The development of the pituitary-adrenal axis in the guinea pig

The concentration of cortisol and ACTH in the umbilical vein plasma of the foetal guinea pig has been measured over the later half of gestation. Between 55--60 days there is a large increase in the concentration of both. Injection of ACTH1-24 into the foetal guinea pig elicits a relatively small increase in plasma cortisol at 46--55 days, a somewhat greater response at 56--60 days and a large response at 61--66 days. Between 47--55 days and 58--63 days cells prepared from the foetal adrenal show a large increase in their responsiveness to ACTH stimulation as measured by cortisol and androstenedione output. However, there was no clear difference in responsiveness between 58--60 days and 61--63 days. Thus between 56--60 days the foetal adrenal appears less responsive to ACTH stimulation in vivo than in vitro. ACTH in the circulation of the guinea pig is present in high-molecular-weight and low-molecular-weight forms and the ratio of concentration of ACTH1-39 to that of the high-molecular-weight forms rises after 60 days. It is proposed that this is an important factor in inducing an increased steroid response from the foetal adrenal.