Carvedilol is an adrenoceptor antagonist which modulates the activity not only of β1 and β1 but also of α1 adrenergic receptors present on the cell surface membrane of the human cardiac myocyte. In the heart, carvedilol has approximately 7 times higher potency for 13, and 132 adrenoceptors, but in the doses 50–100 mg. day−1 used in clinical practice, it is essentially non-selective. In human myocardial preparations and in cultured heart cells, carvedilol has no intrinsic sympathomimetic activity but is able to identify high affinity agonist-binding receptors whose pharmaco logical signature is reduction in binding by incubation with guanine nucleotides (guanine nucleotide-modulatable binding). This property is more prominent for the human β2 than for the β1adrenoceptor. The property of gaunine nucleotide-modulatable binding for carvedilol and structurally related bucindolol correlates with their ability to directly down-regulate β1-like receptors present in cultured chick myocytes, and with a lack of reversal of down-regulation of cardiac β-receptors in patients with heart failure. Carvedilol does not exhibit high levels of inverse agonist activity, which may contribute to its good tolerability in subjects with heart failure. These data indicate that carvedilol produces a high degree of adrenergic receptor blockade in the failing human heart, and does not re-sensitize the β-receptor pathway to stimulation by adrenergic agonists.
Background The regulation of angiotensin II receptors and the two major subtypes (AT 1 and AT 2 ) in chronically failing human ventricular myocardium has not been previously examined. Methods and Results Angiotensin II receptors were measured by saturation binding of 125 I-[Sar 1 ,Ile 8 ]angiotensin II in crude membranes from nonfailing (n=19) and failing human left ventricles with idiopathic dilated cardiomyopathy (IDC; n=31) or ischemic cardiomyopathy (ISC; n=21) and membranes from a limited number of right ventricles in each category. The AT 1 and AT 2 fractions were determined by use of an AT 1 -selective antagonist, losartan. β-Adrenergic receptors were also measured by binding of 125 I-iodocyanopindolol with the β 1 and β 2 fractions determined by use of a β 1 -selective antagonist, CGP20712A. AT 1 but not AT 2 density was significantly decreased in the combined (IDC+ISC) failing left ventricles (nonfailing: AT 1 4.66±0.48, AT 2 2.73±0.39; failing: AT 1 3.20±0.29, AT 2 2.70±0.33 fmol/mg protein; mean±SE). The decrease in AT 1 density was greater in the IDC than in the ISC left ventricles (IDC: 2.73±0.40, P <.01; ISC: 3.89±0.39 fmol/mg protein, P =NS versus nonfailing). β 1 but not β 2 density was decreased in the failing left ventricles. AT 1 density was correlated with β 1 density in all left ventricles ( r =.43). AT 1 density was also decreased in IDC right ventricles. In situ reverse transcription–polymerase chain reaction in sections of nonfailing and failing ventricles indicated that AT 1 mRNA was present in both myocytes and nonmyocytes. Conclusions AT 1 receptors are selectively downregulated in failing human ventricles, similar to the selective downregulation of β 1 receptors. The relative lack of AT 1 downregulation in ISC hearts may be related to differences in the degree of ventricular dysfunction.
In both cell culture based model systems and in the failing human heart, β-adrenergic receptors (β-AR) undergo agonist-mediated down-regulation. This decrease correlates closely with down-regulation of its mRNA, an effect regulated in part by changes in mRNA stability. Regulation of mRNA stability has been associated with mRNA-binding proteins that recognize A + U-rich elements within the 3′-untranslated regions of many mRNAs encoding proto-oncogene and cytokine mRNAs. We demonstrate here that the mRNA-binding protein, AUF1, is present in both human heart and in hamster DDT1-MF2 smooth muscle cells and that its abundance is regulated by β-AR agonist stimulation. In human heart, AUF1 mRNA and protein was significantly increased in individuals with myocardial failure, a condition associated with increases in the β-adrenergic receptor agonist norepinephrine. In the same hearts, there was a significant decrease (~50%) in the abundance of β1-AR mRNA and protein. In DDT1-MF2 cells, where agonist-mediated destabilization of β2-AR mRNA was first described, exposure to β-AR agonist resulted in a significant increase in AUF1 mRNA and protein (~ 100%). Conversely, agonist exposure significantly decreased (~40%) β2-adrenergic receptor mRNA abundance. Last, we demonstrate that AUF1 can be immunoprecipitated from polysome-derived proteins following UV cross-linking to the 3′-untranslated region of the human β1-AR mRNA and that purified, recombinant p37AUF1 protein also binds to β1-AR 3′-untranslated region mRNA. In both cell culture based model systems and in the failing human heart, β-adrenergic receptors (β-AR) undergo agonist-mediated down-regulation. This decrease correlates closely with down-regulation of its mRNA, an effect regulated in part by changes in mRNA stability. Regulation of mRNA stability has been associated with mRNA-binding proteins that recognize A + U-rich elements within the 3′-untranslated regions of many mRNAs encoding proto-oncogene and cytokine mRNAs. We demonstrate here that the mRNA-binding protein, AUF1, is present in both human heart and in hamster DDT1-MF2 smooth muscle cells and that its abundance is regulated by β-AR agonist stimulation. In human heart, AUF1 mRNA and protein was significantly increased in individuals with myocardial failure, a condition associated with increases in the β-adrenergic receptor agonist norepinephrine. In the same hearts, there was a significant decrease (~50%) in the abundance of β1-AR mRNA and protein. In DDT1-MF2 cells, where agonist-mediated destabilization of β2-AR mRNA was first described, exposure to β-AR agonist resulted in a significant increase in AUF1 mRNA and protein (~ 100%). Conversely, agonist exposure significantly decreased (~40%) β2-adrenergic receptor mRNA abundance. Last, we demonstrate that AUF1 can be immunoprecipitated from polysome-derived proteins following UV cross-linking to the 3′-untranslated region of the human β1-AR mRNA and that purified, recombinant p37AUF1 protein also binds to β1-AR 3′-untranslated region mRNA.
