Increase of the 40,000-mol wt pertussis toxin substrate (G protein) in the failing human heart

Effect of epinine on tension of human renal arteries

BACKGROUND

The present study aimed to characterize the effects of epinine, the active metabolite of ibopamine on tension development in human renal arteries.

METHODS AND RESULTS

Experiments were performed on isolated human renal arteries rings obtained during surgery due to kidney tumors (n = 12). Epinine concentration-dependently relaxed isolated precontracted (PGF2 alpha) human renal artery rings (P < 0.05) in the presence of phentolamine, as effectively (epinine -30 +/- 4 mN, dopamine -31 +/- 5 mN) and with the same potency as dopamine (epinine EC50 0.7 mumol/l (0.4-1.2 mumol/l), dopamine 0.5 mumol/l (0.2-1.7 mumol/l). This effect was antagonized by the specific D1-receptor-antagonist SCH 23390. Effective beta-adrenoceptor antagonistic concentrations of propranolol did not affect epinine-induced vasorelaxation. In the absence of alpha- and beta-adrenoceptor-antagonists the potency of epinine to contract renal artery rings was significantly higher compared to dopamine indicating a higher affinity of epinine to alpha-adrenoceptors.

CONCLUSION

The present study provides evidence for direct vasodilatory effects of epinine via activation of D1-receptors on human renal arteries.

Aberrant adenylyl cyclase/cAMP signal transduction and G protein levels in platelets from hypertensive patients improve with antihypertensive drug therapy

We have previously demonstrated a decreased expression of Gi alpha 2 protein in platelets from spontaneously hypertensive rats that was associated with an altered responsiveness of adenylyl cyclase to hormone stimulation and inhibition. In the present studies, we have used platelets from hypertensive patients and examined the hormonal regulation of adenylyl cyclase as well as the levels of G proteins and their modulation by antihypertensive drug therapy. We performed these studies in platelets from four groups of subjects: normotensive subjects (group 1), untreated mildly essential hypertensive patients (group 2), and treated moderately to severely hypertensive patients whose blood pressure was uncontrolled (group 3) or controlled with drug treatment (group 4). GTP gamma S, 5'-(N-ethylcarboxamido)adenosine (NECA), and prostaglandin E1 stimulated adenylyl cyclase activity to a greater extent in hypertensive patients (group 2). This effect was partially corrected (by approximately 50% to 80%) in the patients under antihypertensive drug therapy (groups 3 and 4). In addition, inhibition of adenylyl cyclase mediated by a ring-deleted analogue of atrial natriuretic factor (C-ANF4.23) observed in control normotensive subjects was blunted in hypertensive patients (group 2) and was not corrected in treated patients. Gi alpha levels determined by immunoblotting were in the same range for the four groups, whereas Gi alpha 2 and Gi alpha 3 levels were decreased by 70% and 60%, respectively, in hypertensive patients (group 2) compared with normotensive subjects. Antihypertensive drug therapy (groups 3 and 4) partially restored Gi alpha 2 levels toward normal (group 1) by about 60% and 70%, respectively; however, the reduced Gi alpha 3 levels in group 2 hypertensive patients were not improved in group 3 but were raised toward normal levels in group 4 by about 55%. These results suggest that the altered responsiveness of platelet adenylyl cyclase to hormones in hypertension and the normalization of the response with antihypertensive drug therapy could partly be due to the ability of the latter to modulate Gi alpha protein expression. These effects on platelet function may underlie the beneficial effects of antihypertensive agents on some of the complications of hypertension.

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.

Sarcoplasmic reticulum Ca2+ATPase and phospholamban mRNA and protein levels in end-stage heart failure due to ischemic or dilated cardiomyopathy

Abnormalities in intracellular Ca2+ handling play a crucial role in the pathogenesis of heart failure. The reduced capacity of failing human myocardium to restore low resting Ca2+ levels during diastole has been explained by the impairment of Ca2+ uptake into the sarcoplasmic reticulum (SR) via the SR Ca2+ATPase. It is unclear whether Ca2+ATPase function, protein levels, and mRNA steady-state levels correspond to one other, and whether the cause of heart failure, namely idiopathic dilated or ischemic cardiomyopathy, produces different changes. The present study examined SR Ca2+ATPase activity and both mRNA and protein levels of SR Ca2+ATPase, phospholamban, and Gi alpha 2 in left ventricular myocardium from eight nonfailing hearts, from eight hearts of patients with idiopathic dilated cardiomyopathy (DCM), and from six hearts from patients with ischemic cardiomyopathy (ICM). Compared to nonfailing myocardium, the activity of the SR Ca2+ATPase was significantly reduced in failing myocardium from patients with DCM (36%, P < 0.01) and from patients with ICM (37%, P < 0.001). Significantly lower levels of SR Ca2+ATPase mRNA levels (55% and -56%, P < 0.001 for DCM and ICM, respectively) and phospholamban mRNA (45%, P < 0.001 for DCM; 31%, P < 0.05 for ICM) were observed in failing than in nonfailing myocardium. In contrast, no significant changes were observed at the level of proteins, Gi alpha 2 mRNA and protein levels were both significantly increased in failing myocardium. There were no differences between idiopathic dilated and ischemic cardiomyopathy concerning the examined parameter. It is concluded that reduced SR Ca2+ATPase activity contributes to an altered intracellular Ca2+ handling by the SR in both dilated and ischemic cardiomyopathic hearts. However, changes in SR Ca2+ATPase and phospholamban steady-state protein levels do not contribute to these alterations. The dissociation between protein and mRNA levels provides evidence for a posttranscriptional or post-translational regulation of these proteins. The observed alterations are not dependent on the underlying cause of end-stage heart failure.

Adrenergic and muscarinic receptor regulation and therapeutic implications in heart failure

In end-stage heart failure the expression of different myocardial regulatory proteins involved in the beta-adrenergic cAMP signalling pathway is altered. The downregulation of beta 1-adrenoceptors and their uncoupling from the effector as well as an increased expression of the inhibitory GTP-binding protein seem to be the most important alterations. Since catecholamine levels are elevated in these patients and since some alterations can be 'restored' after treatment with beta-adrenoceptor antagonists it was hypothesized that excessive beta-adrenergic stimulation could be involved in these alterations. In this article the changes of beta-adrenergic receptors, GTP-binding proteins, sarcoplasmic reticulum Ca(2+)-ATPase and of phospholamban found in heart failure are addressed with its possible therapeutic implications.

G-proteins and adenylyl cyclase signalling in hypertension

The present studies were undertaken to examine if adenylyl cyclase activity and the levels of G-proteins (Gs alpha and Gi alpha) are altered in cardiovascular tissues in hypertension. Adenylyl cyclase activity and its responsiveness to stimulatory and inhibitory hormones as well as the expression of G-proteins (Gs and Gi) were determined at protein and mRNA levels by using specific antibodies and cDNA probes in hearts and aorta from 12 week old spontaneously hypertensive rats (SHR) and their age-matched control Wistar Kyoto (WKY) rats. The stimulatory effects of guanine nucleotides, isoproterenol, glucagon etc. on adenylyl cyclase activity were decreased in SHR rats as compared to the WKY rats, whereas, the inhibitory hormones inhibited enzyme activity to a greater extent in SHR rats as compared to WKY rats. Furthermore, the levels of Gi alpha-2 and Gi alpha-3 proteins and Gi alpha-2 and Gi alpha-3 mRNA as determined by immunoblotting and Northern blotting techniques respectively were higher in SHR as compared to WKY rats. However, the levels of Gs alpha were unaltered in SHR. To further investigate if these alterations are the cause or effect of hypertension, the SHRs at various ages of development of blood pressure (3-5 days, 2, 4 and 8 weeks) and their age-matched WKY were used for G-protein expression and adenylyl cyclase activity. The increased expression of Gi alpha-2 and Gi alpha-3 protein and mRNA levels in hearts and aorta were observed as early as in 2-weeks old SHR as compared to WKY, when the blood pressure was still normal. However, the levels of Gs alpha in SHR were not different from WKY rats. In addition, the altered responsiveness of adenylyl cyclase to hormone stimulation and inhibition was also observed as early as in 2 week old SHR. These results suggest that the increased expression of Gi alpha-2 and Gi alpha-3 and decreased levels of cAMP precedes the development of blood pressure and may be one of the contributing factors in the pathogenesis of hypertension.

