AJP - Endocrinology and Metabolism, Vol 264, Issue 2 E208-E214, Copyright © 1993 by American Physiological Society
Activation of central alpha 1-adrenoceptors in humans stimulates secretion of prolactin and TSH, as well as ACTH
S. al-Damluji and D. Francis
Department of Endocrinology, St. Bartholomew's Hospital, London, United Kingdom.
In normal male volunteers, intravenous infusions of the alpha 1-adrenergic
agonist methoxamine stimulated the secretion of prolactin,
thyroid-stimulating hormone (TSH), and adrenocorticotropic hormone (ACTH),
and the effects were abolished by pretreatment with the alpha 1-antagonist
prazosin. To investigate the site of action of methoxamine, its effects
were compared with those of equipotent doses of norepinephrine, an alpha
1-agonist that reaches the pituitary gland and the median eminence after an
intravenous infusion but, unlike methoxamine, does not cross the
blood-brain barrier. Norepinephrine did not stimulate secretion of
prolactin, TSH, or ACTH, suggesting that the stimulant alpha
1-adrenoceptors are located in the central nervous system and not directly
on the pituitary gland or in the periphery. The alpha 2- and
beta-adrenoceptor agonist properties of norepinephrine could not account
for the differences from methoxamine, as pretreatment with prazosin did not
modify hormone concentrations after norepinephrine. Methoxamine had no
behavioral stimulant effects, as judged by visual analog scales that were
sensitive to physiological changes in behavioral arousal. In four patients
with hypothalamic dysfunction but responsive pituitary corticotrophs,
methoxamine had no stimulant effect on the secretion of ACTH, confirming
that the alpha 1-adrenoceptors that stimulate ACTH secretion are not
located directly on the pituitary. None of the drugs had an effect on the
secretion of growth hormone or the gonadotrophins.
