Further Studies on the Action of Pituitary Thyrotropin on the Individual Phosphatides of Thyroid Tissue

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Abstract
Slices of dog thyroid were incubated in Krebs-Ringer-bicarbonate with orthophosphate-32P to characterize the individual phosphatides of the thyroid in a non-ruminant species and the effects of pituitary thyrotropin (TSH) on phos-pholipid metabolism in vitro. Phosphatidlglycerol, choline plasmagogen, ethanolamine plasmalogen and the recently described "higher" phospho-inositides were identified within the thyroid. Inclusion of glucose in the suspending medium augmented the incorporation of orthophosphate-32P into all the individual phosphatides. In glucose-free, as well as glucose-containing media, more selective enhancements of 32P-incorporation into thyroidal phospholipids were elicited with pituitary throtropin. Long-term exposure to TSH during 2-3 hr. of incubation in vitro caused highly significant increases in the turnover of phosphorus in the acidic phospho-glycerides (particularly monophosphatidylinositol and phosphatidic acid), modest increases in the turnover of phosphatidylcholine, and consistent, but small, increases in the incorporation of 32P-orthophosphate into higher phosphoinositides. Turnovers of phosphatidylglycerol, plasmo-genic phosphatides and sphingomyelin were not affected significantly. Acute exposure of thyroid slices of TSH for 2-10 min. (following 2 hr. of prelabeling with 32P-orthophosphate in vitro) enhanced the turnover of labeled phosphatidic acid and monophosphatidylinositol. No other phosphatide was consistently affected. It has been suggested that the effects of TSH upon selective phospholipids may reflect the cytostructural realignments which others have observed with electron microscopy, and that the extraordinary reactivity of monophosphatidylinositol may be linked to its role in some catabolic organelle.