CHARACTERIZATION OF VOLTAGE-ACTIVATED CURRENTS IN HERMISSENDA TYPE B-PHOTORECEPTORS

Two distinct voltage-dependent K+ currents have been previously identi fied in the type B-photoreceptors of Hermissenda: an early, rapidly in activating K+ current (I-A), and a late, 4-AP- and TEA-resistant volta ge and Ca2+-dependent current (I-k(Ca)). Using conventional two-electr ode voltage-clamp techniques, we have characterized two additional cur rents, a late voltage-dependent outward K+ current (I-k(v)) and a volt age-dependent inward current identified as an inward rectifier (I-ir). In addition, we have further studied the activation-inactivation kine tics of I-A. In O-Na+ ASW, I-ir activates at a potential of >-50 mV, i s steeply voltage-dependent and noninactivating, and reaches steady-st ate within 800 msec to 3 sec at -100 mV. In addition to the variabilit y in activation kinetics, there was also considerable variability in I -ir magnitude(-5 to -80 nA, at -100 mV). I-ir 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-photorecept ors is I-k(v) the delayed rectifier. I-k(v) was isolated after removal of I-A and I-k(Ca). I-k(v) activates at around -25 mV or more positiv e membrane potentials and its activation and inactivation are strongly voltage dependent. I-k(v) inactivation to steady state is reached wit hin 1.5-2.5 sec. The wide range of activation-inactivation rates sugge sts that there may be kinetic subtypes of I-k(v). The proposed ''slow' ' I-k(v) peaked in 50-90 msec at +30 mV, and decayed with a single exp onential component with an average tau(off) of 279 msec. Proposed ''in termediate'' and ''fast'' I-k(v) subtypes peaked within 12-50 msec at +30 mV, and had a decay fitted by two exponentials, with an average ta u(1) Of 147 msec and tau(2) of 275 msec, respectively. I-k(v) exhibite d marked twin-pulse inactivation with a recovery time of 30-40 sec, an d also exhibited time- and voltage-dependent cumulative inactivation t o repeated depolarizing pulses. Both types of inactivation were quickl y removed by a prepulse hyperpolarization. 4-AP (5 mM) produced partia l to complete block of the inactivating component of I-k(v), leaving o nly a residual sustained component. Complete block of the transient an d sustained components of I-k(v) was obtained by 100 mM TEA. Reliable voltage separation of I-A from I-k(v) was achieved by activating I-A i n the range of -50 to -20 mV, from a V-h of -80 mV. Voltage-dependent steady-state inactivation curves for I-A were determined, yielding an average h(0.5) value of -56 mV. The decay of I-A was fitted by two exp onentials with time constants, tau(1) 108 msec and tau(2) 185 msec.