Anomalous influence of reduced internal ATP levels on sodium efflux in Myxicola giant axons.

Giant axons from the marine annelid, Myxicola infundibulum, were internally dialyzed with ATP-free media and with media with lower than normal ATP levels in an attempt to determine quantitatively the ATP requirement of the Na pump in these cells. This was accomplished by using 22Na ions to measure Na efflux. When [ATP]i in dialysis fluid fell to values within the range of 20-40 microM, a marked stimulation of Na efflux was observed even though an essentially normal ouabain sensitivity of Na efflux persisted; when axons were dialyzed with ATP-free solutions with ouabain present in the external medium throughout the dialysis period, the stimulation of Na efflux still occurred. The stimulation of Na efflux produced by low [ATP]i levels could be reversed by reintroducing normal ATP levels into the dialysis medium. Reversibility was complete provided axons were not depleted of ATP for periods longer than about 1 hr. Longer periods of ATP depletion led to larger and ultimately irreversible increases in Na efflux. The increases in Na efflux occasioned by ATP depletion either prevented or obscured the decrease in Na efflux expected to occur from unfueling the Na pump. Since [ATP]i levels required to significantly unfuel the Na pump lie below the levels at which the Na efflux stimulation occurred, it is problematic to quantitatively assess the influence of [ATP]i levels on Na pump rate by measurements of Na efflux in this preparation. Substitutes for ATP failed to prevent increases in Na efflux. The large increases in Na efflux observed at low [ATP]i occurred with no important changes in the resting membrane potential, and also occurred in Na-free and Ca-free external media. At least part of the increased Na efflux under these conditions may be due to a Na/Na exchange component, as a significant dependence of Na efflux on [Na]o appropriate for this kind of exchange was observed in the ATP-depleted axons. Whether the highly reproducible anomalous effect on Na efflux in Myxicola axons has some fundamental significance in its own right is a matter for future investigation. A few possible explanations of the anomalous effect of reduced ATP levels are discussed.