Annexin 5 inhibits thyrotropin releasing hormone (TRH) stimulated prolactin release in the primary culture of rat anterior pituitary cells.

Annexin 5, a novel calcium-phospholipid binding protein, is thought to be involved in hormone secretion by the anterior pituitary gland. Gonadotropin releasing hormone stimulates annexin 5 synthesis, which, in turn, enhances gonadotoropin secretion. On the other hand, annexin 5 was shown to inhibit prolactin release in vitro. To understand the nature of the opposing effects of annexin 5 on these two major pituitary hormones, the present study examines the inhibitory effect of annexin 5 on prolactin release in relation to thyrotropin stimulating hormone (TRH) using primary cultures of anterior pituitary cells of adult female rats. While recombinant rat annexin 5 was found to have little effect on basal prolactin release, it significantly inhibited TRH-stimulated prolactin release. Addition of specific anti-annexin 5 serum to the culture increased basal prolactin release in a concentration dependent manner, and no further increase in prolactin release was observed following application of TRH in the presence of anti-annexin 5. The enhanced basal prolactin release induced by anti-annexin 5 was reversed by the simultaneous administration of indomethacin, an inhibitor of cyclooxygenase. These results demonstrate that endogenous pituitary annexin 5 exerts an inhibitory effect on prolactin release and suggest that this is attained by suppression of eicosanoid synthesis in vitro.