Temporal and spatial correlation of fertilization current, calcium waves and cytoplasmic contraction in eggs ofCiona intestinalis

Abstract

Eggs of the ascidianCiona intestinaliswere loaded with the calcium indicator fura-2 via whole-cell clamp electrodes and changes in cytoplasmic calcium and cell currents were monitored during fertilization either in separate eggs or simultaneously in the same egg. The first indication of egg activation was the fertilization current; which reached peak values around 1 nA after 30 s. A wave of elevated calcium was detectable between 5 s and 30 s (mean = 21 s) after the start of the fertilization current. This wave spread across the egg increasing cytoplasmic calcium levels to at least 10 μm. When the fertilization current and calcium wave were complete and cytoplasmic calcium levels were decreasing to pre-fertilization levels, a cortical contraction wave spread across the egg surface. In eggs showing normal fertilization current, the calcium wave and the contraction wave were in the same direction. A region of elevated calcium persisted at the animal pole. Changing cytoplasmic calcium levels locally by local application of ionophorre A23187 caused a contraction wave originating at the site of ionophore application. Increasing cytoplasmic calcium uniformly by facilitating calcium entry through voltage-regulated channels did not result in a contraction wave.