Increase in G iα Protein Accompanies Progression of Post-Infarction Remodeling in Hypertensive Cardiomyopathy

—Hypertensive cardiac hypertrophy and myocardial infarction (MI) are clinically relevant risk factors for heart failure. There is no specific information addressing signaling alterations in the sequence of hypertrophy and post-MI remodeling. To investigate alterations in β-adrenergic receptor G-protein signaling in ventricular remodeling with pre-existing hypertrophy, MI was induced by coronary artery ligation in Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). Ten weeks after the induction of MI, the progression of left ventricular dysfunction and increases in plasma atrial natriuretic peptide (ANP) and cardiac ANP mRNA were more pronounced in SHR than WKY. In addition, the impaired contractile response to β-adrenergic stimulation was observed in the noninfarcted papillary muscle isolated from SHR. Immunochemical Gsα protein and β-adrenoceptor density were not significantly altered by MI in both strains. However, immunochemical Giα was increased (1.5-fold) in the noninfarcted left ventricle of the SHR in which infarction had been induced when compared with that in SHR that underwent sham operation. This increase was observed especially in rats with a high plasma ANP level. Furthermore, there was a positive correlation between Giα and the extent of post-MI remodeling in WKY. A similar correlation between Giα and the extent of hypertensive hypertrophy was observed in SHR. In conclusion, the vulnerability of hypertrophied hearts to ischemic damage is greater than that of normotensive hearts. An increase in Giα could be one mechanism involved in the transition from cardiac hypertrophy to cardiac failure when chronic pressure overload and loss of contractile mass from ischemic heart disease coexist.