31P-NMR analysis of sea urchin sperm activation : Reversible formation of high energy phosphate compounds by changes in intracellular pH

31P-NMR has been used to estimate the internal pH (pH i ) of sperm from the sea urchin Strongylocentrotus purpuratus. The values for pH i obtained from the chemical shift of inorganic phosphate agree well with those obtained from amine accumulation. At low pH i , when sperm are quiescent (immotile and non-respiring), they accumulate phosphocreatine (PCr), but have a low level of inorganic phosphate (P i ). Conversely, when the pH i is elevated, sperm respiration and motility are activated, PCr is decreased and P i is increased. This change is reversible upon decrease of the pH i , whereupon respiration and motility are arrested, P i disappears and PCr increases. We conclude that the overall balance of energy metabolism, and thus the phosphate potential, of sea urchin sperm are under the control of the pH i .