Pharmacological characterization of Ca²⁺ entry pathway during Ca²⁺ oscillations in fertilized mouse eggs

Abstract

In fertilized mammalian eggs, long-lasting Ca²⁺ oscillations are generated by repetitive Ca²⁺ releases from intracellular Ca²⁺ stores through the IP₃ receptor/Ca²⁺ channel. Although Ca²⁺ entry from outside is also required to maintain Ca²⁺ oscillations, the regulatory mechanism of the Ca²⁺ influx pathway is poorly understood. In the present study, we tested the inhibitory effect of several Ca²⁺ channel blockers on the sperm-induced Ca²⁺ responses in mouse eggs, in order to characterize pharmacologically the Ca²⁺ entry channel responsible for the maintenance of Ca²⁺ oscillations. The Ca²⁺ oscillations were inhibited by Gd³⁺ or La³⁺ at millimolar concentrations, whereas Ni²⁺, Co²⁺ and SKF96365, an agent known to nonspecifically inhibit store-operated channels, were ineffective at all concentrations tested. The capacitative Ca²⁺ entry (CCE) which was activated in the eggs treated with thapsigargin and measured by the Mn²⁺ quenching technique, was inhibited by Gd³⁺ and La³⁺ at similar concentrations. The CCE was inhibited by SKF96365 as well, contrasting to Ca²⁺ oscillations. The difference in the pharmacological profiles suggested that the Ca²⁺ entry through the pathway other than CCE may be responsible for the refilling of Ca²⁺ stores and the maintenance of Ca²⁺ oscillations in the fertilized egg. [J Physiol Sci. 2008;58 Suppl:S200]