake, chronically instrumented pigs and biochemical, cellular mechanisms in sarcolemmal preparations from the stunned hearts using the contralateral non-ischemic zone as a control. Ten min of coronary artery occlusion (CAO) and 30 min coronary artery reperfusion (CAR) resulted in depressed posterior wall-thickening (myocardial stunning). Isoproterenol increased transmural wall thickening more in stunned myocardium than in non-ischemic myocardium. In contrast, the responses of wall thickening to forskolin, actually decreased during stunning compared with control. NKH 477, a water soluble forskolin derivative, that does not activate cardiac nerves, increased wall thickening in non-ischemic tissue similarly to the effects on stunned myocardium. Increasing cardiac neural tone reflexly with inferior venal caval occlusion (IVCO) elicited similar results to forskolin, i.e., stunned myocardium responded with less of an increase in wall thickening as compared with non-ischemic myocardium. Beta-adrenergic receptor density, as determined with 125I-cyanopindolol binding, was significantly increased in stunned subendocardium and subepicardium compared with respective values in non-ischemic myocardium. There were no differences in the response of adenylyl cyclase to isoproterenol in stunned and non-ischemic myocardium. The enhanced responsiveness of the beta-adrenergic receptor to isoproterenol stimulation in stunned myocardium corresponded to the increase in beta-adrenergic receptor density. The combination of enhanced responses to isoproterenol, and decreased responses to forskolin and to IVCO and preserved responsiveness to NKH 477, suggest that stunned myocardium is characterized by transient sympathetic neural stunning. The enhanced sensitivity to beta-adrenergic receptor stimulation has important clinical implications, both in terms of therapy of stunned myocardium and detection of stunned and/or hibernating myocardium, i.e., low dose dobutamine echocardiography.

Adenylyl Cyclases↗

PubMed1997

Effects of cardiac denervation on development of heart failure and catecholamine desensitization.

BACKGROUND: Two signatures of heart failure are activation of the sympathetic nervous system and catecholamine desensitization. However, whether or not the elimination of cardiac nerves affects either the progression of heart failure or catecholamine desensitization is not clear. METHODS AND RESULTS: We studied 8 dogs with selective ventricular denervation (VD) (surgical technique) and 10 intact dogs, chronically instrumented for measurement of left ventricular (LV) and arterial pressures, LV dP/dt, LV internal diameter, and wall thickness before and after heart failure was induced by rapid pacing (240 bpm) for 3 to 4 weeks. VD was confirmed by the absence of reflex effects induced by intracardiac veratrine and depletion of tissue norepinephrine and by supersensitive responses to norepinephrine. During the development of heart failure, LV end-systolic and end-diastolic stresses and heart rate increased, while myocardial contractility, as reflected by LV dP/dt and mean velocity of circumferential fiber shortening corrected for heart rate (Vcf(c)), decreased in both intact and VD dogs. However, the increases in LV end-diastolic stress and decreases in LV dP/dt as well as the relationship between LV systolic stress and Vcf(c) in heart failure were less (P<.05) in VD dogs. The responses of LV dP/dt and heart rate to both isoproterenol and norepinephrine in intact dogs were reduced in heart failure. The physiological desensitization to the inotropic effects of isoproterenol and norepinephrine was less in dogs with VD (P<.05), but chronotropic responses were similar because atrial innervation remained intact. Plasma norepinephrine levels were not different in VD dogs (592+/-79 pg/mL) compared with intact dogs (576+/-81 pg/mL) in heart failure. CONCLUSIONS: Dogs with selective VD tolerated the development of heart failure better than intact dogs and demonstrated significantly less catecholamine desensitization. The latter indicates that intact ventricular innervation is required for physiological expression of catecholamine desensitization despite comparable elevation of plasma catecholamines during the development of heart failure.

Adrenergic alpha-Agonists↗

PubMed1997

Beta-adrenergic receptor signalling in stunned myocardium of conscious pigs.

