776-1 Selective Down-regulation of Angiotensin II AT1 Receptors in Failing Human Heart: Relationship to β1-Receptor Down-regulation

The renin-angiotensin and adrenergic nervous systems exhibit multiple levels of cross-regulation in heart failure. These systems are bidirectionally activated in concert; i.e. activation of one system activates the other. We compared the behavior of angiotensin II AT1 and AT2 receptors with β1, -and, β2-adrenergic receptors in a high-yield crude membrane fraction prepared from nonfailing and failing human ventricular myocardium. Ang II receptors were measured by 125I saralasin binding, with Bmax determined by saturation binding displaceable by 1μM cold saralasin. AT1 and AT2 receptor fractions were determined by the amount of specific binding displaceable by 1μM losartan. β1-adrenergic receptor density was determined by saturation binding of 125I ICYP, with the, β1 fraction determined by binding displaceable by 0.2μM CGP 20712A. Results in end-stage human left ventricular myocardium failing as a result of idiopathic dilated cardiomyopathy were compared to nonfailing controls taken from age- and gender-matched organ donors not used for transplant because of blood type or body size mismatch: (Receptor density is in fmol/mg±SEM)β1-ARβ2-ARAT1AT2Nonfailing (n=6)59.0±9.420.7±4.04.14±0.621.52±0.43Failing (n = 6)28.3±2.8*17.2±2.61.53±0.57*2.68±0.51*p<0.05The down-regulation of β1 AR and AT1 receptors was significantly related (r=0.62, n=12, p<0.05)Conclusions(1) Compared to β adrenergic receptors ang II receptors are very low density in the human heart. (2) The AT1 receptor sUbtype predominates in the nonfailing human heart. (3) ATl but not AT2 receptors are downregulated in failing heart. (4) Down-regulation of Ang-II AT1 receptor is similar in degree to down-regulation of β1-adrenergic receptors. These data suggest that the AT1 and β1 receptors are respectively exposed to increased concentrations of mutually activatedlinduced norepinephrine and Ang-II in the failing human heart