The contribution of cationic conductances to the potential of rod photoreceptors.
The contribution of cationic conductances in shaping the rod photovoltage was studied in light adapted cells recorded under whole-cell voltage- or current-clamp conditions. Depolarising current steps (of size comparable to the light-regulated current) produced monotonic responses when the prepulse holding potential (V (h)) was -40 mV (i.e. corresponding to the membrane potential in the dark). At V (h) = -60 mV (simulating the steady-state response to an intense background of light) current injections <35 pA (mimicking a light decrement) produced instead an initial depolarisation that declined to a plateau, and voltage transiently overshot V (h) at the stimulus offset. Current steps >40 pA produced a steady depolarisation to approximately -16 mV at both V (h). The difference between the responses at the two V (h) was primarily generated by the slow delayed-rectifier-like K(+) current (I (Kx)), which therefore strongly affects both the photoresponse rising and falling phase. The steady voltage observed at both V (h) in response to large current injections was instead generated by Ca-activated K(+) channels (I (KCa)), as previously found. Both I (Kx) and I (KCa) oppose the cation influx, occurring at the light stimulus offset through the cGMP-gated channels and the voltage-activated Ca(2+) channels (I (Ca)). This avoids that the cation influx could erratically depolarise the rod past its normal resting value, thus allowing a reliable dim stimuli detection, without slowing down the photovoltage recovery kinetics. The latter kinetics was instead accelerated by the hyperpolarisation-activated, non-selective current (I (h)) and I (Ca). Blockade of all K(+) currents with external TEA unmasked a I (Ca)-dependent regenerative behaviour.
Moriondo
Andrea
A
Dipartimento di Biologia ed Evoluzione, Sezione di Fisiologia e Biofisica, National Institute of Neuroscience and Neuroscience Center, Università di Ferrara, Via L. Borsari 46, Ferrara, Italy.
Rispoli
Giorgio
G
D000200:Action Potentials / Q000187:drug effects; D000558:Ambystoma mexicanum / Q000502:physiology*; D000818:Animals; D020013:Calcium Signaling / Q000187:drug effects* / Q000502:physiology; D004553:Electric Conductivity / Q000009:adverse effects; D008564:Membrane Potentials / Q000187:drug effects*; D009119:Muscle Contraction / Q000187:drug effects* / Q000502:physiology; D009130:Muscle, Smooth / Q000187:drug effects / Q000502:physiology; D009474:Neurons / Q000187:drug effects; D017948:Retinal Rod Photoreceptor Cells / Q000187:drug effects* / Q000502:physiology; D026403:S-Nitrosothiols / Q000494:pharmacology*; D012964:Sodium / Q000494:pharmacology
D026403:S-Nitrosothiols; D012964:Sodium