Properties and functional roles of hyperpolarization-gated currents in guinea-pig retinal rods

1. The inward rectification induced by membrane hyperpolarization was studied in adult guinea-pig rods by the perforated-patch-clamp technique. 2. CsCl blocked the rectification observed in both voltage- and current-clamp recordings at voltages negative to -60 mV, while BaCl2, blocked the inward relaxation observed at voltages positive to -60 mV. The current activated at -90 mV had a low selectivity between sodium and potassium and reversed at -31.0 mV. 3. These observations suggest that two inward rectifiers are present in guinea-pig rods: a hyperpolarization-activated (I-h) and a hyperpolarization-deactivated (I-kx) current. The functional roles of I-h and I-kx were evaluated bg stimulating rods with currents sinusoidally modulated in time. 4. Rods behave like bandpass amplifiers, with a peak amplification of 1.5 at about 2 Hz. For hyperpolarizations that mainly gate I-kx, amplification and phase shifts are fully accounted for by a rod membrane analogue model that includes an inductance. For hyperpolarizations that also gate I-h, a harmonic distortion became apparent. 5. Bandpass filtering and amplification of rod signals, associated with I-h and I-kx gating by membrane hyperpolarization, are strategically located to extend, beyond the limits imposed by the slow phototransductive cascade, the temporal resolution of signals spreading to the rod synapse.