Characteristics of sodium tail currents in Myxicola axons. Comparison with membrane asymmetry currents

Abstract

Sodium currents after repolarization to more negative potentials after initial activation were digitally recorded in voltage-clamped Myxicola axons compensated for series resistance. The results are inconsistent with a Hodgkin-Huxley-type kinetic scheme. At potentials more negative than -50 mV, the Na+ tails show two distinct time constants, while at more positive potentials only a single exponential process can be resolved. The time-course of the tail currents was totally unaffected when tetrodotoxin (TTX) was added to reduce gNa to low values, demonstrating the absence of any artifact dependent on membrane current. Tail currents were altered by [Ca++] in a manner consistent with a simple alteration in surface potential. Asymmetry current "off" responses are well described by a single exponential. The time constant for this response averaged 2.3 times larger than that for the rapid component of the Na+ repolarization current and was not sensitive to pulse amplitude or duration, although it did vary with holding potential. Other asymmetry current observations confirm previous reports on Myxicola.

Gallamine triethiodide‐induced modifications of sodium conductance in Myxicola giant axons.

“…The inward Na+ current during a pulse to -20 mV is followed by a small, rapidly decaying tail current since the pulse duration was not sufficiently long for complete inactivation to have occurred. The time course of this tail current was comparable to previous observations in untreated axons (Schauf et al 1977). After the pulse to + 100 mV (ENa = +69 mV) a large, slowly decaying inward Na+ tail current was observed which was eliminated by removal of external Na+ or application of tetrodotoxin.…”

“…Fig. 3 of Schauf et al 1977). With gallamine, time to peak tail current was approximately 40-60 ssec at -80 mV, 100 #asec at -40 mV, 200 ,usec at -20 mV, and 350 gsec at 0 mV.…”

“…However forshorter pulses repolarization may occur when GNa is high, and Na+ tail currents are then observed. These tail currents have 6-2 163 164 C. L. SCHAUF AND K. J. SMITH a complex time course (Schauf et al 1977;Goldman & Hahin, 1978) and thus their analysis is often difficult, particularly because the smaller, slow components can be quite labile (Schauf & Bullock, 1981). However, the fastest component is reasonably well defined and has a voltage-dependent time constant which ranges from 35-40 ,usec for V < -100 mV, to approximately 80 jtsec at -80 mV, and 200 ,tsec at -50 mV (Schauf, Bullock & Pencek, 1977).…”

Immobilization of intramembrane charge in Myxicola giant axons.

“…Armstrong have interpreted the slow component of the ON response as being a result of the transition of the sodium channel between two different conducting states (xl and x1y) and have supported their argument by observing that in squid the time constants of Na current tails and asymmetry current OFF responses vary with pulse duration (reflecting a transition between xl and x1y). Since neither the Na current tails or asymmetry current OFF responses are appreciably sensitive to pulse duration in Myxicola (Schauf et al 1977) it may well be that the xly state is either lacking or not substantially occupied at similar potentials in Myxicola, accounting for the lack of a slow component in the ON response.…”

“…The only change from those procedures was to replace the K++TEA+ dialysate used in previous studies by a K+-free solution containing 600 mM-Cs+ and 50 mM-F-(plus 550 mM-glutamate and 1 mM-Hepes) to eliminate completely currents through the K+ channel for long duration, large amplitude depolarizations. The external solution was K+-free artificial sea-water (Schauf et al 1977) in those cases where Na+ currents were measured, while the external Na+ was replaced by Tris (hydroxymethyl) aminomethane with 10-6 M-tetrodotoxin added for the measurement of asymmetry currents. Internal and external pHs were 7-3 ± 0-1 and 7-8 ± 0-1 respectively.…”