AJP - Regulatory, Integrative and Comparative Physiology, Vol 254, Issue 1 23-R26, Copyright © 1988 by American Physiological Society
Control of canine ACTH by corticosteroids: interaction between dose and time
M. Keller-Wood
Department of Physiology, University of Florida College of Medicine, Gainesville 32606.
These experiments were designed to test for interactions between dose and
time of administration of steroids in control of adrenocorticotropin
hormone (ACTH). Five dogs were infused for 30 min with cortisol and
corticosterone in a 2:1 ratio at a total rate of 1.5 or 3.0
micrograms.kg-1.min-1 beginning 120, 90, 60, 30, or 0 min before injection
of insulin (0.5 U/kg). The infusions increased total plasma corticosteroids
to approximately 50 or 80 ng/ml. Inhibition of basal ACTH occurred when
infusion of steroid at either rate began greater than or equal to 60 min
earlier. The degree of inhibition of stimulated ACTH was dependent on both
steroid dose and timing of steroid infusion relative to the stimulus.
Increasing corticosteroids to 50 ng/ml reduced the ACTH response to
hypoglycemia if the infusion began 30 min, but not 60, 90, or 120 min
earlier. Increasing corticosteroids to 80 ng/ml inhibited the response if
the infusion began 120 or 60 min before injection of insulin. The data
suggest that 1) inhibition of stimulated ACTH required less than or equal
to 30 min to appear; 2) the duration of corticosteroid feedback is
steroid-dose related; 3) basal ACTH secretion is more sensitive than
stimulated ACTH to steroid inhibition; 4) small increments in plasma
corticosteroids over short periods of time can inhibit canine ACTH.
