Controls on 22 Na + Influx in Corn Roots

Abstract

We have investigated the effects of hyperpolarization and depolarization, and the presence of K(+) and/or Ca(2+), on (22)Na(+) influx into corn (Zea mays L.) root segments. In freshly excised root tissue which is injured, Na(+) influx is unaffected by hyperpolarization with fusicoccin, or depolarization with uncoupler (protonophore), or by addition of K(+). However, added Ca(2+) suppresses Na(+) influx by 60%. In washed tissue which has recovered, Na(+) influx is doubled over that of freshly excised tissue, and the influx is increased by fusicoccin and suppressed by uncoupler. This energy-linked component of Na(+) influx is completely eliminated by low concentrations of K(+), leaving the same level and kind of Na(+) influx seen in freshly excised roots. The K(+)-sensitive energy linkage appears to be by the carrier for active K(+) influx. Calcium is equally inhibitory to Na(+) influx in washed as in fresh tissue. Other divalent cations are only slightly less effective. Net Na(+) uptake was about 25% of (22)Na(+) influx, but proportionately the response to K(+) and Ca(2+) was about the same.The constancy of K(+)-insensitive Na(+) influx under conditions known to hyperpolarize and depolarize suggests that if Na(+) transport is by means of a voltage-sensitive channel, the rise or fall of channel resistance must be proportional to the rise or fall in potential difference. The alternative is a passive electroneutral exchange of (22)Na(+) for endogenous Na(+). The data suggest that an inwardly directed Na(+) current is largely offset by an efflux current, giving both a small net uptake and isotopic exchange.