AJP - Regulatory, Integrative and Comparative Physiology, Vol 255, Issue 2 344-R349, Copyright © 1988 by American Physiological Society
Evidence for rapid inhibition of ACTH by corticosteroids in dogs
M. Keller-Wood and M. E. Bell
Department of Physiology, University of Florida College of Medicine, Gainesville 32610.
These experiments were designed to determine whether stimulated
adrenocorticotropic hormone (ACTH) secretion in dogs can be rapidly
inhibited by increases in plasma corticosteroid concentrations. Five dogs
were injected with a 2:1 mixture of cortisol to corticosterone
(corticosteroids; total doses of 22.5, 45, or 90 micrograms/kg) or vehicle
simultaneously with the injection of insulin (0.5 U/kg). These dogs were
also injected with corticosteroids (45 micrograms/kg) with ovine
corticotropin-releasing factor (oCRF-41; 1 micrograms/kg) or with the same
dose of corticosteroids alone. Plasma ACTH and corticosteroid
concentrations were measured for 90 min after the injections. The
inhibition of ACTH secretion was significant 10 min after injection of
oCRF-41 and 40 min after injection of insulin. The first significant
increase in ACTH during insulin-induced hypoglycemia does not occur until
30 min, however. Therefore, after both of these stimuli to ACTH, the ACTH
response is inhibited within approximately 10 min of its onset. The results
suggest that canine ACTH responses to stimuli can be rapidly inhibited and
that one site of this inhibition is the pituitary.
