Sodium and potassium fluxes across the dialyzed giant axon ofMyxicola

Abstract

Resting and stimulated fluxes of sodium and potassium across the giant axon of the marine annelid, Myxicola infundibulum, have been characterized using the technique of internal dialysis. In most respects the ion movements were found to be similar to those in squid axons. Sodium efflux and potassium influx were found to be active, cardiac glycoside-sensitive fluxes, with a variable coupling ratio. However, when [ATP]i was lowered to less than 20 microM by treatment with cyanide and continuous dialysis, or to less than 2 microM by dialysis with glucose following injection of hexokinase, Na efflux and K influx were unaltered. The maintained fluxes were not accounted for by an increased passive permeability of the axolemma, although 30-60% of the Na efflux appeared to be due to Na-Na exchange. An altered form of Na pump operation at low [ATP]i is a more likely explanation than an alternate energy source, or an ATP source proximate to the axolemma. The transient response of 22Na efflux to a change in [22Na]i was found to be much slower than in squid, tau = 360 sec. The efflux delay could only be accounted for by an extra-axonal diffusion barrier, which is probably the basement membrane surrounding the ventral nerve cord.

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

“…The last is especially likely for the acidic amino acids because their electrochemical driving forces are very large and directed outward. Forbush (1979) could not demonstrate a reduction in Na efflux when internal ATP was reduced to less than lOuxm, and often observed an increased Na efflux at low ATP levels. He could not account for this response, but certainly stimulation of Na co-transport of amino acids should be considered as a contributing factor.…”

“…However, the net flux estimates are upper bounds, owing to the end effect. Na efflux is either unchanged or depressed slightly in Nao-free sea waters, and only about 500 of Na efflux can be attributed to the Na pump (Abercrombie & Sjodin, 1977;Forbush, 1979). Thus about 10-20 pmol/cm2 .…”

“…Apart from some work by Caldwell & Lea (1978, 1979 and Baker & Potashner (1971, 1973 on amino acid fluxes in squid axon, the unique properties of giant axons have not been exploited in studies of amino acid transport. The giant axon of the marine worm Myxicola infundibulum.…”

“…However, the effects of metabolic poisons on other amino acid levels or on other substances which may regulate transport were not checked. Caldwell & Lea (1978, 1979 concluded that glycine influx and efflux are Na-independent but do depend on ATP. Oubain could inhibit influx but not efflux, probably via indirect effects on unknown, metabolically labile substances which interact with the transport system, because 30-100 min elapsed before the ouabain effect was seen, The unidirectional fluxes measured in squid by these authors are very small, more than an order of magnitude lower than the net effluxes reported here for Myxicola.…”

“…It is not unusual to find two groups of Myxicola axons. Forbush (1979) reported two groups defined by their resting Na effluxes. He could offer no explanation.…”