THE CA2-ATPASE ISOFORMS OF PLATELETS ARE LOCATED IN DISTINCT FUNCTIONAL CA2+ POOLS AND ARE UNCOUPLED BY A MECHANISM DIFFERENT FROM THAT OF SKELETAL-MUSCLE CA2+-ATPASE()

Vesicles derived from the dense tubular system of platelets possess a Ca2+-ATPase that can use either ATP or acetyl phosphate as a substrate . In the presence of phosphate as a precipitating anion, the maximum a mount of Ca2+ accumulated by the vesicles with the use of acetyl phosp hate was only one-third of that accumulated with the use of ATP. Vesic les derived from the sarcoplasmic reticulum of skeletal muscle accumul ated equal amounts of Ca2+ regardless of the substrate used. When acet yl phosphate was used in platelet vesicles, the transport of Ca2+ was inhibited by Na+, Li+, and K+; in sarcoplasmic reticulum vesicles, onl y Na+ caused inhibition. When ATP was used as substrate, the different monovalent cation had no effect on either sarcoplasmic reticulum or p latelet vesicles. The catalytic cycle of the Ca2+-ATPase is reversed w hen a Ca2+ gradient is formed across the vesicle membrane, The stoichi ometry between active Ca2+ efflux and ATP synthesis was one in platele t vesicles and two in sarcoplasmic reticulum vesicles. The coupling be tween ATP synthesis and Ca2+ efflux in sarcoplasmic reticulum vesicles was abolished by arsenate regardless of whether the vesicles were loa ded with Ca2+ using acetyl phosphate or ATP. In platelets, uncoupling was observed only when the vesicles were loaded using acetyl phosphate . In both sarcoplasmic reticulum and platelet vesicles, the effect of arsenate was antagonized by thapsigargin (2 mu M), micromolar Ca2+ con centrations, P-i (5-20 mM), and MgATP (10-100 mu M). Trifluoperazine a lso uncoupled the platelet Ca2+ pump but, different from arsenate, thi s drug was effective in vesicles that were loaded using either ATP or acetyl phosphate. Trifluoperazine enhanced Ca2+ efflux from both sarco plasmic reticulum and platelet vesicles; thapsigargin, Ca2+, Mg2+, or K+ antagonized this effect in sarcoplasmic reticulum but not in platel et vesicles. The data indicate that the Ca2+-transport isoforms found in sarcoplasmic reticulum and in platelets have different kinetic prop erties.