Internal pH can regulate Ca 2+ uptake and the acrosome reaction in sea urchin sperm

Abstract

The egg jelly-induced acrosome reaction of sea urchin sperm is accompanied by intracellular alkalinization and Ca 2+ entry. We have previously shown that in the absence of egg jelly, NH 4Cl, which increases intracellular pH (pH i), induces Ca 2+ uptake and the acrosome reaction in sperm of the sea urchin, Strongylocentrotus purpuratus. Here we show that at a constant concentration of NH 4Cl (20 m M) in seawater, sperm react less as external pH is lowered from the normal 8 to 7.25. The pH dependence of the NH 4Cl response is not very sensitive to temperatures between 12 and 17°C. NH 4Cl (15–50 m M) stimulates Ca 2+ uptake and acrosome reactions in sperm suspended in Na +-free seawater, a condition known to inhibit the inductive effect of jelly. Jelly does not further stimulate Ca 2+ uptake of sperm preincubated in NH 4Cl, indicating that once the permeability to Ca 2+ is increased by raising the pH i, the jelly has no further effect. We have used the membrane potential-sensitive dye 3,3′-dipropylthiadicarbocyanine iodide to follow the membrane potential change that occurs when NH 4Cl is added. Depolarization (25 mV) is associated with the acrosome reaction when either the natural inducer, egg jelly, or NH 4Cl is added to sperm. Response to both inducers is inhibited under conditions known to abolish the acrosome reaction, i.e., low-pH seawater and nisoldipine. These results indicate that the NH 4Cl-induced depolarization that accompanies the reaction is probably due to the opening of channels that allow Ca 2+ to enter the cell and not to the depolarization by NH 4 + ions. High-K + seawater, which depolarizes sperm, and tetraethylammonium, a K + channel blocker, inhibit the jelly-induced depolarization and the acrosome reaction, but do not inhibit NH 4Cl-induced changes. It has already been shown that nigericin promotes Ca 2+ entry and the acrosome reaction in sea urchin sperm. We found that the action of this ionophore depends on the pH of normal seawater. In the absence of external Na + (replaced by choline), nigericin does not induce the reaction and does not stimulate Ca 2+ uptake.