Comparison of metabolic and reproductive endocrine functions and body physical growth between north and south climates of Japan
4.4 Discussion
In natural condition, the comparison between Hokkaido and Miyazaki horses on body growth, metabolic and reproductive hormonal changes revealed that Hokkaido colts and fillies were inferior to the Miyazaki yearling horses. In the present study, all growth parameters (body weight, height, girth and cannon circumferences) in Hokkaido colts and fillies tended to be lower than Miyazaki horses consistent with previous research reporting that growth rate was higher in both Miyazaki colts and fillies compared to Hokkaido yearlings (Mizukami et al. 2015). Interestingly, all growth increments declined dramatically in Hokkaido horses in January followed by the increasing in February to March eventually, whereas the decline of those increments in Miyazaki horses were not notable during January.
This remarkable point indicated that Hokkaido in January, the coldest month possessing lowest minimal temperature was hard time and resulted in slow growth rate in Hokkaido horses. After acclimatizing to severe cold, the growth rate returned to increase during February to March. Contrasting to Miyazaki horses, their growth rate kept increasing and reached maximal rate in January. Then, growth rate tended to slow down to be lesser in February to March. These suggested that different weather conditions led to responding to climate for growth in different way. For hair coat condition, both Hokkaido colts and fillies had hair coat scores lower than that in Miyazaki significantly, being consistent with the lower levels of prolactin in Hokkaido colts and fillies when compared to the Miyazaki. These results were in accordance with previous study that prolactin had responsibility in hair shedding in horses (Nambo et al. 2010; Kunii et al. 2015).
Regarding total T4 in the present study, thyroxine which is synthesized and secreted by thyroid gland plays the key roles in regulating of body temperature, metabolism and growth (Bird et al. 1998; Breuhaus et al. 2006;Chen and Riley 1981). T4 levels in Hokkaido
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colts and fillies tended to be higher than in Miyazaki throughout the periods under natural light. Our results indicated that horses raised in the colder north had higher basal level of thyroid hormone than horses dwelled in the milder south. Nevertheless, the higher level of T4 in Hokkaido horses did not conform to the growth profile. It can be suggested that Hokkaido horses adapted in response to lower ambient temperature for maintenance of body homeostasis in long term survival instead of growing. Moreover, the current study did not find significant differences in T4 levels among periods from winter to early spring however little fluctuations of T4 levels were shown accordantly to other findings (Fazio et al.
2012; Irvine 1967; Johnson 1986; McBride et al. 1985). These may suggest that chronic cold exposure during seasonal change did not affect T4 concentration due to capability in acclimatization of horses. Several studies about thermoregulation of horse in winter weather clarified that T4 secretion was transiently elevated during acute or short cold exposure in adult horses (Irvine, 1967; McBride et al. 1985). For chronic cold weather, serum thyroid hormone and metabolic rate did not change significantly (Mejdell and Bøe 2005). According to the limitation of cold resistance in horses, lower critical temperature (LCT), the temperature which metabolic heat production needs to increase for body core temperature maintenance (Curtis 1983) was estimated to be -11qC in yearling horses with ad lib fed (Cymbaluk 1994; Cymbaluk and Christison 1989a) while it was around 0°C in limit-fed for normal growth in yearlings (Cymbaluk 1994; Cymbaluk 1990). Typically, horses need 10-21 days to acclimatize to cold weather and require another 10-21 days for more diminished temperature (Cymbaluk 1994). When reaching LCT, physiological, metabolic and behavioral changes would occur to reduce heat loss and conserve energy (Cymbaluk 1994). Generally, in other species for long term exposure to low temperature, thyroid hormone levels were increased resulting in increasing of metabolism and heat production in sows (Anderson
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2000), sheep (Sano et al. 199) and cattle (Christopherson et al. 1979) contrary to horses employing a state of hypometabolism to keep body warm (Brinkmamm et al. 2012, 2016;
McBride et al. 1985).
In the present study, circulating IGF-1 concentrations in Hokkaido colts were significantly lower than Miyazaki colts consistent to previous study (Mizukami et al. 2015).
IGF-1 was mainly secreted by liver and induced by growth hormone from pituitary gland. It plays a role in contribution for body growth in animals including the period of embryonic development (Doherty et al. 1994; Herrler et al. 1998; Fabian et al. 2004; Wang et al. 2009).
Moreover, IGF-1 was involved in steroidogenesis in Leydig cells of various species (Lin et al.
1986a,b; Kasson and Hsueh 1987; Gelber et al. 1992; Wang and Hardy 2004; Bernier et al.
1986; Perrard-Sapori et al. 1987), and promoted follicular growth and enhanced ovarian activity in mares (Derar et al. 2005; Hammond et al. 1991, Derar et al. 2011). According to the reproductive endocrine function, the levels of progesterone in fillies and testosterone in colts of both Hokkaido and Miyazaki were low from December and then increased to reach over 1 ng/ml at early April and 0.5 ng/ml around late February, respectively. These showed that young horses seemed to have first ovarian and testicular activities in early April for fillies and late February for colts, respectively. Furthermore, circulating estradiol-17E concentration in the present study was similar to previous report (Mizukami et al. 2015) showing that the levels of estradiol-17E were lower in Hokkaido fillies compared to Miyazaki.
