Glial control of neuronal excitability in mammals: II. Enzymatic evidence : Two molecular forms of the (Na(+),K(+))-ATPase in brain.

The arguments supporting the existence of two molecular forms of the (Na(+), K(+))-ATPase in mammalian brain are reviewed. 1. Bulk isolated glial cells (Na(+), K(+))-ATPase activities are highly stimulated by K(+) ions between 5 and 20 mM. This phenomenon was reproduced with plasma membranes prepared from these cells but did not exist in neuronal (i.e. perikarial cells, neuronal membranes or synaptosomes) preparations. 2. This glial enzyme characteristic was not observed in young animals with histoenzymologically immature astrocytes. 3. The glial K(+)-dependent-paranitrophenylphosphatase (K(+)-pNPPase) was also activated by [K(+)](o) from 5 to 20 mM while no significant modification was shown for neuronal enzyme. 4. Kinetic analysis of the (Na(+), K(+))-ATPase activities, on the basis of the hysteretic model, demonstrated that the so-called physiological efficiency (V(max)/K(m) app) of glial enzyme was highly increased by [K(+)](o) from 5 to 20 mM. This phenomenon was not observed in neuronal preparations. These data indicated the existence in glial membranes of a (Na(+), K(+))-ATPase different from the neuronal enzyme and, because its particular structure, highly activated in presence of elevated [K(+)](o) (20 mM) the evolutionnary significance of this phenomenon is an active control of [K(+)](o) by glia.