R. E. Hershberger

A1-adenosine receptor inhibition of adenylate cyclase in failing and nonfailing human ventricular myocardium.

BackgroundReceptors that couple via the stimulatory G protein, Gs to adenylate cyclase and to a positive inotropic response have been extensively investigated in failing human heart. In contrast, much less is known about receptors, such as the A1-adenosine receptor, that couple to adenylate cyclase via the inhibitory G protein, Gi, to give a negative inotropic response. Activation of such Gi-coupled receptors might worsen heart failure. Furthermore, caG; is increased in failing human ventricular myocardium, which may enhance inhibitory receptor coupling to adenylate cyclase. Methods and ResultsA1-Adenosine receptor inhibition of adenylate cyclase was examined in crude particulate preparations derived from 12 nonfailing and 12 failing human left ventricles. Experimental conditions were designed for maximal inhibitory responses. Dose-response curves were performed with the selective A1-adenosine receptor agonist R-phenylisopropyladenosine (R-PIA). No differences in nonfailing versus failing heart were observed for basal adenylate cyclase activity (49.0±4.1 versus 45.7±2.6 pmol cyclic AMP/min/mg), maximal R-PIA-mediated inhibition (31.1±2.6 versus 30.2±1.6 pmol cyclic AMP/min/mg), ED50 (R-PIAx 10-7 M, 1.28±0.10 versus 1.36±0.08), or slope (1.06±0.06 versus 1.03±0.10), respectively. Furthermore, fluoride, forskolin, and manganese adenylate cyclase activation were not different in failing heart, which is consistent with no change in the catalytic unit of adenylate cyclase. The inhibitory G protein aGi, as quantitated by pertussis toxin-catalyzed ADP-ribosylation, was increased in failing heart (105.7±5.8 versus 132.7 + 3.4 optical density units, p < 0.003). Basal adenylate cyclase activity was reduced in failing heart (7.8 ±0.8 versus 4.5 ±0.4 pmol cyclic AMP/min/mg, p < 0.00S) with assay conditions designed to assess G protein effects. ConclusionsThe A1-adenosine receptor pathway exerts a major inhibitory effect on human myocardial adenylate cyclase activity. Although aGi was increased in failing heart, A1- adenosine receptor inhibition of adenylate cyclase was not altered in preparations of failing versus nonfailing human ventricular myocardium.

Effect of Therapeutic Dopamine Administration on Myocardial Catecholamine and Neuropeptide Y Concentrations in the Failing Ventricles of Patients with Idiopathic Dilated Cardiomyopathy

Summary: The purpose of this study was to investigate the relationship between dopamine (DA) exposure and myocardial catecholamine and neuropeptide Y (NPY) concentrations in patients with severe congestive heart failure due to idiopathic dilated cardiomyopathy (IDC). Both nonfailing (NF) and failing (F) hearts were obtained in collaboration with the Utah Cardiac Transplantation Program and the Intermountain Organ Recovery System. The patients were stratified into five groups according to their preoperative exposure to dobutamine (DBT) and/or DA. Compared to 12 untreated, NF control hearts, norepinephrine (NE) concentrations were significantly decreased in 30 untreated F hearts obtained from patients with IDC. Norepinephrine concentrations were also significantly decreased in DA-treated NF hearts and in DAtreated F hearts compared to untreated NF and untreated or DBT-treated failing hearts, respectively. NPY concentrations were significantly decreased in untreated F hearts and were further decreased in dopamine-treated NF and DA-treated F hearts compared to untreated NF and untreated or DBT-treated F hearts. Thus, NE and NPY depletion related to DA administration was evident in both NF and F myocardium and was specific for DA in that it was not evident in patients who received the direct-acting β-agonist inotrope DBT. These data suggest that the major inotropic mechanism of action of DA is through cardiac adrenergic neurotransmitter release. The data also provide further support for the concept that indirect acting inotropes such as DA may have limited inotropic potential in F hearts where neuronal NE has been depleted

Heterogeneous regulatory changes in cell surface membrane receptors coupled to a positive inotropic response in the failing human heart.

The failing human ventricular myocardium undergoes heterogeneous changes at the receptor level that have some impact on the ability of the failing myocardium to respond to inotropic stimuli. In the failing human ventricular myocardium the beta 1-adrenergic receptor is profoundly down-regulated, the beta 2-adrenergic receptor is only slightly decreased, alpha 1-adrenergic receptors are unchanged and VIP receptors appear to be increased in density or affinity. These changes have implications for therapeutic strategies for heart failure and for the natural history and pathogenesis of heart muscle disease.

Mr 40,000 and Mr 39,000 pertussis toxin substrates are increased in surgically denervated dog ventricular myocardium

To test the general hypothesis that cardiac inneervation may participate in myocardial G protein regulation, we examined the effects of complete intrapericardial surgical denervalion or sham operation in dogs. In particulate fractions of dog left ventricular (I.V) myocardium harvested 28—33 days after denervation or sham operation. M1 40,000 and M1 39,000 pertussis toxinsensitive substrates (G proteins) were increased by 31 (1.31 ± 0.084 vs 1.0 ± 0.058 OD, arbitrary units, p 0.01) and 4 (1.40 ± 0.1 vs. 1.000 ± 0.084 OD, arbitrary units, p ≤ 0.02), respectively, as compared With sham-operated controls. The M1 40,000 pectussis toxinsensitive band comigrated with a pertussis toxin-sensitive substrate in human crythrocyte membranes known to contain an G1 species. In these same preparations basal. GIP and GppNHp stimulated adenylate cyclase activities were decreased in denervated heart by 20, 26, and 19, respectively, consistent with increased activity of an inhibitory G protein. In contrast. G function was not altered, because cyct membranes reconstituted with membrane extracts and fluoride and β-receptor-stimulated adenylate cyclase activity were not different between groups. Furthermore, adenylate cyclase catalytic subunit function as assessed with forskolin and manganese stimulation wits not different between preparations of control and denervated heart. We conclude that in preparations of surgically denervated dog myocardium M1 40,000 and M1 39,000 pertussis toxin-sensitive G proteins are increased by 31 and 40, respectively, and that functional alterations in adenylate cyclase activity exist, consistent with increased inhibitory G-protein function.