Excessive Sarcoplasmic/Endoplasmic Reticulum Ca ²⁺ -ATPase Expression Causes Increased Sarcoplasmic Reticulum Ca ²⁺ Uptake but Decreases Myocyte Shortening

Abstract

Background-Increasing sarcoplasmic/endoplasmic reticulum (SR) Ca 2ϩ -ATPase (SERCA) uptake activity is a promising therapeutic approach for heart failure. We investigated the effects of different levels of SERCA1a expression on contractility and Ca 2ϩ cycling. We tested whether increased SERCA1a expression levels enhance myocyte contractility in a gene-dose-dependent manner. Methods and Results-Rabbit isolated cardiomyocytes were transfected at different multiplicities of infection (MOIs)

Determinants of frequency-dependent contraction and relaxation of mammalian myocardium

“…However, if the SR is already functioning at its maximal store capacity, an increase in frequency is not sufficient to further increase this load/release process. This interpretation is supported by the findings that a large increase in SERCA pump activity, achieved by SERCA overexpression similarly can show a negative or blunted force frequency response [4,21,22]. Thus, such muscles are already contracting at maximal rate and may not be responsive to increased frequency.…”

“…A decrease in SERCA protein and PLB phosphorylation has been correlated with a negative force frequency relationship in human hearts [24]. The role of the SERCA pump has been studied by several laboratories both in genetically manipulated animal models [28][29][30] or gene transfer either in vivo or in cultured myocytes [21,[31][32][33], as well as in the context of human heart failure [34]. In general, a decrease in SERCA pump levels is correlated with a negative FFR.…”

“…As stated above, an increase in SERCA activity well beyond the normal spectrum may also result in negative frequency-dependent behavior, because already at low frequency the baseline SR load is higher, and thus there is less room for further enhancement. In addition, if re-uptake of calcium becomes too fast, the cytoplasmic calcium concentration near the myofilaments will decline very rapidly, preventing appropriate activation of the myofilaments, hindering adequate force development [4,21,22]. Combined, these studies indicate that SERCA activity has an optimal range, rather than a direct linear correlation to contractile performance.…”

Recirculating cardiac delivery of AAV2/1SERCA2a improves myocardial function in an experimental model of heart failure in large animals

“…Myocardial SERCA expression has been increased using gene therapy-based approaches in a number of small animal models, with varying success. 4,8,11 In conjunction with the improvement in contractile function, some deleterious aspects of increased calcium cycling have been observed, 12 such as impaired myocyte shortening at high SERCA1a activity, indicating the potential importance of dose optimization. Our study supports the notion of a beneficial effect of augmented SERCA expression in the failing heart.…”

Targeting Altered Calcium Physiology in the Heart: Translational Approaches to Excitation, Contraction, and Transcription

“…Thus manipulations of SERCA activity seem to require exact titration and probably timing, complicating their potential use as targets in gene therapeutic trials. This has also been described in an in vitro study, where exaggerated levels of SERCA overexpression led to enhanced cycling of Ca yet reduced systolic activation of myofilaments, possibly due to overly fast reuptake of Ca into the SR (47). Thus future gene therapy approaches in humans will need to take into account promoter control strategies, in addition to the demand of directed insertion and a high level of tissue specificity.…”