Phosphorylation of phospholamban and troponin I in beta-adrenergic-inducedacceleration of cardiac relaxation

Activation of cAMP-dependent protein kinase A (PKA) in ventricular myocytes by isoproterenol (Iso) causes phosphorylation of both phospholamban (PLB) and troponin I (TnI) and accelerates relaxation by up to twofold. Because P LB phosphorylation increases sarcoplasmic reticulum (SR) Ca pumping and TnI phosphorylation increases the rate of Ca dissociation from the myofilament s, both factors could contribute to the acceleration of relaxation seen wit h PKA activation. To compare quantitatively the role of TnI versus PLB phos phorylation, we measured relaxation rates before and after maximal Iso trea tment for twitches of matched amplitudes in ventricular myocytes and muscle from wild-type (WT) mice and from mice in which the PLB gene was knocked o ut (PLB-KO). Because Iso increases contractions, even in the PLB-KO mouse, extracellular [Ca] or sarcomere length was adjusted to obtain matching twit ch amplitudes (in the presence and absence of Iso). In PLB-KO myocytes and muscles (which were allowed to shorten), Iso did not alter the time constan t (tau) of relaxation (similar to 29 ms). However, with increasing isometri c force development in the PLB-KO muscles, Iso progressively but modestly a ccelerated relaxation (by 17%). These results contrast with WT myocytes and muscles where Iso greatly reduced tau of cell relaxation and intracellular Ca concentration decline (by 30-50%), independent of mechanical load. The Iso treatment used produced comparable increases in phosphorylation of TnI and PLB in WT. We conclude that the effect of beta-adrenergic activation on relaxation is mediated entirely by PLB phosphorylation in the absence of e xternal load. However, TnI phosphorylation could contribute up to 14-18% of this lusitropic effect in the WT mouse during maximal isometric contractio ns.