ROLE FOR HYDROLYSIS OF INOSITOL 1,4,5-TRISPHOSPHATE IN TERMINATING THE RESPONSE TO INOSITOL 1,4,5-TRISPHOSPHATE AND TO A FLASH OF LIGHT IN LIMULUS VENTRAL PHOTORECEPTORS

Injection of inositol 1,4,5-trisphosphate (Ins 1,4,5-P-3) into Limulus ventral photoreceptors produces excitation similar to that produced b y light. One process which might contribute to rapid termination of th e responses to Ins 1,4,5-P-3 and to light is the hydrolysis of Ins 1,4 ,5-P-3 by an InsP(3)-5-phosphatase to form inositol 1,4-bisphosphate. Inositol 2,4,5-trisphosphate (Ins 2,4,5-P-3) is known to be less hydro lysable by the InsP(3)-5-phosphatase than is Ins 1,4,5-P-3. Whereas ve ntral photoreceptors respond to an injection of Ins 1,4,5-P-3 with a s ingle wave of depolarization, the response to Ins 2,4,5-P-3 is a burst of waves of depolarization. Our hypothesis is that it is the resistan ce to hydrolysis by the InsP(3)-5-phosphatase which accounts for the b urst of waves produced by Ins 2,4,5-P-3. To test this idea we injected ventral photoreceptors with Ins 1,4,5-P-3 in the presence of the non- specific phosphatase inhibitors, vanadate and fluoride, which prolong the response to a flash of light in ventral photoreceptors (D.W. Corso n, A. Fein, W.W. Walthall, J. Gen. Physiol. 82 (1983) 659-677). In the presence of fluoride or vanadate the response to Ins 1,4,5-P-3 was co mposed of a burst of waves rather than a single wave of depolarization . We conclude that hydrolysis of Ins 1,4,5-P-3 by the InsP(3)-5-phosph atase plays a role in terminating the ventral photoreceptors response to Ins 1,4,5-P-3 and also to Light. (C) 1997 Elsevier Science B.V.