Journal of Neuroscience, Vol 15, 319-332, Copyright © 1995 by Society for Neuroscience
Characterization of voltage-activated currents in Hermissenda type B- photoreceptors
J Acosta-Urquidi and T Crow
Department of Neurobiology and Anatomy, University of Texas Medical School, Houston 77225.
Two distinct voltage-dependent K+ currents have been previously identified
in the type B-photoreceptors of Hermissenda: an early, rapidly inactivating
K+ current (IA), and a late, 4-AP- and TEA- resistant voltage and
Ca(2+)-dependent current (IK(Ca)). Using conventional two-electrode
voltage-clamp techniques, we have characterized two additional currents, a
late voltage-dependent outward K+ current (IK(v)) and a voltage-dependent
inward current identified as an inward rectifier (Iir). In addition, we
have further studied the activation-inactivation kinetics of IA. In 0-Na+
ASW, Iir activates at a potential of > -50 mV, is steeply
voltage-dependent and noninactivating, and reaches steady-state within 800
msec to 3 sec at - 100 mV. In addition to the variability in activation
kinetics, there was also considerable variability in Iir magnitude (-5 to
-80 nA, at - 100 mV). Iir was blocked by external 4-AP (5 mM), external and
internal TEA, internal Cs+, but not external Ba2+. The major component of
outward K+ current in type B-photoreceptors is IK(v), the delayed
rectifier. IK(v) was isolated after removal of IA and IK(Ca). IK(v)
activates at around -25 mV or more positive membrane potentials and its
activation and inactivation are strongly voltage dependent. IK(v)
inactivation to steady state is reached within 1.5-2.5 sec. The wide range
of activation-inactivation rates suggests that there may be kinetic
subtypes of IK(v). The proposed "slow" IK(v) peaked in 50-90 msec at +30
mV, and decayed with a single exponential component with an average tau off
of 279 msec. Proposed "intermediate" and "fast" IK(v) subtypes peaked
within 12-50 msec at +30 mV, and had a decay fitted by two exponentials,
with an average tau 1 of 147 msec and tau 2 of 275 msec, respectively.
IK(v) exhibited marked twin-pulse inactivation with a recovery time of
30-40 sec, and also exhibited time- and voltage- dependent cumulative
inactivation to repeated depolarizing pulses. Both types of inactivation
were quickly removed by a prepulse hyperpolarization. 4-AP (5 mM) produced
partial to complete block of the inactivating component of IK(v), leaving
only a residual sustained component. Complete block of the transient and
sustained components of IK(v) was obtained by 100 mM TEA. Reliable voltage
separation of IA from IK(v) was achieved by activating IA in the range of
-50 to -20 mV, from a Vh of -80 mV. Voltage-dependent steady-state
inactivation curves for IA were determined, yielding an average h0.5 value
of -56 mV.(ABSTRACT TRUNCATED AT 250 WORDS)
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