Depressed adenyl cyclase activity in the failing guinea pig heart

Effects of beta-adrenoceptor stimulation on pacing-induced failure of dog hypertrophic hearts

1. We tested the hypothesis that the transition to pacing-induced failure in hypertrophic hearts would result in reduced functional and metabolic responses to beta-adrenoceptor stimulation. 2. Isoproterenol (ISO; 0.1 microg/kg per min) was infused into a coronary artery in five anaesthetized open-chest control, five aortic stenosis-induced left ventricular hypertrophy (LVH) and five LVH pacing-induced failure dogs. 3. In both control and LVH dogs, but not in failure dogs, ISO significantly increased local regional work (1,923+/-665 vs 2,656+/-715, 1,185+/-286 vs 1,906+/-562 and 835+/-106 vs 849+/-216g.mm/min, respectively), force (11.1+/-1.4 vs 16.9+/-2.6, 8.6+/-1.5 vs 13.7+/-2.3 and 12.2+/-1.1 vs 11.0+/-1.8g, respectively) and myocardial O2 consumption (7.3+/-2.0 vs 10.0+/-1.5, 8.2+/-1.6 vs 11.6+/-2.6 and 4.4+/-1.5 vs 5.5+/-1.8 mL O2/min per 100 g, respectively). 4. Isoproterenol also significantly increased cAMP in control and LVH dogs (474+/-67 vs 600+/-91 and 473+/-34 vs 619+/-53 pmol/g, respectively). In heart failure, cAMP was significantly lower and there was no significant increase in cAMP in response to ISO (245+/-43 vs 314+/-40pmol/g, respectively). 5. We conclude that there were no significant myocardial functional, O2 consumption or cAMP responses to ISO after the transition from hypertrophy to cardiac failure.

Mechanisms of desensitization to a PDE inhibitor (milrinone) in conscious dogs with heart failure

The goal of this study was to determine the extent to which the effects of milrinone were desensitized in heart failure (HF) and to determine the mechanisms, i.e., whether these effects could be ascribed to changes in cAMP or phosphodiesterase (PDE) activity in HF. Accordingly, we examined the effects of milrinone in seven conscious dogs before and after HF was induced by rapid ventricular pacing at 240 beats/min. The dogs were chronically instrumented for measurements of left ventricular (LV) pressure and first derivative of LV pressure (dP/dt), arterial pressure, LV internal diameter, and wall thickness. Milrinone (10 micrograms . kg-1. min-1 iv) increased LV dP/dt by 1,854 +/- 157 from 2,701 +/- 105 mmHg/s (P < 0.05) before HF. After HF the increase in LV dP/dt in response to milrinone was attenuated significantly (P < 0.05); it increased by 615 +/- 67 from 1,550 +/- 107 mmHg/s, indicating marked desensitization. In the presence of ganglionic blockade the increases in LV dP/dt (+445 +/- 65 mmHg/s) in response to milrinone were markedly less (P < 0.01), and milrinone increased LV dP/dt even less in HF (+240 +/- 65 mmHg/s). cAMP and PDE activity were measured in endocardial and epicardial layers in normal and failing myocardium. cAMP was decreased significantly (P < 0.05) in LV endocardium (-26%) but not significantly in LV epicardium (-14%). PDE activity was also decreased significantly (P < 0.05) in LV endocardium (-18%) but not in LV epicardium (-4%). Thus significant desensitization to milrinone was observed in conscious dogs with HF. The major effect was autonomically mediated. The biochemical mechanism appears to be due in part to the modest reductions in PDE activity in failing myocardium, which, in turn, may be a compensatory mechanism to maintain cAMP levels in HF. Reductions in cAMP and PDE levels were restricted to the subendocardium, suggesting that the increased wall stress and reduced coronary reserve play a role in mediating these changes.

Pressure overload-induced cardiac hypertrophy with and without dilation

OBJECTIVES

The present study was designed to produce a small animal model showing compensated hypertrophy followed by congestive heart failure within a reasonable time period.

BACKGROUND

Although there are various large animal experimental models of hypertrophy and heart failure, the occurrence of these two stages within a reasonable time period has not been shown very successfully in small animals.

METHODS

A mildly constricting band was placed around the ascending aorta of very young guinea pigs (mean age 25 +/- 3 days) to impose a gradually increasing pressure overload. The animals were examined at different postoperative intervals up to 20 weeks.

