The role of calcium ions during mitosis

Abstract

Calcium-containing solutions were microinjected into dividing PtK₁ cells to assess the effect of calcium ion concentration on the morphology and physiology of the mitotic spindle. Solutions containing 50 μM or more CaCl₂ are immediately and irreversibly toxic to PtK₁ cells. Those containing 5–10 μM CaCl₂ cause reversible reduction in spindle birefringence followed by normal anaphase and cytokinesis. Microinjection of 5 μM or less CaCl₂ into anaphase PtK₁ cells has no detectable effect on the rate or extent of chromosome movement. Metaphase cells tend to enter anaphase 4–5 min after injection with 1–10 μM CaCl₂, compared with an average of 16 min after injection with calcium-free buffer. Reducing the intracellular calcium concentration by injection of EGTA-CaCl₂ buffers increases the lag between injection and anaphase to 20 min or more. Microinjection of calcium solutions does not promote precocious chromatid separation in nocodazole-arrested metaphase cells, indicating that the increase in calcium concentration does not induce centromere separation directly. An increase in the concentration of free calcium ions during metaphase appears to stimulate the onset of anaphase. Such an increase, regulated by the cell itself, may contribute to the initiation of chromosome separation in mammalian cells.