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Adrenocortical Response Profiles to Corticotrophin‐Releasing Hormone and Adrenocorticotrophin Challenge in the Chronically Catheterized Adult Guinea‐Pig

The guinea‐pig has been used extensively to investigate adrenal steroidogenesis. However, very little is known about adrenocortical responses to corticotrophin‐releasing hormone (CRH) and adrenocorticotrophin (ACTH) in this species, in vivo. In the present study, we have developed a stress‐free sampling system, in the chronically catheterized adult guinea‐pig, that has allowed us to investigate basal and activated adrenocortical activity. Indwelling carotid artery and jugular vein catheters were surgically implanted into female guinea‐pigs (n= 5). Each animal was treated with vehicle, human CRH (0·2 or 2 μg kg−1) and ACTH1‐24 (0·2 or 2 μg kg−1), and serial plasma samples removed for analysis of ACTH and cortisol concentrations by radioimmunoassay. There was no effect of serial sampling on pituitary‐adrenocortical activity, indicating that the animals remain in an unstressed state. Basal plasma ACTH and cortisol concentrations were 703·9 ± 24·5 pg ml−1 and 117·9 ± 5·2 ng ml−1, respectively. Both CRH and ACTH significantly increased adrenocortical activity in a dose‐dependent manner. ACTH (2 μg kg−1) was the most potent activator leading to plasma cortisol concentrations of 647 ± 116 ng ml−1. In conclusion, we have shown that basal plasma cortisol concentrations in the guinea‐pig are low compared to those obtained in previous studies by cardiac puncture or following decapitation. However, plasma ACTH concentrations are high compared to other species. We have also shown that human CRH and ACTH1‐24 act as potent activators of the guinea‐pig pituitary‐adrenocortical axis, leading to response profiles consistent with mild cortisol resistance.