Daily endogenous cortisol production and hydrocortisone pharmacokinetics in adult horses and neonatal foals.

To compare daily endogenous cortisol production rate and the pharmacokinetics of an i.v. bolus of hydrocortisone between neonatal foals and adult horses. 10 healthy full-term 2- to 4-day-old foals and 7 healthy adult horses. Blood samples were collected from each horse every 15 to 20 minutes for 24 hours for determination of 24-hour mean cortisol concentration. Afterward, dexamethasone (0.08 mg/kg) was administered i.v. to suppress endogenous cortisol production. Twelve hours afterward, hydrocortisone sodium succinate (1.0 mg/kg) was administered as a rapid i.v. bolus and serial blood samples were collected to determine hydrocortisone pharmacokinetics. Cortisol concentrations, daily cortisol production rate, and hydrocortisone pharmacokinetics were determined, and results were compared between adult horses and foals. The mean ± SD 24-hour cortisol concentration was significantly lower in foals (20 ± 4 ng/mL) than in horses (26 ± 6 ng/mL), but the daily cortisol production rate was significantly greater in foals (6,710 ± 320 ng/kg/d) than in horses (2,140 ± 400 ng/kg/d). For hydrocortisone, foals had a significantly greater volume of distribution at steady state (1.92 ± 1.11 L/kg) and total body clearance (1.39 ± 0.108 L/kg/h) and significantly lower peak plasma concentration (1,051 ± 343 ng/mL) than did horses (0.58 ± 0.15 L/kg, 0.349 ± 0.065 L/kg/h, and 8,934 ± 3,843 ng/mL, respectively). Important differences were detected in cortisol production and metabolism between neonatal foals and adult horses consistent with lower plasma protein binding of cortisol in foals. This decrease may contribute to cortisol insufficiency during prolonged critical illness in neonatal foals.