Annexin VI is a member of a Ca(2+)-dependent, phospholipid-binding protein family. Although functions for this annexin have been proposed from in vitro studies, most remain controversial. Díaz-Muñoz et al. (J Biol Chem 265:15894, 1990) demonstrated that annexin VI modified, in a Ca(2+)-dependent manner, the gating behavior of the sarcoplasmic reticulum Ca(2+)-release channel, reconstituted into artificial bilayers, by increasing both the open probability and the mean open time. This effect was specific to the trans chamber, which represents the luminal side of the sarcoplasmic reticulum. In agreement with those findings, we show herein that annexin VI produced no effect on Ca(2+)-uptake or -release by intact heavy sarcoplasmic reticulum vesicles (analogous to the cis chamber). We also used monospecific antibodies to evaluate the subcellular localization of annexin VI by immunofluorescent microscopy. Studies in rat skeletal muscle suggest that annexin VI is present surrounding individual myofibrils. Double immunolocalization studies with cultured muscle cells (chick myotubes) using anti-annexin VI and anti-SR Ca(2+)-ATPase antibodies demonstrated superimposable staining patterns. In non-muscle tissue (normal rat kidney (NRK) cells), a punctate, perinuclear anti-annexin VI staining pattern was observed. Collectively, these data suggest that annexin VI may play a regulatory role in the Ca(2+)-release/uptake cycle in the sarcoplasmic reticulum as well as in non-muscle organelles, a key process in stimulus-response systems.