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 Toxoplasma gondii occurs when the parasite attaches by its apical end to the target host cell. The contents of apical secretory organelles called micronemes have recently been implicated in parasite apical attachment to host cells. Micronemes are regulated secretory vesicles that discharge in response to elevated parasite intracellular Ca²⁺ levels ([Ca²⁺]_i). In the present study we found that ethanol and related compounds produced a dose-dependent stimulation of microneme secretion. In addition, using fluorescence spectroscopy on tachyzoites loaded with the Ca²⁺-sensitive fluorescent dye fura-2, we demonstrated that ethanol stimulated microneme secretion by elevating parasite [Ca²⁺]_i. Furthermore, sequential addition experiments with ethanol and other Ca²⁺-mobilizing drugs showed that ethanol probably elevated parasite [Ca²⁺]_i by mobilizing Ca²⁺ from a thapsigargin-insensitive compartment of neutral pH. Earlier studies have shown that ethanol also elevates [Ca²⁺]_i in mammalian cells. Thus, because it is genetically tractable, T. gondii might be a convenient model organism for studying the Ca²⁺-elevating effects of alcohol in higher eukaryotes.