Influence of ATP and calcium on the cortical reaction in sea urchin eggs

Abstract

THE prevention of polyspermy in the sea urchin egg is a two-stage process: a rapid reduction in the chances of refertilisation—the ‘fast block’^1,2, which seems to be electrical in nature³—and a slower block that coincides with elevation of a morphological barrier, the fertilisation membrane. The fast block has many features in common with the action potential of excitable tissues, while elevation of the fertilisation membrane seems to involve a classical exocytotic reaction similar to that found at nerve terminals. Lying immediately beneath the plasma membrane of unfertilised eggs is an array of vesicles called cortical granules each about 1 µm in diameter. Shortly after fertilisation the contents of these granules are expelled, the granule membrane becoming incorporated into the plasma membrane of the egg. The trigger for initiating the cortical reaction seems to be a rise in intracellular ionised calcium. Thus granule release can also be promoted by the ionophore A23187 (ref. 4) and measurements in aequorin-loaded fish and sea urchin eggs^5,6 show that exocytosis brought about by both sperm and ionophore is associated with a rise in intracellular ionised calcium. The mechanism of exocytosis in any system is unknown and the cortical granule reaction in sea urchin eggs seems to provide an excellent experimental preparation for its investigation. Here we examine the influence of metabolic poisons and calcium on the cortical reaction. In the presence of ATP exocytosis is activated by micromolar concentrations of calcium; but in the nominal absence of ATP the exocytotic process slowly becomes refractory to calcium.