Two different Na,K-ATPases in the optic nerve: cells of origin and axonal transport

Two molecular forms of Na,K-ATPase can be isolated from the central nervous system. The two forms can be distinguished by their sensitivities to cardiac glycosides and by the electrophoretic mobilities of their catalytic subunits, alpha and alpha(+). Because Na,K-ATPase is a membrane-bound enzyme, it would be predicted to move in the rapid phase of axonal transport, and this was used as a means to determine which form(s) is made by a defined neuron of the central nervous system. Retinal ganglion cells were labeled in vivo by intravitreal injection of [35S]methionine; the Na,K-ATPase that was axonally transported down the optic nerve was purified, and the alpha and alpha(+) forms were separated by electrophoresis and detected by fluorography. The two forms were synthesized in the retina in approximately equal amounts. The alpha(+) form was the predominant form transported from the retinal ganglion cells to the lateral geniculate nucleus and superior colliculus. The oligodendrocytes and other sheath cells of the excised optic nerve, in contrast, synthesized only the alpha form when incubated in vitro with [35S]methionine. The labeled Na,K-ATPase found at the nerve endings always included a small amount of the alpha form in addition to the alpha(+) form. The proportions of the two forms did not change with time after transport, and the presence of labeled alpha was not affected by infusion of cycloheximide to inhibit intracranial protein synthesis. Hence, although alpha(+) is the predominant form, the evidence suggests that small amounts of the alpha form are also made and transported by retinal ganglion cells.