Gene expression and enzyme activities of the sodium pump during sea urchindevelopment: Implications for indices of physiological state

The sodium pump consumes a large portion of the metabolic energy (40%) in s ea urchin larvae. Understanding the developmental regulation of ion pumps i s important for assessing the physiological state of embryos and Larvae. We sequenced a partial cDNA clone (1769 bp) from the sea urchin Strongylocent rotus purpuratus and found it to contain the C-terminal portion of an open reading frame coding for 195 amino acids that exhibited high sequence simil arity (89%) to invertebrate a-subunits of the Na+,K+-ATPase sodium pump. No rthern blots using the 3' untranslated region of this cDNA specifically rec ognized a 4.6-kbp transcript under high stringency. During embryonic develo pment, a rapid increase in levels of this mRNA transcript during gastrulati on (25 h postfertilization) was paralleled by a concomitant increase in the total enzymatic activity of Na+,K+-ATPase. Expression of this subunit duri ng gastrulation increased to a maximum at 36 h, followed by a rapid decline to trace levels by 60 h. The rate of removal of the transcript from the to tal RNA pool after 36 h closely followed a first-order exponential decay mo del (r(2) = 0.988), equivalent to a degradation rate of 7.8% h(-1). By 83 h , transcription of the alpha -subunit gene was low, yet sodium pump activit y remained high. Molecular assays for the expression of this gene would und erestimate sodium pump activities for assessing physiological state because of the temporal separation between maximal gene expression in a gastrula a nd maximal enzyme activities in the later larval stage. This finding illust rates the difficulty of using molecular probes for assessing the physiologi cal state of invertebrate larvae.