Effects of hypophysectomy and beef growth hormone replacement therapy on morphometric and biochemical indicators of growth in the fed versus starved black bullhead ( Ictalurus melas)

Abstract

The relationship between food availability and the somatotropic function of the pituitary gland was studied in the bullhead ( Ictalurus melas) at 24° by comparing the effects of hypophysectomy and beef growth hormone (bGH) replacement therapy in fed versus starved fish. In an experiment lasting 14 days, hypophysectomy arrested the length and weight growth of fed bullheads by greatly depressing food consumption and lowering the efficiency of food conversion. Hypophysectomized fish injected with bGH (1.0 μ/g body wt, every second day) consumed about the same amount of food as sham-operated animals, but the former gained 27 and 89% more length and weight, respectively, owing to a 106% increase in the efficiency of food conversion. In starved bullheads, bGH replacement therapy offset the loss of length, weight, and ponderal condition by 60% or more compared to hypophysectomized controls; size losses in sham-operated, saline-injected animals were similar to those in hypophysectomized fish. Regardless of endocrine manipulation, the degree to which starvation altered the relative weights of various body parts was as follows: liver > intestine ⪢ stomach > eviscerated carcass. In both fed and starved bullheads, bGH replacement therapy stimulated an increase in the size of the eviscerated carcass relative to body length, but a decrease in size relative to total body weight. There was a highly positive correlation between total RNA concentration in muscle and changes observed in body size. Compared to RNA, the concentration of DNA in muscle remained relatively constant. Arguments are made that the bullhead pituitary gland plays, at best, a minor role in conserving body size and protein reserves during prolonged periods of food deprivation; that growth in young bullheads is primarily a function of cell replication; and that GH and variations in feeding level influence overall protein metabolism, in part, by altering the RNA content, and thus the protein synthetic capacity, of muscle tissue.