The primary goal of this study was to compare the effects of isoproterenol which stimulates beta-adrenergic receptors and forskolin, and NKH 477, a water soluble derivative of forskolin, which stimulate adenylyl cyclase in stunned myocardium of conscious pigs, previously instrumented for measurements of left ventricular pressure and dP/dt, arterial pressure, and wall thickening. Ten min of coronary artery occlusion induced transmural reductions in blood flow (radioactive microspheres) in subepicardium (-98 +/- 2%) and subendocardium (-99 +/- 1%). Wall thickening (piezoelectric crystals) fell from 2.50 +/- 0.26 mm to -0.26 +/- 0.26 mm and remained depressed at 1.37 +/- 0.19 mm after 20-30 min coronary artery reperfusion, reflecting myocardial stunning. At that time, isoproterenol (0.2 microgram/kg) increased wall thickening in stunned myocardium (+1.40 +/- 0.16 mm, P < 0.05) more than in control (+0.71 +/- 0.22 mm), while forskolin elicited the opposite effects. NKH 477 (30 micrograms/kg), which does not penetrate the blood-brain barrier, increased systolic wall thickening similarly before (+0.95 +/- 0.25 mm) and during (+1.01 +/- 0.24 mm) myocardial stunning. The reflex inotropic responses to inferior vena caval occlusion on wall thickening were diminished, P < 0.05, in the stunned myocardium (+0.53 +/- 0.05 mm) compared with control (+0.95 +/- 0.07 mm). beta-adrenergic receptor density, which was quantitated with 125I-cyanopindolol binding, was increased transmurally in stunned myocardium compared with non-ischemic myocardium (subepicardium: +23 +/- 5%, subendocardium: +34 +/- 13%, P < 0.05). Basal and forskolin-stimulated adenylyl cyclase activities were decreased slightly, but significantly, in the stunned subendocardium but not in the subepicardium, while isoproterenol stimulation of adenylyl cyclase activity showed no differences. In summary, paradoxical responses to beta-adrenergic receptor stimulation were observed in stunned myocardium, with pharmacological stimulation with isoproterenol evoking enhanced responses, and neural stimulation with inferior vena caval occlusion eliciting depressed responses. The diminished responses to forskolin in vivo, in stunned myocardium were out of proportion to the biochemical measurements, and may be attributed to neurally mediated cardiac effects of forskolin, since the responses to direct stimulation of adenylyl cyclase by NKH 477 were preserved.

Adenylyl Cyclases↗

PubMed1997

Localisation of cardiac GS alpha in transgenic mice overexpressing GS alpha.

The biochemical and physiological effects of GS alpha activation are well known; however, little is known about the anatomical localisation of GS alpha in the myocardium. Knowledge of the localisation might yield insights into G protein function in heart. The utility of immunocytochemistry using immunofluorescent methods is limited in normal hearts because of the low expression of GS alpha. In order to magnify the GS alpha signal, we studied transgenic mice overexpressing myocardial GS alpha. Immunofluorescent techniques with confocal imaging using rabbit antiserum specific for GS alpha were studied in frozen sections of mouse left ventricle. GS alpha labeling appeared to be localised to the T-tubules and intercalated disks in the GS alpha overexpressing mouse hearts, whereas the control mice showed background fluorescence with diffuse faint labeling. The localisation of GS alpha to structures involved in calcium handling and membrane conductance places GS alpha at a focal point in the regulation of these key functions.

Animals↗

PubMed1997

Effects of chronic beta-adrenergic receptor stimulation in mice.