Comparison of calcium-current in isolated atrial myocytes from failing and nonfailing human hearts

To identify possible alterations of the L-type calcium currents (I(Ca),L) in cardiomyopathy, I(Ca),L were recorded in atrial myocytes dissociated from the nonfailing heart (NF) of patients undergoing corrective open-heart surgery and explanted failing heart (FH) of patients with dilated cardiomyopathy undergoing heart transplantation. The patch-clamp technique was applied in the single-electrode whole-cell mode. The electrophysiological properties of I(Ca),L, including cell capacitance and current density, were similar in atrial myocytes from both groups of patients. Further to identify possible alterations of the myocardial beta-adrenergic pathway in cardiomyopathy, we examined the effects of isoproterenol, forskolin, 8-Br-cAMP and IBMX on I(Ca),L in both groups of atrial myocytes. Perfusion of isoproterenol (1 microM) significantly increased the peak I(Ca),L by 515 +/- 44% in 6 atrial myocytes from NF but increased only by 135 +/- 25% in 27 atrial myocytes from FH. However, forskolin (1 microM) or 8-Br-cAMP (0.1 mM) increased the peak I(Ca),L to a similar extent in atrial myocytes from NF and FH. IBMX (20 microM) also induced a comparable increase in the peak I(Ca),L by 213 +/- 31% (n = 5) and 207 +/- 59% (n = 4) in atrial myocytes from NF and FH, respectively. The above findings suggest that in atrial myocytes obtained from FH the beta-adrenoceptor numbers might be decreased but no impairment of the signal transduction cascade occurred beyond the GTP binding proteins level.

Chronic treatment with carbachol sensitizes the myocardium to cAMP-induced arrhythmia

BACKGROUND

The present study investigated biochemical and functional consequences of chronic activation of the inhibitory Gi alpha-coupled adenylyl cyclase pathway in the heart.

METHODS AND RESULTS

Rats (220 to 260 g) were treated with 4-day infusions of the M-cholinoceptor agonist carbachol (9.6 mg/kg per day) or vehicle. An additional group that received the beta-adrenoceptor agonist isoprenaline (2.4 mg/kg per day) served as control. The main finding was that chronic infusion of carbachol led to a marked increase in isoprenaline- or forskolin-induced arrhythmia in electrically driven papillary muscles (in vitro). Compared with control, the potency of isoprenaline and forskolin to induce arrhythmia in cardiac preparations from carbachol-treated rats was increased 36- and 2.2-fold and the efficacy was increased 7.3- and 2.3-fold, respectively. The potency of carbachol to antagonize the isoprenaline- and forskolin-induced arrhythmia was decreased 30-fold. These changes were accompanied by a decrease in left ventricular M-cholinoceptor density by 15% (P < .05) and a decrease in pertussis toxin-sensitive G proteins (Gi alpha) by 26% (P < .05) without a decrease in the corresponding mRNAs. beta-Adrenoceptor density and basal and stimulated adenylyl cyclase activity remained unchanged. In contrast, isoprenaline infusion induced a decrease in arrhythmogenic potency of forskolin (P = NS), which was accompanied by a decrease in beta-adrenoceptor density, an increase in Gi alpha protein and mRNA levels, and a decrease in basal and stimulated adenylyl cyclase activity.

CONCLUSIONS

Chronic parasympathetic activation sensitizes the myocardium to cAMP-induced arrhythmia. These changes may be due to quantitative alterations in functional Gi alpha.

Impaired regulation of cell communication by beta-adrenergic receptor activation in the failing heart

We investigated the influence of beta-adrenergic receptor activation on the control of gap junctional conductance (gj) in the heart of cardiomyopathic hamsters (11 months old). We measured gj in isolated ventricular cell pairs using two voltage-clamp circuits. Administration of isoproterenol (10(-6) mol/L) to the bath had no effect on gj in myopathic cell pairs but increased gj by 45 +/- 3% (+/- SE) in normal hamsters. Moreover, forskolin (10(-7) mol/L), an activator of adenyl cyclase, did not change gj in myopathic cells but enhanced gj by 23 +/- 2.8% in controls. Similar results were obtained with isobutylmethylxanthine (10(-6) mol/L), a phosphodiesterase inhibitor. Dibutyryl-cAMP (10(-6) mol/L), however, increased gj of cardiomyopathic cell pairs by 58 +/- 2.1% within 2 minutes and enhanced gj in controls by 50 +/- 3.6%. The effect of dibutyryl-cAMP on gj of myopathic cells was suppressed by intracellular dialysis of an inhibitor of protein kinase A. These observations indicate that the regulation of gj by the beta-adrenergic receptor-G protein-adenyl cyclase signaling system is greatly impaired in the failing heart but the ability of cAMP to increase gj is still preserved.

Role of G-proteins in altered beta-adrenergic responsiveness in the failing and hypertrophied myocardium

In the heart and other tissues, beta-adrenergic desensitization occurs during treatment with catecholamines. In heart failure, a strong sympathetic activation has been observed and is the cause of beta-adrenergic desensitization in this condition. On the receptor level, there is a downregulation of beta 1-adrenergic receptors as well as an uncoupling of beta 2-adrenoceptors. The latter mechanism has been related to an increased activity and gene expression of beta-ARK2 in failing myocardium leading to phosphorylation and uncoupling of receptors. In addition, an increase of inhibitory G-protein alpha-subunits (Gi alpha) has been suggested to be causally linked to adenylyl cyclase desensitization in heart failure. In contrast, the catalytic subunit of adenylyl cyclase, stimulatory G-protein alpha-subunits and beta gamma-subunits have been observed to be unchanged. Recently, evidence has been raised that increases of Gi alpha also depress adenylyl cyclase in compensated cardiac hypertrophy in monogenic and polygenic as well as in secondary hypertension. These increases of Gi alpha can suppress adenylyl cyclase in the absence of beta-adrenergic receptor downregulation. Since cardiac hypertrophy in pressure overload is a strong predictor of cardiac failure these observations indicate that adenylyl cyclase desensitization by Gi alpha could be a pathophysiologically relevant mechanism to contribute to the progression from compensated cardiac hypertrophy to heart failure.

Mechanisms of beta-adrenergic receptor desensitization: from molecular biology to heart failure

beta-Adrenergic receptors are often studied as prototypes of the large family of G-protein-coupled receptors, which includes many other well-known members such as the muscarinic acetylcholine receptors, but also the receptors for light, taste and olfaction. These receptors are regulated by multiple mechanisms which can affect either their function or their expression to a rapidly changing environment. The most obvious changes are effected by receptor agonists, and this process is called receptor desensitization. On the functional level, the most intriguing and important mechanism of desensitization involves the phosphorylation of beta-adrenergic and homologous receptors by specific receptor kinases, termed the G-protein-coupled receptor kinases (GRKs). This phosphorylation is followed by binding of arrestins to the receptors, which causes uncoupling of receptors and G-proteins and thus results in a loss of receptor function. On the expression level, there appear to be two major pathways leading to a reduction of the receptor number: degradation of the receptors themselves, or reduced receptor synthesis brought about by reduced receptor mRNA levels. Heart failure is accompanied by a markedly reduced responsiveness of the beta-adrenergic receptor system, which in many ways resembles the phenomena seen in agonist-induced receptor desensitization. The levels of beta 1-adrenergic receptors are reduced, and this reduction is paralleled by similar decreases in the levels of the corresponding mRNA. At the same time, the activity and the mRNA levels of one of the GRK-isoforms, GRK2 (which is identical to the beta-adrenergic receptor kinase 1) are increased. These alterations may contribute to the loss of beta-adrenergic receptor responsiveness in heart failure and result in further impairment of cardiac function.

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.