The current study found that the cortisol levels were not different between Hokkaido and Miyazaki horses significantly however Hokkaido horses tended to have the cortisol levels higher than Miyazaki in February and April. Previous research reported that prolactin stimulated ACTH production and cortisol secretion in rat. Some study noted that cortisol was accordant to prolactin secretion. In contrast, our study did not found any relation between
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cortisol and prolactin levels. The present study could suggest that the higher cortisol levels in Hokkaido horses presumably caused from exercising program rather than the cold stress. In February, Hokkaido horses were trained with 800 m flat and slope courses while Miyazaki used only 1600 m flat track. For April, cover snow had melted so that the training program in Hokkaido horses changed to using the 1600 m flat and slope courses whereas Miyazaki still used 1600 m flat track. This indicated that exercising program in Hokkaido seemed to be harder than in Miyazaki.
From all of study results, it was clarified that body growth and development of gonadal function was slower in Hokkaido yearlings compared to Miyazaki colts and fillies.
According to the weather reports from the Japan Meteorological Agency from 2012 to 2014 as the years of this experiment, the mean daily minimal temperature in Hidaka area of Hokkaido was lower than Miyazaki throughout the years (9-10qC approximately), especially in winter season around -9qC in Hokkaido whereas 2-3qC in Miyazaki. In addition, during winter period the daylight hour was about one hour shorter in Hokkaido than in Miyazaki.
These climate distinction in accordance with our results suggested that the differences in climates between the north and south of Japan would affect growth and early reproductive development in young horses.
In conclusion, Hokkaido yearlings seemed to have inferiority of growth and reproductive function to the Miyazaki yearlings under natural condition.
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Fig. 4.4 Means of (A) body weight (BW), (B) height (HT), (C) girth (GC) and (D) cannon bone (CC) circumferences compared between Hokkaido ( ) and Miyazaki ( ) in colts (a) and fillies (b) under natural condition from October to March. Each value is expressed as the mean ± SEM. * Denoted the significant differences between different groups in each period and sex (P<0.05).
400 440 480 520
O N D J F M
400 440 480 520
O N D J F M
4 4
BW (kg)
A
52B
a b
150 155 160 165
O N D J F M
150 155 160 165
O N D J F M
a b
C
170 175 180 185
O N D J F M
HT (cm)GC (cm)CC (cm)
18 19 20 21
O N D J F M 18
19 20 21
O N D J F M
D
2 a ba b
0 0
0 0
170 175 180 185
O N D J F M
0 0
0 0
*
* * * * *
a
Hokkaido Miyazaki
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Fig. 4.5 Increment percent (A) body weight (BW), (B) height (HT), (C) girth (GC) and (D) cannon bone (CC) circumferences compared between Hokkaido (____) and Miyazaki (---) in colts (a) and fillies (b) under natural condition from November to March. Each value is expressed as the mean ± SEM.
A
B
C
D
-1 0 1 2 3 4
N D J F M
-1 0 1 2 3 4
N D J F M
0 0.5 1 1.5
N D J F M
0 0.5 1 1.5
N D J F M
-0.4 0 0.4 0.8 1.2 1.6 2
N D J F M
-0.4 0 0.4 0.8 1.2 1.6 2
N D J F M
-1 -0.5 0 0.5 1 1.5 2 2.5
N D J F M
-1 -0.5 0 0.5 1 1.5 2 2.5
N D J F M
a b
a b
a b
a b
Hokkaido Miyazaki
%BW increment
%HT increment 0
0 0 1 1
%GC increment
0 1
%CC increment
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Fig. 4.6 Comparison of hair coat scores between Hokkaido ( ) and Miyazaki ( ) in colts (a) and fillies (b) at November, January and April under natural condition. Each value is expressed as the mean ± SEM. * Denoted the significant differences between groups in each period and sex (P<0.05).
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Nov Jan Apr
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Nov Jan Apr
a b
Hair coat score
* *
Hokkaido Miyazaki
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Fig. 4.7 Comparison of circulating T4 (A), IGF-1 (B), PRL (C) and Cortisol (D) concentrations (ng/ml) between Hokkaido (____) and Miyazaki (---) in both colts (a) and fillies (b) under natural condition from December to April. Values are expressed as mean ± SEM. * Denotes the significant differences at P<0.05 between different groups in each period and sex.
A
B
C
D
T4 (ng/ml)IGF-1 (ng/ml)PRL (ng/ml)Cortisol (ng/ml)
0 100 200 300
D J F M A
0 100 200 300
D J F M A
100 200 300 400
100 200 300 400
D J F M A
0.0 0.5 1.0 1.5 2.0
0.0 0.5 1.0 1.5 2.0
0 20 40 60
0 20 40 60
*
0 0
* * *
*
*
* *
D J F M A D J F M A
a b
a b
a b
a b
Hokkaido Miyazaki
D J F M A
D J F M A D J F M A
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Fig. 4.8 Comparison of circulating estradiol-17E (A) and progesterone (B) in fillies,
testosterone (C) in colts and ir-inhibin (D) in both colts (a) and fillies (b) concentrations between Hokkaido (____) and Miyazaki (---) under natural condition from December to April.
Values are expressed as mean ± SEM. * Denotes the significant differences at P<0.05 between different groups in each period and sex.
A B
C
D
Estradiol (pg/ml)Inhibin (ng/ml) 0 10 20
30
*
* * *
Progesterone (ng/ml)
D J F M A
0.0 1.0 2.0 3.0 4.0
0.0 1.0 2.0 3.0 4.0
D J F M A D J F M A
a b
0 2 4 6 8 10
0.0 0.5 1.0 1.5
Hokkaido Miyazaki
D J F M A D J F M A
Testosterone (ng/ml)
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