G.E. Mann, G.E. Lamming Animal Reproduction Science 64 2000 171–180 175
2.5. Statistical analysis Plasma concentrations of oestradiol were analysed by repeated sample analysis of vari-
ance with group and day as main factors. Changes in plasma concentration of PGFM follow- ing oxytocin challenge were analysed by repeated sample analysis of variance with group
and time as main factors. Endometrial oxytocin binding results were analysed by analysis of variance with group and day as the main factors. The effects of level of oestradiol on
oxytocin binding were analysed by regression analysis with mean oestradiol concentration on day 0 and oxytocin receptor concentration on day 6 as factors. The effects of level of
oestradiol on oxytocin challenge response on day 6 were analysed by regression analysis with mean oestradiol concentration on day 0 and mean PGFM increase on day 6 expressed
as percentage increase above pre-treatment baseline as factors.
3. Results
3.1. Study 1 Plasma concentrations of oestradiol rose markedly in all groups P 0.01 following
the initiation of oestradiol injections to significantly different P 0.01 concentrations between the three groups Fig. 1. Mean ± S.E.M. concentrations on the second day of
oestradiol treatment, on which all animals exhibited behavioural oestrus, were 12.1 ± 1.0, 4.9±0.5 and 2.9±0.4 pg ml
− 1
in the high, medium and low groups, respectively. Following the last injection, plasma concentration of oestradiol fell markedly P 0.01 in all groups
with values reaching similar levels in the three groups within 72 h. On day 2 following oestrus, oxytocin binding in endometrial membrane preparations
was similar in the three groups Fig. 2. However, by day 4, oxytocin binding had fallen
Fig. 1. Mean ± S.E.M. plasma concentrations of oestradiol generated in ovariectomised cows treated with high
s
; n = 4, medium
h
; n = 4 or low △; n = 4 levels of oestradiol administered by i.m. injection in corn oil at 8 h intervals indicated by ▽.
176 G.E. Mann, G.E. Lamming Animal Reproduction Science 64 2000 171–180
Fig. 2. Mean ± S.E.M. oxytocin binding activity fmol mg protein
− 1
in uterine endometrial biopsies collected on days 2, 4 and 6 following oestrus in ovariectomised cows treated with high
j
; n = 4, medium ; n = 4 or low
h
; n = 4 levels of oestradiol.
significantly P 0.05 in the high oestradiol group, and by day 6 oxytocin binding in the high group was significantly P 0.05 lower than in the other two groups. Furthermore,
there was a significant correlation between mean oestradiol concentration on the second day of treatment day 0 and oxytocin binding on day 6 r
2
= 0.71; P 0.001, higher
oestradiol on day 0 being associated with lower oxytocin binding on day 6. 3.2. Study 2
Plasma concentrations of oestradiol generated by the three treatment regimens were similar to those achieved in Study 1 with concentrations showing a significant rise
Fig. 3. Mean ± S.E.M. plasma concentrations of oestradiol open symbols and progesterone closed symbols generated in ovariectomised cows treated with high
s
; n = 4, medium
h
; n = 4 or low △; n = 4 levels of oestradiol administered by i.m. injection in corn oil at 8 h intervals indicated by ▽ and a standard level of
progesterone
d
; n = 12 cows combined administered by i.m. injection in corn oil at 12 intervals indicated by
.
.
G.E. Mann, G.E. Lamming Animal Reproduction Science 64 2000 171–180 177
Fig. 4. Mean ± S.E.M. plasma concentrations of PGFM in ovariectomised cows treated with high
s
; n = 4, medium
h
; n = 4 or low △; n = 4 levels of oestradiol and administered a 50 i.u. OT challenge at time 0 indicated by arrow.
P 0.01 rising in all groups to significantly different concentrations P 0.001 Fig. 3. Treatment with progesterone resulted in a similar steady increase P 0.001
in the plasma concentration of the hormone in the three groups to similar concentrations Fig. 3. Treatment with oxytocin on day 6 resulted in significantly P 0.01 different
increases in the mean plasma PGFM concentration during the hour following oxytocin between groups with the high, medium and low oestradiol groups exhibiting 10.0 ± 2.2,
21.3 ± 4.3 and 41.3 ± 1.2 pg ml
− 1
increases, respectively Fig. 4. Furthermore, when the three groups were combined there was a significant correlation between oestradiol con-
centration on day 0 and the magnitude of the oxytocin challenge response expressed as a percentage increase over basal r
2
= 0.63; P 0.01; n = 9, higher oestradiol being
associated with lower PGF
2
a
secretion.
4. Discussion
