Cyclic ADP-ribose (cADPR), a metabolite of p-NAD, mobilizes calcium from intracellular stores such as sarcoplasmic reticulum (SR) through ryanodine receptor (RyR) channels in a variety of cell types, including smooth muscle cells. Synthesis of cADPR from p-NAD is catalyzed by ADP-ribosyl cyclase, and cADPR is hydrolyzed to ADPR by cADPR hydrolase. In mammalian cells, both ADP-ribosyl cyclase and cADPR hydrolase enzyme activities are associated with CD38, a transmembrane protein of ~45 kDa in size. CD38 is expressed in many different cell types, such as lymphocytes, pancreatic p cells, neurons, and smooth muscle cells, including that of the myometrium. CD38 expression is regulated by many factors such as hormones, retinoic acid, and cytokines. In this context, we demonstrated in a previous study that estrogen increases CD38 expression. The increased CD38 expression is associated with augmented ADP-ribosyl cyclase, but not cADPR hydrolase, activity. This differential regulation of a bifunctional protein would favor the increased production of cADPR.
The endocrine profiles during pregnancy and parturition reveal that estrogen remains at relatively low levels during pregnancy and reaches high levels at or near parturition. Similarly, progesterone levels that are high during pregnancy reach low levels at the time of parturition in the rat. The myometrium remains quiescent during pregnancy, while it becomes highly contractile at the time of parturition. Changes in hormonal profile during the late stage of pregnancy and parturition affect many signaling molecules, which lead to uterine contraction. In this context, estrogen is known to induce gap junctions between cells and oxytocin receptors in the myometrium. Recent studies have demonstrated that estrogen alters the expression of a variety of genes in the myometrium. This altered gene expression in the myometrium is associated with the changes in uterine contractility during pregnancy and parturition. Elevation of intracellular calcium is required for uterine smooth muscle contraction and studies have demonstrated that the expression of contractile molecules is associated with intracellular calcium regulation. All ryanodine receptor subtypes (types 1, 2, and 3) are expressed in both rat and human myometrium. Expression of type 3 receptor is higher compared with type 1 and type 2 receptors during pregnancy in the rat and human. Although the expression level of type 3 RyR did not change in rat myometrium during pregnancy, it is downregulated in human myometrium at the end of pregnancy. Signaling through CD38/cADPR is involved in the regulation of intracellular calcium concentration. However, changes in the expression of CD38, ADP-ribosyl cyclase, and cADPR hydrolase activities and cADPR levels in the uterine smooth muscle during pregnancy and parturition have not been investigated. In this study, we examined the profile of CD38 expression, ADP-ribosyl cyclase, and c-ADPR hydrolase enzyme activities during pregnancy in the rat myometrium. We also investigated the effects of different estrogen:progesterone ratios on the enzyme activities and CD38 protein expression in myometrium obtained from ovariectomized rats.