Excitation and adaptation of Limulus ventral photoreceptors by inositol 1,4,5 triphosphate result from a rise in intracellular calcium.

Abstract

Single pressure injections of 1-10 pl of inositol 1,4,5 triphosphate (IP3) or inositol 4,5 bisphosphate [I(4,5)P2] excite Limulus ventral photoreceptors by inducing rapid bursts of inward current. After excitation by IP3, responses to subsequent injections of IP3 or light flashes are often reversibly diminished (adapted). Single injections of IP3 and I(4,5)P2 are effective at concentrations in the injecting pipette of 20 microM to 1 mM. Single injections of inositol 1,4 bisphosphate are ineffective at concentrations of 100-500 microM. Excitation by IP3 or I(4,5)P2 is accompanied by a rise in intracellular free calcium, as indicated by aequorin luminescence. Prior injection of calcium buffer solutions containing 100 mM EGTA greatly diminishes the total charge transferred across the plasma membrane during excitation by IP3 or I(4,5)P2, which suggests that a rise in Cai is necessary for excitation by the inositol polyphosphates. Adaptation of the response to light by IP3 is also abolished by prior injection of EGTA. In the same cells, the response to brief light flashes is slowed and diminished in amplitude by the injection of calcium buffer, but the charge transferred during the response is not significantly diminished. This suggests that light has access to a pathway of excitation in the presence of EGTA that is not accessible to intracellularly injected IP3.

A lingering elevation of Cai accompanies inhibition of inositol 1,4,5 trisphosphate-induced Ca release in Limulus ventral photoreceptors.

“…Subsequent injections of 100 mM EGTA, however, steadily attenuated and eventually irreversibly abolished the response to InsPs, as reported previously (Payne et al, 1986b). These experiments indicated that EGTA remaining at the injection site is saturated by the calcium released 750 ms later by a subsequent InsP3 injection and is therefore unable to reduce the consequent depolarization.…”

“…This buffering might mask the relief of feedback inhibition by injections of EGTA. High concentrations of EGTA have clearly been shown to antagonize the response to InsP3 (Rubin and Brown, 1985;Payne et al, 1986b). In the experiments described below, a second injection of InsP3 was delivered 750 ms after the injection of a solution containing 1 mM EGTA.…”

“…Because of their large size, InsP3 can be directly injected into Limulus photoreceptors. The response to a single, brief injection of InsP~ comprises a rapid, transient rise in Cai and a consequent depolarization (Brown and Rubin, 1984;Payne, Corson, Fein, and Berridge, 1986b). Ca directly or indirectly initiates the opening of sodium-permeable ion channels in the plasma membrane (Payne, Corson, and Fein, 1986a).…”

“…Detectable aequorin luminescence was limited to the 1-2 s immediately after an injection of InsP~ (Payne et al, 1986b(Payne et al, , 1990. These studies may have failed to make optimal use of aequorin.…”

Protein Kinase C Activators Inhibit the Visual Cascade inLimulusVentral Photoreceptors at an Early Stage

“…The physiological and anatomical consequences of illumination have been well defined. In Limulus ventral photoreceptors, light-induced production of InsP 3 causes the release of calcium from intracellular stores (Brown and Rubin, 1984;Payne et al, 1986b), leading to the opening of nonspecific cation channels in the plasma membrane and a resulting depolarization of the photoreceptor's membrane potential. Calcium release subsequently reduces the photoreceptors' sensitivity, initiating light adaptation (Brown and Lisman, 1975).…”