Characteristics of the Ca2+ pump and Ca2+-ATPase in the plasma membrane of rat myometrium.
A plasma membrane-enriched fraction from rat myometrium shows ATP-Mg2+-dependent active calcium uptake which is independent of the presence of oxalate and is abolished by the Ca2+ ionophore A23187. Ca2+ loaded into vesicles via the ATP-dependent Ca2+ uptake was released by extravesicular Na+. This showed that the Na+/Ca2+ exchange and the Ca2+ uptake were both occurring in plasma membrane vesicles. In a medium containing KCl, vanadate readily inhibited the Ca2+ uptake (K1/2 5 microM); when sucrose replaced KCl, 400 microM-vanadate was required for half inhibition. Only a slight stimulation of the calcium pump by calmodulin was observed in untreated membrane vesicles. Extraction of endogenous calmodulin from the membranes by EGTA decreased the activity and Ca2+ affinity of the calcium pump; both activity and affinity were fully restored by adding back calmodulin or by limited proteolysis. A monoclonal antibody (JA3) directed against the human erythrocyte Ca2+ pump reacted with the 140 kDa Ca2+-pump protein of the myometrial plasma membrane. The Ca2+-ATPase activity of these membranes is not specific for ATP, and is not inhibited by mercurial agents, whereas Ca2+ uptake has the opposite properties. Ca2+-ATPase activity is also over 100 times that of calcium transport; it appears that the ATPase responsible for transport is largely masked by the presence of another Ca2+-ATPase of unknown function. Measurements of total Ca2+-ATPase activity are, therefore, probably not directly relevant to the question of intracellular Ca2+ control.
Enyedi
A
A
Department of Biochemistry and Molecular Biology, Mayo Clinic, Rochester, MN 55905.
Minami
J
J
Caride
A J
AJ
Penniston
J T
JT
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002118:Calcium / Q000493:pharmacokinetics*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002147:Calmodulin / Q000494:pharmacology; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D005260:Female; D008274:Magnesium / Q000494:pharmacology; D008628:Mercury / Q000494:pharmacology; D009215:Myometrium / Q000187:drug effects / Q000378:metabolism*; D009711:Nucleotides / Q000494:pharmacology; D051381:Rats; D012964:Sodium / Q000494:pharmacology; D014357:Trypsin / Q000494:pharmacology; D014638:Vanadates / Q000494:pharmacology
D002147:Calmodulin; D009711:Nucleotides; D014638:Vanadates; D000255:Adenosine Triphosphate; D012964:Sodium; D014357:Trypsin; D000252:Calcium-Transporting ATPases; D008628:Mercury; D008274:Magnesium; D002118:Calcium