Measurement of free cortisol and the capacity and association constant of cortisol-binding proteins in plasma of foals and adult horses.

A direct method for measuring the capacity of the high-affinity binding protein, CBG, based on charcoal adsorption, was validated for use in the horse. Several unique aspects of cortisol binding in the horse were observed: (1) CBG content at birth was the lowest of any species studied, (2) CBG concentration increased with age whereas in other species it decreases, (3) the plasma of the new born foal has a binding protein, not reported for other species, which binds as much cortisol as does CBG. Its capacity and affinity are intermediate between albumin and CBG. It may be involved prenatally in increasing dam to fetus transfer of cortisol, and post-natally in buffering the effects of the huge stress-related peripartum release of cortisol.