Divergence in murine myometrium spontaneous and oxytocin-stimulated contractile responses to serine/threonine protein phosphatase-1 inhibition

Reversible phosphorylation is essential in regulating uterine contractions. Identification, characterization, and functional understanding of myometri um protein phosphatase(s) are lacking. Okadaic acid (OA), which inhibits pr otein phosphatase-1 (PP1) and PP2A, has been shown to alter uterine contrac tions. Experiments were conducted to determine the 1) identity of the myome trial OA-sensitive PP, 2) influence of OA on spontaneous and oxytocin (OT)- stimulated myometrial contractions, and 3) expression of uterine PPs during sexual development. Western blot analysis indicated the presence of PP1(al pha) and PP2A in immature and mature mice. As determined by immunohistochem istry, gonadotropin-stimulated adult mouse uteri contain PP1(alpha) in long itudinal and circular myometrial layers and endometrial epithelium. Convers ely, PP2A was localized to the endometrial stroma. Cumulative addition of O A (n = 9; 10, 100, 250, 500, 1000 nM) did not significantly alter spontaneo us contractions of mouse uterine horns in comparison to vehicle-treated con trols (n = 9). By the end of the test period OA- and vehicle-treated uteri displayed a comparable decline in uterine contractions to 79.2% and 63.7%, respectively, of basal contractile activity. Pretreatment of uterine tissue with OA (1 mu M; n = 7) significantly reduced contractile response to incr easing concentrations of OT (8, 16, 32, 64 nM) in comparison to vehicle pre treatment (dimethyl sulfoxide; n = 7). At the end of the OT-administration period, contractile activity was 160.4% and 67.3% of basal contractile acti vity for vehicle (no OA) and OA-pretreated groups, respectively. During the early prepubertal period PP1(alpha) was expressed in longitudinal myometri um and absent in circular myometrium; whereas, during the transition to sex ual maturity PP1(alpha) was observed in both the longitudinal and circular myometrium. In summary, these studies have indicated 1) that PP1 is the pri mary myometrial OA-sensitive PP; 2) that inhibition of PP1 had no effect on spontaneous contractions, whereas it markedly inhibited CT-stimulated uter ine contractions; and 3) that PP1 is differentially expressed in the circul ar and longitudinal myometrium in relation to sexual development.