Abstract 876: Overexpression of Endothelial Beta 3-Adrenoceptors Regulates Cardiac Alpha-Adrenergic Expression and Function
Background. In heart, β1- and β2-ARs produce an increase in contractility albeit β3-Ars mediate a negative inotropic effect. In human, β3-ARs have been located both in cardiac and endothelial cells. The aim of this study was to determine whether the endothelial β3-ARs could regulate the cardiac contractility.
Methods. We have developed a transgenic rat (Tgβ3) overexpressing the human β3-AR specifically in endothelial cells under the control of the intercellular adhesion molecule-2 promoter. The transgene expression was evaluated by RT-PCR and the expression of β-AR subtypes in ventricle by real time PCR. The contractile response induced by β-AR agonists was evaluated in papillary muscles stimulated at 1 Hz using a mechanoelectric force transducer.
Results. In wild-type rat (WT) hearts, no endogenous β3-AR transcripts were detected whereas in Tgβ3, human β3-AR mRNA was present in heart preparations. The overexpression of endothelial β3-AR was associated with a significant decrease of the β1-ARs (ratio: 0.72±0.1; n=3 for Tgβ3 vs WT) in the cardiomyocytes, whereas the β2-ARs were unchanged (ratio : 1.00±0.08; n=3 for Tgβ3 vs WT, ns). This result was corroborated by the decrease of isoproterenol-induced contractility in Tgβ3 versus WT (p<0.01). The maximal effect (Emax) obtained at 10 μM was 134±49% in WT (n=6) and 71±40% in Tgβ3 (n=7). The perfusion of BRL37344 (0.1 nM-10 μM), a preferential β3-AR agonist, in the presence of nadolol, a β1,2 antagonist, induced a significant concentration-dependent negative inotropic effect only in Tgβ3 papillary muscles. Emax obtained at 10 μM was -31±4% (n=7, p<0.05). This effect was significantly reduced by pretraitement with a β3-AR antagonist, L-748,337 (Emax = -9±7%, n=6) or a NO synthase inhibitor, L-NMMA (Emax = -14±2%, n=6).
Conclusions. Our study demonstrated for the first time that endothelial β3-AR overexpression decrease cardiac contractility through a paracrine effect of NO. Furthermore, the β3-AR overexpression was associated with a decreased β1-AR expression in cardiomyocytes suggesting that endothelial receptors could modulate the expression of receptors located in cardiomyocytes.