Clonidine or xylazine as provocative tests for growth hormone secretion in the dog.

Base-line values of plasma growth hormone (GH) are low in most species, requiring provocative tests to assess GH deficiency. Clonidine, an antihypertensive drug, and its analogue, xylazine, a sedative hypnotic, were found to stimulate GH secretion. Administration (IV) of clonidine to conscious healthy, dogs at doses of 30, 16.5, and 3 microgram/kg produced significant increases in plasma GH by 15 minutes and the effects subsided by 120 minutes. Plasma glucose concentration increased slowly with all doses, but less so and for shorter duration in dogs given the 3 microgram/kg dose. Xylazine increased plasma GH when injected at doses of 300 and 100 microgram/kg, but not at 30 microgram/kg. Plasma glucose increased only with the 300 microgram/kg dose. The alpha-adrenergic blocker, phentolamine, markedly attenuated these responses. Thus, both clonidine and xylazine, when used at appropriate doses, can stimulate GH secretion, with minimal effect on plasma glucose and without causing significant sedation.