AJP - Heart Calcium Transients and Cell-Sarcomere
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Am J Physiol Heart Circ Physiol 291: H1384-H1394, 2006. First published April 14, 2006; doi:10.1152/ajpheart.00875.2005
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Thyroid hormone downregulates the expression and function of sarcoplasmic reticulum-associated CaM kinase II in the rabbit heart

Mao Jiang, Ande Xu, and Njanoor Narayanan

Department of Physiology and Pharmacology, The University of Western Ontario, London, Ontario, Canada

Submitted 16 August 2005 ; accepted in final form 9 April 2006

Phosphorylation of sarcoplasmic reticulum (SR) Ca2+-cycling proteins by a membrane-associated Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) is a well-documented physiological mechanism for regulation of transmembrane Ca2+ fluxes and the cardiomyocyte contraction-relaxation cycle. The present study investigated the effects of L-thyroxine-induced hyperthyroidism on protein expression of SR CaM kinase II and its substrates, endogenous CaM kinase II-mediated SR protein phosphorylation, and SR Ca2+ pump function in the rabbit heart. Membrane vesicles enriched in junctional SR (JSR) or longitudinal SR (LSR) isolated from euthyroid and hyperthyroid rabbit hearts were utilized. Endogenous CaM kinase II-mediated phosphorylation of ryanodine receptor-Ca2+ release channel (RyR-CRC), Ca2+-ATPase, and phospholamban (PLN) was significantly lower (30–70%) in JSR and LSR vesicles from hyperthyroid than from euthyroid rabbit heart. Western immunoblotting analysis revealed significantly higher (~40%) levels of sarco(endo)plasmic reticulum Ca2+-ATPase isoform 2 (SERCA2) in JSR, but not in LSR, from hyperthyroid than from euthyroid rabbit heart. Maximal velocity of Ca2+ uptake was significantly increased in JSR (130%) and LSR (50%) from hyperthyroid compared with euthyroid rabbit hearts. Apparent affinity of the Ca2+-ATPase for Ca2+ did not differ between the two groups. Protein levels of PLN and CaM kinase II were significantly lower (30–40%) in JSR, LSR, and ventricular tissue homogenates from hyperthyroid rabbit heart. These findings demonstrate selective downregulation of expression and function of CaM kinase II in hyperthyroid rabbit heart in the face of upregulated expression and function of SERCA2 predominantly in the JSR compartment.

ryanodine receptor-calcium release channel; sarcoplasmic reticulum calcium-adenosine triphosphatase; phospholamban


Address for reprint requests and other correspondence: N. Narayanan, Dept. of Physiology and Pharmacology, Health Science Center, The Univ. of Western Ontario, London, ON, Canada N6A 5C1 (e-mail: njanoor.narayanan{at}schulich.uwo.ca)






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