The effects of carbon dioxide, oxygen and pH on spreading depression in the isolated chick retina

In the isolated chick retina, the propagation velocity of Spreading Depression (SD) was approximately doubled and the frequency of spontaneous waves was substantially increased with solutions bubbled with 5% CO2 instead of air, at constant pH (7.5-7.6). There was no effect on SD of raised pO2. Large changes of pH (to 6.4 and 9.4) produced, respectively, decreases and increases of velocity; but there was no clear pH dependence with changes less than 0.5 pH units. The resting [K+]0 and the elevation during SD, measured with K+ sensitive micro-electrodes, were not consistently altered with 5% CO2 when there was faster conduction. The effect of raised pCO2 is opposite to that observed previously in rat cortex in vivo, which afforded evidence for a similarity between SD and the disturbance in the aura phase of migraine attacks. The effects in vivo must presumably be due to factors not acting in the isolated chick retina.