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

Abstract

Two molecular forms of Na,K-ATPase can be isolated from the CNS. The 2 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 CNS. 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 seaprated by electrophoresis and detected by fluorography. The 2 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 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 maount of the .alpha. form in addition to the .alpha.(+) form. The proportions of the 2 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. Although .alpha.(+) is the predominant form, the evidence suggest that small amounts of the .alpha. form are also made and transported by retinal ganglion cells.