Submitted on August 18, 2003
Accepted on April 19, 2004
Co-ordinate down regulation of CaM kinase II and phospholamban accompanies contractile phenotype transition in the hyperthyroid rabbit soleus
Mao Jiang1, Ande Xu1, Douglas L Jones1, and Njanoor Narayanan1*
1 Physiology and Pharmacology, The University of Western Ontario, London, Ontario, Canada
* To whom correspondence should be addressed. E-mail: njanoor.narayanan{at}fmd.uwo.ca.
This study investigated the effects of L-thyroxine-induced hyperthyroidism on Ca2+/calmodulin-dependent protein kinase (CaM kinase II)-mediated sarcoplasmic reticulum (SR) protein phosphorylation, SR Ca2+ pump (Ca2+-ATPase) activity and contraction duration in slow-twitch skeletal (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 muscle of the hyperthyroid, compared with euthyroid rabbit. Western blotting analysis revealed strikingly higher levels of SERCA1 (~ 150%) Ca2+ pump isoform, unaltered levels of SERCA2 Ca2+ pump isoform, and significantly lower levels of PLN (~ 50%) and 
-, 
-, 
-CaM kinase II (40~70%) in soleus muscle of the hyperthyroid rabbit. SR vesicles isolated from hyperthyroid rabbit soleus displayed ~ 2-fold higher ATP-energized Ca2+ uptake and Ca2+-stimulated ATPase activities, compared to that from euthyroid control. The maximal velocity of Ca2+ uptake [Vmax(nmol Ca2+/mg SR protein/min): euthyroid, 818 ± 73; hyperthyroid, 1649 ± 90] but not the apparent affinity of the Ca2+-ATPase for Ca2+[K0.5 for Ca2+ (µM): euthyroid, 0.97 ± 0.02, hyperthyroid, 1.09 ± 0.04] differed significantly between the two groups. Stimulation of the Ca2+ sequestering activity of soleus muscle SR resulting from activation of endogenous CaM kinase II was ~ 60% lower in the hyperthyroid, compared with euthyroid. Isometric twitch force of soleus muscle (normalized per unit muscle mass) measured in situ, was significantly greater (~ 36%), and the time to peak force and relaxation time were significantly lower (~ 30-40%) in the hyperthyroid, compared with euthyroid rabbit. These results demonstrate that thyroid hormone-induced transition in contractile properties of the rabbit soleus muscle is associated with co-ordinate down regulation of the expression and function of phospholamban and CaM kinase II, and selective upregulation of the expression and function of SERCA1 but not SERCA2 isoform of the SR Ca2+ pump. The findings also imply unique, interactive roles for phospholamban and CaM kinase II in the SERCA2 isoform-specific physiological regulation of SR Ca2+ pump function.
