Abstract
This chapter discusses the role of Ca 2+ -ATPases in maintaining Ca 2+ homeostasis in the cell. The chapter focuses on sarcoplasmic reticular (SR) Ca 2+ -ATPase(s), which is the primary regulator of the Ca 2+ levels and thus, contractility in muscle. Ca 2+ is a physiological regulator for the contractile proteins and several other enzymes and processes in muscle. The chapter discusses sarcoplasmic reticular (SR) Ca 2+ -ATPase, and several other ATPases. Sarcoplasmic reticular (SR) Ca 2+ includes properties of SR Ca 2+ -ATPase, regulation of SR Ca 2+ -ATPase by phospholamban, and SR Ca 2+ -ATPase in cardiac diseases. Regulation of SR Ca 2+ -ATPase by phospholamban further includes, structure of phospholamban, in vitro studies on regulation of SR Ca 2+ -ATPase, in vivo studies on regulation of SR Ca 2+ -ATPase. SR Ca 2+ -ATPase in cardiac diseases includes SR Ca 2+ -ATPase in hyperthyroidism and hypothyroidism, SR Ca 2+ -ATPase in cardiomyopathies, and SR Ca 2+ -ATPase in ischemia. The SR in both skeletal and cardiac muscles contains an acidic protein, calsequestrin, which binds 40–50 mol of Ca 2+ per mol of protein. In cardiac muscle, slow-twitch skeletal muscle, and smooth muscle, the SR contains the low-molecular-weight protein phospholamban, which can be phosphorylated by various protein kinases. Phosphorylation of phospholamban occurs by cAMP dependent, cGMP-dependent, Ca 2+ -calmodulin-dependent, and Ca 2+ -phospholipid-dependent protein kinases in vitro. The complex regulation of the SR function clearly indicates that even small disturbances in SR Ca 2+ handling may result in profound changes and deterioration of normal myocardial function. Thyroid hormones are important regulators of myocardial contractility and relaxation.. The general properties and structure of plasma membrane Ca 2+ -ATPase(s) are portrayed in detail. It is mentioned that there is a growing interest in further use of molecular biological approaches and specifically site directed mutagenesis for these enzymes to obtain more information about their structural-functional relationships.
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