In vitro and in vivo regulation of chloroplast glyceraldehydes-3-phosphate dehydrogenase isozymes from Chenopodium rubrum. I. Purification and properties of isozymes

Chloroplast glyceraldehyde-3-phosphate dehydrogenase (GPD, EC 1.2.1.13) was purified from leaves of Chenopodium rubrum L. Aggregated (≥ 10⁶) and disaggregated (165 × 10³) molecular weight forms were obtained by gel filtration in the presence of NAD⁺ and NADP⁺, respectively. The disaggregated enzyme was separated into two isozymes by inverse ammonium sulphate gradient solubilization: "NADP-GPD I" was homotetrameric (subunit molecular weight 39 × 10³); "NADP-GPD II" was heterotetrameric (subunit molecular weights 39 × 10³ and 43 × 10³). Isoelectric focusing of the isozymes, both aggregated and disaggregated, revealed two isoelectric forms in each case, at 4.3 and 7.7. Chloroplast GPD was "NADP-suppressed" in crude extracts due to partial oxidation, incubation with dithioerythritol restored full activity.     

In vitro and in vivo regulation of chJoroplast glyceraldehyde-3-phosphate dehydrogenase isozymes from Chenopodium rubrum. III. The molecular basis of the aggregation phenomenon: chloroplast glyceraldehyde-3-phosphate dehydrogenase as an ambiquitous enzyme

The nature of the aggregated form of chloroplast glyceraldehyde-3-phosphate dehydrogenase isozymes (GPD, EC 1.2.1.13) from Chenopodium rubrum leaves was investigated. After disaggregation of the isozymes in NADP ⁺ buffer, and resuspension of the disaggregated isozymes in NAD⁺ buffer, complete reaggregation could only be achieved by remixing the enzyme with a high molecular weight fraction, from which the isozymes had dissociated during the NADP⁺ filtration. After separation of the isozymes by inverse ammonium sulphate gradient solubilization, spontaneous extensive reaggregation of each isozyme was observed in NAD⁺ buffer. The high molecular weight material consisted of ribonucleoprotein, and RNase treatment impaired its ability to promote reaggregation of chloroplast GPD. It is proposed that pyridine nucleotide-controlled aggregation and binding to ribonucleoprotein in vitro are artifacts which reflect an in situ binding to cellular components. Since uncontrolled NAD⁺-linked activities of the bifunctional isozymes in the chloroplast would lead to an equalization of the NAD ⁺ and NADP ⁺ redox couples, it is suggested that the reversible binding of the isozymes forms the basis of a regulatory system in vivo.     

Effects of nitrate on influx, efflux and translocation of potassium in young sunflower plants

Influx, efflux and translocation of K⁺(⁸⁶Rb) were studied in the roots of sunflower seedlings ( Helianthus annuus L. cv. Uniflorus) treated with 0–4.0 m M NO₃⁻ during a 9 day growth period or a 24 h pretreatment period. Roots treated with high levels of NO₃⁻ absorbed and translocated more K⁺(⁸⁶Rb) than seedlings treated with low levels of NO₃⁻. The content of K⁺ in the shoots was, however, higher in seedlings treated with low levels of NO₃⁻, indicating a low rate of retranslocation of K⁺ in those plants. K⁺(⁸⁶Rb) efflux was highest into the low-NO₃⁻ solutions. All effects on K⁺(⁸⁶Rb)-fluxes were more obvious in high-K plants than in low-K plants. The results are discussed in relation to the Dijkshoorn-Ben Zioni hypothesis for K⁺+ NO₃⁻-uptake and translocation in plants.     

Influence of light and darkness and the experimental design on kinetics of K+ (86Rb) influx in roots of intact spring wheat

Seedlings of spring wheat ( Triticum aestivum L. cv. Svenno) were cultivated at 20°C in continuous light or darkness with the roots in nutrient solutions for six days. The plants were starved for K⁺ during different periods of time to produce plants with various K⁺ status. In one cultivation light-grown plants were pretreated in darkness, and vice versa, before the uptake experiment. In all experiments, roots were put in a complete nutrient medium containing 2.0 m M K⁺ radiolabelled with ⁸⁶Rb. The uptake time was varied (5, 60 or 120 min).
The K⁺ concentration in the roots, [K⁺]_root, increased during the course of the uptake experiments, especially in light and at initially low [K⁺]_root, At the same time K⁺ (⁸⁶Rb) influx in the roots decreased. The simoidal relationship obtained between K⁺ (⁸⁶Rb) influx and [K⁺]_root was affected by these changes, and Hill plots gave various Hill coefficients, n_H, depending on the duration of the uptake experiments. n_H from three apparently straight line segments of the same plot, in different [K⁺]_root - intervals, indicated a falling degree of interaction between the binding sites as [K⁺]_root increased. For the dark-grown plants negative cooperativity could not be demonstrated.     

