Induced overexpression of Na+/Ca2+ exchanger transgene: altered myocyte contractility, [Ca2+]i transients, SR Ca2+ contents, and action potential duration

doi:10.1152/ajpheart.00190.2009

Induced overexpression of Na⁺/Ca²⁺ exchanger transgene: altered myocyte contractility, [Ca²⁺]_i transients, SR Ca²⁺ contents, and action potential duration

JuFang Wang,¹^,2^,* Tung O. Chan,²^,* Xue-Qian Zhang,¹^,2 Erhe Gao,² Jianliang Song,¹^,2 Walter J. Koch,² Arthur M. Feldman,² and Joseph Y. Cheung¹^,2

¹Division of Nephrology and ²Center of Translational Medicine, Department of Medicine, Jefferson Medical College, Thomas Jefferson University, Philadelphia, Pennsylvania

Submitted 26 February 2009 ; accepted in final form 8 June 2009

We have produced mice in which expression of the rat cardiacNa⁺/Ca²⁺ exchanger (NCX1) transgene was switched on when doxycyclinewas removed from the feed at 5 wk. At 8 to 10 wk, NCX1 expressionin induced (Ind) mouse hearts was 2.5-fold higher but proteinlevels of sarco(endo)plasmic reticulum Ca²⁺-ATPase, ${alpha}$ ₁- and ${alpha}$ ₂-subunitsof Na⁺-K⁺-ATPase, phospholamban, ryanodine receptor, calsequestrin,and unphosphorylated and phosphorylated phospholemman were unchangedcompared with wild-type (WT) or noninduced (non-Ind) hearts.There was no cellular hypertrophy since WT, non-Ind, and Indmyocytes had similar whole cell membrane capacitance. In Indmyocytes, NCX1 current amplitude was $~$ 42% higher, L-type Ca²⁺current amplitude was unchanged, and action potential durationwas prolonged compared with WT or non-Ind myocytes. Contractionand intracellular Ca²⁺ concentration ([Ca²⁺]_i) transient amplitudesin Ind myocytes were lower at 0.6, not different at 1.8, andhigher at 5.0 mM extracellular Ca²⁺ concentration ([Ca²⁺]_o)compared with WT or non-Ind myocytes. Despite similar Ca²⁺ currentamplitude and sarcoplasmic reticulum (SR) Ca²⁺ uptake, SR Ca²⁺content at 5.0 mM [Ca²⁺]_o was significantly higher in Ind comparedwith non-Ind myocytes, indicating that NCX1 directly contributedto SR Ca²⁺ loading. Echocardiography demonstrated that heartrate, left ventricular mass, ejection fraction, stroke volume,and cardiac output were similar among the three groups of animals.In vivo close-chest catheterization demonstrated similar contractilityand relaxation among the three groups of mice, both at baselineand after stimulation with isoproterenol. We conclude that inducedexpression of NCX1 transgene resulted in altered [Ca²⁺]_i homeostasis,myocyte contractility, and action potential morphology. In addition,heart failure did not occur 3 to 5 wk after NCX1 transgene wasinduced to be expressed at levels found in diseased hearts.

tet-off; excitation-contraction; fura-2; isolated mouse myocytes; electrophysiology; sarcoplasmic reticulum; intracellular Ca²⁺ concentration

Address for reprint requests and other correspondence: J. Y. Cheung, Div. of Nephrology, Jefferson Medical College, 833 Chestnut St., Ste. 700, Philadelphia, PA 19107 (e-mail: joseph.cheung{at}jefferson.edu)