OVEREXPRESSION OF THE RAT SARCOPLASMIC-RETICULUM CA2+ ATPASE GENE IN THE HEART OF TRANSGENIC MICE ACCELERATES CALCIUM TRANSIENTS AND CARDIAC RELAXATION

The Ca2+ ATPase of the sarcoplasmic reticulum (SERCA2) plays a dominan t role in lowering cytoplasmic calcium levels during cardiac relaxatio n and reduction of its activity has been linked to delayed diastolic r elaxation in hypothyroid and failing hearts, To determine the contract ile alterations resulting from increased SERCA2 expression, we generat ed transgenic mice overexpressing a rat SERCA2 transgene, Characteriza tion of a heterozygous transgenic mouse line (CJ5) showed that the amo unt of SERCA2 mRNA and protein increased 2.6-fold and 1.2-fold, respec tively, relative to control mice. Determination of the relative synthe sis rate of SERCA2 protein showed an 82% increase, The mRNA levels of some of the other genes involved in calcium handling, such as the ryan odine receptor and calsequestrin, remained unchanged, but the mRNA lev els of phospholamban and Na+/Ca2+ exchanger increased 1.4-fold and 1.8 -fold, respectively, The increase in phospholamban or Na+/Ca2+ exchang er mRNAs did not, however, result in changes in protein levels, Functi onal analysis of calcium handling and contractile parameters in isolat ed 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 als o significantly faster, In isolated papillary muscle from SERCA2 trans genic mice, the time to half maximum postrest potentiation was signifi cantly shorter than in negative littermates. Furthermore, cardiac func tion measured in vivo, demonstrated significantly accelerated contract ion and relaxation in SERCA2 transgenic mice that were further augment ed in both groups with isoproterenol administration. Similar results w ere obtained for the contractile performance of myocytes isolated from a separate line (CJ2) of homozygous SERCA2 transgenic mice, Our findi ngs suggest, for the first time, that increased SERCA2 expression is f easible in vivo and results in enhanced calcium transients, myocardial contractility, and relaxation that may have further therapeutic impli cations.