Sodium‐potassium exchange in sea urchin egg. II. Ionic events stimulating the Na+‐K+ pump activity at fertilization

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Abstract
The preceding paper (Ciapa et al., 1984) provided biochemical and kinetic characterization of the Na+‐K+ exchange in Paracentrotus lividus eggs. The present work is a study of the ionic events involved in the stimulation of the Na+‐K+ transporter after fertilization. Fertilization in low Na+‐external medium containing amiloride (0.1 mM) suppresses the stimulation of the net efflux of H+ and 86Rb uptake. Activation of eggs with the ionophore A23187 leads to stimulation of both Na+‐H+ exchange and ouabain‐sensitive 86Rb influx. When eggs were activated with A23187 in artificial seawater, 86Rb uptake and 24Na influx showed similar saturable kinetics with respect to the external Na+. A23187 treatment of eggs in Na+ ‐free artifical seawater did not stimulate the Na+‐K+ exchange until 10 mEq Na+ was added. Activation of eggs by NH4Cl (5 mM) stimulated 86Rb influx and Na+ exit; both fluxes were ouabain sensitive. Monensin increased cell Na+ of unfertilized eggs without any significant increase in intracellular pH: a condition in which 86Rb influx was not markedly stimulated. Addition of 10 mEq Na+ to unfertilized eggs in Na+‐free artificial seawater stimulated 86Rb uptake but to a lower extent that did 10 mEq Na+ plus sperm. It is concluded that (1) the stimulation of the Na+‐K+ pump at fertilization has an absolute requirement for the Na+‐H+ exchange; (2) the alkalinization of eggs resulting from the acid efflux is a prerequisite for the enhancement of the Na+‐K+ pump; (3) the amount of Na+ entering eggs at fertilization determines the intensity of the Na+‐K+ exchange; (4) early events of fertilization such as exocytosis and calcium release which may be involved in the stimulation of the Na+‐K+ pump must necessarily be coupled to cell alkalinization.