title: Spontaneous beta(2)-adrenergic signaling fails to modulate L-type Ca(2+) current in mouse ventricular myocytes.
abstract: A receptor can be activated either by specific ligand-directed changes in conformation or by intrinsic, spontaneous conformational change. In the beta(2)-adrenergic receptor (AR) overexpression transgenic (TG4) murine heart, spontaneously activated beta(2)AR (beta(2)-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 beta(2)-R* differs from that of a ligand-elicited beta(2)AR activation (beta(2)-LR*). In ventricular myocytes from TG4 mice, the properties of L-type Ca(2+) current (I(Ca)), a major effector of beta(2)-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 wild-type (WT) cells. Although the contractile response to beta(2)-R* in TG4 cells was abolished by a beta(2)AR 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 beta(2)-R*, while increasing contraction amplitude, does not affect I(Ca) characteristics. In contrast, the beta(2)AR agonist, zinterol elicited a substantial augmentation of I(Ca) in both TG4 and WT cells (pertussis toxin-treated), indicating that L-type Ca(2+) 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 Ca(2+) channel to cAMP-dependent modulation remains intact in TG4 cells. Thus, we conclude that beta(2)-R* bypasses I(Ca) to modulate contraction, and that beta(2)-LR* and beta(2)-R* exhibit different intracellular signaling and target protein specificity.
authors.affiliation: Laboratory of Cardiovascular Science, Gerontology Research Center, National Institute on Aging, National Institutes of Health, Baltimore, Maryland, USA.
authors.forename: Y Y
authors.initials: YY
authors.lastname: Zhou
authors.forename: H
authors.initials: H
authors.lastname: Cheng
authors.forename: L S
authors.initials: LS
authors.lastname: Song
authors.forename: D
authors.initials: D
authors.lastname: Wang
authors.forename: E G
authors.initials: EG
authors.lastname: Lakatta
authors.forename: R P
authors.initials: RP
authors.lastname: Xiao
mesh_terms: D037361:Adenylate Cyclase Toxin; D000818:Animals; D015220:Calcium Channels / Q000378:metabolism*; D020746:Calcium Channels, L-Type; D006352:Heart Ventricles / Q000187:drug effects; D066298:In Vitro Techniques; D051379:Mice; D009200:Myocardial Contraction / Q000187:drug effects; D009206:Myocardium / Q000378:metabolism*; D037342:Pertussis Toxin; D011487:Protein Conformation; D018343:Receptors, Adrenergic, beta-2 / Q000737:chemistry / Q000378:metabolism* / Q000502:physiology; D015398:Signal Transduction / Q000502:physiology*; D016276:Ventricular Function; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
chemical_list: D037361:Adenylate Cyclase Toxin; D015220:Calcium Channels; D020746:Calcium Channels, L-Type; D018343:Receptors, Adrenergic, beta-2; D010566:Virulence Factors, Bordetella; D037342:Pertussis Toxin