title: Effect of temperature on the reversal of the calcium ion pump in sarcoplasmic reticulum.
abstract: Sarcoplasmic-reticulum vesicles were actively loaded with Ca2+ in the presence of phosphate, and the ADP-induced Ca2+ efflux and ATP synthesis were measured as a function of temperature. Arrhenius plots show break points for both processes at about 18 and 37 degrees C. Between 18 and 37 degrees C, Ca2+ efflux and ATP synthesis occur with an activation energy of 67.2-71.4 kJ/mol, whereas it is about 189-210 kJ/mol for temperatures below 18 degrees C. Above 37 degrees C, the rates of ADP-induced Ca2+ release and of ATP synthesis sharply decline until the temperature reaches about 42 degrees C. Above this temperature, the Ca2+ efflux increases again even in absence of ADP, although the synthesis of ATP is inhibited, which reflects leakiness of the vesicles. The results show that the transition temperatures for ADP-induced Ca2+ efflux and for ATP synthesis resemble those for active Ca2+ uptake, which indicates that the same coupling mechanism is involved during the inward and outward Ca2+ translocations across the membrane.
mesh_terms: D000244:Adenosine Diphosphate / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000096:biosynthesis; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*; D013053:Spectrophotometry; D013696:Temperature
chemical_list: D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D002118:Calcium