Spontaneous β₂-adrenergic signaling fails to modulate L-type Ca²⁺ current in mouse ventricular myocytes

A receptor can be activated either by specific ligand-directed changes in conformation or by intrinsic, spontaneous conformational change. In the β₂-adrenergic receptor (AR) overexpression transgenic (TG4) murine heart, spontaneously activated β₂AR (β₂-R*) in the absence of ligands has been evidenced by elevated basal adenylyl cyclase activity and cardiac function. In the present study, we determined whether the signaling mediated by β₂- R* differs from that of a ligand-elicited β₂AR activation (β₂-LR*). In ventricular myocytes from TG4 mice, the properties of L-type Ca²⁺ current (I(Ca)), a major effector of β₂-LR* signaling, was unaltered, despite a 2.5-fold increase in the basal cAMP level and a 1.9-fold increase in baseline contraction amplitude as compared with that of wildtype (WT) cells. Although the contractile response to β₂-R* in TG4 cells was abolished by a β₂AR inverse agonist, ICI118,551 (5 X 10^-7 M), or an inhibitory cAMP analog, Rp- CPT-cAMPS (10^-4 M), no change was detected in the simultaneously recorded I(Ca). These results suggest that the increase in basal cAMP due to β₂-R*, while increasing contraction amplitude, does not affect I(Ca) characteristics. In contrast, the β₂AR agonist, zinterol elicited a substantial augmentation of I(Ca) in both TG4 and WT cells (pertussis toxin- treated), indicating that L-type Ca²⁺ channel in these cells can respond to ligand-directed signaling. Furthermore, forskolin, an adenylyl cyclase activator, elicited similar dose-dependent increase in I(Ca) amplitude in WT and TG4 cells, suggesting that the sensitivity of L-type Ca²⁺ channel to cAMP-dependent modulation remains intact in TG4 cells. Thus, we conclude that β₂-R* bypasses I(Ca), to modulate contraction, and that β₂-LR* and β₂-R* exhibit different intracellular signaling and target protein specificity.