Synthesis of leukotrienes in frog retina and retinal pigment epithelium

Leukotrienes (LT) were identified in the intact frog (Rana pipiens) retinal pigment epithelium (RPE), retina, isolated rod outer segments (ROS), and ROS-free (neuronal) retina. Levels of endogenous LTC4 were measured by radioimmunoassay (RIA) in both unpurified incubation medium and in incubation medium purified by reverse-phase high-performance liquid chromatography (HPLC). The purified LTC4 exhibited characteristic ultraviolet absorption spectrum with lambda max at 280 nm. The Ca2+ ionophore A23187 (10 microM) increased LTC4 production in intact and ROS-free retina and RPE but had no effect on LTC4 levels in isolated ROS. This lack of effect suggests that LTC4 is present but not synthesized in ROS. Synthesis of radiolabeled LTC4 in frog retina and RPE prelabeled in vivo by intravitreal injection of [1-14C]arachidonic acid (20:4, n-6) provided an additional verification of the presence of LTC4 in these tissues. The physiological significance of the presence of these biologically active derivatives of arachidonic acid in photoreceptor cells and retinal pigment epithelium may be related to the interactions between these cells, consisting of photoreceptor membrane shedding and phagocytosis.

Ganglioside administration in retinitis pigmentosa

The retinitis pigmentosa process typically causes variably progressive visual loss due to retinal photoreceptor and pigment epithelial cell deterioration. No effective agent either to retard or stop the deterioration has been known. Because gangliosides have been shown to have a variety of trophic effects on peripheral nervous tissue, we administered a daily 40 mg intramuscular dose in a prospective, double-blind, randomized, placebo-controlled trial in 30 persons with retinitis pigmentosa with or without congenital deafness. When we compared baseline performance on the two principal study outcomes, visual field area and electroretinographic response amplitudes, there was a marginally statistically significant increase in visual field area in the ganglioside-treated group. The subgroup of subjects who had recordable electroretinograms at baseline and who received the drug showed in three of five cases an increase in amplitude to all stimuli. These results appear encouraging; however, the data must be interpreted cautiously because the numbers of individuals are very small. Further study of the possible benefits of ganglioside administration to retinitis pigmentosa patients appears warranted.

Inhibition of the alkali burn-induced lipoxygenation of arachidonic acid in the rabbit cornea in vivo by a platelet activating factor antagonist

We have evaluated the involvement of 1-0-alkyl-2-acetyl-sn-glycero-3-phosphocholine (platelet activating factor, PAF), a membrane derived lipid mediator, in the inflammatory response of the anterior segment of the rabbit eye after corneal injury. The action of BN 52021, an antagonist of PAF receptor, was tested in vivo in models of corneal injury by alkali burn. The sodium salt of 1-14C-arachidonic acid was injected in the anterior chamber and the production of labeled prostaglandins and hydroxyeicosatetraenoic acid (HETE) was monitored as an index of the activation of the cyclooxygenase and lipoxygenase pathways. Topically applied BN 52021 produced a specific inhibition of the formation of labeled 5- and 12-HETE. This inhibition was maximal when the drug was applied during the first few hours after the injury. The drug does not affect the increased protein concentration in the aqueous humor after injury. These results suggest involvement of PAF in the inflammatory response in the cornea that could be blocked by the antagonist.

Decreased content of docosahexaenoate and arachidonate in plasma phospholipids in Usher's syndrome

Docosahexaenoate and arachidonate were found to be significantly decreased in plasma phospholipids from Usher's syndrome patients. The fatty acid content of plasma triacylglycerols was not changed in these patients. Usher's syndrome, an autosomal recessive disorder, involves an inherited visual cell degeneration. Photoreceptor membranes are richly endowed with docosahexaenoate and arachidonate, and a metabolic defect affecting these polyunsaturated fatty acids may occur. Moreover, blindness may be due, at least partially, to an alteration in the unsaturated phospholipids of photoreceptor membranes.

