Use of pharmacological agents to implicate a role for phosphoinositide hydrolysis products in malaria gamete formation

Abstract

The kinetics of phosphoinositol 4,5 bisphosphate hydrolysis products in activated Plasmodium falciparum gametocytes suggests a role for inositol trisphosphate [Ins(l,4,5)P3] and diacylglycerol (DAG) in the signal transduction pathway of malaria gametocytes. To investigate further this role, compounds that have an effect on the metabolism and biologic functions of these second messengers were tested in an in vitro system. Gentamycin, 2,3 diphosphoglycerate (2,3 DPG) and magnesium ion (Mg2+), inhibitors of Ins(1,4,5)P35'phosphatase, all stimulated gametocytes to exflagellate in suspended animation buffer, pH 7.4, at room temperature. In addition, methylxanthines, caffeine and theobromine, calcium ionophore (A-23187), and external calcium also stimulated exflagellation. In contrast, neomycin. an aminoglycoside that inhibits phospholipase C activity, and heparin, an antagonist of Ins(1,4,5)P3 binding to its receptor, inhibited microgamete formation. Quinine and chloroquine which can inhibit both phospholipase A and C activity also inhibited gametocyte exflagellation. The consistent manner in which these various compounds affect gametocyte activation further implicates phosphoinositol turnover in the signal transduction pathway of falciparum gametocytes. Abbreviations

PI, phosphoinositol 4.5 bisphosphate; PL-C, phospholipase C; Ins(1,4,5)P3, inositol trisphosphate; DAG, diacylglycerol; [Ca2+]i, intracellular calcium; [Ca2+]o, extracellular calcium; OAG, l-oleoyl-2-acetyl-glycerol; PL-A phospholipase A; COM, centers of movement; PDE, phosphodiesterase; Ins(1.3,4,5)P4, inositol tetrakis-phosphate; gpip2, glycerophosphatidylinositol 4,5 bisphosphate.