ACTIVIN INHIBITS BUT INHIBIN ACTIVATES MOUSE PLACENTAL LACTOGEN-II SECRETION

The regulation of mouse placental lactogen (mPL)-I and mPL-II secretio n by activin and inhibin and the expression of activin and inhibin sub unit mRNAs in the mouse decidua were examined. Activin-A at a concentr ation of 10 nM/l significantly inhibited mPL-II secretion by placental cells from days 9 and 12 of pregnancy. However, activin-A did not aff ect mPL-I secretion by cells from days 7 and 9 of pregnancy nor mPL-II secretion by cells from day 7 of pregnancy. By contrast, 10 nM/l inhi bin activated mPL-II secretion by cells from day 12 of pregnancy. Thes e effects of activin and inhibin on mPL-II secretion were dose-depende nt. Follistatin, which binds to activin and blocks its bioactivity, co mpletely eliminated the inhibitory effect of activin on mPL-II secreti on. Incubation of placental cells from day 12 of pregnancy with activi n-A resulted in a significant reduction of the mPL-II mRNA level asses sed by Northern blot analysis. Northern blot analysis using poly(A) ()RNA extracted from the decidua indicated that mouse decidua, as well as the placenta, express all activin and inhibin subunits and that the ir gene expressions increased during gestation. The expression of thes e mRNAs in the decidua was much higher than those in the placenta. The se findings suggest that activin and inhibin regulate mPL-II secretion and suggest the presence of an autocrine or paracrine regulation of m PL-II secretion in mouse placenta by activin and inhibin after mid-pre gnancy in vivo.