Different triggers for calcium oscillations in mouse eggs involve a ryanodine-sensitive calcium store

Relative intracellular free Ca2+ concentrations ([Ca2+]i) were monitored in mature unfertilized mouse eggs by measuring fluorescence of intracellular fluo3. A number of different agents were found to cause sustained repetitive transient [Ca2+]i oscillations. These were microinjection of a cytosolic sperm factor, sustained injection of Ins-(1,4,5)P1, or extracellular addition of the thiol reagent thimerosal. Stimulating G-protein activity by injection of guanosine 5'-[gamma-thio]triphosphate plus application of carbachol also caused [Ca2+]i oscillations, but less reliably than other stimuli. A role for Ca(2+)-induced Ca2+ release and a ryanodine-sensitive Ca2+ channel in mouse eggs was suggested by the finding that microinjection, or external addition, of ryanodine also caused [Ca2+]i increases. Furthermore, ryanodine, along with thimerosal, increased the sensitivity of eggs to Ca(2+)-induced [Ca2+]i oscillations. When ryanodine was added to eggs oscillating in response to the sperm factor, InsP3 or thimerosal, it caused a decrease in amplitude of oscillations and eventually a block of [Ca2+]i oscillations associated with a sustained elevation of [Ca2+]i. These data suggest that a ryanodine-sensitive Ca(2+)-release mechanism exists in mouse eggs and that a ryanodine-sensitive Ca2+ store plays a role in generating intracellular [Ca2+]i oscillations.