The aim of this study was to investigate the effects of an increase in
the saturation of blood oxygen (SaO2) and/or serum glucose
on photoreceptor sensitivity in normal subjects and in patients with
diabetes mellitus. We monitored cone and rod sensitivity by recording
dark-adaptation curves to both green and red test stimuli while inhaling
either air (20% O2 + 80% N2) or 100% oxygen in 12
normal subjects and 12 diabetic patients with no (10) or mild (2)
retinopathy. We also repeated the experiment in 10 of the normal subjects
under hyperglycemia (mean serum glucose: 161 mg/dl). Results show that
in normal subjects the dark-adapted cone sensitivity is improved by an
increase in SaO2 or by hyperglycemia. Final rod sensitivity is
unchanged during hyperoxia and during hyperglycemia when measured with a
green test spot. However the kinetics of dark adaptation are altered
during hyperglycemia, and an increase in final sensitivity is observed
when measured with the red test spot. Inhalation of oxygen during
hyperglycemia in normal subjects reduces cone sensitivity compared to that
found during hyperglycemia alone (Pasteur effect). In diabetic subjects
the dark-adapted cone threshold is comparable to that found in normal
subjects, and sensitivity also increases with an increase in
SaO2. The final rod threshold, however, is impaired compared to
that of the control group, and rod sensitivity is improved by increasing
the SaO2. The results suggest that the metabolism of rods and
cones may differ in normal subjects: in cones, the rate of metabolism can
be augmented by increasing the available oxygen or glucose, whereas rods
appear more insensitive to increased blood oxygen saturation and
hyperglycemia. In diabetic subjects, both cone and rod metabolism can be
increased by supplemental oxygen, indicative of an early rod deficit. The
study lends weight to the hypothesis that dark-adapted rods in diabetics
are hypoxic before the onset of retinopathy.