Investigations on bone metabolism in intact and ovariohysterectomised miniature pigs

Biochemical markers of bone metabolism have become a common form of
analysis for bone metabolism. A number of factors, however, cause
fluctuations in these parameters such as day-to-day changes,
seasonal changes and age. In women, studies have confirmed
parameter fluctuations during the menstrual cycle (Zittermann et
al., 2000). Up until now, the coherence hormone status and bone
marker activity have been scanty in animal models. The objective of
this study was to examine the dependence of bone marker activity on
the estrous cycle in the Dresdner miniature pig. The present study
included the bone formation marker osteocalcin, and bone resorption
marker serum crosslaps. The estrous cycle dependency was determined
by comparing the development of marker concentrations with
progesterone and estradiol in the respective cycle stages of each
pig after synchronisation and at particular sampling points of
time. Furthermore the behaviour of biochemical markers, osteocalcin
and crosslaps were examined at and after ovariohysterectomy (OVX)
to determine the effect of OVX on these specific bone markers.
Another aspect of this study was to determine the influence of
calcium content in the diet on bone metabolism before and after
castration. Sixteen animals were divided into three groups, two
control groups and a OVX group. One control group received a 0.99%
calcium diet, while the OVX and the second control group received a
0.7% calcium diet. The animals in the OVX group were
ovariohysterectomised, while the pigs in the control and the second
control groups were not manipulated. The trial consisted of two
periods; the pre-castration period, lasting 65 weeks and a
post-castration period lasting 8 weeks. Blood samples were drawn
and analysed at selected extraction times. The bone marker
development pre-castration showed age-related and cycle-related
changes. Osteocalcin showed definite increased concentrations, at
the age of 9-13 months. If 10 months were seen as a late prepubal
stage (Tanner stage) in the minipig, osteocalcin reacts more slowly
in minipigs as it does in humans. Assuming that female minipigs in
the present study can be compared with female humans during
puberty, the present study shows that osteocalcin concentrations
are still high at late puberty, unlike those in humans, which are
high at midpuberty. Crosslaps increased up until the age of 10
months and then showed a constant decrease in concentrations. This
would be the equivalent reaction of resorption markers in humans.
Osteocalcin concentrations increased with an increase in estradiol
concentrations, so estradiol stimulates bone formation in miniature
pigs. The resorption marker serum crosslaps also showed cyclic
fluctuations. High crosslaps concentrations were found in the
estrus cycle stage or late follicular cycle stage; whereas low
crosslaps were found in the luteal phase. As was often the case in
other studies, the effect of a calcium reduced diet was not evident
between the control and second control group. Resorption and
formation markers showed changes post-castration in the OVX group,
which shows that this is a calcium diet that can be used in further
studies where induced osteopenia is required. Osteocalcin showed a
definite concentration increase 4 weeks after castration. Serum
crosslaps increased 2 weeks post operation. These increased
concentrations were not significant, which could be due to the
small animal numbers included in this study. Both markers reacted
within a short time after ovariohysterectomy, which is a useful
aspect for further osteoporosis research. This study portrayed the
usefulness of the minipig as an animal model, not only for further
studies on postmenopausal osteoporosis, but also verified the use
of osteocalcin and serum crosslaps as biochemical parameters of
bone metabolism in this species, to have age-related and
cycle-related fluctuations, which should be considered in further
studies.