Altered inotropism in the failing human myocardium

Beta-adrenoreceptor-cAMP-dependent inotropic interventions lose their effectiveness depending on the degree of myocardial failure. This blunted effect of beta-adrenoreceptor-dependent stimulation might be due to a downregulation of beta-adrenoreceptors and an increase of inhibitory G-proteins leading to decreased intracellular cAMP-concentrations. However, the maximal positive inotropic effect elicited by elevation of the extracellular [Ca2+] does not differ between failing and nonfailing human myocardium, indicating that terminally failing human myocardium is effective to increase force of contraction to the same degree as nonfailing tissue. Agents which increase force of contraction primarily via increasing the intracellular [Na+], e.g., cardiac glycosides and the Na(+)-channel activator BDF 9148, exert a higher potency in failing myocardium than in nonfailing tissue to increase force of contraction. This could result from an enhanced protein expression of the Na+/Ca(2+)-exchanger observed in diseased human hearts. Alterations in the intracellular Ca(2+)-homeostasis reported in failing myocardium lead to a negative force-frequency-relationship and a prolonged relaxation. As the protein expression of SERCA IIa and phospholamban seems to be similar in NYHAIV and nonfailing tissue, the reduced Ca(2+)- uptake may result from an altered regulation of these proteins, e.g., reduced phosphorylation of phospholamban or the SERCA IIa. After inhibition of the Ca(2+)-ATPase of the sarcoplasmic reticulum with the high specific inhibitor cyclopiazonic acid the former positive force-frequency-relationship became significantly less positive even in the nonfailing tissue and twitch course became similar to diseased hearts. These findings may be indicative for the importance of the Ca(2+)-reuptake mechanism into the sarcoplasmic reticulum in addition to the regulatory control at the site of the contractile apparatus for the regulation of contraction and relaxation in human myocardium.

beta-Adrenoceptor regulation in rat heart, lung and skin after chronic treatment with (--)-tertatolol or (--)-propranolol

1. The effect of long-term treatment with the beta-adrenoceptor antagonists (--)-tertatolol and (--)-propranolol was studied. Sprague-Dawley rats were treated with either (--)-tertatolol (50 micrograms kg-1 hr-1), (--)-propranolol (250 micrograms kg-1 hr-1) or vehicle (1 mM HCl) for 14 days with osmotic minipumps implanted subcutaneously. 2. The mean daily systolic blood pressure and heart rate of rats treated with either (--)-tertatolol (108 +/- 1 mmHg/330 +/- 3 bpm) or (--)-propranolol (103 +/- 1 mmHg/330 +/- 2 bpm) were lower than in the control (126 +/- 1 mmHg/405 +/- 3 bpm, P < 0.001, n = 8-10) indicating the effectiveness of drug delivery. 3. Autoradiographic studies in areas of heart, lung and skin showed that beta-adrenoceptor populations were not significantly affected by the drug treatment (all regions P > 0.05). Nevertheless, the receptor population in the homogenates of (--)-tertatolol treated lung were halved (194 +/- 28 fmol mg protein-1 compared with a control value of 388 +/- 54 fmol mg protein-1, P < 0.01, n = 6). 4. In the presence of CGP 20712A, the left atrial inotropic and right atrial chronotropic responsiveness to (--)-isoprenaline were hypersensitive in both (--)-tertatolol and (--)-propranolol-treated groups (P < 0.005, ANCOVA). 5. (--)-Propranolol treated left ventricular free wall had lower basal [3H]-forskolin binding to adenylate cyclase (14.45 +/- 1.20 fmol mg protein-1 compared with a control value of 18.91 +/- 0.78 fmol mg protein-1, P = 0.01, n = 6). (--)-Tertatolol treatment had no effect on the basal binding. In the presence of the G-protein activators NaF and Gpp(NH)p, the enhancement of [3H]-forskolin binding did not differ between control and the drug treated groups. 6. Chronic (--)-tertatolol or (--)-propranolol treatment therefore did not produce an increase in receptors in heart, lung or skin but the beta-adrenoceptor-mediated responses were enhanced. In addition, [3H]-forskolin binding did not increase suggesting that the hypersensitivity was not due to changes in the number of receptors or adenylate cyclase. Hypersensitivity following beta-adrenoreceptor antagonist administration may therefore involve enhanced coupling of receptors to G-proteins.

Dose-dependent dissociation of ACE-inhibitor effects on blood pressure, cardiac hypertrophy, and beta-adrenergic signal transduction

BACKGROUND

Dose-dependent effects of ACE inhibitors on blood pressure, cardiac hypertrophy, and beta-adrenergic signal transduction were examined in an animal model with beta-adrenergic desensitization, which has been identified in failing hearts and in hypertensive cardiac hypertrophy. It is unknown whether beneficial ACE-inhibitor effects are due to an unloading of the failing heart or a reduction of neuroendocrine activation with beta-adrenergic resensitization.

METHODS AND RESULTS

Low-dose (LD, 1 mg/kg) and high-dose (HD, 25 mg/kg) fosinopril treatment was performed in spontaneously hypertensive rats (SHR) and control (WKY) rats. Myocardial norepinephrine concentrations, adenylyl cyclase activity, beta-adrenergic receptors (radioligand binding), Gs alpha (functional reconstitution), and Gi alpha (pertussis toxin labeling) were determined. Ventricular weights and blood pressures were measured. HD but not LD reduced blood pressure and left ventricular weights in SHR. Isoprenaline- and guanylylim-idodiphosphate-stimulated adenylyl cyclase activities as well as beta 1-adrenergic receptors were reduced in SHR. The catalyst and Gs alpha were unchanged, but Gi alpha and norepinephrine concentrations were increased. Both LD and HD treatments restored beta-adrenergic alteration.

CONCLUSIONS

LD treatment with ACE inhibitors restored beta-adrenergic signal transduction defects independently of regression of cardiac hypertrophy. This could contribute to the effects of ACE inhibitors in patients, who are often treated with nonhypotensive doses.

G protein alterations in hypertension and aging

Defective vasodilator function could be important in the pathogenesis and/or maintenance of the hypertensive state and the predisposition of the elderly to hypertension. Impaired beta-adrenergic-mediated vasodilation and reduced lymphocyte beta-adrenergic activation of adenyl cyclase have been demonstrated both in aging and with hypertension. The cellular mechanisms responsible for these alterations remain unclear. To determine if these defects may be due to alterations in guanine nucleotide regulatory proteins (G proteins) that link receptor activation with effector function, we assessed (1) human lymphocyte adenyl cyclase activity, (2) stimulatory G proteins by cholera toxin-mediated [32P]ADP ribosylation and, in hypertensive subjects, with alpha s-specific and beta-subunit antisera, and (3) inhibitory G proteins by pertussis toxin-mediated [32P]ADP ribosylation and, in older subjects, with alpha i,1,2- and beta-subunit-specific antisera. Lymphocytes from older subjects and from hypertensive subjects demonstrated a comparable reduction in isoproterenol-stimulated adenyl cyclase. However, aluminum fluoride-stimulated activity was reduced only in lymphocytes from hypertensive subjects. Furthermore, aluminum fluoride-stimulated activity was inversely correlated with mean arterial pressure. In lymphocytes from younger hypertensive subjects, cholera toxin-mediated labeling was significantly increased. In contrast, inhibitory G protein labeling by immunodetection was unaltered. In lymphocytes from older subjects, cholera toxin-mediated labeling was not altered; however, pertussis toxin-mediated labelling was significantly increased. In contrast, inhibitory G protein labeling by immunodetection was unaltered. Overall, the study suggests alterations of G protein function of adenyl cyclase is impaired. However, these defects are associated with divergent alterations in stimulatory and inhibitory G proteins.

Signalling pathways in cardiac failure

1. Cardiac failure in humans and in animal models is associated with a marked desensitization of the catecholamine signalling pathway. 2. Beta 1- and beta 2- and possibly beta 3-adrenoceptors (beta-AR) are found in the hearts of humans and common laboratory animals such as rats and guinea-pigs. In rats and guinea-pigs chronic stimulation of cardiac beta-AR leads to a rapid loss of beta 2-AR whereas heart failure in humans is associated with a loss of beta 1-AR or beta 1-AR and beta 2-AR. 3. Desensitization is also associated with phosphorylation of beta-AR by beta-AR kinase (beta-ARK) and uncoupling of receptors from the signalling pathway. Beta-ARK but not beta-arrestin activity and mRNA are markedly increased in heart failure. 4. Chronic beta-AR stimulation and heart failure are associated with increases in Gi alpha but little if any change in Gs alpha. 5. The roles of beta gamma subunits of G-proteins, adenylate cyclase subtypes and cAMP dependent protein kinase A in heart failure are unclear at present.

