Effect of Pituitary Thyrotrophin on the Diglyceride/Triglyceride Interrelationships in Thyroid Tissue

Abstract

IN surviving cellular preparations of thyroid tissue, intermediary metabolism is altered profoundly when ‘work’ is induced by the addition of pituitary thyrotrophin (TSH)¹. Heightened phospholipid metabolism is one of the basic features. Thus, TSH augments the incorporation of orthophosphate-³²P into certain phospholipids of thyroid slices even in the absence of exogenous organic substrates^2,3. In the presence of substrate quantities of glucose-¹⁴C or albumin-complexed ¹⁴C-fatty acids (as potential carbon donors for the glycerophosphate or acyl residues of thyroidal lipids), TSH preferentially channelizes radioactivity into phospholipids rather than neutral lipids^1,4. Since diglycerides may constitute communal precursors for both phospholipids and triglycerides⁵, investigations were instituted to assess whether the disparate activation of phospholipogenesis is accompanied by detectable changes in the diglyceride fraction. The experiments have indicated that the incorporation of carbon atoms into the individual neutral lipid components of thyroid slices during 1–4 h of incubation is altered by TSH. Although TSH does not systematically change the total labelling of neutral lipids from glucose-¹⁴C, the triglyceride-¹⁴C/diglyceride-14C ratio is reduced, possibly connoting a new ‘steady-state’ favouring phospholipogenesis.