Periodic increase of cytoplasmic free calcium in fertilized hamster eggs measured with calcium-sensitive electrodes

Abstract

The hamster egg at fertilization shows transient, periodic hyperpolarizing responses due to a Ca2+-activated K+ conductance. The suggested increase of the intracellular Ca2+ concentration ([Ca2+]i) was measured with Ca2+-sensitive microelectrodes in combination with the voltage-clamp technique. Calculated values of the basal [Ca2+]i before fertilization averaged in the range 0.2-0.4 microM. Insemination of eggs induced periodic Ca2+ transients with exact one-to-one correspondence to periodic, Ca2+-activated K+ currents. They were recorded anywhere in the cytoplasm, even far from the site of sperm-egg fusion, indicating that [Ca2+]i increases in the whole egg upon each response. During a series of responses after insemination the increase in [Ca2+]i reached 1-2 microM in the first three responses and the peak value decreased to 0.7-0.8 microM in the later responses. The rise and decay times of each Ca2+ transient were 6.8 +/- 2.8 and 22 +/- 6 s (mean +/- S.D., n = 36), respectively. No significant difference in the amplitude and time course of the Ca2+ transient was detected in relation to the depth of the cytoplasm about 3-30 micron from the surface. The basal [Ca2+]i increased gradually from 0.39 to 0.56 microM (average in fourteen eggs) during the series of Ca2+ transients. The biological significance and mechanism of the periodic increase in [Ca2+]i and the response time of the Ca2+ electrode are discussed.