title: Magnesium and manganese ions modulate Ca2+ uptake and its energetic coupling in sarcoplasmic reticulum.
abstract: ATP hydrolysis, either coupled or uncoupled from Ca2+ uptake by sarcoplasmic reticulum (SR), is essentially independent of Mg2+ (millimolar range) up to 50 mM. Conversely, a sharp enhancement of Ca2+ uptake by Mg2+ is observed with a consequent increase of pumping efficiency (Ca2+ per ATP). Therefore, Mg2+ modulates pumping efficiency through the molecular mechanism of the pump itself. Manganese ions also stimulate Ca2+ uptake with an apparent efficiency lower than that of Mg2+. Additionally, Mn2+ competes with Ca2+ for the pump system and is accumulated into SR vesicles. Although the affinity of the pump is about three orders of magnitude higher for Ca2+ than for Mn2+, the capacity of the vesicles for Mn2+ is about three times that commonly observed for Ca2+. It is concluded that Mg2+ (millimolar range) couples ATP hydrolysis to Ca2+ uptake and that active transport of cations (Ca2+ and Mn2+) can proceed without a compensatory countertransport of a divalent cation. Finally, it is suggested that the SR pump operates physically as general cation translocator instead of as a Ca2+-specific pump, as commonly assumed.
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D066298:In Vitro Techniques; D007700:Kinetics; D008274:Magnesium / Q000502:physiology; D008345:Manganese / Q000502:physiology; D009132:Muscles / Q000502:physiology*; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*
chemical_list: D008345:Manganese; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D002118:Calcium