Changes in annexin (lipocortin) content in human amnion and chorion at parturition

Arachidonic acid is mobilized from fetal membrane phospholipids at paturition leading to increased production of oxytocic prostaglandins which may initiate or maintain myometiral contractions. Phospholipid mobilization requires activation of phospholipase A₂ or C both of which require calcium for activity. The annexins (lipocortins) are a superfamily of proteins which bind to calcium and phospholipids thereby may alter phospholipase activity through two mechanism: modulation of intracellular free Ca²⁺ concentration or regulation of the accessibility of phospholipids to hydrolyzing enzymes. Using Western immunoblotting with monospecific polyclonal antibodies, annexins I–VI were identfied in human amnion and chorion/decidua at term in tissues obtained from patients in labor or not in labor. Each annexin was present in two distinct pool: a pool which only associated with the membrane in the presence of calcium (calcium‐dependent pool) and a calcium‐independent pool that remained membrane bound in the presence of calicium chelators. Annexin I was present as two species, resolving at 36 kDa and 68 kDa. The total concentration of annexin I in both amnion and chorion/decidua was significantly decreased with labor, while the total concentration of annexin V in chorion significantly increased with labor. The size of individual pools of annexins also changed with labor: the calcium‐dependent pool of annexins I and II in both amnion and chorion significantly decreased; the calcium‐dependent pool of annexin V increased in chorion; and calcium‐independent pools of annexin I in amnion and annexins I, II, and V in chorion significantly decreased with labor. The decrease in totoal annexin I concentration with labor in amnion reflects a substantial decrease (80–90%) in the pool tightly bound to the membrane in a calcium‐independent manner. The striking change distinguishes annexin I as a potential candiate inhibitor which is specially downgregulated a parturition, potentially leading to increased access of phospholipases to substrate phospholipids and increased prostglandins production at labor. © 1992 Wiley‐Liss, Inc.