Ethanol and acetaldehyde elevate intracellular [Ca2+] and stimulate microneme discharge in Toxoplasma gondii

One of the first steps in host-cell invasion by the protozoan parasite Toxo plasma gondii occurs when the parasite attaches by its apical end to the ta rget host cell. The contents of apical secretory organelles called micronem es have recently been implicated in parasite apical attachment to host cell s. Micronemes are regulated secretory vesicles that discharge in response t o elevated parasite intracellular Ca2+ levels ([Ca2+](i)), In the present s tudy we found that ethanol and related compounds produced a dose-dependent stimulation of microneme secretion. In addition, using fluorescence spectro scopy on tachyzoites loaded with the Ca2+ sensitive fluorescent dye fura-2, we demonstrated that ethanol stimulated microneme secretion by elevating p arasite [Ca2+](i). Furthermore, sequential addition experiments with ethano l and other Ca2+-mobilizing drugs showed that ethanol probably elevated par asite [Ca2+](i) by mobilizing Ca2+ from a thapsigargin insensitive compartm ent of neutral pH. Earlier studies have shown that ethanol also elevates [C a2+](i) in mammalian cells. Thus, because it is genetically tractable, T, g ondii might be a convenient model organism for studying the Ca2+-elevating effects of alcohol in higher eukaryotes.