Configuration of light responses in isolated retinal rods

The whole-cell patchclamp technique was employed to investigate the light responses of single retinal rods of the frog (Rana esculenta and R. temporaria). In the majority of experiments, completely isolated cells were studied. Coupling with neighboring cells gave rise to a more complex response configuration. Responses were recorded under voltage-clamp and under current-clamp conditions. Stimulus response curves were measured in experiments with local stimuli illuminating only parts of the outer segment. Metabolic factors such as cGMP, GTP and ATP were also tested and were found to have specific and different influences on the response configurations. When the recording pipette was filled with an intracellular medium devoid of nucleotides, a retardation in the recovery of the light responses was observed during the course of an experiment. Addition of 1 mM ATP to the pipette medium prevented the larger part of the retardation, while 1 mM GTP accelerated the response recovery at the beginning of an experiment but did not prevent a subsequent retardation. Micromolar concentrations of cGMP were sufficient to elicit both a depolarization of the photoreceptor membrane and an increase in the response duration. These results show that, in single photoreceptors, the configuration of light responses not only depends on the stimulus parameters but also on those properties of the cells that are directly controlled by their nucleotide metabolism.