Localized cAMP-dependent signaling mediates beta 2-adrenergic modulation of cardiac excitation-contraction coupling.
Recent studies have shown that beta 2-adrenergic receptor (beta 2-AR)-stimulated increases in the intracellular Ca2+ (Cai) transient and contraction in cardiac myocytes are dissociated from the increase in adenosine 3',5'-cyclic monophosphate (cAMP) level and are not accompanied by an increase in phospholamban phosphorylation, an acceleration in relaxation, or a reduction in myofilament Ca2+ response. Thus we hypothesized that the beta 2-AR modulation of cardiac excitation-contraction (EC) coupling may be mediated by either a cAMP-independent mechanism or a compartmentalized cAMP pathway. To directly distinguish between these two possibilities, the responses of the L-type Ca2+ current (ICa), Cai transient, and contraction to beta 2-AR as well as to beta 1-AR stimulation were examined in rat ventricular myocytes in the presence or absence of specific inhibitory cAMP analogs, Rp diastereomers of adenosine 3',5'-cyclic monophosphothioate (Rp-cAMPS) and 8-(4-chlorophenylthio)-cAMP (Rp-CPT-cAMPS). As expected, the positive inotropic effect induced by an adenylyl cyclase activator, forskolin (2 x 10(-7) M), or a beta 1-AR agonist, norepinephrine (5 x 10(-8) M) plus prazosin (10(-6) M), was completely blocked by Rp-CPT-cAMPS. More importantly, the responses of ICa, Cai transient, and contraction to beta 2-AR stimulation by zinterol (10(-5) M) or isoproterenol plus a selective beta 1-AR antagonist, CGP-20712A, were also entirely abolished by Rp-cAMPS (in the patch-pipette solution) or Rp-CPT-cAMPS (in the bath solution). In pertussis toxin-treated cells, although the response of cAMP was not altered, the beta 2-AR-stimulated increase in contraction amplitude was markedly enhanced and accompanied by a hastened relaxation, resulting in a tight association between cAMP and contraction. These results indicate that beta 2-AR modulation of cardiac excitation-contraction coupling requires cAMP. The dissociation of beta 2-AR-stimulated cAMP production and regulation of myofilament and sarcoplasmic reticulum functions is attributable to a functional compartmentation of the cAMP-dependent signaling due to an activation of beta 2-AR-coupled Gi and/or G(o).
Zhou
Y Y
YY
Laboratory of Cardiovascular Science, National Institute on Aging, Baltimore, Maryland 21224, USA.
Cheng
H
H
Bogdanov
K Y
KY
Hohl
C
C
Altschuld
R
R
Lakatta
E G
EG
Xiao
R P
RP
D000318:Adrenergic beta-Agonists / Q000494:pharmacology; D000319:Adrenergic beta-Antagonists / Q000494:pharmacology; D000818:Animals; D002118:Calcium; D015220:Calcium Channels / Q000502:physiology; D020746:Calcium Channels, L-Type; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000031:analogs & derivatives / Q000378:metabolism* / Q000494:pharmacology; D004983:Ethanolamines / Q000494:pharmacology; D006321:Heart / Q000502:physiology*; D006352:Heart Ventricles; D007093:Imidazoles / Q000494:pharmacology; D066298:In Vitro Techniques; D007545:Isoproterenol / Q000494:pharmacology; D009200:Myocardial Contraction; D009206:Myocardium / Q000166:cytology / Q000378:metabolism; D009638:Norepinephrine / Q000494:pharmacology; D011224:Prazosin / Q000494:pharmacology; D051381:Rats; D018343:Receptors, Adrenergic, beta-2 / Q000502:physiology*; D015398:Signal Transduction; D013873:Thionucleotides / Q000494:pharmacology
D000318:Adrenergic beta-Agonists; D000319:Adrenergic beta-Antagonists; D015220:Calcium Channels; D020746:Calcium Channels, L-Type; D004983:Ethanolamines; D007093:Imidazoles; D018343:Receptors, Adrenergic, beta-2; D013873:Thionucleotides; D005576:Colforsin; C016957:adenosine-3',5'-cyclic phosphorothioate; C002904:zinterol; C049763:CGP 20712A; D000242:Cyclic AMP; D007545:Isoproterenol; D002118:Calcium; D009638:Norepinephrine; D011224:Prazosin