A quantitative comparison of the effects of intracellular calcium injection and light adaptation on the photoresponse of Limulus ventral photoreceptors.

Abstract

Ca2+ were iontophoretically injected into ventral photoreceptors of Limulus by passing current between 2 intracellular pipettes. Changes in sensitivity and photoresponse time course were measured for both light adaptation and Ca2+ injection. For some photoreceptors there was no significant difference in the photoresponse time course for desensitization produced by light adaptation or by Ca2+ injection. In others the time delay of the photoresponse for Ca2+ injection was slightly longer than for light adaptation. The variability of threshold response amplitude and time delay decreased when the photoreceptor was desensitized by light adaptation or Ca2+ injection. The peak amplitude vs. log stimulus intensity relationships for controls, light adaptation, and Ca2+ injection all could be described very closely by a single template curve shifted along the log intensity axis. A 40- to 50-fold change in sensitivity was associated with a 2-fold change in photoresponse time delay for both light adaptation and Ca2+ injection.