Diacylglycerols interfere in normal phase HPLC analysis of lipoxygenase products of docosahexaenoic or arachidonic acids

A methodological problem with the normal phase high performance liquid chromatography (HPLC) of hydroxylated products of docosahexaenoic and arachidonic acids is described. Diacylglycerols present in lipid extracts of rat retina co-elute with monohydroxy derivatives of docosahexaenoic or arachidonic acid, when samples are applied to uPorosil columns and eluted with hexane/isopropanol/acetic acid. Analysis of fatty acid composition of diacylglycerols which were acetone-extracted from the incubation medium showed a profile similar to diacylglycerols extracted from the tissue by hexane/isopropanol, although acetone extraction resulted in extremely variable recovery of diacylglycerols. This co-elution of diacylglycerols with monohydroxy polyunsaturated fatty acids can lead to a significant error in estimation of lipoxygenation activity by conversion of radiolabeled precursors, because the incorporation of fatty acids into diacylglycerols is very active in many tissues. An alternative extraction method and reverse phase HPLC procedures that result in the complete separation of hydroxy fatty acids and diacylglycerols are described.

Arachidonic acid metabolism to eicosanoids in herpes virus-infected rabbit cornea

The metabolism of the polyunsaturated fatty acid, arachidonic acid (20:4, n-6) in rabbit cornea with varying severities of herpes simplex viral infection was investigated. The results indicate an active synthesis of the lipoxygenase and cyclooxygenase reaction products of arachidonic acid in central cornea and corneal-scleral rim. Stimulation of 12-hydroxyeicosatetraenoic acid (12-HETE) production in herpes-infected cornea was correlated positively with the severity of infection. Other eicosanoids were increased maximally in moderately infected corneas. The stimulation of eicosanoid synthesis was more evident in central cornea as compared to corneal-scleral rim. Herpes infection also caused a decline in the incorporation of radiolabeled arachidonic acid into membrane glycerolipids. These data indicate that the production of eicosanoids from arachidonic acid is stimulated in herpes-infected cornea. The stimulation may reflect the presence of phagocytic cells in the infected cornea, an enhanced capacity of the cornea itself to produce eicosanoids, or a combination of these effects. Decreased acylation of membrane lipids may be the result of infection-induced activation of fatty acid release mechanisms, which would lead to degradation of cell membranes. The presence of lipoxygenase reaction products in the herpes-infected cornea introduces a new factor for consideration in the design of therapeutic regimens for this disease.

The effect of phorbol esters on the chloride secreting epithelium of the rabbit cornea

Tumor-promoting phorbol esters have been shown to modulate a number of physiological events in various cell types, and these events are associated with the activation of protein kinase C. The purpose of this study was to examine the effects of phorbol ester stimulation on protein kinase C activity and epithelial ion transport in the Cl- secreting rabbit corneal epithelium. We report here that nanomolar concentrations of tumor-promoting phorbol ester, 12-0-tetradecanoyl-phorbol-13 acetate (TPA), stimulate active ion transport via a putative receptor located in the corneal epithelium. This stimulation is a Cl- -dependent process, but is independent of beta-adrenergic receptor activation and prostaglandin formation. Biochemical data support the idea that TPA activates Ca++/phospholipid-dependent protein kinase C in the corneal epithelium. Therefore, we suggest that protein kinase C may have a role in the regulation of membrane Cl- transport in the mammalian corneal epithelium.

Fatty acid composition and arachidonic acid metabolism in vitreous lipids from canine and human eyes

About 55% of the acyl groups of dog and human vitreous are unsaturated fatty acids. The major components are oleate (18:1, n-9) and arachidonate (20:4, n-6) with moderate amounts of linoleate (18:2, n-6) and docosahexaenoate (22:6, n-3). Palmitate (16:0) and stearate (18:0) are the major saturated fatty acids. There are no significant changes between ages 37-82 years in the fatty acyl group content and composition of human vitreous. In vitreous from Irish setters with hereditary rod-cone dysplasia (RCD) the levels of oleate are decreased with a concomitant increase in arachidonate. [1-14C]Arachidonic acid was actively incorporated into canine vitreous glycerolipids both in vitro and in vivo. The incorporation was mainly into phosphatidylinositol, triacylglycerol, phosphatidylcholine and phosphatidylethanolamine. There were some differences in the pattern of incorporation between human and dog and between in vivo and in vitro incubations of canine vitreous. Glycerolipid acylation was significantly increased in phosphatidylinositol and phosphatidylcholine in RCD canine vitreous. The pattern of incorporation of [U-14C]docosahexaenoic acid into vitreous glycerolipids was different from arachidonic acid incorporation. Although vitreous did not produce any measurable enzymatic synthesis of cyclooxygenase and lipoxygenase products from [1-14C]-arachidonic acid in vitro, there was significant generation of autooxidation products. These results suggest an active lipid metabolism in vitreous.

