MECHANISMS OF CALCIUM-RELEASE AND SEQUESTRATION IN EGGS OF CHAETOPTERUS-PERGAMENTACEUS

Increases in the intracellular free calcium concentration are of great importance to the initiation of development in deuterostomes. Their i nvolvement has not yet been clearly defined in protostomes. We used en dogenous ligands (IP3, cADPR, ryanodine and NAADP) and pharmacological agents (thapsigargin [Tg], thimerosal, caffeine and heparin) to study smooth endoplasmic reticulum Ca2+ pump and release mechanisms in eggs of an annelid, Chaetopterus. Oocyte homogenates effectively sequester ed Ca2+ and released it in response to IP3 in a concentration-dependen t manner. Repeated additions of IP3 were unable to cause further relea se. Heparin inhibited Ca2+ release in response to IP3. The homogenates also released Ca2+ in response to thimerosal, and this release was se nsitive to heparin. Two antibodies to IP3 receptors recognized an appr opriate band in Chaetopterus egg lysates. These results indicate that the oocytes possess type-1 IP3-gated Ca2+ channels. Neither calcium it self, nor strontium, cADPR, ryanodine, caffeine nor NAADP released app reciable Ca2+. At low concentrations, Tg caused a slow release of Ca2; at higher concentrations, it elicited a rapid release. Release of Ca 2+ by Tg activated development. Since one theory of fertilization invo kes the introduction of a Ca2+ releasing soluble protein into the egg upon sperm-egg fusion, we also tested whether soluble extracts of Chae topterus sperm could stimulate Ca2+ release in Chaetopterus egg homoge nates. There was no Ca2+ release when the sperm extract was added to t he homogenate; however, homogenates exposed to sperm extract became re fractory to IP3. Thus, Ca2+ release at fertilization in these oocytes occurs through IP3-gated channels.