THE LATENCY OF THE RESPONSE OF LIMULUS PHOTORECEPTORS TO INOSITOL TRISPHOSPHATE LACKS THE CALCIUM-SENSITIVITY OF THAT TO LIGHT

The latent period before depolarization of Limulus ventral photorecept ors by light flashes was compared with that following brief, intracell ular, pressure-injection of d-myo-inositol 1,4,5 trisphosphate. At tem peratures between 18-degrees-C and 22-degrees-C and with an extracellu lar calcium concentration of 10 mM, the responses of 4 cells to light and to injections of 100-mu-M inositol trisphosphate displayed average latencies of 71 and 56 ms, respectively. The latencies of responses t o InsP3 included an estimated 20 ms dead-time inherent in the injectio n method. Reducing the temperature lengthened the latency of the respo nse to light (Q10 approximately 3.2 between 7 and 22-degrees-C) more t han that to inositol trisphosphate (Q10 approximately 2.3). Bathing th e photoreceptors in seawater containing no added calcium and 1 mM of t he calcium chelator EGTA greatly increased the latency of the light re sponse at all temperatures, but did not increase the latency of the re sponse to inositol trisphosphate. We conclude that the response to ino sitol trisphosphate lacks the calcium- and temperature-sensitive laten t period which characterizes the response to light. If inositol trisph osphate acts, via the release of stored calcium, to stimulate an inter mediate in the visual cascade, then that intermediate would appear to be downstream from the latency-generating mechanism.