Submitted to: Growth Hormone-Insulin Like Growth Factor-I Society Conference
Publication Type: Abstract Only
Publication Acceptance Date: 5/12/2002
Publication Date: 9/5/2002
Citation: Simpson, P.R., Peterson, B.C., Cain, K.D., Hardy, R.H., Overturf, K., Ott, T.L. 2002. Physiological effects of recombinant bovine somatotropin (rbst)in rainbow trout (oncorhynchus mykiss). Growth Hormone-Insulin Like Growth Factor-I Society Conference. Boston, MA. p. 234. Interpretive Summary:
Technical Abstract: Many fish species, including rainbow trout (Oncorhynchus mykiss), exhibit indeterminate growth, with both hypertrophic and hyperplastic muscle growth throughout their lives. Recombinant bovine somatotropin (rbST) treatment increases growth rates of rainbow trout 100-200%, and previous work with rbST treatment showed increased hyperplastic muscle growth. Therefore, an experiment was conducted to examine the growth-promoting effects of rbST in rainbow trout. Sixty-six rainbow trout (mean weight 550 +/- 10 g) received either an intraperitoneal injection of rbST suspended in sesame oil, commercially available as Posilac (Monsanto Agriculture; St. Louis, MO), at 120 mg/g body weight (treatment, T) or sesame oil at the same volume (control, C). Fish received an injection every 21 days beginning on day 0. Blood and tissue samples were collected on days 0, .5, 1, 3, 7, and 28 (n = 12/day) and assayed for changes in circulating hormones and steady-state gene expression levels. Rainbow trout somatotropin (rtST), recombinant bovine somatotropin, and insulin-like growth factor-I in circulation were assayed by RIA. Steady-state gene expression levels were examined using quantitative real-time reverse-transcription polymerase chain reaction (QRT RT-PCR) and Western blot assays. As expected, rbST levels increased (P < 0.05) in circulation 12 hours after treatment (8500 ng/ml) and continued to increase (P < 0.05) to 10400, 14000, and 6900 ng/ml at 1, 3, and 7 days after treatment, respectively. Recombinant bST treatment increased serum IGF-I levels (P = 0.006) one week after treatment (7 d), while endogenous somatotropin was unaffected over time (P = 0.287). Steady-state levels of muscle myosin mRNA were unaffected (P = 0.57) by rbST treatment, while steady-state liver IGF-I mRNA levels increased (P < 0.05). Liver IGF-I mRNA increased (P = 0.004) 1 day post-treatment and remained elevated throughout the trial (day 28). Steady-state muscle IGF-I mRNA levels were unaffected (P > 0.05) by rbST treatment, which is in contrast to previous work showing increased hyperplastic growth in fish species. These results suggest an apparent reduction in the endogenous negative control loop within the GH-IGF-I axis, as rtST was unaffected by increased circulating rbST and IGF-I, in contrast to an expected down-regulation of endogenous somatotropin. It is also suggested that rbST acts on muscle growth via endocrine pathways, as local IGF-I mRNA was unaffected in muscle tissue but circulating IGF-I was increased. Since most fish species, including rainbow trout, exhibit extra-pituitary production of somatotropin, it is now speculated that local ST production may be involved in muscle growth via autocrine and paracrine pathways.