IONIC CONDUCTANCES OF MONKEY SOLITARY CONE INNER SEGMENTS

1. The membrane properties of cone inner segments dissociated enzymati cally from monkey retina were studied under voltage-clamp conditions u sing patch pipettes in the whole-cell clamp configuration. 2. A nonina ctivating, voltage-gated calcium current was evoked at potentials posi tive to -60 mV and peaked between -30 and -20 mV when barium was subst ituted for calcium. Cadmium (50 mu M) but not nickel (50 mu M) blocked the current. 3. A large calcium-activated anion current (I-An) was ob served when the membrane potential was set to a level between -60 and 30 mV. The reversal potential of I-An was O mV with chloride as the so le anion and about -30 and -40 mV when methanesulfonate and D-aspartat e, respectively, replaced intracellular chloride to set the equilibriu m potential for chloride at -50 mV. I-An inactivated and oscillated wh en the membrane potential was maintained at depolarized levels, contra ry to calcium-activated anionic currents seen in photoreceptors of oth er species. 4. A sustained-type potassium current was activated by dep olarizations positive to -50 mV. The time course of activation and dea ctivation were voltage dependent. This potassium current was partially blocked by 20 mM tetraethylammonium ions. 5. A transient potassium cu rrent was activated by depolarizations positive to -20 mV. This curren t was blocked by 4-amino-pyridine (2 mM) and inactivated with a time c onstant of similar to 500 ms. The amplitude in response to voltage ste ps to 45 mV was decreased by prepulses to voltages more positive than -30 mV. 6. Hyperpolarization negative to -65 mV activated an inward cu rrent that was completely blocked by external cesium (10 mM). The reve rsal potential suggested a conductance mechanism permeable to both sod ium and potassium ions. 7. A calcium-activated potassium current, whic h was found in salamander photoreceptors, was not detected. 8. The pre sence of these conductances is expected to influence the membrane pote ntial and the time course of the light response in monkey cones.