Role of Inositol Cyclic Phosphate in Stimulated Tissues

Abstract

THYROID-STIMULATING hormone markedly stimulates phosphatidylinositol turnover in sheep thyroid gland. The enzyme catalysing the degradation of phosphatidylinositol in animal tissues can split the phospholipid, with the formation of a diglyceride and a new cyclic ester, D-myoinositol-1: 2-cyclic phosphate as well as D-myoinositol-1-phosphate^2,3. Increased phosphatidylinositol turnover has been described in a variety of conditions in which cells are stimulated to secrete or into other hyperactivity. The physiological possibilities of this degradation are obvious, and it was suggested^2,3 that the catabolism of phosphatidylinositol and the release of inositol cyclic phosphate might be more directly relevant to the cell stimulation than to the biosynthesis involved in the turnover. Such an hypothesis is attractive, for an accumulation of phosphatidylinositol has never been demonstrated during stimulated conditions. In addition Ca²⁺ is an essential requirement for the formation of inositol cyclic phosphate as well as playing an important part in secretory processes⁴.