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Increase of Gi alpha in human hearts with dilated but not ischemic cardiomyopathy
M Böhm1, P Gierschik, K H Jakobs
1Medizinische Klinik I, Universität München, FRG.
Insights
Dilated cardiomyopathy (DCM) shows increased Gi alpha-protein, linked to reduced heart function. Ischemic cardiomyopathy (ICM) does not show this Gi alpha increase, suggesting different mechanisms in heart failure.
Area of Science:
- Cardiology
- Molecular Biology
- Biochemistry
Background:
- Cardiomyopathies, including dilated cardiomyopathy (DCM) and ischemic cardiomyopathy (ICM), are major causes of heart failure.
- G-protein signaling pathways play a critical role in regulating cardiac function and contractility.
- Dysregulation of G-protein alpha subunits, particularly Gi alpha, has been implicated in cardiac dysfunction.
Purpose of the Study:
- To investigate the role of Gi alpha-protein in the pathophysiology of DCM and ICM.
- To assess the impact of altered Gi alpha levels on adenylate cyclase activity and receptor function in failing hearts.
- To determine the functional relevance of Gi alpha changes in the reduced inotropic responses observed in DCM and ICM.
Main Methods:
- Quantification of Gi alpha-protein using 32P-ADP-ribosylation and immunoblotting in myocardial samples.
- Measurement of basal and stimulated adenylate cyclase activity.
- Assessment of beta-adrenoceptor, m-cholinoceptor, and A1-adenosine receptor density and function.
- Evaluation of inotropic responses to various agonists (carbachol, R-PIA, isoprenaline, milrinone).
Main Results:
- A significant 37% increase in Gi alpha-protein was observed in DCM but not in ICM compared to nonfailing hearts.
- Basal and Gpp(NH)p-stimulated adenylate cyclase activity was reduced in DCM, correlating with increased Gi alpha.
- Beta-adrenoceptor numbers were reduced in both DCM and ICM, while m-cholinoceptor and A1-adenosine receptor alterations were absent.
- Positive inotropic responses to isoprenaline and milrinone were markedly reduced in both ICM and DCM, with a further significant reduction in DCM.
Conclusions:
- The increase in Gi alpha-protein in DCM is associated with reduced adenylate cyclase activity and contributes to impaired cardiac contractility.
- Alterations in m-cholinoceptors and A1-adenosine receptors are not the primary cause of functional deficits in these cardiomyopathies.
- The heightened Gi alpha levels in DCM are functionally relevant, explaining the diminished response to catecholamines and cAMP-dependent agents, unlike in ICM.
Abstract:
In myocardial membranes from hearts with dilated cardiomyopathy (DCM), there was a 37% increase of the Gi alpha-protein as measured by 32P-ADP-ribosylation of a approximately 40 kDa pertussis toxin substrate. Immunoblotting techniques also showed increased amounts of Gi alpha in DCM. In hearts with ischemic cardiomyopathy (ICM), Gi alpha was not altered compared with nonfailing myocardium (NF). Basal and Gpp(NH)p-stimulated adenylate cyclase activity was reduced in DCM but not in ICM. The number of beta-adrenoceptors was similarly reduced both in DCM and ICM compared with NF. Alterations of m-cholinoceptors or A1-adenosine receptors did not occur. Consistently, "indirect" negative inotropic effects of the m-cholinoceptor agonist carbachol and the A1-adenosine receptor agonist R-PIA were not different in ICM, DCM, and nonfailing myocardium. In ICM and DCM, there was a marked reduction of the positive inotropic responses to isoprenaline and milrinone. However, there was a further reduction in DCM compared with ICM. It is concluded that the increase of Gi alpha is accompanied by a reduction of basal and guanine-nucleotide-stimulated adenylate cyclase activity. Alterations of m-cholinoceptors and A1-adenosine receptors do not appear to be involved. The further decrease of the positive inotropic effects of isoprenaline and milrinone in DCM provides evidence that the increase of Gi alpha is functionally relevant in DCM but not ICM and hence might contribute to the reduced effects of endogenous catecholamines and exogenous cAMP-dependent positive inotropic agents in the former but not the latter condition.