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Calcium‐activated potassium current clamps the dark potential of vertebrate rods

2001; Wiley; Volume: 14; Issue: 1 Linguagem: Inglês

10.1046/j.0953-816x.2001.01605.x

1460-9568

Andrea Moriondo, Bruna Pelucchi, Giorgio Rispoli,

Neuroscience and Neuropharmacology Research

Abstract Vertebrate photoreceptors respond to light with a graded hyperpolarization from a membrane potential in the dark of ≈ −35 mV. The present work investigates the physiological role of the Ca 2+ ‐activated K + current in the photovoltage generation in mechanically isolated rods from salamander retina. Membrane current or voltage in isolated rods was recorded from light‐ and dark‐adapted rods under voltage‐ or current‐clamp conditions, respectively. In light‐adapted rods of the salamander, selective blockade of Ca 2+ ‐activated K + channels by means of charybdotoxin depolarized the plasma membrane of current‐clamped rods by ≈ 30 mV, from a resting potential of ≈ −35 mV. A similar depolarization was observed if external Ca 2+ (1 m m ) was substituted with Ba 2+ or Sr 2+ . Under control conditions, the injection of currents of increasing amplitude (up to −100 pA, to mimic the current entering the rod outer segment) could not depolarize the membrane potential beyond a saturating value of ≈ −20 mV. However, in the presence of charybdotoxin, rods depolarized up to +20 mV. In experiments with dark‐adapted current‐clamped rods, charybdotoxin perfusion lead to transient depolarizations up to 0 mV and steady‐state depolarizations of ≈ 5 mV above the dark resting potential. Finally, the recovery phase of the voltage response to a flash of light in the presence of charybdotoxin showed a transient overshoot of the membrane potential. It was concluded that Ca 2+ ‐activated K + current is necessary for clamping the rod photovoltage to values close to the dark potential, thus allowing faithful single photon detection and correct synaptic transmission.