The influence of electrical stimulation on the level of intracellular Ca
2+ in bovine oocytes, as well as activation and extent of parthenogenetic development, was investigated. Mature oocytes were electrically stimulated at 29 hr of maturation, and intracellular Ca
2+ concentration was determined with the Ca
2+ indicator fura-2 dextran (fura-2 D). The Ca
2+ response of oocytes to a given electrical pulse was variable. Oocytes responded with either no Ca
2+ rise from baseline (≈? 12 nM), a short-duration Ca
2+ rise (from 12 nM to 300 nM) that returned to baseline within 2 min of the pulse, or a long-duration Ca
2+ rise (from 12 nM to 1,000–2,000 nM) that never returned to baseline during the 8 min period over which the oocytes were monitored. In these oocytes, Ca
2+ level returned to baseline when oocytes were removed from 0.30 M mannitol and placed in an ionic medium. Increasing field strength or pulse duration tended to increase the proportion of oocytes displaying a Ca
2+ rise, and at 1.0 kVcm
?1 for 40 μsec, all oocytes displayed a long-duration Ca
2+ elevation. Direct transfer of oocytes from culture medium to mannitol also triggered a Ca
2+ rise. Multiple stimulations, either electrical or by transferring to mannitol, produced multiple Ca
2+ rises. This mannitol-induced Ca
2+ rise could be inhibited by first washing the oocytes in medium containing equal parts of 0.30 M mannitol and phosphate buffered saline (PBS). The level of Ca
2+ stimulation affected activation and development of oocytes. Insufficient, or, conversely, excessive Ca
2+ stimulation impaired development. Optimum development was obtained with (1) three pulses of 0.2 kVcm
?1 for 20 μsec, each pulse 22 min apart, after direct transfer of oocytes from culture medium to mannitol (22% blastocysts) or (2) three pulses of 1.0 kVcm
?1 for 20 μsec after transfer of oocytes from culture medium to medium containing equal parts mannitol and PBS, then to mannitol (24% blastocysts). This procedure avoided induction of a Ca
2+ rise prior to the pulse. The results indicate that the level of Ca
2+ stimulation can be regulated by incubation conditions prior to the pulse and, to some extent, by field strength and pulse duration. The level of electrical stimulation influenced oocyte Ca
2+ response, activation, and parthenogenetic development. © 1993 Wiley-Liss, Inc.
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