Sarcoplasmic reticulum calcium uptake

Mercuric chloride is thought to gain access to the intracellular compartment through Na + and Ca2 + channels in the membrane [ 100]. Sulphydryl reagents, including Hg2 +, could inhibit K +-stimulated uptake of Ca2+ into rat brain synaptosomes in vitro [101]. In muscle sarcoplasmic reticulum, Hg2+ causes inhibition of ATP-dependent Ca2 + uptake and loss of accumulated calcium [ 102,103]. However, HgCl2 has been found to inhibit ATP-dependent calcium uptake more strongly than it inhibits potassium-stimulated uptake [ 104],... 

The sarcoplasmic reticulum (SR) functions to uptake calcium from the sarcoplasm (the cytoplasm of muscle fiber). The sarcoplasmic reticulum... 

The sER also functions as an intracellular calcium store, which normally keeps the Ca level in the cytoplasm low. This function is particularly marked in the sarcoplasmic reticulum, a specialized form of the sER in muscle cells (see p. 334). For release and uptake of Ca " ", the membranes of the sER contain signal-controlled Ca channels and energy-dependent Ca ATPases (see p. 220). In the lumen of the sER, the high Ca " " concentration is buffered by Ca -binding proteins. 

It has been postulated that 2-PAM exerts its cardiac action in rabbit atria through its alteration of calcium metabolism. The relaxation phase of skeletal muscle contraction seems to be directly affected by the sarcoplasmic reticulum because of its ability to sequester calcium actively.29,46 a similar role has been suggested for the sarcoplasmic reticulum in cardiac muscle. 6,83 The onset of muscle contraction takes place when calcium reaches a crit-cal concentration. This contraction is later reduced by the increased calcium-sequestering activity of the sarcoplasmic reticulum. Thus, 2-PAM can affect this process by decreasing the rate of calcium uptake by the sarcoplasmic reticulum, which results in increasing the time required to reduce the calcium concentration enough to allow relaxation to take place. This was demonstrated by the Increase in the relaxation phase. It was suggested that this... 

Fig. 5. Calcium uptake of sarcoplasmic reticulum vesicles supported by different substrates followed by calcium release after substrate depletion113. The time courses of calcium uptake and calcium release were measured by flow dialysis. The reaction chamber contained 0.4 mg protcin/ml, 5 mM MgCl2,...

The uptake of calcium is usually initiated by the addition of calcium ions to the otherwise complete assay medium. Before calcium is added, ATP is split with a low rate by most sarcoplasmic reticulum preparations. This ATPase which is active in the absence of calcium ions has been named basic ATPase . 

Fig. 10. The effect of different concentrations of the ionophore X 537 A on calcium release, by sarcoplasmic reticulum vesicles11S. The reaction mixture contained 20 mM Tris-maleate pH 6-8, 50 mM KC1, 10 mM MgClj, 0.1 mM CaCl2, 0.1 mM murexide and 0.27 mg protein/ml. Calcium uptake and release were followed by monitoring the changes in the absorbance undergone by murexide. The measurements were performed with a filter dual wave length (540—507 nm) double beam spectrophotometer...

The best studied example of a Group IIA cation transport system is the calcium pump of the sarcoplasmic reticulum of skeletal muscle. Indeed, the calcium pump and the sodium pump represent the most studied of all transport processes. The calcium pump involves a membrane-bound (Ca2+, Mg2+)-ATPase and uptake of Ca2+ is associated with hydrolysis of ATP. While the... 

In muscle cells, the contraction is induced by Ca2+ release from the sarcoplasmic reticulum, as a result of membrane depolarization and activation of RyRl receptors located at the surface of the SR. The subsequent transport of cytoplasmic Ca2+ back into the lumen of the sarcoplasmic reticulum restores low resting calcium levels and allows muscle relaxation. In fast-twitch skeletal muscle fibers, Ca2+ uptake is mediated by the sarco(endo)plasmic reticulum Ca2+ ATPase SERCA1 which represents more than 99% of SERCA isoforms in these muscle fibers. 

It has become increasingly clear that there is a non-mitochondrial, intracellular calcium pool which plays an important role in cell activation in a large number of nonmuscle cells as well as in smooth and skeletal muscle. This pool is relatively enormous in skeletal muscle, provides the bulk of the Ca2+ needed to regulate skeletal muscle contraction, and is located in a distinct organelle, the sarcoplasmic reticulum. The pool is smaller in non-muscle and in smooth muscle cells, and its location less obvious [10,11]. To fill the pool requires ATP, i.e., uptake of Ca2+ into the pool is driven by a distinct Ca2+-ATPase, an enzyme which purifies with the mi-... 

The sarcoplasmic reticulum is an organelle of muscle cells that resembles the ER in structure. It is used for rapid release and uptake of calcium ions. The resultant changes in cytosolic Ca " concentrations control muscle contraction,... 

An early report suggested that suramin can block the nucleotide-dependent calcium pump of rabbit skeletal sarcoplasmic reticulum by inhibition of the calcium uptake and the ATTase activity [51]. These results have been confirmed by Emmik et al. [52]. Baumert and Heider [53] found that pyridoxal-5-phosphate and a series of its analogues (for chemical structure. 

Protein kinase C activation has been implicated in augmenting the transmembrane flux of calcium ions, the activities of the Na K -ATPase, as well as the activity of the Na /H -antiporter. Substantial evidence suggests that protein kinase C augments the uptake of calcium released during cellular stimulation, thereby limiting the temporal duration of its activation (e.g., phorbol esters activate the sarcoplasmic reticulum Ca -ATPase). 

Mermier P, Hasselbach W. 1976. Comparison between strontium and calcium uptake by the fragmented sarcoplasmic reticulum. Eur J Biochem 69 79-86. 

Figure 7.2. Some intracellular processes that may be affected by calcium channel blocking drugs. Calcium channel blocking drugs inhibit calmodulin-dependent sarcolemmal Ca2 -A TPase (7), myosin light-chain kinase (MLCK) (2) and phosphodiesterase (PDE) (7). Passive Na -Ca2 exchange (4) may also be inhibited, whilst (Na +K )-ATPase (J) is stimulated. Ca2 release from mitochondria (MIT) in exchange for Na ( ) may be inhibited, but the effect of calcium channel blocking drugs on Ca2 uptake into sarcoplasmic reticulum (SR) via Ca2 -ATPase (7) is variable.

Anesthetic Actions on Calcium Uptake and Calcium-Dependent Adenosine Triphophatase Activity of Cardiac Sarcoplasmic Reticulum Ning Miao, Martha J. Frazer, and Carl Lynch III... 

The biochemical mechanism of action of digitalis is associated with (A) A decrease in calcium uptake by the sarcoplasmic reticulum... 

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