The inositide cycle in bovine photoreceptor membranes

Various enzymic steps in the inositide cycle were investigated in purified bovine retinal rod outer segments (ROS). Incubation of ROS with [gamma-32P]ATP resulted in a rapid labeling of phosphatidic acid and phosphatidylinositol 4-phosphate (PIP), while little radio-tracer was recovered from phosphatidylinositol 4,5-bisphosphate (PIP2). This can be explained by the relatively low activity of PIP-kinase activity in ROS as compared to the remainder of the retina. Similarly, relatively little phosphodiesteratic activity degraded PIP2 and PIP in ROS when 32P labeled phosphoinositides in synaptic membranes (heat-treated to inactivate endogenous enzymes) were used. Although light exposure of ROS did cause rapid rhodopsin phosphorylation, no enzymic steps of the cycle were changed, even when ROS were obtained from retinas excised from cows dark-adapted by unilateral eye patching the day prior to kill. These studies do not support the view that light is an agonist of the inositide cycle in mammalian photoreceptors.

Enhanced inositide turnover in brain during bicuculline-induced status epilepticus

Because brain inositides are enriched in the 1-stearoyl-2-arachidonoyl species, they form a likely source for the tetraenoic free fatty acids (FFA) and diacylglycerols (DG) that are accumulated during seizures. To study inositide turnover during bicuculline-induced seizures, rats were injected intraventricularly and bilaterally with 10-20 microCi 32P, mechanically ventilated and sacrificed by 6.5 KW head-focused microwave irradiation. Seizure activity was recorded by electroencephalography. Bicuculline-induced seizure activity resulted in: a) almost 50% increase in 32P labeling of phosphatidic acid (PA); phosphatidylinositol (PI) and phosphatidylinositol 4,5-bisphosphate (PIP2) also increased (24% and 36%, respectively); b) no change in other lipids; and c) water-soluble phosphodiesteratic degradation products, analyzed by high voltage paper electrophoresis, increased 24% in the amount of radiotracer recovered as inositol 1,4-bisphosphate (IP2) and by 44% in the amount recovered as inositol 1,4,5-trisphosphate (IP3). These data indicate that during experimental status epilepticus the cerebral inositide cycle is accelerated: PIP2----(IP3----IP2----IP----I) + DG----PA----PI----PIP----PIP2.

Enhanced synthesis of prostaglandins and hydroxyeicosatetraenoic acids in retina from a canine model of Batten's disease

The metabolism of [1-14C]arachidonic acid (20:4, n-6) was studied in intact retina and retinal pigment epithelial cells from normal English setters and English setters affected with hereditary canine ceroid lipofuscinosis. Acylation of arachidonic acid into membrane glycerolipids and oxygenation by lipoxygenase and cyclooxygenase to eicosanoids were measured by radiochromatographic techniques. In addition, the histopathology of accumulated ceroid particles in retinal ganglion cells and pigment epithelial cells was studied by electron microscopy. Synthesis of prostaglandins and hydroxyeicosatetraenoic acids was increased in canine ceroid lipofuscinosis retina, but not in retinal pigment epithelium. Prostaglandin D2, the putative neuronal eicosanoid, was increased nearly eightfold, whereas other eicosanoids increased two- to threefold. Ultrastructural studies revealed accumulation of ceroid and deterioration of neuronal and pigment epithelial cell architecture. These experiments demonstrate that, although lipopigment accumulates in both tissues, alterations of eicosanoid synthesis are specific for the retina, a neuronal tissue. The specific increase in prostaglandin D2 and the specificity of changes for the retina indicate that enhanced eicosanoid synthesis may be a result of an impairment of the control of oxygenation of arachidonic acid in neurons.

