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

Abstract

. Altered dose response to ␤-agonists in SERCA1a-expressing hearts ex vivo and in vivo. Am J Physiol Heart Circ Physiol 283: H958-H965, 2002; 10.1152/ ajpheart.00078.2002. -In this study we evaluated the contractile characteristics of sarco(endo)plasmic reticulum Ca 2ϩ -ATPase (SERCA)1a-expressing hearts ex vivo and in vivo and in particular their response to ␤-adrenergic stimulation. Analysis of isolated, work-performing hearts revealed that transgenic (TG) hearts develop much higher maximal rates of contraction and relaxation than wild-type (WT) hearts. Addition of isoproterenol only moderately increased the maximal rate of relaxation (ϩ20%) but did not increase contractility or decrease relaxation time in TG hearts. Perfusion with varied buffer Ca 2ϩ concentrations indicated an altered dose response to Ca 2ϩ . In vivo TG hearts displayed fairly higher maximal rates of contraction (ϩ 25%) but unchanged relaxation parameters and a blunted but significant response to dobutamine. Our study also shows that the phospholamban (PLB) level was decreased (Ϫ40%) and its phosphorylation status modified in TG hearts. This study clearly demonstrates that increases in SERCA protein level alter the ␤-adrenergic response and affect the phosphorylation of PLB. Interestingly, the overall cardiac function in the live animal is only slightly enhanced, suggesting that (neuro)hormonal regulations may play an important role in controlling in vivo heart function. transgenic mice; contractility; sarco(endo)plasmic reticulum calcium adenosinetriphosphatase; phospholamban SEVERAL STUDIES HAVE SHOWN that sarco(endo)plasmic reticulum Ca 2ϩ -ATPase (SERCA) activity is decreased in animal models of cardiac hypertrophy and failure as well as in human heart failure (11). This decrease in sarcoplasmic reticulum (SR) Ca 2ϩ transport function was thought to contribute to abnormalities in Ca 2ϩ handling and contractile dysfunction in failing hearts (14). Adenoviral gene transfer of SERCA is currently being explored as a new therapeutic approach for the treatment of heart failure (7,(26)(27)(28)(29).Recent studies have shown that the fast-twitch skeletal muscle isoform SERCA1 has a higher Ca 2ϩ uptake activity than the cardiac isoform SERCA2a because of a higher Ca 2ϩ turnover rate when expressed in cardiac myocytes (34). To test whether SERCA1a can functionally substitute for SERCA2a and increase cardiac contractility, we have developed a transgenic (TG) mouse model expressing SERCA1a in the heart (16,19,23). In this model, SERCA1a expression resulted in a 2.5-fold increase in total SERCA pump protein level, but the expression of the endogenous SERCA2a isoform was reduced to ϳ50% in TG hearts. As a consequence 20% of the total number of SERCA pumps in TG hearts were type 2a whereas the majority of 80% were type 1a pumps. The contractility of isolated TG hearts was clearly increased despite the reduced expression of SERCA2a (23). SERCA1a pump expression was well tolerated without any signs of hypertrophy or other pathophysiological changes in the...

Limited functional and metabolic improvements in hypertrophic and healthy rat heart overexpressing the skeletal muscle isoform of SERCA1 by adenoviral gene transfer in vivo

“…The accelerated relaxation (ϪdP/dt) is consistent with a faster removal of calcium from the cytosol as the number of SERCA pumps are overexpressed. These results compared favorably with functional data reported for isolated, work-performing hearts from SERCA1 transgenic mice (17). Huke et al (17) also reported a 20% increase in LVDP, whereas ϩdP/dt and ϪdP/dt nearly doubled, and HR was unchanged.…”

“…This contrasts with the earlier transgenic mouse model and our own cardiomyocyte data (5,30). In transgenic mice expressing SERCA1, endogenous SERCA2a expression was reduced by 50% and total SERCA levels increased by 2.5-fold (17,25,30). In cardiomyocytes, total SERCA levels increased by three-to fourfold, whereas endogenous SERCA2a was reduced by 30 -60% (5, 18).…”

Advantages of Catheter-Based Adenoviral Delivery of Genes to the Heart for Studies of Cardiac Disease

“…The accelerated relaxation (-dP/dt) is consistent with a faster removal of calcium from the cytosol as the number of SERCA pumps are overexpressed. These results compared favorably with functional data reported for isolated, work-performing hearts from SERCA1 transgenic mice (Huke et al, 2002). Huke and colleagues also reported a 20% increase in LVDP, whereas ±dP/dt nearly doubled, and HR was unchanged.…”

“…Huke and colleagues also reported a 20% increase in LVDP, whereas ±dP/dt nearly doubled, and HR was unchanged. However, much like our own in vivo data, they did not find dramatic changes in vivo (Huke et al, 2002). They suggested that the differences between ex vivo and in vivo data reflect the presence of in vivo compensatory mechanisms that "normalize" cardiac function in transgenic mice.…”

“…Indeed, this level of expression is lower than the level reported for the SERCA1 transgenic mouse model and our own earlier cardiomyocyte data (Cavagna et al, 2000;Loukianov et al, 1998). In transgenic mice expressing SERCA1, total SERCA levels increased by 2.5-fold and endogenous SERCA2a expression were reduced by 50% (Huke et al, 2002;Lalli et al, 2001;Loukianov et al, 1998). In cardiomyocytes, total SERCA levels increased by 3 fold and SERCA2a dropped by 50%.…”