Vitamin D-3-induced proliferative lesions in the rat adrenal medulla

Adrenal medullary hyperplasia and pheochromocytomas are induced in rats by a variety of non-genotoxic agents, and we have hypothesized that these agen ts induce lesions indirectly by stimulating chromaffin cell proliferation. Vitamin D-3, which has not been previously associated with adrenal medullar y proliferative lesions, is the most potent in vivo stimulus to chromaffin cell proliferation yet identified. The present investigation utilized the v itamin D-3 model to prospectively test the relationship between mitogenicit y and focal proliferative lesions in the adrenal medulla and to determine e arly events in the pathogenesis of these lesions. Charles River Crl:CD BR r ats were treated with 0; 5000; 10,000; or 20,000 IU/kg/day of vitamin D3 in corn oil (5 ml/kg) by oral intubation. Rats were killed after 4, 8, 12, or 26 weeks of treatment, following a final week of labeling with bromodeoxyu ridine (BrdU) using a minipump. Adrenal sections were double-stained for Br dU and phenylethanolamine-N-methyl transferase (PNMT) to discriminate epine phrine (E) from norepinephrine (NE) cells or far vesicular acetycholine tra nsporter (VAchT) to identify cholinergic nerve endings. Vitamin D-3 caused a 4-5-fold increase in BrdU labeling at week 4, diminishing to a 2-fold inc rease by week 26. An initial preponderance of labeled E cells gave way to a preponderance of labeled NE cells. By week 26, 17/19 (89%) animals receivi ng the 2 highest doses of vitamin D-3 had focal adrenal medullary prolifera tive lesions, in contrast to an absence of lesions in control rats. The les ions encompassed a spectrum including BrdU-labeled "hot spots'" not readily visible on H and E sections, hyperplastic nodules, and pheochromocytomas. Lesions were usually multicentric, bilateral, and peripheral in location, a nd almost all were PNMT-negative. The lesions were not cholinergically inne rvated, suggesting autonomous proliferation. Hot spots, hyperplastic nodule s, and pheochromocytomas appear to represent a continuum rather than separa te entities. Their development might involve selective responses of chromaf fin cell subsets to mitogenic signals, influenced by both innervation and c orticomedullary interactions. A number of non-genotoxic compounds that indu ce pheochromocytomas in rats are known to affect calcium homeostasis. The r esults of this study provide further evidence to support the hypothesis tha t altered calcium homeostasis is indirectly involved in the pathogenesis of pheochromocytomas, via effects on chromaffin cell proliferation.