Regulation of myocardial Ca2+-ATPase and phospholamban mRNA expression in response to pressure overload and thyroid hormone.

Abstract

The sarcoplasmic reticulum (SR) and the contractile protein myosin play an important role in myocardial performance. Both of these systems exhibit plasticity-i.e., quantitative and/or qualitative reorganization during development and in response to stress. Recent studies indicate that SR Ca2l uptake function is altered in adaptive cardiac hypertrophy and failure. The molecular basis (genetic and phenotypic) for these changes is not understood. In an effort to determine the underlying causes of these changes, we characterized the rabbit cardiac Ca2+-ATPase phenotype by molecular cloning and ribonuclease A mapping analysis. Our results show that the heart muscle expresses only the slow-twitch SR Ca2+-ATPase isoform. Second, we quantitated the steady-state mRNA levels of two major SR Ca2+ regulatory proteins, the Ca2+-ATPase and phospholamban, to see whether changes in mRNA content might provide insight into the basis for functional modification in the SR of hypertrophied hearts. In response to pressure overload hypertrophy, the relative level of the slow-twitch/cardiac SR Ca2+-ATPase mRNA was decreased to 34% of control at 1 week. The relative Ca2+-ATPase mRNA level increased to 167% of control after 3 days of treatment with thyroid hormone. In contrast, in hypothyroid animals, the relative Ca2+-ATPase mRNA level decreased to 51% of control at 2 weeks. The relative level of phospholamban mRNA was decreased to 36% in 1-week pressure overload. Hyperthyroidism induced a decrease to 61% in the phospholamban mRNA level after 3 days of treatment, while hypothyroidism had virtually no effect on phospholamban mRNA levels. These data indicate that the expression of SR Ca2+-ATPase and phospholamban mRNA may not be coordinately regulated during myocardial adaptation to different physiological conditions.Myocardial hypertrophy is an adaptive response to sustained stress where survival depends on the increase in muscle mass. In addition to an increase in cell size there is significant reorganization of the myocardial cell that results in alteration in the function of the contractile proteins and the sarcoplasmic reticulum (SR). The major changes in the myocardial contractile proteins are attributable to altered phenotypic expression (1). The objective of the present study is to discover the basis for the change in myocardial SR function in hypertrophied hearts.The SR of striated muscle is a highly organized intracellular membrane system that plays a critical role in the contractionrelaxation cycle of the myocardium by regulating the cytosolic Ca2+ level (2). Ca2+ pumping by the SR membranes is mediated by a Ca2+-dependent ATPase. The Ca2+-ATPase of skeletal muscle SR has been studied extensively with respect to structure and mechanism ofaction (3, 4). The structure and function of the Ca2+-ATPase phenotype(s) expressed in the heart muscle, however, have been less well studied. Recently, MacLennan and co-workers have described the existence of four distinct Ca2+-ATPase isoforms (5-8, 40). Two of these are alternatively ...

Membrane proteins of the myocytes in cardiac overload.

“…Data from both our laboratory (Lompre et al, 1988) and from another (Nogai et al, 1989) suggest that, both by immunoquantitation and by using a specific cDNA probe, the density of the (Ca +)-ATPase of the sarcoplasmic reticulum is reduced, i.e. that the number of molecules of the enzyme per gram of tissue or per myocyte is diminished.…”

Cardiac Transcriptome Analysis in Obesity-Related Hypertension

“…11 Thyroid hormones, which are also increased during caloric overfeeding, 39 have also been shown to increase PLB mRNA levels. 40 However, the relevance of these changes to explain changes in atrial gene expression in obesity-related hypertension remains to be elucidated. In obese dogs, we also observed a specific 9-fold reduction of N-acetyl galactosaminyl transferase mRNA levels in atrium by Northern blot analysis, whereas its expression was not significantly modified in left ventricle.…”

Sarcolipin and phospholamban as regulators of cardiac sarcoplasmic reticulum Ca2+ ATPase

“…In hyperthyroidism, a decrease in both inhibitors PLB [6,[32][33][34] and SLN [36] relative to SERCA2a is seen. In hypothyroidic hearts, SERCA levels are decreased, whereas PLB and SLN levels are unchanged [6,32,33,36]. These studies suggest that regulation of SERCA2a function can contribute to the altered contractile function in hypothyroid and hyperthyroid hearts.…”

“…SERCA2a promotes muscle relaxation by lowering the cytosolic Ca 2+ concentration through active Ca 2+ transport into the SR and, thereby, restores the intracellular Ca 2+ needed for the next contraction cycle [1]. The expression and regulation of SERCA2a activity have been widely investigated, emphasizing its central role in the regulation of Ca 2+ homeostasis during development and under a variety of patho-physiological conditions [3][4][5][6]. Studies from a variety of animal models of heart disease [7][8][9] and end stage human heart failure [10,11] suggest that defects in SR Ca 2+ uptake function is one of the major contributing factors for the progression of heart failure.…”