Stimulation of Na+,K+-ATPase activity, increase in potassium uptake, and enhanced production of ouabain-like compounds in ammonia-treated mouse astrocytes

Active potassium (K+) uptake and Na+,K+-ATPase activity were measured in primary cultures of mouse astrocytes. Both parameters were virtually unaffected by acute ammonia treatment but increased after chronic exposure to pathophysiologically relevant concentrations of ammonia (0.3 or 3 mM) for 1-4 days. The increased Na+,K+-ATPase activity after chronic treatment with ammonia was further enhanced in the acute presence of 12 mM K+. Based on these observations and literature data it was hypothesized that the direct effect of ammonia is formation of easily diffusible compound(s) with ouabain-like effect, that upregulation occurs of Na+,K+-ATPase activity and K+ uptake in response to the resulting ATPase inhibition, and that the washing procedure preceding the uptake experiments and the determination of Na+,K+-ATPase activity unmasks the upregulation. To test this hypothesis, the content of compounds with ouabain-like action was measured in media in which astrocytes had been incubated in the presence of 3 mM ammonia for 4 days and in controls to which an additional 3 mM NaCl had been added instead of ammonia. An endogenous, compound with ouabain-like activity was demonstrated both under control conditions and in the ammonia-treated cultures, and the content of this compound was increased by 50% in the ammonia-treated cultures. Preliminary experiments showed that at least part of the released ouabain-like compounds cross-react with authentic ouabain.