Slowly inactivating outward currents in a cuticular mechanoreceptor neuron of the cockroach (Periplaneta americana)

1. Although rapid adaptation is a widespread feature of sensory receptors, its ionic basis has not been clearly established in any touch receptor, because their small sizes have severely restricted the range of experiments tat can be performed. In the cockroach tactile spine, intracellular voltage-clamp recordings are now possible. 2. The basic electrophysiological properties of the cockroach femoral tactile spine neuron were studied using discontinuous (switching) single-electrode current- and voltage-clamp recordings. A slowly inactivating voltage-sensitive K+ outward current was detected after the major inward currents were blocked with tetrodotoxin. 3. The total outward current activated in < 1 ms at voltages above 0 mV. At moderate depolarizations it did not inactivate, but at higher depolarizations an inactivation time constant of approximately 260 ms was measured. Some recordings also revealed an additional, slower inactivation time constant of approximately 2.5 s. 4. More than half of the voltage-sensitive K+ outward current could be blocked with the Ca2+ channel blockers Co2+ and Cd2+. Tetraethylammonium chloride (TEA) also reduced the amplitude of the outward current to about half of its original amplitude. The actions of both blockers were reversible and probably reflect overlapping blockades of two separate outward currents. 5. The reversal potentials of the currents that remained after block with Co2+ (-91.7 mV) or TEA (-86.8 mV) were both near the K+ equilibrium potential expected for the tactile spine neuron. The voltage dependencies of activation of the Co(2+)- and TEA-resistant currents were both well fitted by Boltzmann distributions, giving values of half maximal activation (V50) equal to -34.5 mV for the Co(2+)-resistant current and -51.3 mV for the TEA-resistant current. 6. Current-clamp recordings revealed that the TEA-sensitive K+ current was the major component of action potential repolarization but that it did not effect the frequency of action potentials evoked by steady depolarization. On the other hand, blockers of Ca(2+)-sensitive K+ currents (Cd2+, Co2+, or charybdotoxin) reduced adaptation and increased the frequency of action potentials significantly but did not effect the duration or amplitude of individual action potentials.

Two outward K+ currents in bovine pigmented ciliary epithelial cells: IK(Ca) and IK(V)

Pigmented ciliary epithelial cells were studied using the whole cell voltage-clamp technique. Depolarizing steps from a holding potential of -80 mV resulted in a small initial inward current followed by a large outward current. Prolonged depolarizing voltage steps revealed inactivating and noninactivating components of outward current. Outward current was sensitive to the level of Ca2+ in the pipette and was increased by the calcium ionophore A23187; it was blocked by tetraethylammonium (TEA+), quinine, and 4-aminopyridine (4-AP). 4-AP blocked 70% of the outward current with a Ki of 7 x 10(-5) M, and part of the remaining current was abolished by Ni2+. Ni2+ caused a reduction in outward current by blocking IK(Ca) indirectly via decreasing Ca2+ entry through T-type Ca2+ channels. Separating Ni(2+)-sensitive from -insensitive outward conductance gives components that correspond notionally to IK(Ca) and IK(V), respectively. On this basis IK(Ca) represents approximately 28% of K+ outward current. Charybdotoxin blocked 26% of the outward conductance at very depolarized voltage steps as calculated from the slope of the current-voltage curve in this region. It is concluded that there are two major components to the outward current: IK(V), an inactivating voltage-sensitive K+ current, and IK(Ca), which is dependent on the entry of Ca2+ through T-type Ca2+ channels and comprises approximately a quarter of the total K+ outward current under the conditions described. Because of their relative voltage-activation properties, IK(Ca) will be the more important in terms of K+ transport and the secretion of aqueous humor by the ciliary epithelium.