Partial characterization of a thyrotropin releasing hormone-sensitive glycosyl-phosphatidylinositol in pituitary lactotrophes

Abstract

Metabolic labelling experiments performed with cultured pituitary lactotrophes revealed the presence of a glycosyl-phosphatidylinositol (GPtdlns) structurally related to GPtdlns lipids isolated from other cell types as demonstrated by: (i) metabolic incorporation of [3H]galactose, [3H]glucosamine and [3H]inositol into the polar inositolphosphoglycan moiety (InsPG) and [3H]myristate and [3H] palmitate into the diacylglycerol (DAG) backbone of GPtdlns; (ii) sensitivity of the [3H]labelled GPtdlns to nitrous acid deamination and; (iii) sensitivity of GPtdlns to phosphatidylinositol (Ptdlns)-specific phospholipase C (PLC) hydrolysis. In cultured pituitary cells labelled to isotopic steady state with 10 μCi/ml of [3H]glucosamine, treatment with hypothalamic TRH (10−6 M) induced a rapid and transient hydrolysis (ca. 50%) of the labelled GPtdlns. Moreover, as demonstrated in [3H]inositol labelled cells, treatment with thyrotropin releasing hormone (TRH) elicited the cleavage of [3H]GPtdlns in a similar manner, and this effect was followed by the phosphoinositide (Ptdlns, PtdInsP and PtdInsP2) hydrolysis 30 s later. These results suggest that the phosphodiesterase cleavage of GPtdlns could be an early event implicated in TRH action in pituitary lactotrophes.