Temperature effects on the membrane current of retinal rods of the toad.

Thermal effects on the visual transduction mechanism of toad rods were examined by recording the membrane current of a single outer segment while changing the temperature within the range 15-30 degrees C. Warming increased the amplitude rmax of the saturating flash response. This effect had a Q10 of about 1.8 and may result from an increase in the light-sensitive conductance. The flash sensitivity decreased with increasing temperature, while the half-saturating flash intensity increased. There was no evidence of a temperature effect on the probability that an incident 500 nm photon triggered an electrical response. Together with the results in (2) and (3) this indicates that at higher temperature a successfully absorbed photon blocked a smaller fraction of the light-sensitive conductance. Upon warming, the time scale of the flash response shortened but the characteristic wave form was preserved. The speed of the dim flash response, measured by the reciprocal of its time-to-peak, had a Q10 of 2.7 and an apparent activation energy of 16.8 kcal mole-1. The power spectrum of the continuous component of the dark noise could be predicted at different temperatures by assuming that the underlying event was shaped by two of the four delays required to fit the light response. This behaviour is consistent with the notion that the continuous noise arises within the cascade of processes controlling the internal transmitter concentration of the outer segment.