Effeects of hypophysectomy and beef growth hormone replacement therapy, over time and at various dosages, on body weight and total RNA-DNA levels in the black bullhead ( Ictalurus melas)

Abstract

Experiments were conducted on the bullhead ( Ictalurus melas) at 24°: first, to define the effects of hypophysectomy and beef growth hormone (bGH) replacement therapy, over time, on body weight and the concentrations and ratios of RNA and DNA in skeletal muscle and liver; second, to evaluate tissue RNA concentrations and RNA-DNA ratios as biochemical indicators of growth; and third, to assess the effect of various bTH doses on the hypophysectomized bullhead. Hypophysectomy arrested growth in weight by the 12th day of a 26-day experiment. At the same time, the concentration of RNA and the RNA DNA ratio in muscle were reduced relative to the levels seen in sham-operated controls. In the latter, muscle RNA concentration, after an initial increase, remained constant throughout the experiment, whereas muscle RNA DNA rose progressively, due to a decline in the concentration of DNA. The livers of hypophysectomized bullheads had lower RNA and RNA DNA levels than the livers of controls by the fourth day after surgery. In animals that had been hypophysectomized for 2 weeks, bGH (at 40 mIU/fish, every second day) stimulated significant increases in body weight and hepatic RNA concentration within 4 days, after only two injections. Muscle RNA and RNA DNA levels were also elevated by bGH injection, but the effects were not statistically meaningful until the sixth day of treatment. After 8 days, linear relationships were demonstrated for percentage gain in body weight and muscle RNA concentration, each plotted against the logarithm of bGH dose (range tested, 5.7 to 320 mIU/fish, every second day). Collectively, the data indicate that GH has a controlling influence over nucleic acid metabolism in the bullhead; that hypophysectomy and GH replacement therapy can exert their effects rather quickly in fish; and that biochemical as well as morphometric responses can probably be used in assaying for teleost GHs.