Measurements of ooplasmic pCa in fertilized eggs of the locust, Locusta migratoria, and a role for external calcium in experimental activation of unfertilized eggs

The role of Ca2+ in activation and early development of locust eggs was examined through measurement of ooplasmic Ca2+ levels before and after fertilization, and through experimental activation of unfertilized eggs. Ooplasmic pCa (i.e. the negative logarithm of Ca2+ activity) measured in intact eggs decreased from 5.35 before fertilization, to 4.77 and 3.00 by 1 day and 3 days after fertilization, respectively. pCa was also determined for samples of ooplasm collected by rupturing eggs under paraffin oil. The pCa was 5.10 in ooplasm isolated from unfertilized eggs, and 3.84 in ooplasm collected from eggs within 4 h of fertilization. Ooplasmic pCa remained between 3.97 and 3.12 from 1–6 days after fertilization. Since a decline in pCa indicates an increase in ooplasmic Ca2+ activity, the data suggest that regulation of ooplasmic Ca2+ during post-fertilization development involves release of Ca2+ from internal stores. Experimental egg activation was examined in eggs dissected from the oviducts before fertilization and incubated on moist filter paper. Some eggs were first immersed in experimental solutions for 30–60 minutes before incubation. The presence of an embryo 2 or 4 days after fertilization or experimental treatment was used as an indicator of egg activation. Activation occurred in 92% and 12% of fertilized and untreated eggs, respectively. The percentage of unfertilized eggs which activated increased to 47% if eggs were soaked 30–60 minutes in physiological saline, and to as much as 65%-68% if eggs were injected with Ca2+ buffers or if a Ca2+ action potential was evoked. Up to 36% and 42% of unfertilized eggs activated after incubation in Ca2+-free salines or in the presence of the Ca2+-channel blocker Cd2+, respectively. Taken together, the results suggest that entry of external Ca2+ through voltage dependent channels increases the proportion of eggs which activate, but is not an absolute requirement for activation.