Calcium Wave in Activating Hamster Eggs

¹ Department of Physiology, Tokyo Women's Medical College, Kawada-cho, Shinjuku-ku, Tokyo, 162, Japan

The activating hamster egg shows a series of periodic increases in intracellular free calcium concentration ([Ca²⁺]_i) (Igusa and Miyazaki, 1986). The spatial distribution of the Ca²⁺ transients can be analyzed in single, aequorin-injected eggs by the photon-counting imaging method with a supersensitive TV camera system (Miyazaki et al., 1986). A propagating increase in [Ca²⁺]_i is observed in the first response, starting from the sperm attachment site and spreading over the entire egg within 4-7 s. The Ca wave is repeated in the second and sometimes the third response, starting from the same focus, but spreading more rapidly (2 s). In subsequent responses, [Ca²⁺]_i increases synchronously in the whole egg within 1 s.

A Ca²⁺ transient is induced in an all-or-none fashion by injection of either inositol 1,4,5-trisphosphate (IP₃) or Ca²⁺ ions into unfertilized eggs. The Ca²⁺ transient is due to release of Ca²⁺ from intracellular stores, and it is followed by a refractory period of 60-120 s. Injection of guanosine-5'-0-(3-thiotriphosphate) (GTPS) causes a series of Ca²⁺ transients with a pattern similar to that upon insemination, but with remarkable attenuation of the [Ca²⁺]_i rise after 2-3 responses. Signal transduction involving activation of a GTP-binding protein and IP₃- and/or Ca²⁺ -induced Ca²⁺ release is considered to be responsible for the repeated Ca²⁺ transients.

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