The light-induced rise in cytosolic calcium starts later than the receptor current of the Limulus ventral photoreceptor.

The intracellular arsenazo signal indicating the transient light-evoked change in cytosolic Ca2+ (or Sr2+) concentration was measured in Limulus ventral photoreceptors simultaneously with the receptor current under voltage clamp conditions at 15 degrees C. The latency of the light-evoked arsenazo response was consistently more than 25 msec longer than the latency of the electrical light response (receptor current or -potential). Replacing calcium by strontium in the superfusate caused, within 30-40 min, reversible changes: an enlargement of the arsenazo response and a considerable prolongation of both latencies, that of the electrical and that of the arsenazo response; the difference between the two latencies, however, stayed essentially constant.