INHIBITION OF THE FIRST COMPONENT OF THE RECEPTOR CURRENT IN LIMULUS PHOTORECEPTOR

THE effect of l-cis-diltiazem on the light-activated current was inves tigated in Limulus ventral nerve photoreceptor. Intracellularly inject ed diltiazem blocked the first component, reduced the second one to ab out 50%, but hardly changed the third component of the receptor curren t. Extracellularly applied diltiazem elicited a similar effect, howeve r, only after about 60 min incubation suggesting that the substance mu st diffuse into the cell, probably though the cut end of the axon, to be effective. Results suggest that the ion channels underlying the fir st component are similar to the cGMP-gated channels of rods and thus c an be moderately permeable to calcium ions. This calcium influx is suf ficient to regulate the gain of the second component which is activate d by the inositol phosphate cycle,