Changes in triacylglycerol, diacylglycerol and free fatty acids after fertilization in developing toad embryos

The content and fatty-acid composition of triacylglycerols, diacylglycerols and free fatty acids were analyzed from the unfertilized oocyte stage to the gastrula stage in the toad Bufo arenarum Hensel. Fertilization triggered a 30% and a 40% decrease in triacylglycerol and diacylglycerol, respectively. In contrast, free fatty acid increased continuously from oocyte to gastrula stage with an accumulation of palmitate predominating. However, the observed increase in free fatty acid was too small to account for the decreases in both neutral glycerides. The decrease in triacylglycerol might be a reflection of the activation of lipolytic enzymes and the subsequent oxidation of fatty acids to meet the increased metabolic energy requirements brought on by fertilization. The diminished diacylglycerol content due to fertilization may be accounted for, at least in part, by the utilization of diacylglycerol in the synthesis of membrane phospholipids, inasmuch as their decrease occurred simultaneously with an increase in phosphatidic acid. When cell-free homogenates taken from the three stages of development (unfertilized, fertilized and gastrula) were incubated in Tris-Ringer buffer for 90 min, free fatty acid accumulated. Triacylglycerol and diacylglycerol did not change substantially during this incubation period. This fact indicates that the free fatty acid released during incubation was not derived from neutral glycerides, but probably from membrane phospholipids. The release of free fatty acid was significantly greater in samples from the fertilized oocyte stage. The results described in this paper suggest that the synthesis of membrane phospholipids, including an enhanced turnover of the acyl moiety, plays a significant role in the metabolic events activated by fertilization.

Reduced labeling of brain phosphatidylinositol, triacylglycerols, and diacylglycerols by [1-14C]arachidonic acid after electroconvulsive shock: potentiation of the effect by adrenergic drugs and comparison with palmitic acid labeling

The effect of electroconvulsive shock on the labeling of phospholipids and neutral lipids in mice brains was examined after intracerebral injection of [1-14C] arachidonic acid or [1-14C]palmitic acid. Electroconvulsive shock reduced greatly the removal of radiolabeled arachidonic acid from the free fatty acid pool. At the same time, the incorporation of arachidonic acid was partially inhibited in triacylglycerol, diacylglycerol, and phosphatidylinositol, whereas the incorporation of [1-14C]palmitic acid was not affected. Pretreatment with desipramine and pargyline potentiated the lipid effect of electroconvulsive shock in neutral glycerides. These electroconvulsive shock-induced changes reflect alterations in the metabolism of intracerebrally injected arachidonic acid, but not of similarly injected palmitic acid. From the available data whether decreased ATP, enzyme inhibition or other factors are involved cannot be ascertained. Moreover, the electroconvulsive shock-enhanced endogenous free arachidonic acid may possibly dilute the injected radiolabeled fatty acid, thus decreasing its availability for arachidonoyl-coenzyme A synthesis. Hence, a partial inhibition of the activation-acylation of these fatty acids, primarily arachidonic acid, also may be involved in the seizure-induced accumulation of free fatty acids in the brain.

Cationic amphiphilic drugs perturb the metabolism of inosititides and phosphatidic acid in photoreceptor membranes

Incubation of purified bovine photoreceptor rod outer segments with [gamma-32P]ATP resulted in the labeling of phosphatidylinositol 4-phosphate (PIP) and phosphatidic acid (PA) with little labeling of phosphatidylinositol 4,5-bisphosphate (PIP2). Propranolol inhibited in a dose-dependent manner the labeling of PA and enhanced that of PIP. Various cationic amphiphilic drugs also were tested for these effects. Propranolol had the same effects on high-speed rat brain particulate material. While this particular preparation displayed more labeling of PIP2, propranolol was ineffective, as it was on retinal PIP-kinase. Ca2+-activated polyphosphoinositide phosphodiesterase activity in nerve-ending membranes also was inhibited by propranolol. It is concluded that cationic amphiphilic drugs can inhibit diacylglycerol kinase and the polyphosphoinositide phosphodiesterase and stimulate the phosphatidylinositol-kinase (but not PIP-kinase).

Endogenous fatty acids are covalently and noncovalently bound to interphotoreceptor retinoid-binding protein in the monkey retina

Interphotoreceptor retinoid-binding protein (IRBP) purified from monkey interphotoreceptor matrix contains relatively high concentrations of endogenous fatty acids, 6.51 mol/mol of protein. Sixty-five percent of the total fatty acid bound to IRBP was found to be noncovalently attached, with the remainder covalently bound. The fatty acids are not residual components of phospholipids or neutral lipids, as judged by microchemical methods. The major fatty acids bound to IRBP are: palmitic (35%), stearic (21%), palmitoleic (7%), oleic (29%), linoleic (6%) and docosahexaenoic acids (2%). These fatty acids account for about 90% of the total fatty acid bound to interphotoreceptor matrix proteins extracted with organic solvents. Thus, IRBP may function as an intercellular fatty acid carrier and may depend on the covalently bound fatty acids for anchoring in the outer leaflet of cell membranes.

