Secretion of melanocyte-stimulating hormone and adrenocorticotropin from transplanted pituitary pars intermedia in stressed and nonstressed rats.

Rats bearing kidney grafts of the pituitary pars intermedia were divided into three groups: unstressed, acutely stressed, and chronically stressed. Corresponding sham-operated rats were used for comparisons. Twenty days after grafting, the rats were sacrificed and alpha-melanocyte-stimulating hormone (alpha-MSH), adrenocorticotropin (ACTH), and corticosterone were estimated in plasma. The adrenal/body weight ratio and DNA content of the glands were also investigated. The following results were obtained: MSH was found not to be increased in unstressed rats, but it was in grafted animals subjected to acute and chronic swimming stress. ACTH and corticosterone rose in all three groups. Adrenal/body weight ratio and DNA content increased only in grafted chronically stressed rats. Moreover, plasma corticosterone was found higher in grafted hypophysectomized rats than in non-grafted hypophysectomized animals. Administration of ergocryptine to nonstressed grafted rats induced a decrease in the blood content of ACTH and MSH, indicating that the grafts were the source of a part of the circulating ACTH. On the other hand, the fall in MSH levels could show the effect of the drug upon the pars intermedia. Comparison of the ratios of both hormones released in incubations showed that grafts secreted more ACTH than MSH; on the other hand, when intact neurointermediate lobes were incubated, MSH predominated over ACTH. For the first time it is demonstrated that the pars intermedia can secrete ACTH in vivo. Nevertheless, the ability to secrete this hormone is not a property of normal intact pars intermedia, but it manifests in the transplantations probably due to the overactivity of light cells induced by chronic stoppage of dopaminergic inhibition.