Search Results

ABSTRACT

In previous studies we have found that the cholecystographic contrast agent ipodate induced a rapid, sustained and reversible inhibition of thyroxine (T₄) secretion from perfused dog thyroid lobes. This type of inhibition of thyroid secretion has not been observed previously. To evaluate whether this effect is unique for ipodate, ten other iodine-containing radiographic contrast agents were tested. The four agents used for cholecystography (iocetamate, iodipamide, ioglycamate and iotroxate) all induced rapid inhibition of T₄ secretion from TSH-stimulated perfused dog thyroid lobes, while none of six agents predominantly excreted through the kidneys (amidotrizoate, metrizamid, metrizoate, iodamide, diodone and ioxithalamate) influenced T₄ secretion significantly. All the cholecystographic agents inhibited T₄ deiodinases from dog thyroid and liver. Diodone also inhibited the deiodinases while none of the other compounds tested had any effect.

The results indicate that the structure necessary to inhibit thyroid secretion is common to a number of cholecystographic agents and that it could be related to the structure responsible for the inhibitory effect of cholecystographic agents on T₄ deiodinases.

J. Endocr. (1987) 112, 387–390

ABSTRACT

Cholecystokinin (CCK) is a heterogeneous gut hormone which is also synthetized in extra-intestinal endocrine cells and neurones. In order to examine the possibility that CCK peptides are local modulators of calcitonin secretion, we have studied the structure– activity relationship on calcitonin secretion from perfused canine thyroid lobes as well as the presence and molecular nature of CCK in the thyroid. Peptides containing the intact COOH-terminal tetrapeptide amide of CCK (CCK-4, CCK-5, pentagastrin, CCK-8 and gastrin-17) all induced dose-dependent (0·1, 3 and 100 nmol/l) increases in calcitonin release (P<0·05, n =4) with biphasic secretion during 6-min infusion periods. The deamidated tetrapeptide and the COOHterminal tripeptide were without effect. Gel chromatography of neutral water and acid–ethanol extracts of thyroid tissue, monitored by sequence-specific CCK and gastrin radioimmunoassays, disclosed a variety of CCK and gastrin peptides of which a predominant form resembled small molecular forms like CCK-4 and CCK-5. The presence in the thyroid of small CCK-like peptides and the pronounced effect of such peptides on calcitonin secretion suggest that calcitonin secretion is modulated by local release of small CCK peptides. They could originate from intrathyroidal nerves or from sub-populations of C-cells.

J. Endocr. (1987) 115,77–82

ABSTRACT

Characterization of iodothyronine-deiodinating enzymes has been difficult due to loss of enzyme activity during purification. To obtain a new tool for studying these enzymes we investigated the possibility of developing monoclonal antibodies (MAbs) against iodothyronine-5′-deiodinase (5′-D). Two specific and sensitive solid-phase microassays were developed for screening hybridoma supernatants for the presence of antibodies inhibiting rat kidney 5′-D. and antibodies binding to but not inhibiting the enzyme.

BALB/c mice were immunized with a 3-((3-cholamidopropyl) -dimethylammonio) -1- propanesulphonate (CHAPS)-solubilized 5′-D-rich membrane preparation from rat kidney cortical tissue. Spleen cells were fused with NSI-Ag 4/1 mouse myeloma cells by means of polyethylene glycol.

Two hybridoma cell lines (AF5 and BE8) secreting MAbs specifically binding to without inhibiting 5′-D were produced. The AF5 antibody was of the IgG_2a subclass and the BE8 antibody of the IgG_2b subclass. Binding of one of the antibodies to the enzyme inhibited binding of the other in both an enzyme-linked immunosorbent assay (ELISA) and a specific enzymebinding assay. CHAPS-solubilized kidney microsomal fraction was chromatographed on a Sepharose 6B column. Elution profiles of 5′-D activity and MAb-binding antigens, as measured by ELISA with both AF5 and BE8, were identical.

Monoclonal antibodies should be valuable probes in the further elucidation of the nature of the iodothyronine-deiodinating activity in various tissues.

J. Endocr. (1988) 118, 439–445