GENERALANDCOMPARATIVEENDOCRINOLOGY~~, 493-497(1984) Stimulation of Growth Hormone Secretion in Dwarf Chickens by Thyrotrophin-Releasing Hormone (TRH) or Human Pancreatic Growth-Hormone-Releasing Factor (hpGRF) S. HARVEY,~ C. G. SCANES, AND J. A. MARSH* Department of Animal Sciences, Rutgers, The State University, New Brunswick, New Jersey 08903, and *Department of Poultry and Avian Sciences, Cornell University, Ithaca, New York, 14853 Accepted November 29, 1983 The basal plasma growth hormone (GH) level in adult sex-linked dwarf hens was elevated in comparison with autosomal dwarf hens and with control (Cornell K strain) laying hens. The iv administration of thyrotrophin-releasing hormone (TRH) (10 ug/kg) had no effect on GH secretion in control hens but slightly (1.2-fold) and transiently (for 10 min) increased the GH level in the autosomal dwarfs and greatly (8.7-fold) increased the GH level in the sex-linked dwarfs, in which it remained elevated for at least 30 min after injection. The iv administration of human pancreatic GH-releasing factor (hpGRF) (10 kg/kg) stimulated GH release in each strain. The response in the sex-linked dwarfs was greater than that in the autosomal dwarfs and the control hens but less than that elicited by TRH. These results suggest that the increased basal GH level in the sex-linked dwarfs results from an increased responsiveness to provocative stimulation. Dwarf strains of chickens are potentially valuable models for studying the physi- ology of growth and the interrelationships between the hormones involved in the con- trol of growth. Two distinct dwarf strains of chickens have been described: a sex- linked dwarf (Hutt, 1959) and an autosomal dwarf (Cole, 1969). These dwarfs are thy- roid-deficient, having low levels of plasma triiodothyronine (Ts) and, in the case of the autosomal dwarfs, low thyroxine (T4) levels @canes et al., 1983b). Although human dwarfism can result from hypopituitary function and impaired growth hormone (GH) secretion (Daughaday, 1972; Daugh- aday et al., 1975) the basal plasma GH level in these dwarfs is paradoxically elevated, especially in the sex-linked dwarfs @canes et al., 1983b). The stimulation of GH secretion in the autosomal and sex-linked dwarf chickens may result from their thyroid deficiency, since the basal rate of GH secretion is ele- ’ Permanent address: Wolfson Institute, University of Hull, Hull HU6 7RX, United Kingdom. vated in hypothyroid chicks @canes et al., 1976; Chiasson et al., 1969; Harvey et al., 1983) and depressed by the administration of T, or T, (Harvey, 1983). Thyroidecto- mized birds are similarly more responsive to GH secretagogues than intact birds (Harvey et al., 1983) and exogenous T, or T, blunts the in viva GH response to pro- vocative stimuli (Harvey, 1983). Thyrotrophin-releasing hormone (TRH) stimulates GH secretion in immature chickens (Harvey et al., 1981) and although the adult pituitary remains responsive to TRH stimulation in vitro (Harvey et al., 1978), adult birds are insensitive to in vivo TRH treatment (Harvey et al., 1981; Scanes et al., 1981). Preliminary data how- ever (Harvey et al., 1983) suggest that the stimulatory effect of TRH on in vivo GH secretion can be observed in adults, if they are thyroidectomized neonatally. The pos- sibility that adult thyroid-deficient dwarf chickens may be more responsive to GH- releasing stimuli than euthyroid birds has therefore been investigated in the present 493 0016-6480184 $1.50 Copyright 0 1984 by Academic Press. Inc. All rights of reproduction in any form reserved.