Specific Activity of Na⁺ K⁺ ATPase Is Not Altered in Response to Changing Salinities during Early Development of the Brine Shrimp Artemia franciscana

Abstract

The effects of salinity on the specific activity of ouabain-sensitive Na⁺ K⁺ ATPase were investigated in Artemia franciscana embryos and nauplii. Whole organismal specific activity of the enzyme was measured in organisms exposed to chronic (constant) salinity conditions of 5-, 12-, 32-, or 50-ppt seawater. During emergence and throughout early stage 1 naupliar development, the specific activity of Na⁺ K ⁺ ATPase increased at all external salinities. No significant differences in enzyme activities were observed among the salinity treatments. The pattern and levels of enzyme activity suggest a genetically determined program of enzyme synthesis that is not influenced by the external salinity. The specific activity of the enzyme was also measured after exposure of stage 1 nauplii to acute hypoosmotic and hyperosmotic conditions during both the early and late stages of the observed increase in enzyme activity. The apparent synthesis of Na⁺ K⁺ ATPase was not influenced when stage 1 nauplii were exposed to an abrupt salinity change. These results suggest that, in stage 1 nauplii, this ontogenetic program of enzyme synthesis overrides any acute osmotic stress responses that might involve changes in the total amount of the enzyme. During early naupliar development, modulation of the preexisting enzyme may be a more important mechanism of ion regulation than is modulation of the enzyme's synthesis and degradation.