Kinetics and components of the flash photocurrent of isolated retinal rods of the larval salamander, Ambystoma tigrinum

1. Membrane currents initiated by intense, 20 μs flashes (photocurrents) were recorded from isolated salamander rods by combined extracellular suction electrodes and intracellular tight-seal electrodes either in current or voltage clamp mode. The magnitudes (mean±2 S.E.M.) of the maximal photoresponses recorded by the suction and by the intracellular electrode respectively were 40±5 pA (n = 18) and 35±7 mV (n = 8) for current clamp at zero current; 43±9 pA and 66±13 (n = 11) pA for voltage clamp at the zero-current holding potential, -24±3 mV. 2. Photocurrents initiated by flashes isomerizing 0.1% or more of the outer segment's rhodopsin achieved a saturated velocity and were 95% complete in less than 50 ms. The effect of incrementing flash intensity above 0.1% isomerization can be described as a translation of the photocurrent along the time axis towards the origin. Within the interval 0-50 ms the latter two-thirds of the velocity-saturated photocurrent is well described as a single-exponential decay. The decay was much faster in voltage clamp (2.8±1.2 ms, n = 11) than in current clamp mode (17±5 ms, n =17). 3. The initial third of the velocity-saturated photocurrent, occurring over the interval from the flash to the onset of exponential decay, followed about the same time course in current and voltage clamp. The time interval occupied by this initial 'latent' phase decreased with increasing flash intensity and attained an apparent minimum of about 7 ms in response to flashes isomerizing 10% or more of the rhodopsin at ca. 22°C.