Inhibition of the first component of the receptor current in Limulus photoreceptor.

The effect of l-cis-diltiazem on the light-activated current was investigated in Limulus ventral nerve photoreceptor. Intracellularly injected diltiazem blocked the first component, reduced the second one to about 50%, but hardly changed the third component of the receptor current. Extracellularly applied diltiazem elicited a similar effect, however, only after about 60 min incubation suggesting that the substance must diffuse into the cell; probably though the cut end of the axon, to be effective. Results suggest that the ion channels underlying the first component are similar to the cGMP-gated channels of rods and thus can be moderately permeable to calcium ions. This calcium influx is sufficient to regulate the gain of the second component which is activated by the inositol phosphate cycle.