Effects of hypoxia and hypercapnia on the light peak and electroretinogram of the cat

The effects of systemic hypoxia and hypercapnia on the standing potential, light peak, and electroretinogram (ERG) of the intact cat eye were studied. DC recordings were made in the vitreous humor. The amplitude and waveform of the light peak were surprisingly sensitive to hypoxia. The light peak began to decrease at an arterial oxygen tension (P(a)O₂) of 60 to 80 mmHg, and was reduced to 25 to 60% of the control amplitude at a P(a)O₂ of 40 mmHg. Increases in c-wave amplitude were also observed during hypoxia, beginning at about the same P(a)O₂ as decreases in the light peak. In contrast, the b-wave and ERG threshold were generally unchanged when the P(a)O₂ was above 40 mmHg. The light peak and c-wave were also more sensitive than the b-wave and ERG threshold to hypercapnia. Decreases in light peak amplitude and increases in c-wave amplitude began at an arterial pH of about 7.3. The b-wave was reduced, and ERG threshold was elevated, beginning at a pH of about 7.2 The standing potential of the eye, recorded in darkness, generally increased in both hypoxia and hypercapnia. A common factor in the generation of the light peak and c-wave is that both involve changes in membrane potential of retinal pigment epithelial (RPE) cells. These events were affected much more by hypoxia than were the b-wave and ERG threshold, suggesting that the RPE is unusually sensitive to hypoxia. Similarly, the earliest effect of retinal hypercapnia appears to be on the RPE rather than on the neural retina. Furthermore, hypoxia and hypercapnia may have acted on the RPE through similar mechanisms, because their effects on the light peak, c-wave, and standing potential were in the same direction.