The goal of the present study was to determine the effects of chronic beta-adrenergic receptor stimulation with isoproterenol (ISO) on cardiac tissue, systemic trophic changes and on beta-adrenergic receptor desensitization in mice. Mice (n=36) received continuous ISO (30 microg/g/day) via osmotic minipump for 13 days. Left ventricle (LV)/body weight (BW) ratio was increased by 27% in ISO v control (CON) mice. The extent of cardiac hypertrophy induced by chronic ISO was offset in part by concomitant increases in body weight, which were greater in ISO than CON mice (22 v 8%), and occurred with increases in both muscle mass and brown fat to BW ratios. Histological analysis of mice revealed a three-fold increase in subendocardial interstitial connective tissue with no evidence of acute cellular necrosis or chronic inflammation. Acute i.v. ISO challenges induced dose-dependent increases in LV fractional shortening (FS) and ejection fraction (EF) using echocardiography (9 MHz), which were attenuated after chronic ISO, i.e. physiological desensitization was observed. Cellular mechanisms of beta-adrenergic receptor desensitization included decreases in beta-adrenergic receptor density (-49%) and decreased basal (-45%) and ISO-stimulated (-61%) adenylyl cyclase activities. Lesser decreases in forskolin-stimulated adenylyl cyclase activity (-16%) and adenylyl cyclase mRNA levels for both type V (-17%) and type VI (-23%) isoforms were observed following chronic ISO. Thus, chronic ISO (30 microg/g/day) induced cardiac hypertrophy without cellular necrosis, increased weight gain and clear physiological desensitization in mice, with more extensive biochemical mechanisms than expected from simple catecholamine-specific (homologous) desensitization.

Adenylyl Cyclases↗

PubMed1997

Downregulation of caveolin by chronic beta-adrenergic receptor stimulation in mice.

Caveolae, flask-shaped invaginations of cell membranes, are believed to play pivotal roles in transmembrane transportation of molecules and cellular signaling. Caveolin, a structural component of caveolae, interacts directly with G proteins and regulates their function. We investigated the effect of chronic beta-adrenergic receptor stimulation on the expression of caveolin subtypes in mouse hearts by immunoblotting and Northern blotting. Caveolin-1 and -3 were abundantly expressed in the heart and skeletal muscles, but not in the brain. Continuous (-)-isoproterenol, but not (+)-isoproterenol, infusion via osmotic minipump (30 micrograms.g-1.day-1) for 13 days significantly downregulated both caveolin subtypes in the heart. The expression of caveolin-1 was reduced by 48 +/- 6.1% and that of caveolin-3 by 28 +/- 4.0% (P < 0.01, n = 8 for each). The subcellular distribution of caveolin subtypes in ventricular myocardium was not altered as determined by sucrose gradient fractionation. In contrast, the expression of both caveolin subtypes in skeletal muscles was not significantly changed. Our data suggest that the expression of caveolin subtypes is regulated by beta-adrenergic receptor stimulation in the heart.

Adrenergic beta-Agonists↗

PubMed1997

Cardiomyopathy induced by cardiac Gs alpha overexpression.

The goal of this study was to determine whether chronic endogenous sympathetic stimulation resulting from the overexpression of cardiac stimulatory G protein alpha subunit (Gs alpha) in transgenic mice (15.3 +/- 0.1 mo old) resulted in a clinical picture of cardiomyopathy. The left ventricular ejection fraction, measured by echocardiography, was reduced in older mice with Gs alpha overexpression (50.4 +/- 5.4%) compared with age-matched control mice (70.9 +/- 1.6%; P < 0.05). When ejection fractions were compared at similar heart rates, the Gs alpha mice exhibited a greater left ventricular end-diastolic dimension than control mice (4.3 +/- 0.2 vs. 3.7 +/- 0.1 mm; P < 0.05). Baseline heart rates were elevated in conscious Gs alpha mice (722 +/- 27 beats/min; n = 5) compared with control mice (656 +/- 28 beats/min; n = 5). Moreover, electrocardiographic monitoring demonstrated a high incidence of arrhythmias. Increased mortality compared with control mice (31.6 vs. 3.0%; P < 0.01) was also observed. Thus older mice with Gs alpha overexpression exhibit many of the features of dilated cardiomyopathy. This study supports the concept that chronic sympathetic stimulation over an extended period of time, i.e., over the life of an animal, is deleterious and actually may result in cardiomyopathy.

Animals↗

PubMed1997

Consequences of growth hormone deficiency on cardiac structure, function, and beta-adrenergic pathway: studies in mutant dwarf rats.

