Effect of zinc chelation on the aspartate-isolated a-wave. A. Electroretinograms (ERGs) recorded from a single skate eyecup preparation in response to a 1 second flash of light plotted with pre-flash baselines superposed. The ERG recorded in normal skate Ringer (upper trace) exhibits a small a-wave at light onset that is quickly obscured by a prominent, vitreal-positive transient b-wave, and followed by the slowly developing c-wave. After 30 minute superfusion with 75mM Na L-aspartate the b-wave and c-waves are absent, but the amplitude of the a-wave (the photoreceptor-generated ERG component) appears significantly greater (middle trace). Adding the zinc chelator histidine (100μM) to the aspartate solution for an additional 30 minutes (following 40 minutes in aspartate) produced a 2-fold increase in the a-wave amplitude (lower trace, in red). B. ERG intensity-response series recorded from one eyecup showing histidine enhancement of the isolated a-wave response. The ERG recorded at the Log intensity (Log I) indicated on the left after 30 minutes in 75mM Na L-aspartate is shown in black. Responses obtained after adding 100μM histidine to the aspartate solution for an additional 30 min (following 40 minutes in aspartate) are superposed in red. Records at each intensity are shifted vertically for clarity of presentation.

Zinc attenuation of ERG a-wave response amplitude is reduced by histidine. The mean ΔV_max obtained from the relationship above fitted to each dataset for 75mM Na L-aspartate to which 100μM histidine had been added for 30 minutes is shown as a percent increase from the a-wave amplitude after 30 minutes in aspartate alone (left column, ±SEM, N=5). The mean ΔV_max of similar curves fitted to normalized responses recorded after adding 50μM zinc (middle column, ±SEM, N=4) or 50μM zinc plus 100μM histidine (right column, ±SEM, N=4) for the final 30 minutes following 40 minutes in aspartate show that the a-wave amplitude attenuation induced by zinc is reduced in the presence of histidine.

Intensity-response data of ERG a-wave peak amplitudes recorded from five preparations confirm a histidine enhancement effect. Responses from each preparation were normalized to the response amplitude at Log I = -2.5 after 30 minutes in 75mM Na L-aspartate. The mean of responses at each intensity obtained from all five preparations after 30 minutes in aspartate (filled squares, ±SEM, N=5)) were fitted (lower curve) by the equation V/V_max = Iⁿ/(Iⁿ + I₅₀) where I₅₀ is the intensity at which the amplitude of the response is ½V_max. Following addition of 100μM histidine to the aspartate superfusate for 30 minutes, there was a significant enhancement of the a-wave amplitude (circles, ±SEM, N=5) having a V_max (upper curve) over 50% greater than control.