This study was performed to investigate the effects of recombinant bovine somatotropin (rBST) administration on residual BST and insulin-like growth factor-I (IGF-I) levels in various tissues. Two experiments, (A) and (B) were conducted as follows. (A) Beef cattle (average b.w. of 450kg) were divided into three groups: control (CONT), low-dose (LOW) and high-dose (HIGH) groups. The CONT group received neither rBST nor vehicle. The LOW and HIGH groups were treated for 20 weeks by s.c. injection

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... ith 250 mg rBST at one-week intervals and 500mg rBST at two-week intervals, respectively. (B) Beef cattle were divided into four groups: control (CONT) group, as described in experiment (A), and sustained-release, low-dose (SR-L), sustained-release, medium-dose (SR-M) and sustained-release, high-dose (SR-H) groups. SR-L, SR-M and SR-H were treated at two-week intervals for 24 weeks by s.c. injection with doses of 0.42mg rBST/kg b.w. (0.03mg rBST/kg b.w./day), 0.84mg rBST/kg b.w. (0.06 mg rBST/kg b.w./day) and 1.26mg rBST/kg b.w. (0.09mg rBST/kg b.w./day), respectively. In both experiments, animals were killed two weeks after the final treatment. In experiment (A), residual BST and IGF-I levels in muscle tissues of the LOW and HIGH groups were not affected by rBST administration. In experiment (B), residual BST and IGF-I levels in muscle, fat, liver and kidney tissues of treated groups were not different from that of the CONT group, either. These results suggest that levels of BST and IGF-I in various tissues are not increased at two weeks after rBST administration.