To evaluate GH's role in cardiac physiology and its interrelationship with the beta-adrenergic system, we studied GH-deficient dwarf (dw/dw) and control rats in 4 groups of 20 each: dwarf group receiving placebo, dwarf-GH group receiving 2 mg/kg GH, dwarf-GH-propranolol group receiving 2 mg/kg GH and 750 mg/liter propranolol, and a control group of Lewis rats receiving placebo. Dwarf rats showed reduced left ventricular weight and myocyte cross-sectional area, and impaired cardiac performance in vitro. Left ventricular pressure-volume curves showed a shift upward and leftward, indicating reduced distensibility. These abnormalities reversed after GH treatment regardless of concomitant propranolol administration. Although isoproterenol responsiveness was reduced in dwarf rats, there were no differences in beta-adrenergic receptor density, affinity, Na+,K+-adenosine triphosphatase activity, or adenylyl cyclase activity. In summary, myocyte size, cardiac structure, myocardial contractility, and distensibility are abnormal in GH deficiency. The effects of GH are not mediated by the beta-adrenergic pathway, which, in turn, is unaffected by changes in the GH-insulin-like growth factor I axis. Thus, GH plays a regulatory role in normal cardiac physiology that is independent of the beta-adrenergic system.

Adrenergic beta-Agonists↗

PubMed1996

Physiological and biochemical evidence for coordinate increases in muscarinic receptors and Gi during pacing-induced heart failure.

BACKGROUND: It is not clear whether the increase in the myocardial guanylyl nucleotide inhibitory protein (Gi), frequently observed in heart failure, is associated with any functional effects. METHODS AND RESULTS: Eight sham-operated dogs and 10 dogs were studied with pacing-induced heart failure (240 bpm for 4 to 7 weeks), characterized by reduced (P<.05) left ventricular dP/dt (from 2926+/-99 to 1303+/-126 mm Hg/s). The muscarinic agonist acetylcholine (10 micrograms/kg IV) in the presence of ganglionic blockade reduced left ventricular dP/dt more (P<.05) in heart failure (-23+/-2%) than before heart failure (-8+/-2%), despite lesser reductions in arterial pressure. Gi alpha2 was increased by 55% in heart failure. Dose-response curves for carbachol (10-8 to 10-3 mol/L) inhibition of isoproterenol-stimulated adenylyl cyclase demonstrated significantly greater (P<.05) inhibition in heart failure compared with sham-operated dogs. These changes were associated with a coordinate increase in muscarinic receptor density, determined by antagonist binding with 3H-quinuclidinyl benzilate, in heart failure (153+/-6.2 fmol/mg protein) compared with sham-operated dogs (124+/-7.4 fmol/mg protein). Agonist binding with carbachol also revealed an increase in total muscarinic receptors in heart failure without a change in fraction of high- and low-affinity receptors. CONCLUSIONS: These data, in the aggregate, provide physiological and biochemical evidence to support the concept that the coordinate increases in muscarinic receptor number and Gi levels in heart failure are coupled to increased inhibition of adenylyl cyclase activity and an increased inhibition of myocardial contractility.

Acetylcholine↗

PubMed1996

Identification and functional role of beta-adrenergic receptor subtypes in primate and rodent: in vivo versus isolated myocytes.

