Endoplasmic reticulum, heart

This gene is broadly distributed in skeletal muscle, heart, uterus, and in a variety of non-muscle cells. The mRNA levels are particularly high in intestine, lung and spleen, whereas they are very low in liver, testes, kidney and pancreas. In the muscle tissue SERCA3 may be confined primarily to non-muscle cells (vascular smooth muscle, endothelial cells, etc.). The C-terminus of SERCA3 is Asp-Gly-Lys Lys-Asp-Leu-Lys (Table I) it may serve as a sorting signal for retention of the enzyme in the endoplasmic reticulum [57]. 

As mentioned in Box 6-D, mitochondria sometimes take up calcium ions. The normal total concentration of Ca2+ is 1 mM and that of free Ca2+ may be only 0.1 pM 22e f However, under some circumstances mitochondria accumulate large amounts of calcium, perhaps acting as a Ca2+ b u ffer.22 2 The so called ryanodine receptors (Fig. 19-21), prominent in the endoplasmic reticulum, have also been found in heart mitochondria, suggesting a function in control of calcium oscillations.221) On the other hand, accumulation of calcium by mitochondria may be pathological and the activation of Ca2+-dependent proteases may be an initial step in apoptosis.2211 221 ... 

The enzymes have molecular weights ranging from about 47 kDa (mouse and rhesus monkey) to 65 kDa (human), and can be found in liver, kidney, small intestine, heart, muscle, lung and other respiratory tissues, adipose tissue, CNS, and blood. Within the blood, CES have been found both in plasma as well as within leukocytes. Species differences in their presence in neuronal tissue and capillary endothelial cells of the CNS have led to some equivocation about their contribution to the blood—brain barrier. That is, the extent to which these enzymes might limit access of drugs into the brain by hydrolyzing them before they can diffuse from the blood into the brain isn t aU that clear. CES are localized both in the cytosol, endoplasmic reticulum (ER), and in lysosomes. 

Ca -ATPases located in the plasma membrane pump Ca - out of the cell. Ca -ATPases in the endoplasmic reticulum, and in the sarcoplasmic reticulum of heart and other muscles, sequester Ca within the membranes, where it is bound by a low-affinity binding protein. Ca is released from the sarcoplasmic reticulum in response to a nerve impulse, which signals contraction, and the increase of Ca stimulates both muscle contraction and the oxidation of fuels. Within the heart, another Ca transporter protein, the NaVCa exchange transporter, coordinates the efflux of Ca in exchange for Na, so that Ca is extraded with each contraction. 

Wang X, Ni L, Yang L, Duan Q, Chen C, Edin ML, Zeldin DC, Wang DW (2014) CYP2J2-derived epoxyeicosatrienoic acids suppress endoplasmic reticulum stress in heart failure. Mol Pharmacol 85 105 115... 

---