ABBREVIATIONS

beta-ARK - beta-adrenoceptor kinase AR - adrenoceptor G-protein - GTP binding protein

Cardiac beta-adrenergic neuroeffector systems in acute myocardial dysfunction related to brain injury. Evidence for catecholamine-mediated myocardial damage

BACKGROUND

Ten percent to 20% of potential cardiac donors with brain injury and no previous cardiac history have myocardial dysfunction. We assessed components of the beta-receptor-G-protein-adenylyl cyclase complex as well as the contractile response in 10 explanted acutely failing human hearts (donor heart dysfunction [DHD]) and compared the results with 13 age-matched nonfailing (NF) organ donor controls.

METHODS AND RESULTS

As measured by echocardiography, all DHD hearts exhibited a decreased shortening fraction (16 +/- 2%, mean +/- SEM). Although total and subpopulation beta-receptor densities measured by [125I]iodocyanopindolol (ICYP) were similar in the DHD and NF groups, DHD hearts exhibited a 30% decrease in maximum isoproterenol-stimulated adenylyl cyclase activity and a 50% decrease in the maximal response to zinterol. DHD hearts also exhibited decreases in adenylyl cyclase maximal stimulation by forskolin (211 +/- 25 [DHD] versus 295 +/- 23 [NF] pmol cAMP.min-1.mg-1, P < .05) and 5'-guanylylimidodiphosphate (12.5 +/- 1.8 [DHD] versus 19.6 +/- 3.2 [NF] pmol cAMP.min-1.mg-1, P < .05), but there was no significant decrease in adenylyl cyclase stimulation by Mn2+, a direct activator of adenylyl cyclase. Right ventricular trabeculae removed from DHD hearts exhibited a profound decrease in the contractile response to isoproterenol (8.7 +/- 1 [DHD] versus 22 +/- 2 [NF] mN, P < .001) as well as reduced calcium responses (7.2 +/- 1.6 [DHD] versus 14 +/- 3 [NF] mN, P = .03). Morphological examination of two hearts revealed some ultrastructural evidence suggestive of catecholamine-mediated injury, but there was no difference in tissue creatine kinase activity between the two groups.

CONCLUSIONS

Compared with NF hearts, DHD hearts exhibit marked uncoupling of beta 1- and beta 2-adrenergic receptors from adenylyl cyclase and contractile response stimulation as well as decreased intrinsic systolic function. Thus, acute myocardial dysfunction accompanying brain injury is characterized by marked alterations in beta-adrenergic signal transduction as well as changes in the contractile apparatus, and this profile is markedly different from what occurs in the chronically failing human heart.

Enrichment of G protein alpha-subunit mRNAS in the AV-conducting system of the mammalian heart

We investigated the expression pattern of the heterotrimeric G proteins Gs alpha, Gi alpha-2 and Go alpha in rat and guinea-pig heart by in situ hybridization. Cryosections were hybridized with single-stranded 35S-cRNA probes complementary to subtype-specific sequences of the respective mRNAs. Hybridization signals were visualized by exposition to X-ray films and dipping autoradiography. The rank order of abundance was Gi alpha-2 approximately Gs alpha >> Go alpha. In general, G protein alpha-subunit mRNAs were evenly distributed in the heart including endo- and epicardium, large vessels and valves. Go alpha-mRNA levels were significantly higher in atria than in ventricles. In contrast to the rather uniform labeling of working myocardium, expression of all three G proteins was enriched in small intramural blood vessels and in subendocardial Purkinje fibers of septum and papillary muscles. A more marked enrichment of Gs alpha-, Gi alpha-2- and especially Go alpha-mRNA was seen in neuronal ganglionic cells in the atrial septum and posterior regions of the atrium. The main finding, however, was an enrichment of all three G protein mRNAs in the atrioventricular conductive tissue. The accumulation was strictly co-localized with acetylcholinesterase-positive regions identified as the atrioventricular node, the bundle of His and the right and left bundle branches and was seen similarly in rat and guinea-pig hearts. Quantitative in situ hybridization revealed Gs alpha-, Gi alpha-2- and Go alpha-mRNA levels in the bundle of His to be 206 +/- 0.13%. 191 +/- 0.15% and 165 +/- 0.06%, respectively, of that in the surrounding interventricular working myocardium. These findings indicate that heterotrimeric G proteins play an important role in modulation of electrical conductance in the heart.

Decrease in beta 1-adrenergic and M2-muscarinic receptor mRNA levels and unchanged accumulation of mRNAs coding for G alpha i-2 and G alpha s proteins in rat cardiac hypertrophy

During compensatory cardiac hypertrophy in the rat, hemodynamic overload induces a parallel decrease in the densities of both beta 1-adrenergic (beta 1-AR) and M2-muscarinic (M2-MR) receptors in the left ventricle, but the total number of receptors remains unchanged. It is not known whether this reduction is transcriptionally or translationally regulated, or if the functionally closely linked alpha-subunits of G protein (G alpha s and G alpha i-2) partake in this regulation. In order to resolve these questions, the absolute concentrations of mRNAs for both receptors and for G alpha s and G alpha i-2 were quantified by slot blot analysis of the left ventricles of adult rats 5 weeks after aortic banding. The results showed a significant decrease of both receptor mRNA levels in hypertrophied left ventricle (beta 1-AR: -48%; M2-MR: -42%) that paralleled the reduction in receptor protein densities and was negatively correlated with the left ventricular weight/body weight ratio (LVW/BW). By contrast, the relative levels of G alpha s and G alpha i-2 mRNAs remained unchanged, and both accumulated proportionally to the increase in LVW/BW. These results show that the beta 1-AR and the M2-MR were pretranslationally regulated. This suggests the hypothesis that the corresponding genes do not follow the general increase in transcriptional activity. By contrast, the genes coding for G alpha s and G alpha i-2 may follow the general pattern of activation during hypertrophy. Receptors and coupling proteins belong to two different groups of genes that are controlled by distinct mechanisms of regulation.

Adrenergic desensitization in left ventricle from streptozotocin diabetic swine

Patients with diabetes mellitus that exhibit cardiac pump failure display compromised stroke volume, ejection fraction, and slower rates of rise and fall of left ventricular (LV) dP/dt in the absence of ischemic injury. We hypothesized that diabetic cardiomyopathy may involve decrements in adrenergic sensitivity, with specific molecular alterations in the beta-adrenergic receptor (beta AR)- G protein- adenylyl cyclase (AC) signal transduction system. We assessed the effects of 3 months of streptozotocin-induced diabetes (125 mg/kg i.v.; DIAB, n = 10) on myocardial signal transduction in mini-pigs. DIAB were hyperglycemic compared to controls (CON, n = 10; 20.92 +/- 2.64 v 5.24 +/- 0.35 mM glucose), and had lower fasting insulin levels (6.46 +/- 0.97 v 13.68 +/- 3.91 microU/ml). Transmural LV free wall homogenates from DIAB exhibited similar beta AR density as CON, but decreased cAMP production (pmol cAMP/mg prot.min) using these pharmacological stimulators: 10 microM Isoproterenol plus 100 microM GTP (74 +/- 5 v 97 +/- 11); 100 microM Gpp(NH)p (116 +/- 7 v 161 +/- 17); 10 mM fluoride ion (266 +/- 16 v 324 +/- 25). No differences between DIAB and CON were observed when stimulated by 100 microM forskolin (440 +/- 20 v 429 +/- 33), suggesting no alterations in the catalytic subunit of AC. In DIAB, quantitative immunoblotting indicated slightly depressed levels of Gs (552 +/- 44 v 630 +/- 59 pmol/g ww; NS), but a significant redistribution of alpha s from the sarcolemma to the cytosol (32.7 +/- 0.82% v 25.9 +/- 1.7%). Significantly elevated levels of cardiac Gi were seen in DIAB homogenates compared to CON ventricles (2326 +/- 145 v 1522 +/- 181 pmol/g ww), with no alpha i subunit redistribution. We conclude that despite maintained beta AR density, receptor-dependent and G protein-dependent stimulation of AC is depressed so that streptozotocin-induced diabetic LV is affected by increased cardiac Gi, redistribution of Gs alpha to the cytosol, and an increase in the Gi/Gs ratio. These results help explain depressed catecholamine responsiveness and cardiac performance exhibited by diabetic patients.

