Functional characteristics of transplantable rat pituitary tumor MtT SA5 inducing pronounced adrenal hyperplasia

Transplantable rat pituitary tumor MtT SA5 is characterized by the induction of pronounced adrenal enlargement. In spite of remarkable elevation of plasma ACTH levels, serum corticosterone levels in hypophysectomized tumor-bearing rats were lower than those in intact control rats. No immunohistochemical alterations were seen in corticotrophs of the pituitary in tumor-bearing rats. Ratio of bioactive ACTH (Bio-ACTH) to immunoreactive ACTH (Ir-ACTH) in the tumor tissues was 0.2%, being extraordinarily lower than in the pituitary tissues (23%). Gel chromatography of the tumor tissue showed large molecular weight forms of Ir-ACTH, which seem to result from abnormal processing of proopiomelanocortin, and are responsible for the lower levels of Bio-ACTH compared to Ir-ACTH. The MtT SA5 tumor is suggested to secrete ACTH-related peptides which induce pronounced adrenal enlargement with little or no stimulation of glucocorticoid production.