In the present study, we determined the profiles of CD38 expression and ADP-ribosyl cyclase activities during pregnancy. In myometrium obtained from term rats, there was increased CD38 mRNA and protein expression compared with levels at Days 14-17 of pregnancy. This increased CD38 protein expression was associated with significantly increased ADP-ribosyl cyclase, but not cADPR hydrolase, activity. The cADPR levels were significantly higher in term-rat uterine smooth muscles compared with that in uterine smooth muscles from preterm rats. The increased CD38 expression and differential regulation of the enzyme activities associated with this protein in uterine smooth muscle at term are similar to changes we reported in a previous study in response to estrogen in ovariectomized rats. On the other hand, the CD38 expression and enzyme activities in uterine smooth muscle obtained from preterm rats were similar to those in control ovariectomized rats or those treated with estrogen along with high doses of progesterone. These results indicate that, in rat uterine smooth muscle, the expression of CD38, differential regulation of its enzyme activities, and cADPR content are regulated during gestation and by estrogen:progesterone ratio.

Changes in hormonal profile and cell signaling during pregnancy lead to increased uterine motility, which initiates labor. The myometrium is quiescent during pregnancy, when progesterone level is high and estrogen level is low. However, uterine motility increases at the end of pregnancy, when estrogen levels sharply increase and progesterone levels decrease. The myometrium is a main target for estrogen action, although effects of estrogen have been shown in other tissues, such as bone marrow, kidney, and adipose tissues. Changes in the estrogen:progesterone ratio are reported to lead to many structural changes, such as increase in number of oxytocin receptors, extracellular matrix deposition, cellular remodeling, and myometrial cell growth. Furthermore, the expression of proteins such as c-fos, connexin-43, and receptors for oxytocin and prostaglandin F2a in the myometrium is upregulated prior to labor. These studies provide evidence for changes in the expression of genes involved in cell signaling during the onset of labor.

CD38/cADPR signaling is well documented in the regulation of intracellular calcium in a variety of smooth muscle cells. In the present study, we dem onstrated that the expression of CD38 is higher at parturition compared with Days 14-17 of pregnancy. The higher CD38 expression would result in increased cADPR production and release of intracellular calcium, which would favor uterine motility. Recently, a study by Barata et al. demonstrated a role for the CD38/cADPR signaling pathway in intracellular calcium release and myometrial contraction in human myometrial cells and muscle strips. The switch from low CD38 expression and enzyme activities to high expression and differential regulation of the two enzyme activities happens very close to term in the rat. In this species, there is a switch from a low estrogen:pro-gesterone ratio during gestation to a sharp increase in the ratio at term. This increase in estrogen level is most likely involved in the regulation of CD38 expression and differential regulation of its enzyme activity. Estrogen palindromic sequences have been identified in the promoter region of the CD38 gene, which may account for the estrogen effects in the myometrium.

The mechanism by which progesterone affects the expression of CD38 in uterine smooth muscle is not known. In an attempt to address the effect of progesterone on CD38 expression, we administered different doses of progesterone along with a fixed dose of estrogen. In ovariectomized rats, earlier studies demonstrated that estrogen treatment resulted in higher CD38 expression and ADP-ribosyl cyclase activity in the rat myometrium. Furthermore, we also demonstrated in that study that cADPR hydrolase activity decreases during estrogen treatment, reflecting a posttranslational modification of the CD38 protein. In the present study, we demonstrate that estrogen-induced changes in CD38 protein expression and ADP-ribosyl cyclase activity are inhibited by 10 and 20 mg, but not by 1 mg, of progesterone. In the presence of high doses of progesterone, there were significant decreases in CD38 expression and enzyme activities. These results demonstrate an apparent inhibitory effect of progesterone on estrogen effects. In this context, several studies have reported the inhibitory effects of progesterone on estrogen-induced gene expression, resulting in delayed labor. For example, the expression levels of calcium channel protein, connexin 43, receptors for oxytocin and PGF2a, c-fos, c-jun, junD, and junB are inhibited by progesterone. Furthermore, in the presence of a progesterone antagonist, there is increased expression of genes that are normally upregulated during late gestation. The increased CD38 expression, ADP-ribosyl cyclase activity, and cADPR levels and the decreased cADPR hydrolase activity observed in the present study at parturition could also arise from withdrawal of the inhibitory effect of progesterone. Progesterone appears to not only inhibit estrogen stimulation of CD38 expression but also the post-translational modification that results in the differential regulation of the enzyme activities.

In summary, in the present study, we demonstrate increased CD38 expression and ADP-ribosyl cyclase activity in myometrium from rats at parturition compared with at Days 14-17 of pregnancy. These changes are also associated with increased cADPR levels. The effects of progesterone on CD38 expression and function are similar to those seen during midpregnancy, when progesterone levels are relatively high. The precise mechanism by which progesterone inhibits estrogen effects on CD38 expression and the differential regulation of enzyme activities needs further investigation.