We have recently shown that, following delivery of a large bleach, the sensitivity of human cone photoreceptors measured using the ERG a-wave recovers approximately exponentially, with time constant ~1.5 min (Paupoo et al. 2000). We now examine recovery from a range of bleaching exposures.
We recorded the corneal ERG from human subjects using a conductive fibre electrode. Approval was obtained from the Cambridge Human Biology Ethics Committee, and subjects gave informed consent. We delivered all bleaching and test stimuli in a ganzfeld apparatus using the natural pupil (Paupoo et al. 2000). Bleaching was achieved by exposing the subject’s eye to intense white light for durations ranging from 5 to 80 s. The cone a-wave was measured before, and for 5 min after, the bleach, in response to dim yellow flashes (0.9 cd m-2 s).
At early times after the bleaching exposures the dim-flash response amplitude was reduced significantly from its dark-adapted level (see Fig. 1). The amplitude then increased at a rate that appeared approximately constant with time, and independent of bleaching level.The amplitude of the response to a dim flash is thought to be proportional to the product of three factors: the circulating current, the amplification constant of transduction, and the fraction of unbleached photopigment (Thomas & Lamb, 1999). We have recently shown that cone circulating current recovers within seconds following bleaching exposures of these levels. Assuming the amplification to be unchanged, the recovery of dim-flash response amplitude then reflects the fractional regeneration of photopigment.
Hence our results suggest that, following bleaches ranging from ~30 to 90 %, human cone pigment regenerates at an approximately constant rate; i.e. that regeneration is rate limited. Previous studies have usually fitted the recovery with a single exponential, but the parallel nature of the recoveries that we observe following bleaches of different levels is not consistent with a first-order process. Rate limiting does not appear to have been reported previously for the human cone system.
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| Figure 1. Dim-flash response amplitudes following three representative bleaching exposures of 5, 15 and 80 s duration at an intensity of 9000 cd m-2 (pupil diameter ~1.9 mm). Amplitudes were measured 17 ms after flash delivery. Points are means ± S.E.M. (n = 30-60) for 2-4 bleach repetitions, with sets of 15 flash trials (presented at 0.5 s intervals and repeated every 10 s), and are normalized to the dark-adapted level. Lines plot linear recovery at a common rate of 0.4 min-1, beginning from initial reductions of 0.3, 0.6 and 0.87. |