Overexpression of the rat sarcoplasmic reticulum Ca2+ ATPase gene in the heart of transgenic mice accelerates calcium transients and cardiac relaxation.

Abstract

The Ca 2 ϩ ATPase of the sarcoplasmic reticulum (SERCA2) plays a dominant role in lowering cytoplasmic calcium levels during cardiac relaxation and reduction of its activity has been linked to delayed diastolic relaxation in hypothyroid and failing hearts. To determine the contractile alterations resulting from increased SERCA2 expression, we generated transgenic mice overexpressing a rat SERCA2 transgene. Characterization of a heterozygous transgenic mouse line (CJ5) showed that the amount of SERCA2 mRNA and protein increased 2.6-fold and 1.2-fold, respectively, relative to control mice. Determination of the relative synthesis rate of SERCA2 protein showed an 82% increase. The mRNA levels of some of the other genes involved in calcium handling, such as the ryanodine receptor and calsequestrin, remained unchanged, but the mRNA levels of phospholamban and Na ϩ /Ca 2 ϩ exchanger increased 1.4-fold and 1.8-fold, respectively. The increase in phospholamban or Na ϩ /Ca 2 ϩ exchanger mRNAs did not, however, result in changes in protein levels. Functional analysis of calcium handling and contractile parameters in isolated cardiac myocytes indicated that the intracellular calcium decline ( t 1/2 ) and myocyte relengthening ( t 1/2 ) were accelerated by 23 and 22%, respectively. In addition, the rate of myocyte shortening was also significantly faster. In isolated papillary muscle from SERCA2 transgenic mice, the time to half maximum postrest potentiation was significantly shorter than in negative littermates. Furthermore, cardiac function measured in vivo, demonstrated significantly accelerated contraction and relaxation in SERCA2 transgenic mice that were further augmented in both groups with isoproterenol administration. Similar results were obtained for the contractile performance of myocytes isolated from a separate line (CJ2) of homozygous SERCA2 transgenic mice.

Constitutive Cardiac Overexpression of Sarcoplasmic/Endoplasmic Reticulum Ca 2+ -ATPase Delays Myocardial Failure After Myocardial Infarction in Rats at a Cost of Increased Acute Arrhythmias

“…LV papillary muscles showed faster contraction and relaxation rates and increased R 1 , indicative of an increased SR Ca 2ϩ -uptake function, in good agreement with the 37% increase in SERCA2a protein expression. These data are in keeping with previous findings in SERCA2a TG mice 18 and in the same rat line. 7 In our study, LV global systolic and diastolic function, as assessed by echocardiography, were not different between TG and WT rats, probably because these Abbreviations as in Table 1.…”

Adenoviral gene transfer of Akt enhances myocardial contractility and intracellular calcium handling

“…In fact, SERCA-2 overexpression is known to increase Ca 2+ transport and cardiac function, and is mainly characterized by increased systolic Ca 2+ levels, shorter transients, enhanced contractility with accelerated rates of contraction and relaxation, and no evidence of diastolic Ca 2+ overload consistent with the enhanced capacity of the SR to sequester calcium. 25,26 The mechanisms of SERCA-2 are likely complex, but do not involve significant changes of phospholamban, calsequestrin, and ryanodine receptor protein and transcripts levels, confirming previous data obtained in Akt-overexpressing mice. 11 However, the possibility of a change in the extent of phospholamban phosphorylation and its effect on SERCA-2 cannot be excluded.…”

Altered dose response to β-agonists in SERCA1a-expressing hearts ex vivo and in vivo

“…In mice with moderately increased levels of the endogenous cardiac isoform SERCA2a (ϩ20% in SERCA protein level), in vivo hemodynamic measurements revealed significantly increased maximal ϩdP/dt and ϪdP/dt, similar to what was observed in isolated myocytes and isolated papillary muscles (13). Similarly, PLB-null hearts were hypercontractile and had a severely blunted response to stimulation with ␤-agonists ex vivo as well as in vivo (21,25).…”