Differential Regulation of mRNA Specific forβ1- andβ2-adrenergic Receptors in Human Failing Hearts. Evaluation of the Absolute Cardiac mRNA Levels by Two Independent Methods

In human heart failureβ-adrenergic receptors are downregulated which contributes to the reduced responsiveness to positive inotropicβ-agonists in the diseased heart. The present study addressed the question whether the number ofβ-adrenergic receptors in the failing human heart is regulated at the level of the mRNA and whether the absolute steady-state levels of subtype-specific mRNAs mirror the expression of receptor-subtype proteins in human heart. In a collaborative effort, two different and independent methods, performed in two independent laboratories, reverse transcription followed by polymerase chain reaction (RT-PCR) and RNase protection assays, were used to determine the absolute steady-state levels ofβ1- andβ2-adrenergic receptor mRNAs in control (NF) and in failing human hearts. As determined by quantitative RT-PCR theβ1-mRNA was significantly reduced from 0.98 ±0.12 (n=10) to 0.49 ±0.11 pg /μg total RNA in dilated cardiomyopathy (dCMP,n=7) and to 0.40 ±0.11 pg /μg total RNA in ischemic cardiomyopathy (iCMP,n=8). The steady-state levels of mRNA specific forβ2-adrenergic receptors also tended to be decreased but without reaching significance (NF: 0.16 ±0.05, dCMP: 0.11 ±0.03, iCMP: 0.13 ±0.04 pg /μg total RNA). RNase protection assays revealed similar values.β1-mRNA was found to be significantly reduced from 1.22 ±0.22 in NF (n=10) to 0.63 ±0.14 pg /μg total RNA in dCMP (n=5) and to 0.52 ±0.1 pg /μg total RNA in iCMP (n=8). Theβ2-mRNA also tended to be lower in dCMP and in iCMP as compared to NF but again without reaching significance (NF: 0.14 ±0.02, dCMP: 0.099 ±0.02, iCMP 0.107 ±0.02 pg /μg total RNA). This is the first study to demonstrate in parallel by two different methods performed independently in two laboratories that the ratio ofβ1- andβ2-adrenergic receptor densities in the left ventricle of the normal human heart of about 80 /20 is closely related to the absolute steady state concentrations of their specific mRNA. In addition, the magnitude of the decrease in mRNA-levels ofβ1- andβ2-adrenergic receptors in the failing human heart closely correlates with the decrease of the respective receptor proteins. These data suggest that the predominant regulation ofβ-adrenergic receptors occurs at the mRNA level.