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 meas ured in primary cultures of mouse astrocytes. Both parameters were virtuall y unaffected by acute ammonia treatment but increased after chronic exposur e to pathophysiologically relevant concentrations of ammonia (0.3 or 3 mM) for 1-4 days. The increased Na (+),K (+) -ATPase activity after chronic tre atment with ammonia was further enhanced in the acute presence of 12 mM K ( +). Based on these observations and literature data it was hypothesized tha t the direct effect of ammonia is formation of easily diffusible compound(s ) with ouabain-like effect, that upregulation occurs of Na (+),K (+) -ATPas e activity and K (+) uptake in response to the resulting ATPase inhibition, and that the washing procedure preceding the uptake experiments and the de termination of Na (+),K (+) -ATPase activity unmasks the upregulation. To t est 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 h ad 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 oua bain, (C) 2000 Elsevier Science Ltd. All rights reserved.