We determined the relationship between the beta 1- and beta 2-adrenergic receptor subtypes in isolated myocytes and their physiological responsiveness in chronically instrumented conscious baboons and rats. In conscious baboons, isoproterenol (ISO) (0.02 microgram/kg) increased left ventricular (LV) dP/dt by 89 +/- 6.7% from 2898 +/- 370 mmHg/s and only by 13 +/- 3.3% from 2491 +/- 146 mmHg/s after beta 1-adrenergic receptor blockade, indicating that the predominant physiological response was mediated by beta 1-adrenergic receptors. Decreases in mean arterial pressure (-11 +/- 0.5 mmHg v -16 +/- 4.6 mmHg) and coronary vascular resistance (-3.1 +/- 0.4 v -3.6 +/- 0.4 mmHg/ml/min) induced by ISO were not different before and after beta 1-blockade, indicating that beta 2-adrenergic receptors were not blocked. In conscious rats, ISO (0.4 microgram/kg) increased LV dP/dt by 50 +/- 4.9% from 13252 +/- 2002 mmHg/s and only by 10 +/- 3.9% from 10793 +/- 1364 mmHg/s after beta 1-adrenergic receptor blockade: whereas decreases in mean arterial pressure induced by ISO were not different before and after beta 1-blockade (-19 +/- 2.4 mmHg v -16 +/- 2.2 mmHg), i.e. very consistent with the physiological responses in baboons. In vitro studies of isolated myocytes, using radioligand binding with 125I-cyanopindolol (125I-cyp) and the subtype beta 1-selective antagonist betaxolol and the beta 2-selective antagonist ICI 118551 indicated that the beta 1/beta 2 ratio of rat myocytes was 92/8: whereas baboon myocytes were more equally distributed (59/41). Thus, in both species the preponderance of effects of ISO on ventricular function was beta 1-adrenergic receptor mediated, which is consistent with the beta 1/beta 2 ratio in rat myocytes but not in baboon myocytes, where a significant fraction of beta 2-adrenergic receptors does not appear to exert an effect on conctractility in vivo.

Adrenergic beta-Antagonists↗

PubMed1996

Beta-adrenoceptor desensitization during the development of canine pacing-induced heart failure.

1. The goal of this review is to emphasize four major points regarding the development of catecholamine desensitization in heart failure (HF). 2. Catecholamine desensitization occurs prior to the development of HF (i.e. after 1 day of rapid pacing, physiological responses to beta-adrenoceptor stimulation are depressed by over 50%, yet no evidence of HF is observed for 3-4 weeks of rapid pacing). 3. Multiple mechanisms in the beta-adrenoceptor cascade are involved. In HF there are decreases in beta 1-adrenoceptors, high affinity beta-adrenoceptors, adenylyl cyclase activity and messenger RNA and increases in Gi. 4. Not all mechanisms appear simultaneously (i.e. early decreases occur in high affinity beta-adrenoceptors and adenylyl cyclase; late increases in Gi and decreases in beta-adrenoceptor density evolves). 5. Mechanisms distal to cAMP generation also play a role (i.e. alterations in ryanodine receptor binding and excitation-contraction coupling also occur).

Adrenergic beta-Agonists↗

PubMed1996

Prolonged exercise alters beta-adrenergic responsiveness in healthy sedentary humans.

To examine whether beta-adrenergic desensitization occurs after prolonged exercise, echocardiograms, heart rate responses to isoproterenol, plasma catecholamines, and circulating lymphocyte beta-adrenergic receptors were examined in 10 sedentary normal subjects at rest and after brief (10 min) and exhaustive (mean duration 95 min) cycle exercise. Resting end-diastolic volume and ejection fraction were significantly reduced after exercise (from 120 +/- 34 to 100 +/- 26 ml and from 60 +/- 0.4 to 54 +/- 0.6%, respectively; both P < 0.05). The amount of isoproterenol needed to increase heart rate 15 and 25 beats/min increased in a dose- (exercise duration) related fashion, and the increase in amount of isoproterenol needed after prolonged exercise was closely related to the decrease in ejection fraction (r2 = 0.67, P = 0.004). Circulating lymphocyte beta-receptor density and affinity, agonist binding, and adenylylcyclase levels were unchanged with prolonged exercise. In conclusion, prolonged exercise in sedentary normal subjects resulted in reduced cardiac chronotropic responsiveness to isoproterenol that was not reflected in peripheral lymphocyte beta-adrenergic-receptor downregulation.

Adenylyl Cyclases↗

PubMed1996

Adverse effects of chronic endogenous sympathetic drive induced by cardiac GS alpha overexpression.