Photosystem I acceptor side limitation is a prerequisite for the reversible decrease in the maximum extent of P700 oxidation after short-term chilling in the light in four plant species with different chilling sensitivities

Changes in the extent of P700 oxidation (P700⁺) were investigated after chilling of barley, rice, pumpkin, and cucumber leaf segments at 4°C for 1 h under light with various photon flux densities. At 50 µmol photons m⁻² s⁻¹, the decrease in P700⁺ was observed only in cucumber, but at 150 µmol photons m⁻² s⁻¹, it was found in all plants except barley, revealing their expected chilling sensitivities. However, the decrease in P700⁺ by this short-term chilling was reversible in the presence of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea or methyl viologen, and it did not show any causal relationship with the decrease in the electron transfer rate nor with the down-regulation of photosystem II through the accumulation of zeaxanthin and the development of non-photochemical quenching. These results led to the suggestion that photosystem I (PSI) acceptor side limitation is a prerequisite for the decrease of P700⁺. Furthermore, PSI acceptor side limitation could be mainly due to limitation of electron-sink pathways such as CO₂ assimilation and ascorbate–glutathione cycle, because treatment with glycolaldehyde which inhibits the former pathway, and with KCN which inhibits both pathways, decreased P700⁺ by 20–30% in barley leaves after chilling in the light.     

Vitamin D3 affects growth and Ca2+ uptake by Phaseolus vulgaris roots cultured in vitro

Vitamin D₃ at low concentration (10⁻⁹ M) inhibited the growth of Phaseolus vulgaris L. (cv. Contrancha) roots in vitro as measured by elongation (14 h) and [³H]-leucine incorporation into protein (2 h), and increased their labelling with ⁴⁵Ca²⁺ (2 h). Cycloheximide and puromycin (50 u.M) blocked vitamin D₃ stimulation of root ⁴⁵Ca²⁺ labelling, indicating that it is mediated by de novo protein synthesis. The calcium ionophore X-537A (10⁻⁵JW) induced similar changes both in root elongation and ⁴⁵Ca²⁺ uptake (14 h). This may indicate that the inhibitory effects of the sterol on root growth are mediated by changes in Ca²⁺ fluxes. However, this interpretation should be further strengthened by additional studies as the ionophore may have acted on root growth, affecting physiological processes other than Ca²⁺ transport.     

Contribution of current photosynthates to root respiration of non-nodulated Medicago sativa: effects of light and nitrogen supply

Abstract: The effects of light (PFD) and nitrogen (N) supply on root respiration of new C (currently assimilated carbon, R _new) and old C ( R _old) were analysed in non-nodulated Medicago sativa . Plants were pre-treated with high/low PFD and high/low N supply with a regular 16/8 h light/dark cycle. Five to eight weeks after planting current photosynthates were labelled with ¹³C and their contribution to root respiration was continuously measured during a 24 h day/night cycle. PFD conditions during labelling were either those of the pre-treatments (control, 25 or 6 mol m^-2 d^-1) or, for high PFD plants, 6 mol m^-2 d^-1 by shortening the photoperiod or reducing irradiance. The fraction of new C in the respiratory CO₂ increased during the light period, but remained constant in the dark period. In control plants, R _new contributed 40 % to the daily root respiration in high PFD/high N conditions. Continuously low PFD increased (50 %) and low N decreased (26 %) the contribution of R _new. Exposing plants from high PFD pre-treatments to a short photoperiod or to low PFD stimulated R _old, indicating mobilisation of reserve C. This stimulation was more pronounced in plants with high N supply than in those with low N supply. Comparison with other legumes suggested that R _new in root respiration was mainly defined by the ratio between the assimilatory capacity of the shoots and the maintenance costs of roots with a short-term capacity of buffering respiratory demand by mobilisation of reserves in situations of fluctuating PFD.     

Effect of temporary changes in light intensity on carbon transport, partitioning and respiratory loss in young tomato seedlings raised under different light intensities

Tomato plants were grown under light intensities of 36 or 90 W m⁻² [photosynthetically active radiation (PAR)], and then the light intensity was changed to 36, 90 or 180 W m⁻² for 8 h to investigate the effect of temporary changes in light intensity on the carbon budget of photoassimilates from the third leaf using a ¹⁴CO₂ steady-state feeding method. In the plants that were raised under 90 W m⁻², the photosynthetic rate increased when the light intensity was increased to 180 W m⁻², whereas no increase occurred in the plants that were raised under 36 W m⁻². Although the total amount of carbon fixed during the 8-h light period showed a large difference between plants grown at the two initial light intensities, the proportion of carbon exported during the light period did not differ apparently, irrespective of the change in light intensity. However, the amount of carbon exported during the time course was higher in plants that were raised under 90 W m⁻² than those raised under 36 W m⁻², irrespective of the change in light intensity. The partitioning pattern of ¹⁴C-photoassimilates was not changed by the change in light intensity, irrespective of whether the light intensity was increased or not. However, the amount of ¹⁴C-photoassimilates accumulated in each part differed according to the two initial light intensities. The carbon transport from a source leaf was also investigated through a quantitative analysis of carbon balance.     

