Measurement of intracellular pH in sea urchin eggs by ³¹P NMR

The intracellular pH (pH(i)) of sea urchin eggs was measured using ³¹P nuclear magnetic resonance (NMR) spectroscopy. The mean value of pH(i) of unfertilized eggs was about 6.2 (H. pulcherrimus and A. crassispina) to 6.3 (P. depressus). In contrast to results of other studies, pH(i) of sea urchin eggs was not changed after fertilization. During exposure of the eggs to NH₄Cl or procaine-containing natural sea water (NSW), however, pH(i) rose about 0.4-0.8 pH units; the pH(i) fell to its initial value upon washing the eggs with weak base-free NSW. These changes of pH(i) by weak base treatment agreed well with the data obtained by other workers. In order to understand the discrepancy of pH(i) changes in fertilized eggs between NMR data and other measuring procedures, we measured acid production and O₂ uptake, so that CO₂ accumulation and proton release did not result in alkalinization. The invariance of the fertilized eggs under anaerobic conditions; otherwise NMR showed a different answer from other measuring procedures, because of its particular characteristics such as non-destructivity and compartmentation of pH.