Direct measurement of intracellular pH during metabolic derepression of the sea urchin egg

Abstract

SEA URCHIN eggs are shed in a metabolically repressed state and are derepressed by fertilisation. This derepression occurs in two phases, the first of which is caused by an intracellular release of Ca²⁺ (refs 1,2). It has been postulated that the second phase of derepression is accomplished by an increase of intracellular pH^3–5. Furthermore, attempts to measure intracellular pH by the use of cell homogenates have supported the idea that intracellular pH is increased during egg activation^5,6. We report here direct measurements of intracellular pH of sea urchin eggs during fertilisation and activation by ammonia. These measurements demonstrate the postulated increase of intracellular pH resulting from fertilisation or exposure to NH₄Cl, although the changes in pH differ from those reported in homogenates and seem to be regulated in a more complex manner than originally anticipated.