Two latent period processes in Limulus ventral photoreceptors

The latent period of Limulus ventral photoreceptor potentials consists of two contiguous time segments, S 1 and S 2, which exhibit the following properties: (1) the duration of S 1 is inversely dependent on the incident energy provided by the first stimulating light flash; (2) the duration of S 1 is prolonged in the presence of GTP-γ-S [guanosine-5′-0- (3-thiotriphosphate)] or papaverine; (3) the duration of S 1 is abbreviated in the presence of sodium vanadate or chlorobutanol; (4) the duration of S 2 is abbreviated by light adaptation; and (5) the duration of S 2 is prolonged more than S 1 in a low temperature environment. The existence of these two components of the Limulus ventral photoreceptor potential latent period is demonstrable both in dark adapted photoreceptors, using single constant intensity light pulses of varying duration, or in partially light adapted photoreceptors, using a stimulating conditioning pulse-test pulse sequence. On the basis of these results it is tentatively concluded that the two segments of the latent period of Limulus ventral photoreceptor potentials are occupied by different processes. The first process, occupying the S 1 segment, generates a critical concentration of a photoproduct which, directly or indirectly, eventually alters the conductance of the active photoreceptor membrane. The second process, occupying the S 2 segment, is concerned with the initiation of the SPFs which sum to produce the receptor potential. It is possible that the hydrolysis of GTP actively participates in the first process.