title: Parvalbumins and muscle relaxation: a computer simulation study.
abstract: The distribution of Ca2+ and Mg2+ among the 'regulatory' cation binding sites of troponin (T-sites) and the strong, Ca2+-Mg2+ binding sites of troponin and parvalbumins (P-sites) in the sarcoplasm of a muscle was calculated. At rest, 60% of the T-sites were metal free, while 92% of the P-sites were loaded with Mg2+. In response to a Ca2+ pulse, troponin-calcium (T-Ca) complexes were rapidly formed, while the binding of Ca2+ to P-sites was limited by the slow rate of dissociation of the parvalbumin-magnesium (P-Mg) complexes. Muscle activation was not prevented by a high content of parvalbumins. Parvalbumin and the sarcoplasmic reticulum (SR) pump were complementary relaxing factors that removed Ca2+ from the cytosol and from the T-sites. Parvalbumins dominated the first part of relaxation, while the action of the SR was essential to ensure the return to a very low level of free Ca2+ ion and of T-Ca. After relaxation, a large fraction of the Ca2+ pulse was still bound to parvalbumins and returned slowly to the SR during the recovery. When the SR activity was reduced, the presence of parvalbumins preserved a fast rate of relaxation, at least for a few contractions. This may have a high adaptive value in cold-blooded animals.
mesh_terms: D000663:Amphibians; D000818:Animals; D002118:Calcium / Q000378:metabolism; D003201:Computers; D005399:Fishes; D008274:Magnesium / Q000378:metabolism; D008433:Mathematics; D009119:Muscle Contraction; D009124:Muscle Proteins / Q000378:metabolism*; D009126:Muscle Relaxation; D010320:Parvalbumins / Q000378:metabolism*; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D013997:Time Factors; D014018:Tissue Distribution; D014336:Troponin / Q000378:metabolism
chemical_list: D009124:Muscle Proteins; D010320:Parvalbumins; D014336:Troponin; D008274:Magnesium; D002118:Calcium