Figure 4.

Time-evolving ACTH secretory dynamics modeled via individual and combined CRH and AVP feedforward under noncompetitive cortisol (Cort) feedback. A, Data from one horse administered metyrapone. B, Data from another horse subjected to hypoglycemia. Because cortisol concentrations change much more slowly than CRH or AVP secretion (Sec) rates, overall time was divided into successive 40-min windows each advanced by a 10-min increment (left to right and top to bottom). Data contained in each window were used to estimate CRH and AVP drive of ACTH secretion through logistical dose-response functions. Therefore, consecutive dose-response surfaces reflect the effects of time and changing concentrations of cortisol, as stated above each window.