Inhibitory Effect of Omeprazole, a Specific Inhibitor of H⁺, K⁺‐ATPase, on Spicule Formation in Sea Urchin Embryos and in Cultured Micromere‐Derived Cells.

<jats:p>Embryos kept with omeprazole, a specific H<jats:sup>+</jats:sup>, K<jats:sup>+</jats:sup>‐ATPase inhibitor, in a period of development between the mesenchyme blastula and the pluteus corresponding stage became abnormal plutei having quite small spicules, somewhat poor pluteus arms and apparently normal archenterons. In micro‐mere‐derived cells, kept with omeprazole at pH 8.2 in a period between 15 and 40 hr of culture at 20°C, omeprazole strongly inhibited spicule formation but did not block the outgrowth of pseudopodial cables, in which spicule rods were to be formed. These indicate that omeprazole probably exerts no obvious inhibitory effects other than spicule rods formation. Omeprazole‐sensitive H<jats:sup>+</jats:sup>, K<jats:sup>+</jats:sup>‐ATPase, an H<jats:sup>+</jats:sup>pump, seems to be indispensable for CaCO<jats:sub>3</jats:sub> deposition (formation of spicule rod) in these spicule forming cells. H<jats:sup>+</jats:sup>, produced in overall reaction for CaCO<jats:sub>3</jats:sub> formation: Ca<jats:sup>2+</jats:sup>+ CO<jats:sub>2</jats:sub>+H<jats:sub>2</jats:sub>O°CaCO<jats:sub>3</jats:sub>+2H<jats:sup>+</jats:sup>, is probably released from the cells by this H<jats:sup>+</jats:sup>pump and hence, this reaction tends to go to CaCO<jats:sub>3</jats:sub> production to form spicule rods. Omeprazole, known to become effective following its conversion to a specific inhibitor of H<jats:sup>+</jats:sup>, K<jats:sup>+</jats:sup>‐ATPase at acidic pH, is able to inhibit formation of spicule rod at alkaline pH in sea water. This is probably due to an acidification of sea water near the cell surface by H<jats:sup>+</jats:sup>ejection in H<jats:sup>+</jats:sup>, K<jats:sup>+</jats:sup>‐ATPase reaction.</jats:p>