Progesterone and progesterone receptors in reptiles

The role of progesterone (P) has been most extensively studied in the female reproductive tissues (ovary, reproductive tract, mammary gland) and in the brain, in which it is an important regulator and modulator in conjunction with estradiol (E). In nonmammalian vertebrate species, less work has been done on P metabolites involved in ovulation. In addition, P induces the expression of egg-white proteins, decreases myometrial contractility, and facilitates processing of eggs, formation of eggshell, and deposition of egg-white proteins. Actions of P may be synergistic with, or antagonist to, the actions of E, depending on hormone ratios, timing of exposure, and physiological state. These effects of P are mediated through progesterone receptor isoform A (PRA), a general transcription inhibitor of P target genes, and isoform B (PRB), a specific transcriptional stimulator of some reproductive tract genes. In this review, we focus on P action and PR expression in the liver and reproductive tissues of several reptilian species, especially Chelonia, since most of the data obtained are from this group. We also present novel data showing the expression of PR in nonreproductive tissues (such as kidney, spleen, gastrointestinal tract) in the painted turtle, Chrysemys picta.