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PJ Burton and BJ Waddell

The enzyme 11 beta-hydroxysteroid dehydrogenase type 2 (11 beta-HSD2), which reduces glucocorticoid potency in target cells by metabolism of active glucocorticoids, is expressed in the non-pregnant rat uterus in an oestrogen-dependent manner. Because glucocorticoids appear to facilitate parturition in many species, expression of 11 beta-HSD2 in pregnant myometrium is likely to influence pregnancy maintenance and possibly the onset and progression of labour. The present study therefore examined myometrial 11 beta-HSD2 mRNA, protein and bioactivity across rat pregnancy, with emphasis on the peripartum period. A single 1.9 kb transcript of 11 beta-HSD2 mRNA was evident in myometrium at all stages, with maximal (P<0.05) levels observed at day 16 (term=day 23). Consistent with this pattern of mRNA expression, Western blot analysis showed the presence of a 40 kDa 11 beta-HSD2 protein at all stages, with the maximal immunoreactive signal also observed on day 16. The 11 beta-HSD2 signal was immunolocalized to myometrial smooth muscle cells and endometrial stromal cells. Bioactivity specific to 11 beta-HSD2 was detectable in myometrium at all stages, but in contrast to the patterns of 11 beta-HSD2 mRNA and protein, the V(max) decreased at the beginning of pregnancy and remained stable until term. The apparent K(m) of 11 beta-HSD2 for corticosterone increased from 47 +/- 11 nM in non-pregnant myometrium to 75 +/- 7 nM by day 10 of pregnancy, and remained high until returning to an intermediate level on the day of delivery (60 +/- 8 nM). Progesterone competitively inhibited 11 beta-HSD2 bioactivity (K(i)=1.75 muM) whereas 20 alpha-hydroxypregn-4-en-3-one, the other major progestin present during rat pregnancy, had no such effect. In conclusion, these data suggest that local levels of active glucocorticoid in the myometrium are determined by the net effects of myometrial 11 beta-HSD-1 and -2 expression across pregnancy. Because the previously reported increase in myometrial 11 beta-HSD-1 near term occurs with little change in myometrial 11 beta-HSD2 bioactivity, this is likely to facilitate parturition by increasing local concentrations of active glucocorticoid.