title: beta2-Adrenergic receptor agonists stimulate L-type calcium current independent of PKA in newborn rabbit ventricular myocytes.
abstract: Selective stimulation of beta(2)-adrenergic receptors (ARs) in newborn rabbit ventricular myocardium invokes a positive inotropic effect that is lost during postnatal maturation. The underlying mechanisms for this age-related stimulatory response remain unresolved. We examined the effects of beta(2)-AR stimulation on L-type Ca(2+) current (I(Ca,L)) during postnatal development. I(Ca,L) was measured (37 degrees C; either Ca(2+) or Ba(2+) as the charge carrier) using the whole-cell patch-clamp technique in newborn (1 to 5 days old) and adult rabbit ventricular myocytes. Ca(2+) transients were measured concomitantly by dialyzing the cell with indo-1. Activation of beta(2)-ARs (with either 100 nM zinterol or 1 microM isoproterenol in the presence of the beta(1)-AR antagonist, CGP20712A) stimulated I(Ca,L) twofold in newborns but not in adults. The beta(2)-AR-mediated increase in Ca(2+) transient amplitude in newborns was due exclusively to the augmentation of I(Ca,L). Zinterol increased the rate of inactivation of I(Ca,L) and increased the Ca(2+) flux integral. The beta(2)-AR inverse agonist, ICI-118551 (500 nM), but not the beta(1)-AR antagonist, CGP20712A (500 nM), blocked the response to zinterol. Unexpectedly, the PKA blockers, H-89 (10 microM), PKI 6-22 amide (10 microM), and Rp-cAMP (100 microM), all failed to prevent the response to zinterol but completely blocked responses to selective beta(1)-AR stimulation of I(Ca,L) in newborns. Our results demonstrate that in addition to the conventional beta(1)-AR/cAMP/PKA pathway, newborn rabbit myocardium exhibits a novel beta(2)-AR-mediated, PKA-insensitive pathway that stimulates I(Ca,L). This striking developmental difference plays a major role in the age-related differences in inotropic responses to beta(2)-AR agonists.
authors.affiliation: Department of Pediatrics, Program in Pediatric Cardiology, New York University School of Medicine, New York, New York, USA.
authors.forename: Leon P
authors.initials: LP
authors.lastname: Collis
authors.forename: Shekhar
authors.initials: S
authors.lastname: Srivastava
authors.forename: William A
authors.initials: WA
authors.lastname: Coetzee
authors.forename: Michael
authors.initials: M
authors.lastname: Artman
mesh_terms: D058666:Adrenergic beta-2 Receptor Agonists; D000318:Adrenergic beta-Agonists / Q000008:administration & dosage*; D000818:Animals; D000831:Animals, Newborn; D002118:Calcium / Q000378:metabolism*; D020746:Calcium Channels, L-Type / Q000187:drug effects / Q000502:physiology*; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology; D002478:Cells, Cultured; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology*; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000502:physiology*; D011817:Rabbits; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology
chemical_list: D058666:Adrenergic beta-2 Receptor Agonists; D000318:Adrenergic beta-Agonists; D020746:Calcium Channels, L-Type; D017868:Cyclic AMP-Dependent Protein Kinases; D002118:Calcium