Regulation and possible functional implications of G-protein mRNA expression in nonfailing and failing ventricular myocardium.
In human end-stage heart failure an increased amount of inhibitory G-protein alpha-subunits (Gi alpha) is assumed to play a role in desensitization of the adenylyl cyclase signaling pathway. In the present study, northern blot experiments with 32P-labeled cDNA probes in ventricular tissue samples from explanted human hearts revealed that Gi alpha-2- and Gi alpha-3- mRNA are the predominant Gi alpha-mRNA subtypes in human ventricles, whereas Gi alpha-1-mRNA was not detectable. The mRNA for the stimulatory G-protein alpha-subunit (GS alpha) consisted of two mRNA sizes. Quantification of mRNA levels revealed a 103 +/- 38% increase in Gi alpha-2-mRNA levels in hearts with idiopathic dilative cardiomyopathy (IDC; n = 8), and a 77 +/- 25% increase in hearts with ischemic cardiomyopathy (ICM; n = 6) as compared to nonfailing controls (NF, n = 8). In contrast, Gi alpha-3- and GS alpha-mRNA levels were similar in failing and nonfailing hearts. To investigate whether or not the increased expression of Gi alpha-2-mRNA might be due to chronically elevated catecholamine levels, we determined the influence of a 4-day infusion of isoprenaline (Iso; 2.4 mg/kg.d), propranolol (Prop; 9.9 mg/kg.d), Iso + Prop or 0.9% NaCl as control (Ctr) on myocardial Gi alpha-mRNA and Gi alpha-protein levels in rats. In Iso-treated rats, hybridization experiments revealed a 49 +/- 18% (n = 7) and 27 +/- 7% (n = 8) increase in Gi alpha-2 and Gi alpha-3-mRNA, respectively. Pertussis toxin-catalyzed ADP-ribosylation revealed a 22 +/- 7% (n = 8) increase in Gi-protein as compared to Ctr (n = 8). These alterations were accompanied by an increased potency for the negative inotropic effect (NIE) of carbachol (mean EC50: 0.04 microM vs. 0.28 microM) in the presence of Iso in isolated electrically driven (1 Hz) papillary muscles. Prop itself had no effect, but it antagonized all Iso-induced effects. We conclude that, in human heart failure due to IDC or ICM, increased Gi alpha-2-, but not Gi alpha-3- mRNA levels accompany the increased amount of Gi alpha-protein, suggesting that this increase is at least in part due to increased de novo synthesis. The experiments in rats demonstrated that chronic beta-adrenergic stimulation leads to an increased expression of Gi alpha-mRNA and -protein, and to an enhanced potency of the negative inotropic effect of muscarinic agonists.(ABSTRACT TRUNCATED AT 400 WORDS)
Eschenhagen
T
T
Abteilung Allgemeine Pharmakologie, Universitäts-Krankenhaus Eppendorf, Hamburg, FRG.
Mende
U
U
Nose
M
M
Schmitz
W
W
Scholz
H
H
Schulte am Esch
J
J
Sempell
R
R
Warnholtz
A
A
Wüstel
J M
JM
D000328:Adult; D000818:Animals; D002217:Carbachol / Q000494:pharmacology; D002311:Cardiomyopathy, Dilated / Q000503:physiopathology*; D003327:Coronary Disease / Q000503:physiopathology*; D005260:Female; D019204:GTP-Binding Proteins / Q000032:analysis* / Q000502:physiology; D005786:Gene Expression Regulation; D006321:Heart / Q000502:physiology; D006801:Humans; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D008875:Middle Aged; D009200:Myocardial Contraction / Q000187:drug effects; D009206:Myocardium / Q000737:chemistry*; D009924:Organ Culture Techniques; D012333:RNA, Messenger / Q000032:analysis*; D051381:Rats; D011919:Rats, Inbred Strains; D015854:Up-Regulation
D012333:RNA, Messenger; D002217:Carbachol; D019204:GTP-Binding Proteins; D007545:Isoproterenol