To study the physiological effect of the overexpression of myocardial Gsalpha (protein levels increased by approximately threefold in transgenic mice), we examined the responsiveness to sympathomimetic amines by echocardiography (9 MHz) in five transgenic mice and five control mice (both 10.3 +/- 0.2 months old). Myocardial contractility in transgenic mice, as assessed by left ventricular (LV) fractional shortening (LVFS) and LV ejection fraction (LVEF) was not different from that of control mice at baseline (LVFS, 40 +/- 3% versus 36 +/- 2%; LVEF, 78 +/- 3% versus 74 +/- 3%). LVFS and LVEF values in transgenic mice during isoproterenol (ISO, 0.02 micrograms/kg per minute) infusion were higher than the values in control mice (LVFS, 68 +/- 4% versus 48 +/- 3%; LVEF, 96 +/- 1% versus 86 +/- 3%; P < .05). Norepinephrine (NE, 0.2 micrograms/kg per minute) infusion also increased LVFS and LVEF in transgenic mice more than in control mice (LVFS, 59 +/- 4% versus 47 +/- 3%; LVEF, 93 +/- 2% versus 85 +/- 3%; P < .05). Heart rates of transgenic mice were higher than those of control mice during ISO and NE infusion. In three transgenic mice with heart rates held constant, LV dP/dt rose by 33 +/- 2% with ISO (0.02 micrograms/kg per minute) and by only 13 +/- 2% in three wild-type control mice (P < .01). NE (0.1 micrograms/kg per minute) also induced a greater effect on LV dP/dt in the three transgenic mice with heart rates held constant compared with three wild-type control mice (65 +/ 8% versus 28 +/- 4%, P < .05). Pathological and histological analyses of older transgenic mouse hearts (16.0 +/- 0.8 months old) revealed hypertrophy, degeneration, atrophy of cells, and replacement fibrosis reflected by significant increases in collagen volume in the subendocardium (5.2 +/- 1.4% versus 1.2 +/- 0.3%, P < .05) and in the cross-sectional area of myocytes (298 +/- 29 versus 187 +/- 12 micron2, P < .05) compared with control mouse hearts. These results suggest that Gsalpha overexpression enhances the efficacy of the beta-adrenergic receptor-Gs-adenylyl cyclase signaling pathway. This in turn leads to augmented inotropic and chronotropic responses to endogenous sympathetic stimulation. This action over the life of the animal results in myocardial damage characterized by cellular degeneration, necrosis, and replacement fibrosis, with the remaining cells undergoing compensatory hypertrophy. As a model, this transgenic mouse offers new insights into the mechanisms of cardiomyopathy and heart failure and provides a new tool for their study.

Animals↗

PubMed1995

Inotropic effects of alpha 1-adrenergic agonists in myocardium from rats with postinfarction heart failure.

In clinical and experimental heart failure, the inotropic response to beta-adrenergic receptor stimulation is depressed. Therefore, non-beta-adrenergic mechanisms may assume increasing importance for summoning inotropic reserve in the failing heart. To test the integrity of the inotropic pathway mediated by alpha 1-adrenergic receptor stimulation in a model of chronic ischemic heart failure, we administered phenylephrine to noninfarcted left ventricular papillary muscles isolated from sham-operated rats (n = 10) and rats with large (> 40% left ventricular circumference) anterior myocardial infarctions (n = 9). Isometric force was monitored, and intracellular Ca2+ (Cai2+) transients were recorded with the bioluminescent protein aequorin. Compared with muscles from sham-operated rats, contractility of muscles from rats with postinfarction heart failure was depressed at extracellular Ca2+ concentrations between 0.5 and 3.0 mM. Phenylephrine produced comparable dose-dependent increases in developed tension (126 +/- 4 vs. 125 +/- 7% of baseline) and peak rate of tension rise (125 +/- 4 vs. 140 +/- 9% of baseline) in muscles from sham and infarcted rats, respectively. There was no significant change in the time course of the isometric twitch or in the time course or amplitude of the Cai2+ transient after phenylephrine administration in muscles from either group. No evidence of Cai2+ overload, as defined by spontaneous Ca2+ release, was observed during phenylephrine administration in muscles from normal or failing hearts. The density of alpha 1-adrenoceptors measured with [3H]prazosin binding in crude membranes isolated from noninfarcted left ventricular tissue was not different in control and infarcted hearts (48 +/- 5 vs. 53 +/- 4 fmol/mg protein). These data indicate that the positive inotropic effect of alpha-agonists may be preserved in chronic ischemic heart failure. In both normal and failing myocardium, the inotropic effects of alpha 1-adrenergic stimulation occurred with little or no increase in Cai2+ availability and no apparent adverse effects on myocardial relaxation. Therefore, agents that act by similar mechanisms may have certain therapeutic advantages over traditional inotropic agents in patients with heart failure.

