Membrane currents of the tunicate egg under the voltage-clamp condition

Area of Science:

• * Cellular electrophysiology
• * Ion channel biophysics
• * Marine invertebrate biology

Background:

• * Understanding ion transport mechanisms in non-neuronal excitable cells is crucial for comparative physiology.
• * The egg membrane of the tunicate Halocynthia roretzi presents a model system for studying fundamental electrical properties.

Purpose of the Study:

• * To investigate the ionic currents present in the egg membrane of Halocynthia roretzi using the voltage-clamp technique.
• * To characterize the properties of transient inward currents, particularly the sodium (Na) current component.
• * To compare the kinetic properties of the egg membrane Na current with those found in other excitable tissues.

Main Methods:

• * Voltage-clamp technique applied to Halocynthia roretzi egg cells.
• * Analysis of transient inward and outward currents during membrane depolarization.
• * Ionic substitution experiments (Na+, choline+, Tris+, Cs+, Li+) to identify current carriers.
• * Kinetic analysis of current activation and inactivation using voltage pulses.
• * Temperature dependence studies (Q10) of kinetic parameters and conductance.

Main Results:

• * Depolarization induced a major transient inward current with characteristics of a Na current, including activation/inactivation kinetics and reversal potential.
• * The Na current was abolished by replacing external Na+ with choline+, Tris+, or Cs+, but not Li+.
• * Kinetic parameters (taum, tauh) of the egg Na current were significantly larger than those in squid axons but showed similar voltage dependence.
• * Temperature dependence (Q10) for Na current kinetics was 4.0, and for conductance was 2.0.
• * Evidence for separate inward and outward rectifying conductances (Ca2+, K+) was also observed.

Conclusions:

• * The major transient inward current in Halocynthia roretzi egg membrane is a Na current, functionally similar to Na channels in other excitable cells.
• * The kinetic properties of this egg Na current are distinct from squid axon Na channels, exhibiting slower activation and inactivation.
• * The study suggests the presence of multiple, distinct ion channel types (Na+, Ca2+, K+) in the tunicate egg membrane.