Effects of temperature on hypophyseal (growth hormone) regulation of length, weight, and allometric growth and total lipid and water concentrations in the black bullhead ( Ictalurus melas)

Abstract

The effects of temperature on the somatotropic function of the pituitary gland were studied in the bullhead ( Ictalurus melas) by comparing the 16-day length, weight, and allometric growth responses at 12, 18, 24, and 29° of hypophysectomized and sham-hypophysectomized fish versus those of hypophysectomized fish injected with beef growth hormone (bGH). In an experiment beginning 6 days after surgery, hypophysectomized, saline-injected bullheads gained small but significant length increments at 18°, but not 12, 24, or 29°: mean total weights increased 9 to 18%, regardless of temperature. Beef GH (1.0 μg/g body weight, every second day) stimulated length increases of 4.93, 7.13, and 6.51% at 18, 24, and 29°, respectively: weight gains of 49 to 53% were made at all three temperatures. Under similar conditions, sham-hypophysectomized, saline-injected fish added length increments of 3.92, 5.64, 3.42% and weight increments of 24 to 28%. Sham-operated bullheads grew little at 12°; GH-treated animals gained weight (20%) but not length. After 16 days of treatment at a given temperature, relative morphometry and total lipid and water concentrations were generally the same in hypophysectomized and sham-operated control groups. Growth hormone, especially at 18°, caused splanchnomegaly of the nonhepatic viscera and stimulated increases in ponderal condition, relative somatic muscle and skeletal mass, and morphometric indexes of body depth and width. In the same animals, tissue water concentrations, measured only at 18 and 24°, were increased: total lipids were decreased at 18° but not 24°. Growth hormone at 12, 18, and 24° slowed the allometric enlargement of the liver caused by hypophysectomy. Arguments are made that temperature may influence rates of pituitary growth-regulating hormone synthesis, secretion, and turnover, as well as alter levels of tissue responsiveness, and that the growth rates or patterns of individual tissues, organs, and body parts may be differentially influenced by variations in their thermalhormonal environment.