Effects of temperature changes on toad rod photocurrents.

Rod current responses were measured over the range 5-30 degrees C. Following a rapid decrease in temperature the amplitude of the dark current decreased without detectable delay (less than 3 s). Over a period of several minutes the amplitude of the dark current sometimes relaxed slightly towards its previous value. The rapid change cannot be accounted for simply by altered activity of the sodium pump and instead indicates that the conductance of the outer segment in darkness changes with temperature. Over the range 10-30 degrees C the amplitude of the dark current increased approximately linearly with temperature, and the straight line of best fit extrapolated to zero current at about 5 degrees C. The few points available below 10 degrees C indicated that the relationship flattened out, but this could not be investigated properly. The kinetics of responses to dim flashes accelerated with a Q10 of about 2.2, and were well described by an Arrhenius plot with an activation energy of 13.8 kcal mol-1 (HEPES Ringer solution). The time course of recovery of dark current following a saturating flash showed a similar temperature dependence to that of the dim flash kinetics. A simple explanation of the previous two findings is that the delays determining the time course of responses to both dim and bright flashes are largely determined by the fluidity of the disk membrane. The sensitivity to dim flashes had a broad peak at about 22 degrees C, decreasing at both lower and higher temperatures. The relative sensitivity to long wave-length light increased slightly with temperature. The sensitivity at 700 nm relative to that at 500 nm increased by 0.225 log10 units (1.68 times) upon a temperature increase from 11.5 to 29.3 degrees C (from approximately -5.0 log10 units to approximately -4.8 log10 units). This change appears to be approximately what would be expected theoretically.