Regulation of phosphoinositide hydrolysis in transformed human endometrial cells

Addition of the cholinergic agents acetylcholine or carbamylcholine (CCh) to suspensions of human endometrial adenocarcinoma cells (Ishikawa line) preincubated with [³H] myoinositol promoted a rapid concentration-dependent hydrolysis of labeled phosphoinositides to inositol tris-, bis-, and monophosphates with EC₆₀ values (mean ± SE) of 3.5 ±1.6 and 26.5 ± 4.8 μM, respectively. Atropine inhibition of the CCh effects (K_i = 1.6 ± 1.3 nM) and the ineffectiveness of nicotinic antagonists indicate involvement of a muscarinic receptor.Both basal and CCh-stimulated production of inositol phosphates were higher in the presence of LiCl. The effect of LiCl on inositol monophosphate accumulation was concentration dependent (1†100 mM). Vasopressin, oxytocin, phenylephrine, histamine, and prostaglandin F_2α, had no apparent affect on inositol phosphate levels. Phorbol esters inhibited up to 35% of the effect of CCh on inositol phosphate accumulation. Triphenylethylene antiestrogens at micromolar concentrations increased inositol phosphate accumulation, but inhibited the effects of CCh. However, the rapid uptake of trypan blue observed after exposure to 10 μM tamoxifen suggests an alteration of the plasma membrane which may affect signal-transducing systems. The effects of CCh on the production of inositol phosphates and the expected concomitant liberation of diacylglycerol by transformed epithelial cells of human endometrium are of potential significance in normal endometrial physiology, since cholinergic innervation of endometrial glands has been reported, and the role of hormonally stimulated phosphoinositide hydrolysis in secretory mechanisms has been demonstrated in many systems.