Effects of pituitary hormones, melatonin, and thyroidal inhibitors on radioiodide uptake by the thyroid glands of larval and adult tiger salamanders, Ambystoma tigrinum (Amphibia: Caudata)

Abstract

Thyroidal uptake of radioiodide 24 hr after intraperitoneal injection of 131I was used as a measure of thyroidal function in the tiger salamander ( Ambystoma tigrinum). Larvae pretreated for 7 days with 2.5 or 25 μg TSH/day exhibited significantly greater 131I uptake than did untreated animals. Spontaneously metamorphosing animals accumulated 131I equivalent to larvae pretreated with 2.5 μg TSH. Fully metamorphosed animals were insensitive to pretreatment with 2.5 μg TSH/day, but did respond to 25 μg TSH/day. PTU or SCN pretreatment of larvae reduced TSH-stimulated 131I uptake to control levels, but prolactin, melatonin, and bovine pineal extract had no effect. Prolactin treatment of larvae (7 or 60 days) did not influence 131I uptake as compared to controls. Thyroidal 131I uptake in metamorphosed animals was not influenced by 7 days of treatment with prolactin. Daily pretreatment (7 days) with 25 μg LH or FSH or 0.25 μg TSH did not increase 131I accumulation by larvae. Mammalian prolactin and gonadotropins apparently are not heterothyrotropic factors in this species. The stimulation of metamorphosis by mammalian gonadotropins (Norris et al., 1973b) can be attributed to the amount of mammalian TSH present in these preparations.