STEADY-STATE CONCENTRATIONS OF MESSENGER-RNA ENCODING 2 INHIBITORS OFPROTEIN-KINASE-C IN-OVINE LUTEAL TISSUE

Prostaglandin F-2 alpha (PGF(2 alpha)) decreases secretion of progeste rone from the corpus luteum in domestic ruminants. However, it is less effective during the early part of the oestrous cycle (Louis et al., 1973) and at the time of maternal recognition of pregnancy (Silvia and Niswender, 1984; Lacroix and Kann, 1986). Decreased luteal. responsiv eness may be due to failure of PGF(2 alpha) to activate fully its norm al second messenger system, protein kinase C (PKC). Alternatively, inc reased resistance of the corpus luteum to PGF(2 alpha) might be attrib utable to greater concentrations of recently identified biological inh ibitors of PKC. These possibilities were addressed by measuring steady -state concentrations of mRNA encoding PGF(2 alpha) receptor and two i nhibitors of PKC, protein kinase C inhibitor-1 (PKCI-1) and kinase C i nhibitor protein-1 (KCIP-1, brain 14-3-3 protein), in corpora lutea co llected from ewes on days 4, 10 and 15 of the oestrous cycle (n = 5 pe r day) and day 15 of pregnancy (n = 7). There were no differences in m ean concentrations of mRNA encoding PGF(2 alpha) receptor among the gr oups. However, concentrations of mRNA encoding both inhibitors of PKC were higher (P < 0.01) on day 4 of the oestrous cycle compared with th e other groups. Treatment of ewes with a luteolytic dose of PGF(2 alph a), which activates PKC, did not change concentrations of mRNA encodin g either PKCI-1 or KCIP-1 up to 24 h later. Luteal expression of mRNA encoding the PKC inhibitors and PGF(2 alpha) receptor was also examine d in ewes treated with oestradiol in viva for 16 h in the midluteal ph ase. High concentrations of oestradiol in serum (20 and 70 pg ml(-1)) did not influence quantities of any of the mRNAs examined. Therefore, an increase in PKC inhibitors may be involved in resistance of the cor pus luteum to PGF(2 alpha) during the early part of the oestrous cycle but does not appear to mediate the increased resistance of the corpus luteum to PGF(2 alpha) during maternal recognition ol: pregnancy. Nei ther PGF(2 alpha) nor oestradiol affected steady-state concentrations of mRNAs encoding PKCI-1 or KCIP-1.