Change in content, incorporation and lipoxygenation of docosahexaenoic acid in retina and retinal pigment epithelium in canine ceroid lipofuscinosis

We have examined the metabolism of docosahexaenoic acid (22:6, n-3) in retina and retinal pigment epithelium of normal dogs and those affected with canine ceroid lipofuscinosis (CCL), a hereditary degenerative neurological disorder. In the CCL retina, there was a decrease in 22:6 content in phospholipids, particularly phosphatidylethanolamine. This decrease in 22:6 was compensated by an increase in arachidonic acid (20:4, n-6). In contrast, CCL retinal pigment epithelium had higher levels of 22:6 and lower levels of 20:4 in phosphatidylethanolamine. The in vitro incorporation of radiolabeled 22:6 into glycerolipids of CCL retina and retinal pigment epithelium was increased as compared to control. The major lipoxygenase reaction product of 22:6, (11-hydroxy-4,7,9(trans)13,16,19)-22:6, increased 31% in CCL retina, but not in the retinal pigment epithelium. This is the first report describing alterations in content, incorporation and lipoxygenation of 22:6 in an animal model of a human disease (Batten's disease).

Synthesis of docosahexaenoyl-, arachidonoyl- and palmitoyl-coenzyme A in ocular tissues

The synthesis of long-chain acyl coenzyme A (CoA) was studied in the cornea, lens, vitreous, retina and pigment epithelium (PE) in the rat using [14C]-labeled palmitic, arachidonic and docosahexaenoic acids as substrates. Except for retina and PE, the ocular tissues studied showed relatively little enzyme activity with the fatty acid substrates. In addition, the enzyme activities were studied in homogenates and microsomal fractions from retina, pigment epithelial cells and choroid of frog, bovine and human eyes. Long-chain acyl CoA synthetase from the microsomal fraction exhibited three- to fivefold greater activity than homogenates in retina and PE. The enzyme activity was highest with palmitic acid, followed by arachidonic acid and docosahexaenoic acid. There were significant differences in enzyme activity between the species. The apparent Km (microM) and Vmax [nmol min-1 (mg protein)-1] values for the enzyme in bovine retinal microsomes were 7.91 +/- 0.39 (S.E.) and 21.6 +/- 1.04, respectively, for palmitic acid substrate and 5.88 +/- 0.25 and 4.58 +/- 0.21, respectively, for docosahexaenoic acid substrate. These values for bovine pigment epithelial microsomes were 13.0 +/- 0.27 and 36.9 +/- 1.18, respectively, for palmitic acid and 15.8 +/- 0.40 and 13.2 +/- 0.56, respectively, for docosahexaenoic acid. The synthesis of acyl CoA may play a central role in controlling the availability of free arachidonic acid for eicosanoid formation and in the retention of polyunsaturated fatty acid families (18:2, n-6 and 18:3, n-3) within cells of ocular tissues, particularly retina and retinal PE.

Cryogenic lesion alters the metabolism of arachidonic acid in rabbit cornea layers

The metabolism of radiolabeled arachidonic acid in epithelium, stroma, and endothelium was studied in normal and cryogenically lesioned rabbit corneas. The synthesis of cyclooxygenase- and lipoxygenase-reaction products, as well as the incorporation of arachidonic acid in phospholipids and neutral lipids, was followed by in vitro incubation (60 min) of corneas obtained 2 hr, 5 days, and 15 days after injury. In unwounded controls, prostaglandin E2 (PGE2) was the major cyclooxygenase product formed in the stroma, whereas thromboxane B2 predominated in the endothelium and epithelium. The major lipoxygenase product detected under these conditions in the epithelium was mono-hydroxyeicosatetraenoic acid (mono-HETE) and in the stroma, 12-HETE. In contrast, lipoxygenase products could not be detected in control endothelium. Two hours after injury, the labeling of lipids in epithelium and endothelium decreased; the largest decrease was in phosphatidylinositol, followed by phosphatidylcholine and phosphatidylethanolamine. At the same time, cyclooxygenase-reaction products in the epithelium increased, particularly PGF2 alpha. Prostaglandin levels in the stroma rose rapidly after injury and remained elevated for 15 days. In the endothelium, increases in PGF2 alpha and PGE2 were the most prominent effects of injury. After wounding, lipoxygenase products appeared for the first time in the endothelium and increased in the stroma and epithelium. Within 2 hr after lesioning, 12-HETE and 5-HETE increased in stroma. These studies show that the metabolism of arachidonic acid is altered by cryogenic injury and that the resulting changes differ in the three layers of the cornea. These changes involve arachidonoyl groups of phospholipids and cyclooxygenase and lipoxygenase products, and it is suggested that they are at least partly due to the migration of inflammatory cells to the wound site.