RESULTS

At 10 weeks, there was a 56% increase in ventricular weight/body weight ratio, a 33% increase in left ventricular wall thickness and a significant increase in left ventricular systolic pressure. The animals with 20 weeks of banding had developed various clinical symptoms of congestive heart failure including dyspnea, cyanotic appearance of the extremities, hydrothorax and ascites. Although at this stage there was 86% hypertrophy, the increase in wall thickness was only 20%, indicating cardiac dilation. Depressed left ventricular systolic pressure and increased left ventricular end-diastolic pressure and the increase in wet weight/dry weight ratio in the lungs and liver at 20 weeks also indicated the occurrence of heart failure. The collagen content in the heart of animals with banding for 10 and 20 weeks was 160% and 240%, respectively, of that in corresponding sham control animals.

CONCLUSIONS

The data suggest that the heart was in a stage of compensated hypertrophy for up to 10 weeks, whereas heart failure was seen at 20 weeks. The two functional stages, compensatory hypertrophy followed by prolonged failure, make this model appropriate for studies on the transition of heart hypertrophy to congestive heart failure.

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

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

Changes in the receptor-G protein-adenylyl cyclase system in heart failure from various types of heart muscle disease

The abnormalities of the receptor-G protein-adenylyl cyclase (RCG) system in failing human myocardium as the result of 1) idiopathic dilated cardiomyopathy (IDC), 2) ischemic dilated cardiomyopathy (ISCDC), and 3) primary pulmonary hypertension (PPH) were investigated. Depending on the etiology of heart failure, abnormalities of the RCG system result from a reduced number of beta 1 receptors, uncoupling of beta 1 or beta 2 receptors, alteration of G protein function, or decreased catalytic subunit activity of adenylyl cyclase. Compared to IDC, beta 1 receptor down-regulation is less pronounced in ISCDC, and slightly more pronounced in PPH. Preliminary data suggest that beta 1 receptor down-regulation results from alteration in steady-state receptor mRNA levels. Increased functional activity of Gi protein, which seems to result from posttranslational modification, is observed in IDC and ISCDC. Altered Gi protein function may be the basis for beta-receptor uncoupling in IDC and ISCDC, whereas in PPH, this phenomenon may result from altered adenylyl cyclase function. Catalytic subunit activity of adenylyl cyclase is decreased in order of increasing pulmonary hypertension in right-ventricular preparations from PPH greater than IDC greater than ISCDC. However, catalytic subunit activity is similar in LV preparations from all three groups. The decrease in adenylyl cyclase catalytic subunit activity may be the result of the marked cellular injury produced by pressure overload. In summary, numerous desensitization phenomena occur in the failing human heart that are etiology- or model-dependent. To a certain extent, these changes are teleologically beneficial, as they are able to partially protect the failing heart from potentially toxic adrenergic stimuli.

Characterization of lymphocyte beta-adrenergic receptors at rest and during exercise in ambulatory patients with chronic congestive heart failure

To investigate beta-adrenergic receptor dysfunction in congestive heart failure (CHF), the density of lymphocyte beta receptors and adenylate cyclase activity was measured at rest and at peak exercise in 30 patients with CHF and 7 age-matched control subjects. At rest, patients with CHF had reduced beta-receptor density (normals 33 +/- 2; CHF 21 +/- 2 fmol/mg protein; p less than 0.01) and isoproterenol-stimulated adenylate cyclase activity (normals 50 +/- 9; CHF 28 +/- 4 pmol/mg protein/min; p less than 0.05). Sodium fluoride-stimulated adenylate cyclase activity was also reduced (normals 98 +/- 17; CHF 48 +/- 12 pmol/mg protein/min; p less than 0.01). In the patients with CHF, there was no significant correlation between receptor density and peak exercise VO2, ejection fraction or resting plasma catecholamines. In the normal subjects, maximal exercise increased beta-receptor density by 100% (rest 33 +/- 2; exercise 67 +/- 7 fmol/mg protein) and isoproterenol-stimulated adenylate cyclase activity by 66% (rest 50 +/- 9; exercise 83 +/- 18 pmol/mg protein/min (both p less than 0.01]. In contrast, patients with CHF exhibited only a 58% increase in beta-receptor density (rest 20 +/- 3; exercise 32 +/- 6 fmol/mg protein; p less than 0.01) and no significant change in isoproterenol-stimulated adenylate cyclase activity (rest 27 +/- 5; exercise 24 +/- 5 pmol/mg protein/min).(ABSTRACT TRUNCATED AT 250 WORDS)

