Beta1 and Beta2 adrenoceptors in human ventricular myocytes: a study of their proportion and signaling pathways.

Alterations in β1 and β2-adrenoceptor (AR) density and function represent a common feature of heart failure. In both human and rat hearts, βAR subtypes are present in different proportion depending on cardiac disease and cell type, and are coupled to different metabolic pathways. The aim of this study was to examine the ratio (β1:β2) and functional activity of β1 and β2AR subtypes in human ventricular myocytes (HuVM) isolated from transplanted failing hearts (1). β1:β2 was measured in cells derived from 10 human hearts using radioligand binding techniques (2). The number of βAR was measured with saturation binding assays of the non selective βAR antagonist 3H-CGP12177; competition for 3H-CGP12177 binding by the selective β1AR antagonist CGP20712A gave biphasic curves which demonstrated two binding sites. β1:β2 was approximately 1:1. βAR signaling pathways were studied by evaluating the effect on the midpoint of activation (VH) for the pacemaker current If in patch-clamped HuVM. β1AR stimulation with 1 µM noradrenaline caused a positive shift of VH (8.9±1.3 mV) significantly larger than that caused by β2AR stimulation obtained with 1 µM isoprenaline plus CGP20712A (0.1 µM) (4.5±1.1 mV). Preincubation of HuVM with pertussis toxin (PTX) significantly increased the β2AR response (11.3±1.4 mV), which became similar to the β1AR one. In fact, the effect of β1AR was unaffected by PTX-treatment (10.5±1.8 mV). Thus, in HuVM β2AR are coupled to both stimulatory and inhibitory G proteins. When the latter are blocked, βAR subtypes, which are present in a similar proportion in these cells, exert an equal effect on If.