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Previous study found that cells isolated from mature follicles
of rabbit ovary could be luteinized by a short-term stimulation
of hCG (hr 0 to hr 24). If the luteinized cells being treated with
an identical stimulation from hr 96 to hr 120, their P4 secreting
ability could be kept for next 10 to 12 days. Thus, P4 secreting
ability of rabbit follicular cells are able to be stimulated, then,
to be maintained in vitro as long as a pseudopregnany with the
2-step hCG treatments. The present study was to observe the
changes of P450scc and StAR proteins in the cell during 3
phases. They were 1> hr 0 to hr 24 for luteinization; 2> hr 96 to
hr 120 during the second hCG treatment; and 3> the period from
hr 216 to hr 408 while the P4 secreting ability being decreasing.
The result indicated that once hCG being added into the culture
media, within 4 hrs, the StAR content of follicular cells increased
dramatically and dropped below basal level at hr 24. However,
P450scc did not alter by the hCG treatment. Therefore, the acute
follicular P4 secretion being seen during preovulatory LH surge
in rabbit was due to acute synthesis of StAR in the cells but not
that of P450scc. The effect of the second hCG treatment differed
from the first stimulation. It was able to significantly increased
the biosynthesis of P450scc in 24 hrs (hr 120) but not StAR,
although cellular StAR contents gradually elevated until hr 216.
This may indicate that infrequent LH secretion, being seen in
rabbit luteal phase, mainly enhances P450scc biosynthesis to
maintain the luteal function. Both cellular P450scc and StAR
level decreased after hr 216. Thus, the decrease of P4 synthesis
involves the degrading of both rate-limiting factors in this
species. Since estrogen has been thought to be an ultimate
luteotropic factor in rabbits, I also added E2 into the culture
environment to test its effect on P450scc, StAR and P4. However,
I did not find any significant estrogenic action on the present
in vitro system.
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