Activation of β 2 -Adrenergic Receptors Hastens Relaxation and Mediates Phosphorylation of Phospholamban, Troponin I, and C-Protein in Ventricular Myocardium From Patients With Terminal Heart Failure

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Abstract
Background —Catecholamines hasten cardiac relaxation through β-adrenergic receptors, presumably by phosphorylation of several proteins, but it is unknown which receptor subtypes are involved in human ventricle. We assessed the role of β 1 - and β 2 -adrenergic receptors in phosphorylating proteins implicated in ventricular relaxation. Methods and Results —Right ventricular trabeculae, obtained from freshly explanted hearts of patients with dilated cardiomyopathy (n=5) or ischemic cardiomyopathy (n=5), were paced at 60 bpm. After measurement of the contractile and relaxant effects of epinephrine (10 μmol/L) or zinterol (10 μmol/L), mediated through β 2 -adrenergic receptors, and of norepinephrine (10 μmol/L), mediated through β 1 -adrenergic receptors, tissues were freeze clamped. We assessed phosphorylation of phospholamban, troponin I, and C-protein, as well as specific phosphorylation of phospholamban at serine 16 and threonine 17. Data did not differ between the 2 disease groups and were therefore pooled. Epinephrine, zinterol, and norepinephrine increased contractile force to approximately the same extent, hastened the onset of relaxation by 15±3%, 5±2%, and 20±3%, respectively, and reduced the time to half-relaxation by 26±3%, 21±3%, and 37±3%. These effects of epinephrine, zinterol, and norepinephrine were associated with phosphorylation (pmol phosphate/mg protein) of phospholamban 14±3, 12±4, and 12±3; troponin I 40±7, 33±7, and 31±6; and C-protein 7.2±1.9, 9.3±1.4, and 7.5±2.0. Phosphorylation of phospholamban occurred at both Ser16 and Thr17 residues through both β 1 - and β 2 -adrenergic receptors. Conclusions —Norepinephrine and epinephrine hasten human ventricular relaxation and promote phosphorylation of implicated proteins through both β 1 - and β 2 -adrenergic receptors, thereby potentially improving diastolic function.

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