Transgenic manipulation of β-adrenergic receptor kinase modifies cardiac myocyte contraction to norepinephrine

To determine the direct functional significance of the β-adrenergic receptor (AR) kinase 1 (βARK1) on myocardial performance in the absence of tonic sympathoadrenal neural activation and mechanical loading, we measured the contractile responses to acute β₁-AR stimulation in left ventricular myocytes isolated from nontransgenic control (NTG) and transgenic mice overexpressing either βARK1 (TGβK12) or a βARK1 inhibitor (TGMini27). Contractile response to five concentrations (10^-8-10^-7 M) of the β₁-AR agonist norepinephrine (NE) plus prazosin (10^-6 M) was measured after a 60- s rest, i.e., rested-state contraction (RSC), and during steady-state contraction (SSC) stimulation at 0.5 Hz (23°C). At baseline, resting cell length was significantly greater in TGβK12 myocytes (P < 0.05); however, there were no significant differences in RSC or SSC among NTG, TGβK12, or TGMini27 mice. On the other hand, both the dose-response curve and kinetics for the NE-induced SSC response normalized to RSC (SSC/RSC) were significantly different among experimental groups (P < 0.001). Specifically, maximal SSC induced by NE in myocytes isolated from TGβK12 was only 70% of the response observed in NTG cells and 50% of the response measured in TGMini27. These data suggest that 1) in the absence of circulating catecholamines or basal sympathetic tone, βARK1 actions in single myocytes are minimal, and 2) substantial functional βARK1 modulation of β₁AR signaling occurs in cardiac myocytes even during short-term β₁-AR stimulation. These results are consistent with a role for agonist-induced phosphorylation and desensitization of cardiac β₁-ARs by βARK1 in single myocytes and highlight the potential functional importance of βARK1 as a critical determinant of the cardiac β₁-AR contractile response.