METABOLIC IMPORTANCE OF NA+ K+-ATPASE ACTIVITY DURING SEA-URCHIN DEVELOPMENT/

Early stages of animal development have high mass-specific rates of me tabolism, The biochemical processes that establish metabolic rate and how these processes change during development are not understood. In t his study, changes in Na+/K+-ATPase activity (the sodium pump) and rat e of oxygen consumption were measured during embryonic and early larva l development for two species of sea urchin, Strongylocentrotus purpur atus and Lytechinus pictus. Total (in vitro) Na+/K+-ATPase activity in creased during development and could potentially account for up to 77 % of larval oxygen consumption in Strongylocentrotus purpuratus (plute us stage) and 80 % in Lytechinus pictus (prism stage), The critical is sue was addressed of what percentage of total enzyme activity is physi ologically active in living embryos and larvae and thus what percentag e of metabolism is established by the activity of the sodium pump duri ng development, Early developmental stages of sea urchins are ideal fo r understanding the in vivo metabolic importance of Na+/K+-ATPase beca use of their small size and high permeability to radioactive tracers ( Rb-86(+)) added to sea water, A comparison of total and in vivo Na+/K-ATPase activities revealed that approximately half of the total activ ity was utilized in vivo, The remainder represented a functionally act ive reserve that was subject to regulation, as verified by stimulation of in vivo Na+/K+-ATPase activity in the presence of the ionophore mo nensin, In the presence of monensin, ill vivo Na+/K+-ATPase activities in embryos of S. purpuratus increased to 94 % of the maximum enzyme a ctivity measured in vitro, Stimulation of in vivo Na+/K+-ATPase activi ty was also observed in the presence of dissolved alanine, presumably due to the requirement to remove the additional intracellular Naf that was cotransported with alanine from sea water, The metabolic cost of maintaining the ionic balance was found to be high, with this process alone accounting for 40% of the metabolic rate of sea urchin larvae (b ased on the measured fraction of total Na+/K+-ATPase that is physiolog ically active in larvae of S. purpuratus). Ontogenetic changes in pump activity and environmentally induced regulation of reserve Na+/K+-ATP ase activity are important factors that determine a major proportion o f the metabolic costs of sea urchin development.