Studies of two thyrotrophin-secreting pituitary adenomas: evidence for dopamine receptor deficiency.
Of 22 previously reported patients with TSH-secreting pituitary adenomas challenged with dopamine agonists, 18 showed no decrease in serum TSH. There have been few in-vitro studies of these rare tumours so the mechanism of the dopaminergic resistance has remained obscure. We describe two further patients with thyrotrophinomas; the first was thyrotoxic (T3 6.1 nmol/l, TSH 7 mU/l) and the second was diagnosed after radioiodine for presumed Graves' disease. The second patient had an alpha-subunit: TSH molar ratio less than unity (0.27). In-vivo TSH responses to TRH, bromocriptine and domperidone were compared with those of the resected tumour cells in vitro, the latter studied using a continuous perifusion system. Dopamine receptors were sought in membranes from each tumour using a radioreceptor assay employing 3H-spiperone. Patient 1 showed significant increases in serum TSH (7 to 13 mU/l) and alpha-subunit (18.7 to 385 ng/ml) after 200 micrograms TRH (i.v.) but patient 2 showed no such increases (TSH: 69 to 72 mU/l, alpha-subunit: 4.9 to 5.2 ng/ml). Neither patient showed a change in serum TSH following bromocriptine 2.5 mg (orally) or domperidone 10 mg (i.v.), though serum PRL responded normally. Serum TSH from patient 1 was of apparently normal molecular size but increased bioactivity (B/I ratio 3.8) and that from patient 2 was of increased molecular size but reduced bioactivity (B/I ratio 0.1). Tumour cells from each patient immunostained for TSH beta and alpha-subunit, and secreted TSH in vitro. The first showed dose-dependent TSH release after TRH (1-100 ng/ml) which could not be inhibited by dopamine (5 mumol/l) but the second was unresponsive to TRH in vitro. Neither tumour showed inhibition of TSH release by dopamine (5 mumol/l) or bromocriptine (0.01-10 nmol/l) and neither contained membrane-bound dopamine receptors. The results suggest that the dopaminergic resistance typical of most TSH-secreting pituitary adenomas may be due to altered or absent membrane-bound dopamine receptors.
Bevan
J S
JS
Department of Endocrinology, Radcliffe Infirmary, Oxford, UK.
Burke
C W
CW
Esiri
M M
MM
Adams
C B
CB
Ballabio
M
M
Nissim
M
M
Faglia
G
G
D000236:Adenoma / Q000378:metabolism* / Q000648:ultrastructure; D000328:Adult; D001681:Biological Assay; D001971:Bromocriptine; D002850:Chromatography, Gel; D004294:Domperidone; D005260:Female; D006801:Humans; D007150:Immunohistochemistry; D008854:Microscopy, Electron; D008875:Middle Aged; D010911:Pituitary Neoplasms / Q000378:metabolism* / Q000648:ultrastructure; D011954:Receptors, Dopamine / Q000187:drug effects / Q000378:metabolism*; D013972:Thyrotropin / Q000097:blood / Q000378:metabolism*
D011954:Receptors, Dopamine; D001971:Bromocriptine; D004294:Domperidone; D013972:Thyrotropin