Membrane currents of the tunicate egg under the voltage‐clamp condition.

Abstract

SUMMARY1. Ionic currents of the egg membrane of a certain tunicate, Halocynthia roretzi Drashe, were studied by the voltage-clamp technique.2. The membrane depolarization beyond -55 mV in standard artificial sea water induced mainly transient inward current and slight outward currents, when the holding potential was kept at -90 mV.3. The transient inward current was composed of two components; the major one showed a faster time course, a more negative critical level of about -55 mV, and a reversal potential around + 60 mV and the minor one showed a slower time course, a less negative critical level of -10 mV, and no definite reversal potential.4. The major component became maximum at about -25 mV with the peak time of 6-9 msec at 15°C, and the maximum currents ranged from 0-5 to 15 x10-5 A/cm2.5. The major component of the inward current was abolished by the replacement of Na with choline or Tris or Cs ions, while it was almost unaltered by the replacement with Li. The minor component was independent of Na concentration in the external solution.6. This major component showed the activation and inactivation identical with those of Na current of other excitable membranes. A conditioning depolarization over -90 mV inactivated the Na current and the half inactivation of the major inward current was obtained by a conditioning pulse to -56 mV, when the pulse duration was 400 msec and the temperature was at 150 C. 6HARUMASA OKAMOTO AND OTHERS 7. The time course of the Na current was formulated with m and h parameters in the following equations: i=a gia(Em -ENa)m h . gNa(Eme-tENa)M2 Ie-Tm)2e-t/7h; M = o+ (m,m-o)(O)e-tIrm), h = h, + (ho -ho)e-tITh. 8. The kinetic parameters Tm and Th of egg Na current were calculated and compared with those of the squid axon. The potential dependence of Tm and Th was almost identical with that of the axon, but the absolute values of both Tm and Th were ten-to twentyfold larger than those of the axon in any range of the membrane potential.9. The temperature dependence of the kinetic parameters Tm, Th and of the chord conductance 9Na was studied. The Q10's for Tm and Th were both 4 0, while the Q10 for 9Na was 2-0 in the temperature range from 5 to 200 C.10. The outward and inward rectifying conductances of egg membrane were remarkably activated at the potential level above + 100 mV and below -70 mV respectively in standard artificial sea water. Both increased currents were subsequently subject to inactivation.11. It was suggested that Na, Ca, K inward rectifying and K outward rectifying conductances all exist separately in the egg cell membrane and the Na current was essentially identical with that through the Na channel in other excitable membranes.

Unitary current through sodium channel and anomalous rectifier channel estimated from transient current noise in the tunicate egg.

“…A depolarizing test pulse applied to the tunicate egg cell membrane activates the Na channel (Okamoto et al 1976). The transient Na current was digitized at fixed intervals of 70-200 gsec.…”

“…Not only Na channels but also voltage-dependent Ca channels elist in the egg cell membrane of the tunicate (Okamoto et al 1976). Both channels are activated by the depolarizing test pulse.…”

A Weakly Inward Rectifying Potassium Channel of the Salmon Brain

“…The presence has also been known in egg cells of starfish (10,11) and tunicates (12), which suggests the functional significance in wide ranged species of the animal kingdom.…”

Neural induction suppresses early expression of the inward‐rectifier K+ channel in the ascidian blastomere.

“…Previously, ion channels in unfertilized eggs (Okamoto, Takahashi & Yoshii, 1976) and their changes after fertilization (Kozuka & Takahashi, 1982) have been described in the ascidian, using the two-electrode voltage-clamp technique and, more recently, the whole-cell patch-clamp technique (Block & Moody, 1987). Membrane properties of ascidian gastrulae have also been studied by intracellular recording from cells in whole embryos in which all the blastomeres are tightly electrically coupled, and the existence of inward-rectifier K+ channels at the gastrula stage has been suggested (Miyazaki, Takahashi, Tsuda & Yoshii, 1974).…”

“…Ionic currents were recorded using a conventional two-microelectrode voltage-clamp method (Okamoto et al 1976). The membrane potential was held at -75 mV.…”