Evaluation of the Effect of Two Doses of ACTH on Serum Free Cortisol Concentrations in Clinically Healthy Dogs
The adrenocortical response of healthy dogs to synthetic ACTH (cosyntropin) has been reported using several doses of intravenous ACTH. Both 0.5 and 5 µg/kg doses cause maximal stimulation with regard to serum total cortisol concentration. The effect of these doses on serum free cortisol concentration, however, has not been evaluated. The purpose of this study was to determine adrenal response to 0.5 and 5 µg/kg ACTH with respect to free cortisol in clinically healthy dogs.
Two dose-response trials were performed in 10 clinically healthy dogs. Each dog was given 0.5 or 5 µg/kg of cosyntropin (Cortrosyn) IV with a 2-week wash out period between doses. Blood samples were obtained before and 60 min after ACTH administration. Samples were centrifuged after clotting, and the serum was separated and stored at -80°C until analysis. Data were analyzed using repeated measures ANOVA on ranks; significance was set at the p<0.05 level.
For measurement of serum free cortisol concentration, samples were assayed by a modified ultrafiltration technique. For validation, free cortisol % and concentration were determined in 53 samples obtained from another population of healthy dogs before and after administration of either 1 µg cortrosyn/dog or 5 µg/kg body weight. A significant (p<0.0001) linear relationship between total serum and % free cortisol was found by linear regression; the regression equation was similar to that found when free cortisol was measured in canine plasma by a centrifugal ultrafiltration-dialysis technique (Kemppainen RJ et al, AJVR, 1991). In addition, a single basal sample from a dog was divided into 6 equal aliquots and the free cortisol concentration measured in all 6 ultrafiltrate portions obtained. The coefficient of variation was 3.2%.
Mean total and free serum cortisol concentration increased significantly after administration of both dosages when compared to baseline (p<0.0001). However, mean post-ACTH total and free cortisol concentration did not differ between doses. For all basal and stimulated serum cortisol concentrations (n=20 each), mean % free cortisol was 5.8% (range 1.1-14.5) and 12.7% (range 8.1-18.0), respectively.
In conclusion, cosyntropin administered at 0.5 and 5 µg/kg intravenously significantly increases total and free serum cortisol concentrations in clinically healthy dogs. These results can be used in subsequent studies to evaluate the hypothalamic-pituitary-adrenal axis in healthy and critically ill dogs.