Evidence against a role of nitric oxide in the indirect negative inotropic-effect of M-cholinoceptor stimulation in human ventricular myocardium

Nitric oxide (NO) has been reported to mediate several effects in response to muscarinic cholinergic stimulation in cardiovascular tissues. Recently, an attenuation of guinea pig cardiac myocyte contraction by NO has been described. The aim of the present study was to determine whether the indirect negative inotropic effect of M-cholinoceptor stimulation in human myocardium is in part due to an effect of endogenous NO. Therefore, the effect of carbachol was studied under control conditions and during inhibition of NO-synthase by pretreatment with NG-monomethyl-L-arginine (NMMA). Functional experiments were performed in isolated, electrically driven (1 Hz, 37 degrees C) left ventricular papillary muscle strips of human myocardium. Since cytokines have been reported to be increased in the serum of patients with heart failure and could induce NO-synthase activity in failing myocardium, we compared samples from nonfailing and terminally failing (classified as NYHA IV) hearts. The indirect negative inotropic effect of carbachol (10 mumol/l) was studied in the presence of the beta-adrenoceptor agonist isoprenaline (0.03 mumol/l). After stimulation with isoprenaline, carbachol significantly (P < 0.05) reduced force of contraction. This effect was diminished in failing myocardium compared to nonfailing, probably due to the diminished inotropic response most likely due to the lower cAMP levels in response to beta-adrenoceptor stimulation in the former condition. Pretreatment with NMMA (100 mumol/l) altered the antiadrenergic effect of carbachol neither in nonfailing nor in failing preparations. Furthermore, inhibition of guanylyl cyclase, the target enzyme of NO, by preincubation with methylene blue (10 mumol/l) for 30 min had no effect on the carbachol-induced decrease in force of contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

EMD 53998 acts as Ca(2+)-sensitizer and phosphodiesterase III-inhibitor in human myocardium

The effect of EMD 53998 (EMD) (0.1-100 mumol/l), chemically a racemic thiadiazinone derivative, suggested to be a potent Ca(2+)-sensitizer, was studied in human failing and nonfailing left ventricular myocardium. For comparison, the effects of the pyridazinone derivative pimobendan (0.1-300 mumol/l), isoprenaline (Iso) (0.001-3 mumol/l) as well as CaCl2 (1.8-15 mmol/l Ca2+) were investigated. The positive inotropic responses were examined in electrically driven (1 Hz, 37 degrees C) human left ventricular papillary muscle strips from terminally failing hearts (NYHAIV, n = 24) and nonfailing donor hearts (NF, n = 9). The effect of EMD on the Ca(2+)-sensitivity of skinned fiber preparations from the very same human failing hearts were studied as well. EMD and pimobendan increased force of contraction (FOC) in a concentration-dependent manner. As judged from the EC50-values, EMD increased FOC more potently than pimobendan. EMD was significantly more effective than pimobendan to increase FOC in papillary muscle strips from NYHA IV (EMD: +2.5 +/- 0.1 mN; pimobendan: +0.8 +/- 0.2 mN) as well as from nonfailing hearts (EMD: +3.1 +/- 0.5 mN; pimobendan: +1.2 +/- 0.2 mN). Only in terminally failing myocardium, EMD increased FOC as effectively as Iso. After inotropic stimulation with EMD, pimobendan, or Iso, carbachol (1000 mumol/l) reduced FOC in left ventricular papillary muscle strips, indicating a cAMP-dependent mode of action. In skinned fiber experiments, EMD increased Ca(2+)-sensitivity significantly more (p < 0.01) than pimobendan.

IN CONCLUSION

EMD increases FOC in human myocardium via sensitizing of the contractile proteins towards Ca2+ and by inhibition of phosphodiesterase III-isoenzymes. EMD is a potent calcium sensitizing agent in human myocardium. Thiadiazinone derivatives could be one step in the evolution to more potent and selective calcium-sensitizers.

The force-interval relationship in human myocardium

The force-interval relationship is an important modulator of contractility in mammalian myocardium. In a number of mammalian species, increasing the frequency of stimulation results in an increase in force of contraction. Over the last 10 years, the effects of atrial pacing have been closely examined in normal human subjects and in patients with dilated cardiomyopathy, and the effects of the stimulation frequency have been investigated in isolated preparations from nonfailing and failing human hearts. An abnormal force-interval relationship in vivo and in vitro has been a consistent finding in patients with dilated cardiomyopathy, whereby an increase in stimulation frequency fails to increase the contractile response. The force-interval relationship of cardiac muscle has been shown to reflect intracellular calcium cycling and sarcoplasmic reticulum function. Therefore, agents that affect excitation-contraction coupling, in particular intracellular calcium mobilization and sarcoplasmic reticulum function, modulate the response of contraction force to stimulation frequency.

Characterization of lymphocyte beta-adrenoceptor activity and Gs-protein in patients with rheumatic heart valvular disease

In order to test whether the beta-adrenoceptor activity in rheumatic heart valvular disease depends on the ventricular load conditions, we determined their density and binding affinity to [125I]-iodocyanopindolol in lymphocytes, as well as plasma catecholamine and cAMP levels in 69 patients with regurgitant and stenotic lesions of the aortic and mitral valves. The patients were classified as having left ventricular pressure overload (LVP), left ventricular volume overload (LVV), mixed lesions (MOL) or right ventricular pressure overload (RVP). The beta-adrenoceptor activity was determined by radioligand binding methods, catecholamines by high performance liquid chromatography using an electrochemical detector and cAMP by radioimmunoassay. The mean beta-adrenoceptor density (Bmax) of the control group was 60.1 +/- 9.5 fmol (n = 29) per 10(6) lymphocytes. In the study population, the density was decreased by 83% in LVP, 78% in LVV, 87% in MOL and 86% in RVP. Plasma norepinephrine was elevated by 89% in LVP and 60% in MOL, epinephrine by 43% in LVP, 50% in VOL, 115% in MOL and 20% in RVP, while dopamine was not significantly changed, and cAMP was slightly elevated in all four groups. Screening for activating mutational changes in the Gs alpha-protein gave negative results, possibly dissociating the elevation in plasma cAMP from stimulatory effects of such abnormalities in the Gs-protein signaling.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations of beta-adrenoceptor-G-protein-regulated adenylyl cyclase in heart failure

Alterations of receptor-G-protein-regulated adenylyl cyclase activity have been suggested to represent an important alteration leading to contractile dysfunction in the failing human heart. Recent experiments suggest that the beta 1-adrenoceptor (beta 1 AR) density and mRNA levels are reduced, while beta 2-adrenoceptors and stimulatory G-proteins are unchanged (mRNA and protein level). Functional assays demonstrated that the catalyst of the adenylyl cyclase is not different between failing and nonfailing myocardium. Inhibitory G-proteins are increased (pertussis toxin substrates, protein and mRNA) and correlate to the reduced inotropic effects of beta-adrenoceptor agonists and of cAMP-PDE inhibitors. Gi alpha-coupled m-cholinoceptors and A1-adrenergic receptors are unchanged in density and affinity. Stimulation of these receptors resulted in an unchanged antiadrenergic effect on force of contraction. In conclusion, a downregulation of beta 1 AR and an increase of Gi alpha have been observed as signal transduction alteration in failing human myocardium. These alterations are due to alterations of gene expression in the failing heart and are related to a defective regulation of force of contraction in heart failure.

