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

Background—Increasing sarcoplasmic/endoplasmic reticulum (SR) Ca -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 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) with adenoviruses encoding SERCA1a (or -galactosidase as control). Myocyte relaxation half-time was decreased by 10% (P 0.052) at SERCA1a MOI 10 and by 28% at MOI 50 (P 0.05). Myocyte fractional shortening was increased by 12% at MOI 10 (P 0.05) but surprisingly decreased at MOI 50 ( 22%, P 0.05) versus control. SR Ca uptake (in permeabilized myocytes) demonstrated a gene-dose–dependent decrease in Km by 29% and 46% and an increase in Vmax by 37% and 72% at MOI 10 and MOI 50, respectively (all P 0.05 versus control). Ca transient amplitude was increased in Ad-SERCA1a–infected myocytes at MOI 10 (by 121%, P 0.05), but at MOI 50, the Ca transient amplitude was not significantly changed. Caffeine-induced Ca transients indicated significantly increased SR Ca content in Ad-SERCA1a–infected cells, by 72% at MOI 10 and by 87% at MOI 50. Mathematical simulations demonstrate that the functional increase in SR Ca -ATPase uptake activity at MOI 50 (and increased cytosolic Ca buffering) is sufficient to curtail the Ca transient amplitude and explain the reduced contraction. Conclusions—Moderate SERCA1a gene transfer and expression improve contractility and Ca cycling. However, higher SERCA1a expression levels can impair myocyte shortening because of higher SERCA activity and Ca buffering. (Circulation. 2004;110:3553-3559.)