Isoproterenol-induced acceleration of cardiac relaxation is mediated entirely by phospholamban phosphorylation without external load, with troponin I contributing 14-18% during isometric contractions.
Beta-adrenergic acceleration of cardiac relaxation is mediated entirely by phospholamban phosphorylation in the absence of external load, while troponin I phosphorylation contributes up to 14-18% during maximal isometric contractions.
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 PLB phosphorylation increases sarcoplasmic reticulum (SR) Ca pumping and TnI phosphorylation increases the rate of Ca dissociation from the myofilaments, both factors could contribute to the acceleration of relaxation seen with PKA activation. To compare quantitatively the role of TnI versus PLB phosphorylation, we measured relaxation rates before and after maximal Iso treatment 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 out (PLB-KO). Because Iso increases contractions, even in the PLB-KO mouse, extracellular Ca or sarcomere length was adjusted to obtain matching twitch 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 constant (tau) of relaxation ( approximately 29 ms). However, with increasing isometric force development in the PLB-KO muscles, Iso progressively but modestly accelerated 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 external load. However, TnI phosphorylation could contribute up to 14-18% of this lusitropic effect in the WT mouse during maximal isometric contractions.
Li et al. (2000) studied this question. Isoproterenol vs. Baseline (absence of isoproterenol) was evaluated on Time constant (tau) of relaxation. Isoproterenol-induced acceleration of cardiac relaxation is mediated entirely by phospholamban phosphorylation without external load, with troponin I contributing 14-18% during isometric contractions.