How Cytotoxic is Zinc? A Study on Effects of Zinc on Cell Proliferation, Endocytosis, and Fine Structure of the Ciliate Tetrahymena

Addition of 0.5-2.0 mM zinc (Zn2+) to proliferating Tetrahymena cultures in 2% proteose peptone (PP) medium induced varying lag periods (~1⁄2-4 h) before proliferation resumed at almost normal (0.5, 0.75 mM) or decreased rates (1, 1.5, 2 mM). Initially cells in 1 mM Zn2+ moved slowly whereas in higher concentrations a dose-dependent number of cells ceased to move exhibiting rocking movements; the cells recovered, however, and regained motility during the lag period. All Zn-treated cells had small refractive granules which increased in number during the lag period, decreased when cell proliferation resumed, and increased again in non-proliferating cells in high density cultures. The endocytic capacity of the cells was elevated in the low concentrations but suppressed dose-dependently in 1-2 mM Zn2+, concommitantly with an increase in cells not forming vacuoles (up to 80%), corresponding to the immobile cells mentioned above. Tetrahymena exposed to Zn2+ in dilute, 1% and 0.5% PP, medium showed the same trend of growth pattern and endocytosis as in 2% PP but at lower Zn concentrations. The maximum sublethal concentrations in 0.5%, 1%, and 2% PP growth medium permitting cell proliferation, were 0.2, 1.0, and 2.0 mM Zn2+, respectively, a linear relationship between the amount of organic matter and toxicity. Fine structurally, Zn2+-treated cells had dilated endoplasmic reticulum (induced protein synthesis), electron-dense granules (fused with food vacuoles), and a variety of vesicles, some containing electron-dense dots (Zn?). Apart from the early strong reaction, Tetrahymena adapted quickly to excess amounts of Zn2+, perhaps because an ion regulation pathway for Zn2+ is already operating in the cells.