Recent development of transgenic mouse models overexpressing G-protein coupled receptors provides a unique new opportunity to study the transmembrane signal transduction induced by spontaneous receptor activation in the absence of an agonist. To determine whether the signaling of unliganded, spontaneously activated beta2-adrenergic receptor (AR) ( beta2-R*) differs from that of ligand-activated beta2AR ( beta2-LR*) in a beta2AR overexpression transgenic murine model (TG4 mice), we examined L-type Ca2+ current (ICa), intracellular Ca2+ transient (Cai) and cell contraction using whole-cell patch clamp and confocal imaging techniques. Surprisingly, while baseline cAMP level, Cai and contraction amplitude were increased by 150%, 35%, and 87%, respectively, in TG4 relative to WT myocytes, due to the presence of beta2-R*s, no difference in basal ICa density, voltage-dependence or inactivation kinetics was observed, suggesting that ICa is unresponsive to signals originating from beta2-R*. The beta2AR inverse agonist, ICI118,551 (5x10-7 M), or an inhibitory cAMP analogue, Rp-CPT-cAMPS (10-4 M), reversed the enhancement of basal Cai and contractility, but did not alter the simultaneously recorded basal ICa. Inhibition of Gi function by pertussis toxin (PTX) did not increase basal ICa, excluding the involvement of Gi proteins in the lack of ICa response to beta2-R*. In contrast, beta2-AR agonist zinterol (10-6 M) in the presence of PTX and an adenylyl cyclase activator forskolin (10-6 M) both elicited ICa responses similar to those observed in WT cells, indicating that L-type Ca2+ channels in TG4 cells are intact in response to cAMP signaling. We concluded that, unlike beta2-LR*, beta2-R* bypasses ICa to modulate cardiac contractility. Thus, beta2-R* may differ from beta2-LR* in their G protein selectivity, intracellular signaling or effector specificity. The novel finding of differential ligand-occupied versus unliganded receptor signaling requires a reformulation of current thought regarding potential mechanism of beta2-AR modulation of cardiac function.
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