Synthesis and processing of pro-ocytocin in bovine corpus luteum and granulosa cells

Bovine corpus luteum is the site of intense production of pro-ocytocin-neurophysin mRNA at day 1 after estrus (Ivell et al. (1985) FEBS Lett. 190, 263-267) which is followed by apparent delayed production of ocytocin. Therefore it is a good model to study both the translational and post-translational production of this neuropeptide in non-hypothalamic tissues and its regulation. In order to assess if this mRNA is translated during the lag period we have analyzed the neurophysin-like species produced in this organ. As early as day 2 after estrus one neurophysin species (pI approximately 4.7) could be detected and was unequivocally identified as pro-ocytocin-neurophysin. In primary cultures of luteinizing granulosa cells, biosynthetic intermediates were characterized, i.e. ocytocin-Gly, ocytocin-Gly-Lys and ocytocin-Gly-Lys-Arg, whereas amidated, fully mature, ocytocin was undetectable. We conclude that translation of pro-ocytocin-neurophysin mRNA takes place soon after transcription and we propose that incomplete processing could be responsible for the low level of ocytocin in the early bovine corpus luteum.