Characteristics of the beta-adrenergic receptor complex in the epicardial border zone of the 5-day infarcted canine heart

BACKGROUND

The effect of isoproterenol on increasing the peak amplitude of the L-type calcium current is reduced in myocytes dispersed from the epicardial border zone (EBZ) of the 5-day infarcted canine heart when compared with control cells from noninfarcted hearts. This suggests that specific alterations in the beta-adrenergic receptor complex develop in this setting. The present study is an examination of individual components of the beta-adrenergic receptor complex with the aim of elucidating the biochemical defect(s) that might be responsible for the diminished beta-adrenergic receptor responsiveness in the myocytes that survive in the infarcted heart.

METHODS AND RESULTS

We compared components of the beta-adrenergic receptor signaling pathway in membranes prepared from the EBZ of the 5-day infarcted heart and a remote, noninfarcted region (RZ) of the same ventricle as well as the corresponding regions of noninfarcted ventricles. Defects in multiple components of the beta-adrenergic receptor complex were confined to the EBZ of the 5-day infarcted heart. These include a decrease in beta-adrenergic receptor density; diminished basal, guanine nucleotide-, isoproterenol-, forskolin-, and manganese-dependent adenylyl cyclase activities; an increase in the EC50 for isoproterenol-dependent activation of adenylyl cyclase; a diminished level of the alpha-subunit of the Gs protein. and an elevated level of the alpha-subunit of the Gi protein.

CONCLUSIONS

Defects in multiple components of the membrane beta-adrenergic receptor complex were identified in the EBZ of the 5-day infarcted canine heart. This constellation of abnormalities would be predicted to impair functional beta-adrenergic responsiveness and contribute to the defect in isoproterenol-dependent stimulation of the L-type calcium current in myocytes isolated from this tissue.

Treatment in hypertensive cardiac hypertrophy, I. Neuropeptide Y and beta-adrenoceptors

In the present study, we investigated serum and myocardial neuropeptide Y concentrations as measures of sympathetic activity as well as myocardial beta-adrenoceptors and beta-adrenoceptor-stimulated adenylyl cyclase activity in spontaneously hypertensive rats (SHR). SHR and control rats at 10 weeks of age were kept on oral treatment with captopril, nitrendipine, or both for 20 weeks. Treatment only slightly reduced but did not normalize blood pressure and cardiac hypertrophy in SHR. The elevated serum concentration of neuropeptide Y, the reduced number of beta-adrenoceptors, and the depressed beta-adrenoceptor-stimulated adenylyl cyclase activity were partly normalized compared with the values observed in control rats. We conclude that antihypertensive treatment, at doses that failed to normalize systolic pressure and to reverse cardiac hypertrophy completely, is able to reduce sympathetic activity in SHR, thereby resensitizing the depressed beta-adrenoceptor-adenylyl cyclase system.

Mutations of signal-transducing G proteins in human disease

Heterotrimeric guanine nucleotide binding proteins (G proteins) couple a large number of cell surface receptors to their intracellular effector molecules, such as enzymes or ion channels. Mutations of G proteins can lead to either activation or inactivation of the corresponding signal transduction pathway and thus cause clinical symptoms. Mutations of heterotrimeric G proteins have been found in a number of endocrine tumors, the McCune-Albright syndrome, Albright's hereditary osteodystrophy, and a combination of precocious puberty and pseudohypoparathyroidism Ia. The identification of the molecular defects underlying the above disorders and the investigation of their functional consequences for metabolism and growth regulation have been the subject of many studies over the past few years. A close understanding of these pathophysiologic mechanisms is crucial for the development of therapeutic strategies.

Treatment in hypertensive cardiac hypertrophy, II. Postreceptor events

We investigated the effect of pharmacological treatment with captopril, nitrendipine, and captopril plus nitrendipine on myocardial heterologous adenylyl cyclase desensitization and the underlying postreceptor defects in spontaneously hypertensive rats (SHR). In myocardial membranes from SHR, stimulation of adenylyl cyclase with guanylylimido-diphosphate (P < .001) and forskolin (P < .05) was significantly reduced, whereas no difference with forskolin was obtained in the presence of manganese chloride. Reconstitution of Gs alpha into Gs alpha-deficient S49 cyc- mouse lymphoma cells revealed no difference between SHR and control rats. In contrast, pertussis toxin labeling of Gi alpha was significantly increased in SHR. The reduction of adenylyl cyclase in SHR was abolished after pertussis toxin treatment of membranes. Treatment with captopril, nitrendipine, or both reduced Gi alpha and increased guanylylimidodiphosphate-stimulated adenylyl cyclase activity in SHR. In summary, heterologous adenylyl cyclase desensitization due to an increase of Gi alpha but in the presence of an unchanged activity of Gs alpha or the catalyst occurs in SHR. This alteration, which could contribute to the progression of contractile dysfunction by producing adrenergic subsensitivity, is sensitive to pharmacological treatment most likely because of a reduction of sympathetic activity.

Decreased adenylate cyclase activity and expression of Gs alpha in human myocardium after orthotopic cardiac transplantation

We studied several aspects of guanine nucleotide-stimulated adenylate cyclase function in patients after orthotopic cardiac transplantation. In 28 patients, adenylate cyclase activity was measured in endomyocardial biopsy samples obtained just before and at monthly intervals after cardiac transplantation. In biopsies obtained > or = 6 months after transplantation, basal adenylate cyclase activity was decreased by 67% (n = 12; P < .05), GTP gamma S-stimulated adenylate cyclase activity was decreased by 78% (n = 12; P < .05), Mn+2+forskolin-stimulated adenylate cyclase activity was decreased by 80% (n = 8; P < .05), and Mn+2-stimulated adenylate cyclase activity (a measure of activity of the catalytic subunit of adenylate cyclase) was decreased by 83% (n = 8, P < .05). Western blot analysis demonstrated that 6 months after cardiac transplantation, the level of Gs alpha protein was decreased by 61 +/- 12% (n = 8; P < .001). There was no change in the level of Gi alpha as assessed by pertussis toxin-catalyzed ADP-ribosylation (n = 4; P = NS). With the use of the quantitative polymerase chain reaction, a 50 +/- 10% (n = 6; P < .001) reduction in the steady-state level of Gs alpha mRNA was observed. There was no change in the level of mRNA for Gi-3 alpha. Thus, after orthotopic cardiac transplantation in humans, guanine nucleotide-stimulated adenylate cyclase activity is decreased in parallel with decreased levels of Gs alpha protein and mRNA.

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.

Increase of myocardial inhibitory G-proteins in catecholamine-refractory septic shock or in septic multiorgan failure

PURPOSE

The aim of the study was to investigate the mechanisms of myocardial catecholamine refractoriness in septic shock.

METHODS

The inhibitory guanine nucleotide-binding proteins (Gi alpha) were studied with pertussis toxin labeling and radioimmunologically in myocardium from patients who died while in catecholamine-refractory septic shock and from patients who died of noncardiac disease.

RESULTS

An increase by 62% (immunological Gi alpha) and 221% (pertussis toxin substrate) of myocardial Gi alpha was observed in patients with catecholamine-refractory shock compared with controls. The increases of Gi alpha were greater than those found in chronic heart failure reported earlier.

CONCLUSIONS

An increase in the expression of Gi alpha could also be important in conditions other than chronic heart failure, eg, septic shock. An increase of Gi alpha could play a pathophysiologically relevant role in catecholamine refractoriness in septic shock and could provide a target for pharmacologic treatment in this condition.

