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luteal tissue mass (Athorn
et al.
, 2012a). These observations indicate that feed restriction
may compromise progesterone secretion capacity, although the mechanism is poorly
understood. Blocking LH secretion using a GnRH antagonist (Antarelix) in the luteal
phase of cyclic gilts reduced luteal tissue mass and systemic progesterone (Quesnel
et al.
,
2000). Whether alterations in feeding level would have similar effects is questionable.
Long term feed restriction may reduce LH secretion (Booth, 1990; Booth
et al.
, 1996),
but short term feed deprivation may not have such an effect (Barb
et al.
, 2001). hese
effects on LH were established in prepubertal animals which have a different LH secretion
pattern than pregnant pigs where progesterone exerts a negative feedback on LH secretion
anyway. Studies in pregnant gilts or during the luteal phase are equivocal, with some
studies reporting no effect on LH (Peltoniemi
et al.
, 1997a; Quesnel
et al.
, 2000), but some
studies reporting a reduction in LH secretion in feed-restricted pregnant gilts (Ferguson
et al.
, 2003; Peltoniemi
et al.
, 1997b). Therefore, if there is any effect of feed level on LH,
the effects on luteal tissue mass would be mild. There may, however, be other pathways
that mediate effects of feed level on luteal tissue formation such as through IGFs and
insulin. These pathways have hardly been studied in luteal tissue formation, although it
is known that porcine luteal tissue is responsive to insulin and IGF-1 (Miller
et al.
, 2003;
Ptak
et al.
, 2004).
Higher feed levels during the established luteal phase may also directly influence
progesterone secretion. Athorn
et al.
(2012b) showed that an increase in feed level from
1.5 to 2.8 kg/d in pregnant gilts increased the number of progesterone pulses in the caudal
vena cava from 3.8±0.7 to 4.9±1.1 per 6 h on day 9 after ovulation. That equates to an
increase by 4.4 pulses over a 24-h period. Progesterone pulses in the caudal vena cava
had an average amplitude of 179 ng/ml, which is 6-fold higher than systemic levels in that
period, and measurement of these pulses are probably more reflective of ovarian secretion
of progesterone. Some of these direct nutritional effects on progesterone may be mediated
by alterations in LH pulses, however, not all LH pulses coincide with a progesterone
pulse (Brüssow
et al.
, 2011; Virolainen
et al.
, 2005a) and, as outlined above, effects of
feed level on LH are equivocal. One could speculate that a low LH secretion is sufficient
for the function of corpora lutea in terms of progesterone secretion. In fact, Easton
et
al.
(1993) suggested that a few pulses of LH, rather than a certain basal level of LH, are
important for luteal function. If so, variation in LH secretion within a physiological range,
as observed in some reports (Peltoniemi
et al.
, 1997a,b; Prunier
et al.
, 1993), would not
have much influence on luteal maintenance. Nevertheless, progesterone concentration
is transiently elevated once LH secretion is stimulated pharmacologically by a GnRH
agonist (Peltoniemi
et al.
, 1995), and treatment with eCG on day 10 of gestation had an
immediate effect on progesterone (O'Leary
et al.
, 2011).
Increases in insulin and IGF-1 may also mediate the direct effects of a higher feed level.
Miller
et al.
(2003) reported that progesterone secretion by the ovaries increased after
infusion with IGF-1 into the ovarian pedicle, and Langendijk
et al.
(2008) found a
positive correlation between systemic IGF-1 and progesterone in the days after ovulation.
A higher feed level generally increases systemic IGF-1 (Ferguson
et al.
, 2003). It is also
interesting to note that in gilts on a high feed level, replacement of starch in the diet with
a fat source increased progesterone secretion (on day 15 of pregnancy), whilst luteal
