Iqbal Khan

Placental-derived regulators and the complex control of luteal cell function.

Publisher Summary The corpus luteum is a transient ovarian endocrine gland that results from a rapid growth and differentiation of follicular theca and granulosa cells. Several types of corpora lutea are formed during the reproductive life of the female rat. They differ in size, life span, and steroidogenic output. While corpora lutea of the cycle and pseudopregnancy do not have a clearly defined role, the corpus luteum of pregnancy is absolutely crucial for normal fetal development and survival in every mammalian species. In the rat, this gland is initially the same size as that of pseudopregnancy; however, instead of involuting, it increases in size and in steroidogenic capacity and remains functional for approximately 20 days. The capacity of this corpus luteum to secrete progesterone and to sustain a normal pregnancy is tightly regulated by hormones generated by the pituitary, the placenta, and by itself. This chapter reviews the hormonal interrelationships that regulate the corpus luteum of pregnancy in the rat, focusing primarily on placental secretion of androgen and the role of placental steroids in luteal steroidogenesis and growth.

Inhibition and stimulation of rat luteal protein phosphorylation by protein kinase effectors

Estradiol-17 beta (E2) predetermined protein phosphorylation systems have been identified recently in midpregnant rat corpus luteum. Major type protein kinase activities in these systems were explored here using as probes protein kinase inhibitors. Luteal nuclear, mitochondrial, microsomal and cytosolic fractions were obtained from rats hysterectomized and hypophysectomized on day 12 of pregnancy and then treated for 72 h with E2. In vitro phosphate transfer from [gamma-32P]ATP was monitored by SDS-PAGE followed by autoradiography. Polymyxin B (PMB), 1-200 microM, a PKC inhibitor, completely blocked, in a dose dependent manner, the Ca2+ phospholipid (PL) stimulated radiolabeling of nuclear fraction Mr 79,000 substrate(s) as expected. Similarly, the calmodulin (CaM) antagonist compound 48/80, 1-20 micrograms/ml, inhibited the Ca2+/CaM-dependent phosphorylation of the microsomal fraction Mr 60,000 and Mr 56,000 proteins. The Ca2+ PL-enhanced labeling of mitochondrial fraction Mr 76,000 substrate(s) was only partially susceptible to inhibition by PMB or compound 48/80. Studies of microsomal fraction phosphoprotein bands not stimulated by added cofactors indicated that the radiolabeling of Mr 75,000 protein(s) was partially blocked by compound 48/80 but not by PMB. Phosphate transfer to Mr 41,000 protein(s) was inhibited by the cAMP-dependent kinase protein inhibitor (PKI), while the phosphorylation of Mr 31,000 protein(s) was refractory to all inhibitors employed here. Surprisingly, regardless of hormonal pretreatment, PMB and compound 48/80 activated in every subcellular fraction the cofactor independent appearance of at least one phosphoprotein band, between Mr 87,000-99,000. This novel observation should be instrumental in understanding the actions of these compounds towards living cells.