Lymphocyte G proteins reflect response to treatment in congestive heart failure

Congestive heart failure is associated with chronotropic and inotropic hyporesponsiveness to adrenergic stimulation. A decrease in Gs alpha or an increase in Gi alpha is associated with a decrease in adenylyl cyclase activity. The current study assessed G proteins in response to treatment with direct-acting vasodilators and correlated changes in lymphocyte beta-adrenergic receptor components with changes in hemodynamic variables. Twenty-three patients with severe chronic congestive heart failure (New York Heart Association functional classes III and IV) were studied. Patients were grouped as responders (n = 10) or nonresponders (n = 13) on the basis of clinical assessment of functional status from questionnaires. Therapy was associated with an increase in cardiac index, a decrease in mean arterial pressure, and a decrease in systemic vascular resistance in all patients. Left ventricular filling pressure significantly decreased in responders (26 +/- 2 mm to 13 +/- 3 mm, p < 0.05) but did not change significantly in nonresponders. Similarly, mean right atrial pressure significantly decreased in responders (11 +/- 2 mm Hg to 4 +/- 1 mm Hg, p < 0.05) but did not change in nonresponders. Plasma norepinephrine increased significantly only in nonresponders (679 +/- 100 pg/ml to 1233 +/- 201 pg/ml, p < 0.05). Whereas lymphocyte beta-adrenergic receptor density and Gs did not significantly change, Gi increased after treatment only in the nonresponder group (23 +/- 5 to 51 +/- 11 fmol/mg, p < 0.05). A poor response to direct-acting vasodilators can be distinguished by reactive increases in plasma norepinephrine and lymphocyte Gi in the absence of a decrease in either left- or right-sided filling pressures.

Effect of captopril on myocardial beta-adrenoceptor density and Gi alpha-proteins in patients with mild to moderate heart failure due to dilated cardiomyopathy

In end-stage heart failure due to idiopathic dilated cardiomyopathy beta 1-adrenoceptors are downregulated and Gi alpha-proteins are upregulated. The aim of the present study was to investigate the influence of the angiotensin-converting enzyme inhibitor captopril on beta-adrenoceptor density and Gi alpha-proteins in sequential endomyocardial biopsies. Nineteen patients with mild to moderate congestive heart failure due to idiopathic dilated cardiomyopathy (NYHA Class II-III) were studied before and after 8-11 weeks of therapy. Patients were randomised into a captopril and a control group; 9 patients received captopril 12.5-50 mg per day, (divided in 2-3 doses) p.o. in addition to "conventional" therapy with digoxin and diuretics, and 10 controls received "conventional" therapy only. Echocardiography, spiroergometry, right heart catheterisation and endomyocardial biopsies were performed before (baseline) and after treatment. Compared to baseline, captopril increased total beta-adrenoceptor density by selectively increasing beta 1-adrenoceptors (31.6 vs 41.2 fmol.mg-1; p < 0.05) but had no significant effect on Gi alpha-proteins. The results indicate that treatment with angiotensin-converting enzyme inhibitors partly restores myocardial beta 1-adrenoceptor density, and this action effect may contribute to the clinical improvement of patients with idiopathic dilated cardiomyopathy treated in this way.

Evidence for reduction of norepinephrine uptake sites in the failing human heart

OBJECTIVES

This study investigated the role of neuronal uptake of norepinephrine (uptake-1) in human heart failure as a local factor for altering concentrations of norepinephrine at the cardiac myocyte membranes.

BACKGROUND

Several beta-adrenergic neuroeffector defects occur in heart failure. Whether an alteration in norepinephrine uptake-1 occurs is still unresolved.

METHODS

The role of norepinephrine uptake-1 was studied in electrically stimulated (1 Hz, 37 degrees C) human ventricular cardiac preparations and isolated myocardial membranes.

RESULTS

The effectiveness of norepinephrine in increasing the force of contraction was decreased in relation to the degree of heart failure. In contrast, the potency of norepinephrine was increased in failing hearts (New York Heart Association functional class IV) in relation to the concentrations producing 50% of the maximal effect (EC50). The EC50 values for isoproterenol, which is not a substrate for norepinephrine uptake-1, were reduced in myocardium in functional classes II to III and IV compared with those in nonfailing myocardium. The uptake inhibitors cocaine and desipramine (3 mumol/liter) potentiated the positive inotropic effects of norepinephrine in nonfailing myocardium (p < 0.05) but not in functional class IV myocardium. Radioligand binding experiments using the uptake inhibitor hydrogen-3 mazindol revealed a significant decrease by approximately 30% in norepinephrine uptake-1 carrier density in functional classes II to III and IV myocardium versus nonfailing myocardium (p < 0.05).

CONCLUSIONS

In human heart failure, there is a presynaptic defect in the sympathetic nervous system, leading to reduced uptake-1 activity. This defect in the failing heart can be mimicked by the effects of uptake blocking agents, such as cocaine and desipramine, in the nonfailing heart only. Compromised norepinephrine uptake-1 in functional class IV cannot be further increased by cocaine and desipramine. The pathophysiologic consequences could be an increased synaptic concentration of norepinephrine predisposing to adenylyl cyclase desensitization.

Beta-adrenergic neuroeffector mechanisms in cardiac hypertrophy of renin transgenic rats

We studied neuroeffector defects in hypertrophied myocardium of hypertensive transgenic rats harboring the mouse Ren-2d gene. In transgenic rats, epinephrine and neuropeptide Y concentrations were reduced. A heterologous desensitization of adenylyl cyclase was observed, which was accompanied by a downregulation of beta 1-adrenergic receptors, an increase of inhibitory G protein alpha-subunits, and a mildly depressed catalyst activity of adenylyl cyclase, whereas the bioactivity of stimulatory G protein alpha-subunits and beta 2-adrenergic receptors was unchanged. Desensitization of adenylyl cyclase was accompanied by a reduced positive inotropic response to isoproterenol, whereas the effect of Ca2+ was unchanged. We conclude that sympathetic neuroeffector defects occur in transgenic rats similar to those observed in human failing myocardium. These alterations occur in the stage of hypertrophy and could contribute to contractile dysfunction in later stages.

Alterations in G-proteins in congestive heart failure in cardiomyopathic (UM-X7.1) hamsters

In order to explain the attenuated sympathetic support during the development of heart failure, the status of beta-adrenergic mechanisms in the failing myocardium was assessed by employing cardiomyopathic hamsters (155-170 days old) at moderate degree of congestive heart failure. The norepinephrine turnover rate was increased but the norepinephrine content was decreased in cardiomyopathic hearts. The number and the affinity of beta receptors in the sarcolemmal preparations were not changed in these hearts at moderate stage of congestive heart failure. While the basal adenylyl cyclase activity was not altered in sarcolemma, the stimulation of enzyme activity by NaF, forskolin, Gpp(NH)p or epinephrine was depressed in hearts from these cardiomyopathic hamsters. Since G-proteins are involved in modifying the adenylyl cyclase activity, the functional and bioactivities as well as contents of both Gs and Gi proteins were determined in the cardiomyopathic heart sarcolemma. The functional stimulation of adenylyl cyclase by cholera toxin, which activates Gs proteins, was markedly depressed whereas that by Pertussis toxin, which inhibits Gi proteins, was markedly augmented in cardiomyopathic hearts. The cholera toxin and pertussis toxin catalyzed ADP-ribosylation was increased by 37 and 126%, respectively; this indicated increased bioactivities of both Gs and Gi proteins in experimental preparations. The immunoblot analysis suggested 74 and 124% increase in Gs and Gi contents in failing hearts, respectively. These results suggest that depressed adenylyl cyclase activation in cardiomyopathic hamsters may not only be due to increased content and bioactivity of Gi proteins but the functional uncoupling of Gs proteins from the adenylyl cyclase enzyme may also be involved at this stage of heart failure.

Alterations in muscarinic K+ channel response to acetylcholine and to G protein-mediated activation in atrial myocytes isolated from failing human hearts

BACKGROUND

A variety of previous studies have demonstrated reduced diastolic potential and electrical activity in atrial specimens from patients with heart disease. Although K+ channels play a major role in determining resting membrane potential and repolarization of the action potential, little is known about the effects of preexisting heart disease on human atrial K+ channel activity.

