The cell membrane: A common site of action of thyrotropin (TSH) and long-acting thyroid stimulator (LATS)

Abstract

We have recently shown that the initial interaction of both thyrotropin (TSH) and long-acting thyroid stimulator (LATS) with the thyroid is rapid, firm binding to a superficial cell site, possibly the cell membrane. In order to evaluate those structural features of the cell membrane requisite for hormonal action in the present study, sheep thyroid slices were treated with phospholipase C, which is known to affect membrane phospholipids, and the subsequent response to TSH or LATS as measured by 32P incorporation into phospholipids was examined. Phospholipase C (5–25 μg./ml.) abolished this response although basal and acetylcholine-stimulated phospholipogenesis were unaffected. Thus, a membrane phospholipid appears important for the action of both TSH and LATS on the thyroid cell. Since phospholipid may play an important role in membrane transport of ions, the effects of cations and ouabain on TSH and LATS stimulation of 32P incorporation into sheep thyroid slice phospholipid were studied. Omission of Na + from the buffer markedly decreased base-line incorporation and abolished the response to both stimulators. Addition of 12 mM Na + restored TSH and LATS action without increasing base-line incorporation. Ca ++ and Mg ++ were not essential for TSH or LATS stimulation of 32P incorporation. Ouabain (10 −4 M) abolished the effects of both stimulators on phospholipogenesis. These findings indicate that both TSH and LATS stimulation of 32P incorporation into phospholipids have an absolute dependence on Na + and are similarly influenced by the ionic composition of the medium. It would thus appear that despite their marked physico-chemical and immunological differences, the in vitro effects of both stimulators are mediated in similar fashion, presumably via interaction with the thyroid cell membrane.