Excitation-Contraction Coupling in Hypertrophied Myocardium

Abstract

In hypertensive cardiac hypertrophy, inotropic responsiveness of alpha and beta adrenergic stimuli is reduced. We have previously shown that hearts from two-kidney, one-clip renal hypertensive rats (RHR) have increased beta-adrenergic receptor density and a defect in the guanine nucleotide regulatory protein, leading to decreased adenylate cyclase activity. In spontaneously hypertensive rats (SHR), beta-receptor density was decreased with no change in adenylate cyclase. In these present experiments, we have shown that the alpha 1-adrenergic receptor changes are in the opposite direction, decreasing in RHR and increasing in SHR. All these changes are reversible within 4 weeks following removal of the clipped kidney in RHR, at which time blood pressure and heart weight have also returned towards normal. Further studies on the excitation-contraction pathway have indicated that c-AMP-stimulated protein kinase is decreased in SHR with no changes seen in RHR. Subcutaneous infusion of epinephrine leads to some increase of cardiac mass associated with decreased beta-adrenergic receptors element and decreased adenylate cyclase activity. However, following angiotensin II infusion, even though hypertrophy is more pronounced, no changes in receptors or cyclase are detected. We conclude that different models of hypertensive cardiac hypertrophy associated with different biochemical defects in the adrenergic excitation response pathway, and that if some of these changes become irreversible, further cardiac deterioration and even heart failure may ensue.