function and has extended these findings by comparing the
effects of feeding protected plant-derived PUFAs from two
different families. Plasma progesterone concentrations were
reduced, particularly in the early luteal phase, in cows fed a
diet supplemented with either LNA (n-3) or LA (n-6) PUFAs.
These results are consistent with the study of Hinckley et al.
(1996), in which the addition of longer chain PUFAs to dispersed
bovine luteal cells decreased progesterone secretion.
The mechanism of this process is unknown, although
four possibilities should be considered. Firstly, the PUFAs
may alter luteal prostaglandin synthesis directly. For example,
PGF2a in the late luteal phase is luteolytic, whereas
PGE2 and 6-keto-PGF1a are luteotrophic in the early luteal
phase (Milvae et al., 1996). A high concentration of LA in
the corpus luteum in the early luteal phase could therefore
decrease production of luteotrophic prostaglandins.
Secondly, there is also growing evidence for a role of
arachidonic acid in steroidogenesis. Arachidonic acid can
regulate luteal expression of StAR protein and thus may
influence LH-stimulated progesterone production, as
transfer of cholesterol to the inner mitochondrial membrane
is a rate-limiting step (Wang et al., 1999). Thirdly, increased
plasma cholesterol concentrations could stimulate progesterone
production, although plasma cholesterol is not
thought to be rate-limiting to ovarian steroidogenesis
(Carroll et al., 1992). Finally, ovulation may be delayed in
the PUFA fed cows, leading to delayed luteal development.
None of these explanations are entirely supported by the
data from the present study, as the effects of both PUFA diets
on progesterone output were fairly similar, but concentrations
of the various possible mediators measured in milk or
plasma differed between the two PUFA diets. The LA (n-6)
supplemented diet increased milk LA and plasma cholesterol,
whereas the LNA (n-3) supplemented diet decreased