Cationic amphiphilic drugs inhibit the synthesis of long-chain fatty acyl coenzyme A in rat brain microsomes

The effect of cationic amphiphilic drugs (CAD) on the synthesis of thiol esters of coenzyme A with long-chain fatty acids was studied in microsomes of rat brain in vitro. The results indicate that propranolol, tetracaine and to a lesser extent, chloroquine, inhibit enzyme activity. Procaine and lidocaine did not inhibit enzyme activity in concentrations up to 0.8 mM. This inhibition seems to be directed primarily to the synthesis of polyunsaturated fatty acyl coenzyme A. The results also suggest that this inhibition may be due to the action of CAD on the microsomal membrane and not to an interaction of these drugs with the fatty acid substrates.

Inflammation-induced stimulation of the synthesis of prostaglandins and lipoxygenase-reaction products in rabbit cornea

The cyclooxygenase and lipoxygenase pathways that produce prostaglandins (PGs) and hydroxyeicosatetraenoic acids (HETEs) were studied in inflamed rabbit cornea. A cryogenic lesion was induced and five days later the epithelium, stroma and endothelium were isolated and incubated with [1-14C]arachidonic acid. After lesioning, the arachidonic acid metabolites, thromboxane B2, PGF2 alpha and 6-keto-PGF1 alpha exhibited the greatest increase in the stroma. Lipoxygenase products were formed in the three layers also, although 12-HETE predominated. The correlation between the synthesis of these compounds and corneal injury is discussed.

Long-chain acyl CoA synthetase in microsomes from rat brain gray matter and white matter

Long-chain acyl coenzyme A (CoA) synthetase in homogenates and microsomes from rat brain gray and white matter was studied. The formation of the thioesters of CoA was studied upon addition of [1-14C]-labeled fatty acids. The maximal activities were seen with linoleic acid, followed by arachidonic, palmitic, and docosahexaenoic acids in both gray and white matter homogenates and microsomes. The specific activities in microsomes were 3-5 times higher than in homogenates. The presence of Triton X-100 in the assay system enhanced the activity of long-chain acyl CoA synthetase in homogenates. The effect was more pronounced in palmitic and docosahexaenoic acid activation. The apparent Km values and Vmax values for palmitic and docosahexaenoic acids were much lower than for linoleic and arachidonic acids. The presence of Triton X-100 in the medium caused a definite decrease in the apparent Km and Vmax values for all the fatty acids except palmitic acid in which case the reverse was true. There were no significant differences observed in the kinetic measurements between gray and white matter microsomes. These findings are similar to those resulting from the known interference of Triton X-100 in the measurement of kinetic variables of long-chain acyl CoA synthetase of liver microsomes. In this work, no correlation was observed between the fatty acid composition of gray and white matter and the capacity of these tissues for the activation of different fatty acids.

Inhibition of phosphatidylinositol-4-phosphate kinase by its product phosphatidylinositol-4,5-bisphosphate

Phosphatidylinositol 4,5-bisphosphate (PIP2) is enzymatically produced when high speed supernatant fraction from bovine retina is incubated with [gamma-32P]ATP and phosphatidylinositol 4-phosphate (PIP) as substrates. Exogenously added PIP2 inhibits PIP kinase activity 50% at equimolar concentrations of product and substrate. Ca2+-dependent phosphodiesteratic activity, resulting in the loss of PIP2 and PIP and concommitant increase in myo-inositol 1,4,5-trisphosphate and myo-inositol 1,4-bisphosphate, was observed when soluble retinal fractions were incubated with heat-inactivated 32P-prelabeled guinea pig nerve ending membranes as substrate. It is suggested that polyphosphoinositides are under stringent and complex control and that upon receptor activation-mediated stimulation of phosphodiesteratic degradation release of the feedback inhibition shown here may occur and result in the synthesis and replenishment of PIP2.