Regulatory and energetic role of Na + in amino acid uptake by fertilized sea urchin eggs

Abstract

Relationships between the Na + dependent amino acid uptake displayed by fertilized sea urchin eggs and the electrochemical gradient of Na + was investigated. The time course of Na + content and valine or alanine uptake was simultaneously monitored in Na + loaded eggs [by fertilization in K +-free artificial sea water (OK-ASW), or by using monensin, antimycin, cyanide, or ciguatoxin]. Our results demonstrate that the uphill amino acid uptake follows the “Na + gradient hypothesis.” Subsequent fertilization of eggs Na + depleted by ammonia for 40 min stimulates to a great extent the development of amino acid uptake as compared with controls eggs. By using simultaneous change of external and intracellular Na + concentration, we studied the specific role of this ion. An increase in internal Na + inhibits the uptake through trans inhibitory action while an increase in external Na + stimulates the efficiency of the uptake system. In eggs fertilized since 30 min, hyperpolarization obtained in K +-free ASW stimulates amino acid uptake while depolarization (transfer from K + free ASW to ASW) inhibits it. This potential-dependent effect developed after fertilization with a time course similar to that the establishment of K + conductance described by R. A. Steinhardt, L. Lundin, and D. Mazia (1971, Proc. Natl. Acad. Sci. USA 68, 2426–2430). In conclusion, our results point out that slight modulations in the activity of the Na + pump can widely affect the amino acid uptake, suggesting that activation of Na + K + ATPase has a key role in the stimulation of amino acid transport.