Sodium and potassium fluxes across the dialyzed giant axon of Myxicola

Direct inhibitory action of EGTA-Ca complex on reverse-mode Na/Ca exchange in Myxicola giant axons

Giant axons from the marine annelid Myxicola infundibulum were internally dialyzed with solutions containing 22Na ions as tracers of Na efflux. In experiments performed in Li-substituted seawater, Na efflux that is dependent on external Ca ion concentration, [Ca2+]o, was measured using dialysis to maintain [Na+]i at 100 mM, which enhances the [Ca2+]o-dependent Na efflux component, (i.e., reverse-mode Na/Ca exchange). When dialysis fluid contained EGTA (1 mM) to buffer the internal Ca concentration, [Ca2+]i, to desired levels, Na efflux lost its normal sensitivity to external calcium. The inhibition was not simply due to the Ca-chelating action of EGTA to produce insufficient [Ca2+]i to activate Na/Ca exchange. The addition of EGTA inhibited Cao-dependent Na efflux even when a large enough excess of [Ca2+]i was present to saturate the EGTA and still produce elevated values of [Ca2+]i. Control experiments showed that these high values of [Ca2+]i resulted in normal Na/Ca exchange in the absence of EGTA. It is concluded that the presence of EGTA itself interferes with the manifestation of reverse-mode Na/Ca exchange in Myxicola giant axons.

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.

Application of the Goldman-Hodgkin-Katz current equation to membrane current-voltage data

A new approach for the analysis of current-voltage (IV) data is presented and applied to a variety of published data collected from various systems. Our analysis of published results shows that our method of analysis can account for the observed IV data. The calculated permeability coefficients are in reasonable agreement with those calculated from ion fluxes. In those cases where two ions are assumed to carry the current, the ratios of the calculated permeability coefficients are in agreement with those ratios determined from the Goldman-Hodgkin-Katz voltage equation. In most cases, the entire IV curve can be accounted for by using our method of analysis. In several examples, only a portion of the IV curve is in agreement with the predictions. We attribute the failure to account for the IV data to reflect the failure of one or more of the assumptions used by the GHK current equation. In other cases, assuming that an additional ion carries the current, the treatment can account for the IV data. However, the identity of the extra ion cannot be established from these published data without additional studies (e.g., ionic replacement studies).

Amino acid transport in Myxicola giant axon: stability of the amino acid pool, taurine efflux, and trans effect of sodium

1. The giant axon of Myxicola infundibulum was assessed for its suitability as a model preparation for study of amino acid transport mechanisms.2. The amino acid composition of axoplasm was measured and compared with those of coelomic fluid, muscle and axon sheath. The axoplasmic composition is unique. Axoplasm/coelomic fluid concentration ratios are all much larger than 1. The axoplasmic amino acid concentrations are (mmol/kg plasm): cysteic acid (104), aspartic acid (75), glutamic acid (10), taurine (64), serine (5), glycine (191) and alanine (5). Other amino acids or primary amines, if present, must have concentrations of less than 1 mm.3. The size of the sheath amino acid pool is 12% or less of the axoplasmic pool.4. The amino acid pool of axons soaked in sea water for up to 24 h is stable. Removal of Na from sea water causes a large increase of net efflux and net production of amino acids.5. Net amino acid production can not be detected in sheath. Metabolic production occurs in axoplasm with little accumulation. Time scales for production and net efflux are therefore similar.6. The Myxicola axon has a vigorous amino acid metabolism and transport systems capable of relatively large fluxes. Homeostasis is strongly linked to Na and may involve Na-coupled co-transport. Conservation of transmembrane amino acid gradients could be promoted in part by trans inhibition of efflux by external Na.7. Taurine is a useful model substrate because it is not catabolized in Myxicola and its net efflux is sensitive to Na. [(3)H]taurine efflux was measured from injected axons. Fluxes and internally recorded action potentials are stable for up to 6 h.8. Axon sheaths take up [(3)H]taurine from 10 mm-taurine sea water with an apparent half-time of 5 h. [(3)H]taurine washout from the apparent extracellular space has a half-time of 5 min. Washout from sheath cells has a half-time of 2-3 h. Sheath is not an important parallel compartment for taurine fluxes in the axon.9. Taurine efflux has a Q(10) of 1.8.10. Taurine efflux is insensitive to external taurine concentrations up to 10 mm.11. Taurine efflux is sensitive to external Na, but only if internal Na is high.12. Taurine is transported by a low-affinity Na-dependent system in Myxicola axon. Results could be explained by a carrier which is more mobile in the empty state than in the substrate-loaded state. Trans inhibition of taurine efflux by external Na is an important property of the system, and contributes to conservation of axoplasmic taurine.