Protective effect of coenzyme Q10 on thyrotoxic heart in rabbits

An excess of thyroid hormone is known to produce cardiac dysfunction and failure, i.e., thyrotoxic heart. We studied the protective effect of coenzyme Q10 (CoQ10) on the thyrotoxic heart in 29 rabbits. A group treated with 1-thyroxine sodium salt (T4; 167 micrograms/kg) for 3 weeks showed marked decreases in the myocardial content of norepinephrine (NE) and ATP (0.5 +/- 0.2 microgram/g wet weight, P less than 0.05 and 31.1 +/- 2.6 nmol/mg protein, P less than 0.05, respectively) as compared with a group treated with CoQ10 solvent (2 ml/kg) for 3 weeks (1.1 +/- 0.1 microgram/g wet weight and 45.7 +/- 4.7 nmol/mg protein). The mitochondrial Ca2+ content of the T4 group showed significant increases (21.3 +/- 0.6 nmol/mg protein, P less than 0.05) compared with the solvent group (18.2 +/- 0.8 nmol/mg protein), while the total tissue Ca2+ content of the T4 group was unchanged compared with the solvent group. These biochemical derangements suggest that T4-treated rabbits were in a state of cardiac dysfunction. In contrast, a group which was assigned to concomitant treatment of T4 and CoQ10 (5 mg/kg) for 3 weeks showed no reductions in NE and ATP (0.9 +/- 0.2 micrograms/g wet weight and 44.6 +/- 1.9 nmol/mg protein, respectively) and protected an increase in the mitochondrial Ca2+ content (18.2 +/- 1.2 nmol/mg protein). A group treated with CoQ10 (5 mg/kg) for 3 weeks showed no changes in myocardial NE, ATP, and Ca2+ content in the mitochondria. These results suggest that exogenously administered CoQ10 may protect against biochemical derangements in the thyrotoxic heart.

Cardiac beta adrenoceptors and adenylate cyclase in normotensive and renal hypertensive rabbits during changes in autonomic activity

The effects of hypertrophy and alterations in cardiac autonomic activity on left ventricular (LV) beta adrenoceptors and adenylate cyclase were measured in rabbits. Normotensive and renal hypertensive animals were exposed to three levels of chronic sympathetic activity: (i) "normal" activity; (ii) reduced activity after 2 weeks treatment with guanethidine; (iii) 2 weeks increased sympathetic activity following sino-aortic denervation. In hypertensive animals with "normal" activity LV beta receptor sarcolemma concentration was reduced by 36+ compared with the normotensive subgroup whilst total LV receptor numbers were unaltered. Isoprenaline activated adenylate cyclase was similarly affected whilst other sarcolemma marker enzymes were unaffected. Chronic guanethidine administration to normotensive rabbits increased beta receptor concentration (16%, P less than 0.05), basal and isoprenaline activated meters were unaffected. Sino-aortic denervation did not significantly affect beta receptor concentration in either group. The small changes in beta receptor concentration during alterations in sympathetic activity suggest that only a small proportion of LV beta receptors appear to be innervated. The reduction in sarcolemma beta receptor concentration in hypertensive animals appears to be a specific effect due to hypertrophy of the cardiocyte.

Investigation of cardiac beta-adrenoceptors using 125I-labelled 1-(4-iodophenoxy)-3-isopropylaminopropan-2-ol

1-(4-iodophenoxy)-3-isopropylaminopropan-2-ol (IIP) is a potent beta-adrenergic antagonist which has been labelled to high specific activity with 125I and used to bind to rat myocardial membranes. The characteristics of binding were consistent with the known properties of beta-receptors. Thus, binding was highly stereospecific for the L-stereoisomer since L-propranolol was two orders of magnitude more potent than the D-isomer in competing for these sites. The beta-adrenergic agonists isoproterenol, epinephrine and norepinephrine competed for binding with potencies paralleling their pharmacological potencies as beta-adrenergic effectors. The dissociation constant for binding of IIP was 4--5 nM as measured either by direct binding studies or by its inhibition of isoproterenol stimulated adenylate cyclase. Binding was saturable with 0.06 pmoles of IIP per mg of membrane protein binding at saturation. 125IIP is a high affinity, high specific activity ligand suitable for use as a selective probe for the detection and quantitation of cardiac beta-receptors. Its introduction should help solve the problems involved in the investigation of myocardial beta-adrenergic receptors.

