Drug action of thapsigargin on the Ca2+ pump protein of sarcoplasmic reticulum.
Thapsigargin is found to be a potent inhibitor of the intracellular Ca2+ pump proteins from skeletal muscle sarcoplasmic reticulum (SR), cardiac SR, and brain microsomes. For skeletal muscle SR, the molar ratio of thapsigargin to Ca2+ pump protein for complete inhibition (MRc) of the Ca2+ loading rate, Ca(2+)-dependent ATPase activity, and formation of phosphorylated intermediate (EP) was approximately 1. When the Ca2+ pump protein of low affinity to Ca2+ (E2 state) was pretreated with thapsigargin, ATP and Ca2+ binding to the Ca2+ pump protein was completely inhibited. In the presence of Ca2+ (E1 state), Ca2+ pump protein was protected from inactivation by thapsigargin with respect to Ca2+ binding and EP formation. The MRc for brain microsomes, which mediate Ca2+ uptake into intracellular (inositol 1,4,5-trisphosphate-releasable) Ca2+ pools, is likewise stoichiometric. Approximately 30% of Ca2+ loading activity of brain microsomes was insensitive to thapsigargin, indicating the presence of other Ca2+ pumping system(s). The MRc for heart is 3.8, indicating that the Ca2+ pump of cardiac SR is less sensitive to thapsigargin. Phosphorylation of cardiac SR with protein kinase A increased the sensitivity to thapsigargin to MRc of 2.8. In summary, we find that: 1) thapsigargin is the most effective inhibitor of the Ca2+ pump protein of intracellular membranes (SR and endoplasmic reticulum); 2) its primary inhibitory action appears to inactivate the E2 form of the enzyme preferentially; 3) cardiac SR shows lesser sensitivity to thapsigargin than skeletal muscle SR and brain microsomes; protein kinase A treatment of cardiac SR enhances the sensitivity to the drug.
Kijima
Y
Y
Department of Molecular Biology, Vanderbilt University, Nashville, Tennessee 37235.
Ogunbunmi
E
E
Fleischer
S
S
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001921:Brain / Q000378:metabolism; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000187:drug effects*; D008861:Microsomes / Q000378:metabolism; D009132:Muscles / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D013729:Terpenes / Q000494:pharmacology*; D019284:Thapsigargin
D013729:Terpenes; D019284:Thapsigargin; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases; D002118:Calcium