Voltage-gated potassium conductances in Gymnotus electrocytes(AB)

Electrocytes are muscle-derived cells that generate the electric organ discharge (EOD) in most gymnotiform fish. We used an in vitro preparation to determine if the complex EOD of Gymnotus carapo was related to the membrane properties of electrocytes. We discovered that in addition to the three Na(+)-mediated conductances described in a recent paper [Sierra F, Comas V, Buño W, Macadar O (2005) Sodium-dependent plateau potentials in electrocytes of the electric fish Gymnotus carapo. J Comp Physiol A 191:1-11] there were four K(+)-dependent conductances. Membrane depolarization activated a delayed rectifier (I(K)) and an A-type (I(A)) current. I(A) displayed fast voltage-dependent activation-inactivation kinetics, was blocked by 4-aminopyridine (1 mM) and played a major role in action potential (AP) repolarization. Its voltage dependence and kinetics shape the brief AP that typifies Gymnotus electrocytes. The I(K) activated by depolarization contributed less to AP repolarization. Membrane hyperpolarization uncovered two inward rectifiers (IR1 and IR2) with voltage dependence and kinetics that correspond to the complex "hyperpolarizing responses" (HRs) described under current-clamp. IR1 shows "instantaneous" activation, is blocked by Ba(2+) and Cs(+) and displays a voltage and time dependent inactivation that matches the hyperpolarizing phase of the HR. The activation of IR2 is slower and at more negative potentials than IR1 and is resistant to Ba(2+) and Cs(+). This current fits the depolarizing phase of the HR. The EOD waveform of Gymnotus carapo is more complex than that of other gymnotiform fish species, the complexity originates in the voltage responses generated through the interactions of three Na(+) and four K(+) voltage- and time-dependent conductances although the innervation pattern also contributes [Trujillo-Cenóz O, Echagüe JA (1989) Waveform generation of the electric organ discharge in Gymnotus carapo. I. Morphology and innervation of the electric organ. J Comp Physiol A 165:343-351].