Relationship of Liver and Skeletal Muscle IGF-1 mRNA to Plasma GH Profile, Production of IGF-1 by Liver, Plasma IGF-1 Concentrations, and Growth Rates of Cattle

Growth hormone (GH), insulin-like growth factor-1 (IGF-1), and thyroid hormone (T3 and T4) concentrations in blood plasma of 18 crossbred cattle (six bulls, six steers, and six heifers) were measured over an 8-hr period. One week later at slaughter, IGF-1 production by liver slices and IGF-1 mRNA concentrations in skeletal muscle and liver were measured. Bulls had higher (P < 0.05) mean plasma GH and GH peak amplitudes (P < 0.01) than heifers, and values for steers were intermediate between bulls and heifers. Baseline GH concentrations and number of GH peaks were not significantly different for the three groups. Bulls had 1.6-fold (P < 0.01) and 3.0-fold (P < 0.01) greater liver IGF-1 mRNA concentrations than steers or heifers, respectively, whereas the steers had 1.8-fold (P < 0.05) greater IGF-1 mRNA in liver than heifers. Production of IGF-1 by liver slices was greater (P < 0.05) in bulls than steers or heifers. Bulls had 1.3-fold greater plasma IGF-1 than steers (P < 0.01), whereas steers had 1.8-fold greater plasma IGF-1 than heifers (P < 0.01). There were no significant differences in concentrations of skeletal muscle IGF-1 mRNA between the three groups of animals. Liver IGF-1 mRNA, liver IGF-1 production, and plasma IGF-1 were all significantly correlated with gain and mean GH peak amplitude, but not with GH baseline, GH peak frequency, or concentrations of T3 and T4. Concentrations of IGF-1 mRNA in skeletal muscle were not correlated to gain or any parameter of the GH profile. Plasma concentrations of T3 were significantly (P < 0.05) negatively correlated to plasma GH baseline concentrations. Muscle IGF-1 mRNA concentration was negatively related to plasma T4 and T3. The results of this study suggest that the cascade of events starting with secretion of GH from the pituitary, expression of liver IGF-1 mRNA, and secretion of IGF-1 by the liver are important phenomena for growth of cattle.