Abstract
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 ≈−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 Ca2+ 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.
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Abbreviations
- C m :
-
Rod membrane capacitance
- ChTx:
-
Charybdotoxin
- I Ca :
-
Voltage-activated Ca2+ current
- I cG :
-
cGMP-gated Na+ and Ca2+ current
- I ClCa :
-
Ca-activated Cl− current
- I h :
-
Hyperpolarisation-activated, non-selective cation current
- I KCa :
-
Ca-activated K+ current
- I Kx :
-
Slow delayed-rectifier-like K+ current
- OS:
-
Isolated rod outer segment
- R m :
-
Rod membrane resistance
- V h :
-
Holding potential
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This work was supported by grants from the Ministero per l’Istruzione, l’Università e la Ricerca (MIUR), Roma.
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Proceedings of the XIX Congress of the Italian Society of Pure and Applied Biophysics (SIBPA), Rome, September 2008.
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Moriondo, A., Rispoli, G. The contribution of cationic conductances to the potential of rod photoreceptors. Eur Biophys J 39, 889–902 (2010). https://doi.org/10.1007/s00249-009-0419-z
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DOI: https://doi.org/10.1007/s00249-009-0419-z