Sarcoplasmic membrane calcium binding

ATP is used not only to power muscle contraction, but also to re-establish the resting state of the cell. At the end of the contraction cycle, calcium must be transported back into the sarcoplasmic reticulum, a process which is ATP driven by an active pump mechanism. Additionally, an active sodium-potassium ATPase pump is required to reset the membrane potential by extruding sodium from the sarcoplasm after each wave of depolarization. When cytoplasmic Ca2- falls, tropomyosin takes up its original position on the actin and prevents myosin binding and the muscle relaxes. Once back in the sarcoplasmic reticulum, calcium binds with a protein called calsequestrin, where it remains until the muscle is again stimulated by a neural impulse leading to calcium release into the cytosol and the cycle repeats. 

Uterine relaxation is mediated in part through inhibition of MLCK. This inhibition results from the phosphorylation of MLCK that follows the stimulation of myometrial (3-adrenoceptors relaxation involves the activity of a cyclic adenosine monophosphate (cAMP) mediated protein kinase, accumulation of Ca++ in the sarcoplasmic reticulum, and a decrease in cytoplasmic Ca. Other circulating substances that favor quiescence of uterine smooth muscle include progesterone, which increases throughout pregnancy, and possibly prostacyclin. Progesterone s action probably involves hyperpolarization of the muscle cell membrane, reduction of impulse conduction in muscle cells, and increased calcium binding to the sarcoplasmic reticulum. 

Studies of the efflux of Ca by stimulated rabbit atria have characterized three calcium pools. Phase I may represent extracellular washout of the Ca that binds to the surface of muscle membrane and is characterized by a high rate constant. Phase II may represent loosely bound calcium present in cell membrane and calcium released at the sarcoplasmic reticulum. Calcium in this pool is directly related to contractility.65,84,93 phase III may represent the tightly bound calcium that exchanges very slotrly and does not play a role in maintaining calcium concentrations. Recent study has shown that the storage or release of calcium at the sarcoplasmic reticulum and other loosely bound calcium sites (cell membrane) that are involved in muscle contractility can be directly affected by 2-PAM.21 These results Indicate that 2-PAM increases the rate of release of Phase II calcium. 

Sarzala, M. G., Zubrzycka, E., and Drabikowski, W. Characterization of the constituents of sarcoplasmic reticulum membrane. In Calcium binding proteins (eds. W. Drabikowski,... 

Relaxation of the muscle is brought about by removal of the ionic calcium from the sarcoplasm. This calcium is transported across the membrane of the sarcoplasmic reticulum, in an energy requiring process. In addition to the calcium pumping ATPase, the sarcoplasmic reticulum also contains a calcium binding protein called calsequestrin (Section 4.3.3). Some of the calcium segregated by the sarcoplasmic reticulum is apparently bound to this protein within the lumen of the sarcoplasmic reticulum. As sequestration of calcium ions into sarcoplasmic reticulum proceeds, more calcium ions dissociate from their binding sites on troponin C, re-... 

Fig. 47.3. Events leading to sarcoplasmic reticulum calcium release in skeletal muscle. 1. Acetylcholine, released at the synaptic cleft, binds to acetylcholine receptors on the sar-colemma, leading to a change of conformation of the receptors such that they now act as an ion pore. This allows sodium to enter the cell and potassium to leave. 2. The membrane polarization that results from these ion movements is transmitted throughout the muscle fiber by the T-tubule system. 3. A receptor in the T-tubules (the dihydropyridine receptor, DHPR) is activated by membrane polarization (a voltage-gated activation) such that activated DHPR physically binds to and activates the ryanodine receptor in the sarcoplasmic reticulum (depolarization-induced calcium release). 4. The activation of the ryanodine receptor, which is a calcium channel, leads to calcium release from the SR into the sarcoplasm. In cardiac muscle, activation of DHPR leads to calcium release from the T-tubules, and this small calcium release is responsible for the activation of the cardiac ryanodine receptor (calcium-induced calcium release) to release large amounts of calcium into the sarcoplasm.

Another mechanism in initiating the contraction is agonist-induced contraction. It results from the hydrolysis of membrane phosphatidylinositol and the formation of inositol triphosphate (IP3)- IP3 in turn triggers the release of intracellular calcium from the sarcoplasmic reticulum and the influx of more extracellular calcium. The third mechanism in triggering the smooth muscle contraction is the increase of calcium influx through the receptor-operated channels. The increased cytosolic calcium enhances the binding to the protein, calmodulin [73298-54-1]. 

Inositol triphosphate (IP3)-gated channels are also associated with membrane-bound receptors for hormones and neurotransmitters. In this case, binding of a given substance to its receptor causes activation of another membrane-bound protein, phospholipase C. This enzyme promotes hydrolysis of phosphatidylinositol 4,5-diphosphate (PIP2) to IP3. The IP3 then diffuses to the sarcoplasmic reticulum and opens its calcium channels to release Ca++ ions from this intracellular storage site. 

Calpactins calectrin Calpain Calsequestrin Membrane-binding proteins (365) Protease controlling cell adhesion (366) Sequesters calcium within the sarcoplasmic reticulum when muscles relax (80,367)... 

The first molecule, the Ca2+ channel, is required for coupling at the triad. Skeletal muscle contains higher concentrations of this L-type Ca2+ channel that can be accounted for on the basis of measured voltage-dependent Ca2+ influx because much of the Ca2+ channel protein in the T-tubular membrane does not actively gate calcium ion movement but, rather, acts as a voltage transducer that links depolarization of the T-tubular membrane to Ca2+ release through a receptor protein in the SR membrane. The ryanodine receptor mediates sarcoplasmic reticulum Ca2+ release. The bar-like structures that connect the terminal elements of the SR with the T-tubular membrane in the triad are formed by a large protein that is the principal pathway for Ca2+ release from the SR. This protein, which binds the... 

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. 

High affinity Ca2+ binding sites amounting to about 10 nmoles per mg of protein are exposed on the exterior of sarcoplasmic reticulum membranes. They can solely be accounted for by the ATPase which binds two moles of Ca2+ per mole and constitutes between 5 and 7 nmoles per mg of intact sarcoplasmic reticulum protein. Inside the vesicles the binding capacities for ATP-involved calcium can be as high as 100 nmoles per mg protein. 

The message of calcium is conveyed from the troponin to which it binds, via tropomyosin to the actin filament193-198).As soon as the nerve impulse ceases, the calcium becomes quickly removed and returned to the storage sites situated in the membranes of the sarcoplasmic reticulum. When Ca2+ concentration in the sarcoplasm reaches 1 x 10-7 molar, the fiber is relaxed. 

Figure 13.4. Structure of SR CA " ATPase. This enzyme, the calcium pump of the sarcoplasmic reticulum, comprises a membrane-spanning domain of 10 a helices and a cytoplasmic headpiece consisting of three domains (N, P, and A). Two calcium ions (green) bind within the membrane-spanning region. The aspartate residue characteristic of this protein family is indicated.

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