HYPERPOLARIZING CURRENT OF THE NA K ATPASE CONTRIBUTES TO THE MEMBRANE POLARIZATION OF THE PURKINJE-CELL IN RAT CEREBELLUM/

The contribution of the Na/K ATPase (pump) current to the polarization of the Purkinje cell has been studied using slices of the rat cerebel lum by blocking the pump with dihydro-ouabain (DHO) while recording th e membrane potential with microelectrodes in the somata. From our reco rdings, it appeared that blocking the pump depolarized the Purkinje ce lls more rapidly than might be expected from shifts in Na+ and K+ conc entrations, suggesting the removal of a hyperpolarizing current. Appli cation of DHO, in the presence of tetrodotoxin (TTX), led to calcium s pike firing and plateau-like discharges suggesting activation of volta ge-dependent calcium channels in the dendrites. Adding 2 mM Co2+ to th e medium did not prevent the depolarizations. Removing calcium from th e bathing medium containing 2 mM Co2+ blocked the spiking activity but DHO application still produced a depolarization. Experiments to measu re the current inhibited by DHO indicated that the Na/K pump supplies a constant current of 240 pA. Substitution of the sodium with choline produced a hyperpolarization, during which DHO had no effect on the me mbrane potential. Substitution of the sodium with lithium produced onl y a slowly developing depolarization. It is concluded that in the cere bellar Purkinje cell, a continuous sodium ion influx activates the pum ps which produce a current that directly contributes to the membrane p olarization. Possible pathways for this sodium influx are discussed.