Coordinate downregulation of CaM kinase II and phospholamban accompanies contractile phenotype transition in the hyperthyroid rabbit soleus

Abstract

This study investigated the effects of l-thyroxine-induced hyperthyroidism on Ca2+/calmodulin (CaM)-dependent protein kinase (CaM kinase II)-mediated sarcoplasmic reticulum (SR) protein phosphorylation, SR Ca2+pump (Ca2+-ATPase) activity, and contraction duration in slow-twitch soleus muscle of the rabbit. Phosphorylation of Ca2+-ATPase and phospholamban (PLN) by endogenous CaM kinase II was found to be significantly lower (30–50%) in soleus of the hyperthyroid compared with euthyroid rabbit. Western blotting analysis revealed higher levels of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) 1 (∼150%) Ca2+pump isoform, unaltered levels of SERCA2 Ca2+pump isoform, and lower levels of PLN (∼50%) and δ-, β-, and γ-CaM kinase II (40 ∼ 70%) in soleus of the hyperthyroid rabbit. SR vesicles from hyperthyroid rabbit soleus displayed approximately twofold higher ATP-energized Ca2+uptake and Ca2+-stimulated ATPase activities compared with that from euthyroid control. The Vmaxof Ca2+uptake (in nmol Ca2+·mg SR protein−1·min−1: euthyroid, 818 ± 73; hyperthyroid, 1,649 ± 90) but not the apparent affinity of the Ca2+-ATPase for Ca2+(euthyroid, 0.97 ± 0.02 μM, hyperthyroid, 1.09 ± 0.04 μM) differed significantly between the two groups. CaM kinase II-mediated stimulation of Ca2+uptake by soleus muscle SR was ∼60% lower in the hyperthyroid compared with euthyroid. Isometric twitch force of soleus measured in situ was significantly greater (∼36%), and the time to peak force and relaxation time were significantly lower (∼30–40%), in the hyperthyroid. These results demonstrate that thyroid hormone-induced transition in contractile properties of the rabbit soleus is associated with coordinate downregulation of the expression and function of PLN and CaM kinase II and selective upregulation of the expression and function of SERCA1, but not SERCA2, isoform of the SR Ca2+pump.