Adrenergic alpha-Agonists↗

PubMed1995

Adrenergic responsiveness is reduced, while baseline cardiac function is preserved in old adult conscious monkeys.

To examine the physiological deficit to adrenergic stimulation with aging, five younger adult (3 +/- 1 yr old) and nine older adult (17 +/- 1 yr old) healthy monkeys were studied after instrumentation with a left ventricular (LV) pressure gauge, aortic and left atrial catheters, and aortic flow probes to measure cardiac output directly. There were no significant changes in baseline hemodynamics in conscious older monkeys. For example, an index of contractility, the first derivative of LV pressure (LV dP/dt) was similar (3,191 +/- 240, young vs. 3,225 +/- 71 mmHg/s, old) as well as in isovolumic relaxation, tau (24.3 +/- 1.7 ms, young vs. 23.0 +/- 1.0 ms, old) was similar. However, inotropic, lusitropic, and chronotropic responses to isoproterenol (Iso; 0.1 micrograms/kg), norepinephrine (NE; 0.4 micrograms/kg), and forskolin (For; 75 nmol/kg) were significantly (P < 0.05) depressed in older monkeys. For example. Iso increased LV dP/dt by by 146 +/- 14% in younger monkeys and by only 70 +/- 5% in older monkeys. Iso also reduced tau more in younger monkeys (-28 +/- 7%) compared with older monkeys (-13 +/- 3%). Furthermore, peripheral vascular responsiveness to Iso, NE, For, and phenylephrine (PE; 5 micrograms/kg) was significantly (P < 0.05) reduced in older monkeys. For example, phenylephrine (5 micrograms/kg) increased total peripheral resistence by 69 +/- 4% in younger monkeys and by only 45 +/- 3% in older monkeys. Thus in older monkeys without associated cardiovascular disease, baseline hemodynamics are preserved, but adrenergic receptor responsiveness is reduced systemically, not just in the heart.

Adrenergic Agents↗

PubMed1995

Differential regulation of G protein expression in rat hearts exposed to chronic hypoxia.

Chronic hypoxia impairs adrenergic responsiveness. A modulation of Gs and/or G1 protein alpha-subunits may be associated with the downregulation of the beta-adrenergic receptors previously found in chronic hypoxia. G protein gene expression and protein level and function in rat hearts exposed to a 30-day hypobaric chronic hypoxia were compared with control rat hearts. No change was observed in G alpha s mRNA levels in either right or left ventricles. In right ventricles, mRNA levels of G alpha i-2 increased by 40% (P < 0.05), but not in left ventricles. In both left and right ventricles, chronic hypoxia did not modify G alpha i-2 and G alpha s protein amounts, but significantly decreased functional activity of G alpha s. In conclusion, gene expression, protein levels of G alpha s and G alpha i-2, and activity of G alpha s do not change in parallel fashion with chronic hypoxia. In chronic hypoxic right ventricles, although the mRNA level of G alpha i-2 is increased, the protein level is unchanged. One potential mechanism of desensitization to catecholamines in chronic hypoxia appears to involve a decreased functional activity of G alpha s in spite of normal mRNA and protein levels.

Animals↗