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Plasma GH profiles and circulating concentrations of plasma insulin-like growth factors-I and -II (IGF-I and -II) were examined in 20 steers on either high (3% dry matter of body weight per day) or low (1% dry matter of body weight per day) planes of nutrition with or without an implant of oestradiol-17β. The response of plasma IGF-I and -II to a bolus injection of bovine GH (bGH) was also investigated.
Reduced feeding significantly (P <0·01) increased the mean concentration, peak height and integrated area of plasma GH. Treatment of steers with oestradiol at low nutrition significantly increased baseline GH concentrations. Treatment of steers with oestradiol at high nutrition significantly (P <0·05) increased mean, baseline, peak height, and integrated area of plasma GH. GH pulse frequency was not changed by either nutritional plane or oestradiol treatment.
Basal concentrations of plasma IGF-I were significantly (P <0·01) decreased by reduced feeding in both the oestradiol-treated and the control group. Treatment with oestradiol increased (P <0·01) basal plasma concentrations of IGF-I at both high and low levels of nutrition. After i.v. injection of bGH (0·1 mg/kg body weight), an increase in plasma IGF-I was observed only in steers at high nutrition. Basal concentrations of plasma IGF-II were not altered by nutritional manipulations but were significantly (P <0·001) increased by oestradiol treatment. After bGH infusion only steers at high nutrition showed an increase in plasma IGF-II. Significant correlations were observed between daily body weight gain and plasma concentrations of IGF-I (r= 0·91, P<0·001, n = 20) and also between the capacity of the high-affinity hepatic somatotrophic receptor and plasma IGF-I (r= 0·89, P <0·001, n= 10).
Decreased plasma concentrations of IGF-I at a low level of nutrition may abolish the growth-promoting activity of circulating GH. The increase in both GH secretion and the number of somatotrophic receptors with oestradiol treatment may represent a coordinated response of the somatotrophic axis leading to enhanced IGF-I and -II production and improved growth rate. The inferential relationships between the capacity of the high-affinity somatotrophic receptor, plasma concentrations of IGF-I and growth rates suggest that active modulation of somatotrophic receptors is an important regulatory constituent of the somatotrophic axis.
J. Endocr. (1988) 118, 243–250
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