Sperm extract and inositol 1,4,5-triphosphate induce cytosolic calcium rise in the central cell of Torenia fournieri

Microinjection of soluble sperm extract and Calcium Green-1 10 kDa-dextran conjugate (CG-1) into the mature central cell of Torenia fournieri induced a significant rise in cytosolic free calcium concentration (Ca²⁺]_i). The rise reached a maximum at 20 min after injection and then steadily declined. Nevertheless, a relatively high level of Ca²⁺]_i was maintained even 40 min after injection. Microinjection of sperm extract of maize into Torenia central cells, however, did not trigger any increase in Ca²⁺]_i, suggesting the possibility of distinct triggers in different species. We also injected caged inositol 1,4,5-triphosphate (InsP₃) and caged cyclic ADP-ribose (cADPR) into Torenia central cells to compare the pattern of Ca²⁺ rise induced by the sperm extract. The results showed that Ca²⁺]_i elevation triggered by the release of InsP₃ after photolysis appears much faster than that induced by sperm extract. The increase in Ca²⁺]_i reached a maximum at 70-80 s and dropped to the resting level within 300 s after photolysis. Microinjection of cADPR, however, did not induce any changes in Ca²⁺]_i. The results indicate that sperm extract might contain factors triggering the release of Ca²⁺ in the central cell.