STIMULATION OF SECRETION BY MEMBRANE DEPOLARIZATION INCREASES EXTRACELLULAR PLASMA-MEMBRANE ASSOCIATION OF ANNEXIN-5 IN THE ANTERIOR-PITUITARY GLAND

Annexin 5, a calcium dependent phospholipid binding protein, was previ ously found to be expressed by rat anterior pituitary glands (Kawamina mi et al, 1992). In this report, the membrane association of annexin 5 and the effect of increased hormone secretion on this association was examined. Annexin 5 was revealed to remain in a particulate fraction of the rat pituitary gland throughout extensive washing and sonicating when extracellular levels of calcium were maintained at higher than n ormal concentrations. Association with the particulate fraction was sh own immunocytochemically to be mainly with the plasma membrane and nuc lear envelope. incubating intact pituitary tissue and dissociated pitu itary cells in a solution of [ethylenebis (oxyethylenenitrilo)] tetraa cetic acid (5 mM) dramatically decreased annexin 5 on the plasma membr ane without effect on the nuclear envelope. Loss of plasma membrane as sociation was into the medium as demonstrated by gel analysis. The che lator extractable annexin 5 was increased by incubation of pituitary t issue in high potassium (50 mM) medium for 2 h, a maneuver which also stimulated the secretion of both prolactin and growth hormone. These r esults reveal that annexin 5, a protein without a signal sequence, nor mally exists on the outside of pituitary cells and this extracellular display is increased by stimulation of hormone secretion.