ATPase cytosolic calcium concentration

O Ruthenium Nitrogen Oxygen Cartxxi Figure 15 Proposed nitric oxide release and vasodilation mechanism induced by the [Ru(tpy)(NH NHq)NO] complex. The colored (different gray shades in the print version) circles represent atoms in the chemical structure. The hydrogen atom has been omitted in the structure. Legend sGC, soluble guanylyl cyclase GK, G Kinase Protein SERCA, sarco/endoplasmic reticulum calcium-ATPase Ca, calcium K, potassium [Ca o cytosolic calcium concentration. 

The cytosolic concentration of free Ca2+ is generally at or below 100 mi, far lower than that in the surrounding medium, whether pond water or blood plasma. The ubiquitous occurrence of inorganic phosphates (Pj and I l ,) at millimolar concentrations in the cytosol necessitates a low cytosolic Ca2+ concentration, because inorganic phosphate combines with calcium to form relatively insoluble calcium phosphates. Calcium ions are pumped out of the cytosol by a P-type ATPase, the plasma membrane Ca2+ pump. Another P-type Ca2+ pump in the endoplasmic reticulum moves Ca2+ into the ER lumen, a compartment separate from the cytosol. In myocytes, Ca2+ is normally sequestered in a specialized form of endoplasmic reticulum called the sarcoplasmic reticulum. The sarcoplasmic and endoplasmic reticulum calcium (SERCA) pumps are closely related in structure and mechanism, and both are inhibited by the tumor-promoting agent thapsigargin, which does not affect the plasma membrane Ca2+ pump. 

We will consider the structural and mechanistic features of these enzymes by examining the Ca2+ ATPase found in the sarcoplasmic reticulum (SR Ca2+ ATPase) of muscle cells. This enzyme, which constitutes 80% of the sarcoplasmic reticulum membrane protein, plays an important role in muscle contraction, which is triggered by an abrupt rise in the cytosolic calcium level. Muscle relaxation depends on the rapid removal of Ca + from the cytosol into the sarcoplasmic reticulum, a specialized compartment for calcium storage, by the SR Ca + ATPase. This pump maintains a Ca2+ concentration of approximately 0.1 iM in the cytosol compared with 1.5 mM in the sarcoplasmic reticulum. 

Calcium ions (Ca ) are important for the mediation of hepatic injury. Cytosolic free calcium is maintained at relatively low concentrations compared to the extracellular levels. The majority of intracellular calcium is sequestered within the mitochondria and endoplasmic reticulum. Membrane associated calcium and magnesium ATPases are responsible for maintaining the calcium gradient (Farrell et ah, 1990). Significant and persistent increases in the intracellular calcium result from nonspecific increases in permeability of the plasma membrane, mitochondrial membranes, and membranes of the smooth endoplasmic reticulum. Calcium pumps in the mitochondrial membrane require NADPH, thus depletion of available NADPH can cause calcium release from mitochondria (Cullen, 2005). 

The Ca -ATPase piays an essential role In the pumping of calcium out of cells, and in the control of its cytosolic concentration. In the heart, the role of the pump is minor with respect to that of the sodium-calcium exchanger, but is most probabiy predominant in skeletal and smooth muscle. The pump is encoded by four independent genes, showing different patterns of tissue-specific expression and alternative splicing of the primary transcripts. The intracellular Ca pump proteins from skeletal muscle sarcoplasmic reticulum (SR), cardiac SR and brain microsomes are similar. Thapsigargin is a potent inhibitor, also lanthanum salts inhibit the pump at most sites. 

In the cytosol of muscle cells, the free Ca " ranges from 10 M (resting cells) to more than 10 (contracting cells), whereas the total Ca concentration In the SR lumen can be as high as 10 M. However, two soluble proteins In the lumen of SR vesicles bind Ca " and serve as a reservoir for intracellular Ca ", thereby reducing the concentration of free Ca " ions In the SR vesicles and conse-quendy the energy needed to pump Ca " ions into them from the cytosol. The activity of the muscle Ca " ATPase increases as the free Ca " concentration in the cytosol rises. Thus in skeletal muscle cells, the calcium pump in the SR membrane can supplement the activity of a similar Ca " pump located in the plasma membrane to assure that the cytosolic concentration of free Ca " in resting muscle remains below 1 jjlM. 

The deterministic dynamics of this cluster model has been investigated in [32, 33]. In addition to IP3 mediated Ca liberation, we considered sarco-endoplasmic reticulum calcium ATPase (SERCA) pumps, which transport Ca from the cytosol to the ER, and a leak flux. The stationary Ca concentration profile that results from these three fluxes is... 

The extracellular concentration of Ca + is approximately 1.5 him. The cytosolic concentration, on the other hand, is only 0.1 hm (note the units see Chemistry 111). This is a gradient of more than 10000 1 in concentration (Fig. 50.1) and again requires active transport, in this case by a Ca +-ATPase. This pumps calcium ions out of the cell across the plasma membrane. Alternatively Ca + can also be pumped out of the cytosol into an internal compartment, the endoplasmic reticulum. In the case of muscle, the highly specialised internal compartment is known as the sarcoplasmic reticulum and plays an important role in triggering contraction. 

---