Gene expression and enzyme activities of the sodium pump during sea urchin development: implications for indices of physiological state.

The sodium pump consumes a large portion of the metabolic energy (40%) in sea urchin larvae. Understanding the developmental regulation of ion pumps is important for assessing the physiological state of embryos and larvae. We sequenced a partial cDNA clone (1769 bp) from the sea urchin Strongylocentrotus purpuratus and found it to contain the C-terminal portion of an open reading frame coding for 195 amino acids that exhibited high sequence similarity (89%) to invertebrate alpha-subunits of the Na+,K+-ATPase sodium pump. Northern blots using the 3' untranslated region of this cDNA specifically recognized a 4.6-kbp transcript under high stringency. During embryonic development, a rapid increase in levels of this mRNA transcript during gastrulation (25 h postfertilization) was paralleled by a concomitant increase in the total enzymatic activity of Na+,K+-ATPase. Expression of this subunit during 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 total RNA pool after 36 h closely followed a first-order exponential decay model (r2= 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 activity remained high. Molecular assays for the expression of this gene would underestimate sodium pump activities for assessing physiological state because of the temporal separation between maximal gene expression in a gastrula and maximal enzyme activities in the later larval stage. This finding illustrates the difficulty of using molecular probes for assessing the physiological state of invertebrate larvae.