Identification of cardiac beta-adrenergic receptors by (minus) [3H]alprenolol binding

(Minus) [3-H] alprenolol, a potent beta-adrenergic antagonist, was used to identify binding sites in a fraction of canine cyocardium. Beta adrenergic agonists and antagonists compete for these binding sites in a manner which directly parallels their known affinity for the cardiac beta-adrenergic receptor. Thus, binding was highly stereo-specific, with the (minus) isomers of beta-adrenergic agonists or antagonists being at least two orders of magnitude more potent than were the (plus) isomers in competing for these sites. The order of potency for inhibition of binding by beta-adrenergic agonists was (minus) isoproterenol greater than (minus) epinephrine greater than (minus) norepinephrine. The dissociation constant (KD) of (minus) alprenolol for the beta-adrenergic receptors was 7-11 nM as determined independently by direct binding studies or by inhibition of isoproterenol-stimulated adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1]. The beta-adrenergic antagonist (minus) propranolol also had high affinity for the binding sites (KD equals 12 nM). The physiologically inactive catechol-containing compounds pyrocatechol and (plus or minus) dihydroxymandelic acid, as well as the metabolite (plus or minus) normetanephrine, and the alpha-adrenergic antagonist phentolamine did not compete for the binding sites at a concentration of 160 muM. Binding was rapid (t1/2 less than 30 sec) and was rapidly reversible (t1/2 less than 15 sec). The binding sites were saturable and bound 0.35 pmol of (minus) [3-H] alprenolol per mg of membrane protein. These characteristics suggest that these binding sites represent the cardiac beta-adrenergic receptors.

Depressed myocardial creatine phosphokinase activity following experimental myocardial infarction in rabbit

Since creatine phosphokinase (CPK) is found predominantly in myocardial and skeletal muscle cells, in contrast to cells participating in the inflammatory response, it was considered likely that measurement of activity of this enzyme in the heart would provide a sensitive and relatively specific index of the extent of ischemic injury following acute coronary artery occlusion. Accordingly, CPK activity was measured serially following coronary artery occlusion in extracts from rabbit myocardium with gross infarction and from normal rabbit left ventricle. In addition, myocardial CPK activity was assayed in extracts from various portions of dog hearts 24 hours after ligation of the coronary artery. CPK activity of rabbit myocardium with infarction was uniformly depressed within 6 hours following coronary occlusion. After 24 hours, activity declined from 15.5±0.9 (mean±SE) to 3.4±0.3. CPK activity in whole left ventricular extracts was depressed and in general, the extent of depression was proportional to the size of the gross infarct. CPK depression in various regions of the dog heart 24 hours after coronary occlusion correlated with the extent of reduction of blood flow determined with the use of radioactively labeled microspheres. Results suggest that depression of myocardial CPK activity may be useful in estimating the extent of tissue damage following experimental coronary artery occlusion and the effect of prophylactic and therapeutic measures on the survival of myocardium in this setting.

Effects of experimental heart failure on the capacity of glucagon to augment myocardial contractility and activate adenyl cyclase

Although glucagon exerts positive inotropic effects in patients with no or mild impairment of cardiac function, similar effects are not consistently observed in patients with chronic heart failure. Accordingly, the inotropic effects of glucagon on papillary muscles from normal cats and cats in which right ventricular failure had been produced for 4-145 days by pulmonary artery banding were compared. At the peak of the concentration-response curve, glucagon increased peak isometric tension (T) in normal muscles from 4.4+/-0.4 to 6.6+/-0.5 g/mm(2) (P <0.001), and maximum rate of tension development (dT/dt) from 16.9+/-0.9 to 25.1+/-1.6 g/sec per mm(2) (P < 0.001). In contrast, glucagon produced no significant increases in T or dT/dt in failure muscles. The percentage increases in T and dT/dt caused by norepinephrine were the same in muscles from normal and failing hearts. Since the cardiac effects of glucagon and norepinephrine may be mediated by adenyl cyclase, responsiveness of adenyl cyclase was determined in particulate fractions of the right ventricle. Glucagon activated adenyl cyclase in normal, but had no effect in failure preparations. Norepinephrine-induced activation of adenyl cyclase, however, was unaltered by failure. Thus, in contrast to norepinephrine, glucagon loses the capacity to augment myocardial contractility and activate adenyl cyclase in hearts derived from cats in chronic failure.