METHODS AND RESULTS

We characterized the inwardly rectifying K+ channel (IKI) and the muscarinic K+ channel [IK(ACh)] in atrial myocytes isolated from patients with heart failure (HF) and compared electrophysiological characteristics with those from donors (control) by the patch-clamp technique. Resting membrane potentials of isolated atrial myocytes from HF were more depolarized (-51.1 +/- 9.7 mV, mean +/- SD, n = 30 patients) than those from donors (-73.0 +/- 7.2 mV, n = 4 patients, P < .001). The action potential duration in HF was longer than that in donors. Although acetylcholine (ACh) shortened the action potential, reduced the overshoot, and hyperpolarized the atrial cell membrane in HF, these effects were attenuated compared with those observed in donors. The whole-cell membrane current slope conductance in HF was small, the reversal potential was more positive, and the sensitivity to ACh was less compared with donors. In single-channel recordings from cell-attached patches, IK1 channel conductance and gating characteristics were the same in HF and donor atria. When ACh was included in the pipette solution, IK(ACh) was activated in both groups. Single-channel slope conductance of IK(ACh) averaged 42 +/- 3 pS (n = 28) in HF and 44 +/- 2 pS (n = 4) in donors, and mean open lifetime was 1.3 +/- 0.3 milliseconds (n = 24) in HF and 1.5 +/- 0.4 milliseconds (n = 4) in donors. These values were virtually identical in the two groups (not significantly different, NS), although both single IK1 and IK(ACh) channel densities were less in HF. Channel open probability of IK(ACh) was also less in HF (4.0 +/- 1.2%, n = 24) than in donors (6.8 +/- 1.1%, n = 3, P < .01). The concentration of ACh at half-maximal activation was 0.11 mumol/L in HF and 0.03 mumol/L in donors. In excised inside-out patches, IK(ACh) from HF required higher concentrations of GTP and GTP gamma S to activate the channel compared with donors. These results suggest a reduced IK(ACh) channel sensitivity to M2 cholinergic receptor-linked G protein (Gi) in HF compared with donors.

CONCLUSIONS

Atrial myocytes isolated from failing human hearts exhibited a lower resting membrane potential and reduced sensitivity to ACh compared with donor atria. Whole-cell and single-channel measurements suggest that these alterations are caused by reduced IK1 and IK(ACh) channel density and reduced IK(ACh) channel sensitivity to Gi-mediated channel activation in HF.

Hemodynamic and renal effects of dopexamine and dobutamine in patients with reduced cardiac output following coronary artery bypass grafting

OBJECTIVE

Dopexamine hydrochloride is a novel synthetic adrenergic agonist that combines the renal effects of dopamine with the hemodynamic effects of dobutatmine. Our study is designed to compare the hemodynamic, diuretic, and natriuretic effects of dopexamine and dobutamine in patients with reduced cardiac index following heart surgery.

DESIGN

Prospectively randomized, blinded study.

SETTING

Operating room and intensive care unit of a large, urban, academic medical center.

PATIENTS

Twenty-eight patients undergoing elective coronary artery bypass grafting (CABG) with preoperative ejection fraction of at least 40 percent gave informed consent. The study group consisted of the ten patients who had a cardiac index < or = 2.5 L/min/m2 (while receiving no inotropic medication) immediately after separation from cardiopulmonary bypass.

INTERVENTIONS AND MEASUREMENTS

Study patients were randomly given a starting dose of either 5 micrograms/kg/min of dobutamine (n = 5) or 2 micrograms/kg/min of dopexamine (n = 5). During the initial 30 min following separation from bypass, dosages were titrated incrementally to maintain cardiac index > or = 3.0/L/min/m2. Further titrations of the drug were done only if cardiac index fell below 3.0 L/min/m2 or if sustained tachycardia occurred during the 24-h study period. Data were collected at 5- and 10-min intervals for the first 30 min after separation from bypass, hourly for the next 8 h, then every 2 h for the remainder of the study period.

RESULTS

Both drugs increased cardiac index by more than 50 percent over baseline (dobutamine 2.2 +/- 0.1 to 3.5 +/- 0.2 [p < 0.05]; dopexamine, 2.3 +/- 0.1 to 3.5 +/- 0.1 [p < 0.05] L/min/m2). The mean dose required to maintain cardiac index > or = 3.0L/min/m2 was 1.5 micrograms/kg/min for dopexamine and 3.5 micrograms/kg/min for dobutamine. There were no significant differences in either urinary output or net sodium excretion in the dopexamine group compared with the dobutamine group, and tachycardia (heart rate > 120 beats/min) was more common in the dopexamine group.

CONCLUSIONS

Our study demonstrates that dopexamine produces hemodynamic, diuretic, and natriuretic effects similar to dobutamine in patients with reduced cardiac index following CABG.

Age-related changes in beta-adrenergic neuroeffector systems in the human heart

BACKGROUND

Aging decreases cardiac beta-adrenergic responsiveness in model systems and in humans in vivo. The purpose of this study was to comprehensively evaluate the age-related changes in the beta-receptor-G protein-adenylyl cyclase complex in nonfailing human hearts.

METHODS AND RESULTS

Twenty-six nonfailing explanted human hearts aged 1 to 71 years were obtained from organ donors and subjected to pharmacological investigation of beta-adrenergic neuroeffector systems. When the population was subdivided into the 13 youngest and 13 oldest subjects, total beta-receptor density assessed by maximum [125I]ICYP binding (beta max) was reduced in older hearts by 37% in left ventricles and 31% in right ventricles (both P < .05), and the downregulation was confined to the beta 1 subtype (r = .78 left ventricle beta 1 density versus donor age). Older donor hearts exhibited a 3- to 4-fold rightward shift of ICYP-isoproterenol (ISO) competition curves and demonstrated 43% fewer receptors in a high-affinity agonist binding state (P < .05). Older hearts exhibited decreased adenylyl cyclase stimulation by ISO, by zinterol (beta 2-agonist), and by the G protein-sensitive probes forskolin, Gpp(NH)p, and NaF. In contrast, there was no change in response to manganese, a specific activator of the adenylyl cyclase catalytic subunit. Toxin-catalyzed ADP ribosylation in membranes prepared from older versus younger hearts revealed a 29% to 30% reduction (P < .05) with cholera toxin (Gs) but no difference with pertussis toxin (Gi). The systolic contractile response of isolated right ventricular trabeculae to ISO was decreased by 46%, with a 10-fold increase in ISO EC50 in older relative to younger donor hearts.

CONCLUSIONS

There is a profound decrease in cardiac beta-adrenergic responsiveness with aging. This occurs by multiple mechanisms including downregulation and decreased agonist binding of beta 1-receptors, uncoupling of beta 2-receptors, and abnormal G protein-mediated signal transduction.

Preserved beta-adrenoceptor-mediated adenylyl cyclase activity despite receptor and postreceptor dysfunction in acute myocardial ischemia

To examine responses of the beta-adrenoceptor guanine nucleotide protein (G protein)/adenylyl cyclase complex to acute myocardial ischemia, we measured adenylyl cyclase activity stimulated at the beta-adrenoceptor and postreceptor levels and compared crude homogenates prepared from ischemic and nonischemic rabbit myocardium obtained after 30 minutes of coronary artery occlusion. Basal adenylyl cyclase activity was unchanged, but enzyme activity stimulated by the guanosine triphosphate analog guanyl-5'-imidodiphosphate (GppNHp) at 10 mumol/L was depressed 63% by ischemia (n = 16, p = 0.001). In contrast, adenylyl cyclase activity stimulated by 1 mumol/L (-)-isoproterenol in the presence of 10 mumol/L GppNHp was not significantly reduced (n = 10), a finding that indicates relative preservation of beta-adrenoceptor-mediated adenylyl cyclase activity in ischemia. The ratio of (-)-isoproterenol-stimulated to GppNHp-stimulated adenylyl cyclase activity increased fourfold in ischemic myocardium (n = 6, p = 0.001), consistent with more efficient beta-adrenergic signal transduction via less functional stimulatory G protein (Gs). These data could not be explained by augmented beta-adrenoceptor density or agonist affinity or by a reduction in inhibitory G protein-mediated inhibition of adenylyl cyclase. Forskolin (1 mmol/L) and Mn2+ (1 mmol/L), agents that directly stimulate the catalytic subunit of adenylyl cyclase, each increased enzyme activity significantly more in ischemic than in nonischemic myocardium. We conclude that preservation of (-)-isoproterenol-mediated adenylyl cyclase activity during acute myocardial ischemia in the rabbit results at least in part from enhanced function of the catalytic subunit of adenylyl cyclase.(ABSTRACT TRUNCATED AT 250 WORDS)

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.