Pituitary, thyroid and adrenal responses to clonidine in dairy cattle.

Clonidine, 2-(2,6-dichlorophenylamine)-2-imidazoline hydrochloride is an alpha - adrenoceptor stimulant. This drug was used to help define the neuroendocrine control of pituitary, thyroid and adrenal gland hormone release in cattle. Clonidine significantly increased radioimmunoassayable concentrations of serum GH, when infused iv into Holstein heifers at a dose of 20 microgram/kg body weight. Serum concentrations of GH were elevated within 5 min after drug infusion. A peak in GH was reached in sera 10 min after drug infusion. A pronounced and significant elevation in serum PRL concentrations were observed 45 min after 2 microgram/kg clonidine. PRL remained elevated until reaching baseline concentrations 150 min after infusion. Serum concentrations of thyroid hormones were not affected by the drug. Clonidine elevated serum concentrations of cortisol; peak concentrations occurring from 45 to 90 min after drug infusion. Clonidine had no significant effect on circulating concentrations of LH. These data suggest that secretion of GH, but not that of LH nor TSH, in cattle may be modulated by noradrenergic receptors.