Influence of some metal chelators and light regimes on bioaccumulation and toxicity of Cd2+ in duckweed (Lemna gibba)

A factorial culture experiment was designed to investigate the influence of light regimes and of some metal chelators on the accumulation of cadmium by Lemna gibba L. The plants were grown in a complete nutrient solution containing Cd²⁺ concentrations ranging from 0 to 27 μ M with or without EDTA, ethylenediamine-N,N'- bis -( o -hydroxyphenylacetic acid) (EDDHA) or salicylic acid. Each experiment was run for eight days in 18 h:6 h light:dark or continuous light. An increase in the Cd²⁺ concentration in plants and a simultaneous drop in accumulation efficiency (ratio of Cd²⁺ concentration in plants to the initial Cd²⁺ concentration in the nutrient solution) with increasing ambient Cd²⁺ levels was best represented by regression power curves. At the lowest Cd²⁺ concentration which caused a significant decrease in the relative growth rate of duckweed, there was a decrease in manganese and zinc and an increase in the iron level in the plants. EDDHA and EDTA protected in some cases against the toxic action of cadmium without preventing its uptake by plants. It was thus observed that 9 μ M or higher levels of Cd²⁺ were toxic to Lemna gibba depending on the chelator and light regime. Duckweed grown in continuous light produced, in general, more dry matter and hence accumulated more cadmium.     

DEFICIENCY OF A Ca2+-ATPase IN BRAINS OF SEIZURE PRONE MICE

Abstract– Ca²⁺-stimulated ATPase activity was studied in membrane enriched preparations from the brains of audiogenic seizure-prone (DBA) and control (C57 and C3H) mice. The animals ranged in age from 7 to 60 days. Na⁺, K ⁺ -ATPase, 5'-nucleotidase and p -nitrophenylphosphatase were assayed to evaluate membrane integrity.
Ca²⁺-ATPase was significantly lower in DBA mice; notably during the period of maximal seizure sensitivity. Mg²⁺ -ATPase somewhat followed the pattern shown by Ca²⁺ -ATPase. Na⁺, K⁺ -ATPase in DBA did not differ significantly from controls and there were no differences in either 5'-nucleotidase or p-nitrophenylphosphatase activities.
Ca²⁺-ATPase kinetics experiments showed even more clearly the difference between DBA and control preparations. V_max was consistently lower in DBA than in controls. The K_m values appeared to fall into groupings suggestive of sequential synthesis of isozymes. Differences in the patterns of DBA and C57 just prior to the time of maximal seizure sensitivity are interpreted as reflecting failure to synthesize an isozyme or delay of its synthesis. The genesis of seizures through such an enzymatic defect may be related to the action of translocated ATP on the plasma membrane.     

The characteristics of light in floral induction in vitro of Cichorium intybus. The possible role of phytochrome

The action of light in the initiation of floral buds in vitro has been studied using monochromatic light qualities on root explants of a long day plant, Cichorium intybus L. cv. Witloof. Red light (660 nm, 0.30 W m^-2) promotes flowering, while far-red (730 nm, 0.31 W m^-2) and irradiation with combined red + far-red (0.20 + 0.41 W m^-2) have no effect. In short day conditions floral response can be obtained in two ways: 1) by interrupting the dark period with 5 brief irradiations of red light (0.45 W m^-2, 12 min) at regular intervals, although these are counteracted by far-red irradiations of equal intensity and duration; 2) by interrupting the long night with 5 h red light applied during the second third of the night, while at the beginning or at the end it is ineffective. Red light efficiency appears to depend on the photosynthetic activity of the tissues, so that flowering increases with increasing intensity of white light and is suppressed if no white light is supplied. The reproductive development is determined by the coordination of proper irradiation conditions with sufficient sensitivity of the perceiving meristematic cells. The period of highest sensitivity to environmental light conditions in the life cycle of a Cichorium root explant occurs between the 8th and the 16th day after the start of the culture. The data strongly suggest that phytochrome is involved in flower induction of Cichorium in vitro.     

Differential effects of chilling on the activity of C4 enzymes in two ecotypes of Echinochloa crus-galli from sites of contrasting climates

Five-week-old plants of Echinochloa crusgalli (L.) Beauv. from Mississippi and from Québec grown under controlled conditions were subjected to dark chilling for 10 h at 5°C or light chilling treatments for 14 h at 7°C under hight light (1 000 μmol m⁻² s⁻¹). The activities of four C₄ enzymes of Québec plants, measured 4 h after the completion of the cold treatment, were not affected by the chilling treatment in the dark. The activities of pyruvate, P_i dikinase (PPDK; EC 2.7.9.1) and NADP⁺-malic enzyme (NADP⁺-ME; EC 1.1.1.40), were significantly reduced in dark-chilled Mississippi plants. Chilling under high light conditions elicited significant levels of reduction in the activities of the four enzymes from both ecotypes but the reductions were significantly less severe for Québec plants. The recovery of activities of phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) and PPDK for both ecotypes was completed within 36 to 60 hours following the chilling treatment, but NADP⁺-malate dehydro-genase (NADP⁺-MDH; EC 1.1.1.82) and NADP⁺-ME activities of chilled Mississippi plants remained below that of control plants at the end of the 5-day monitoring period. PPDK was inactivated in vitro at 0 and 10°C and the rates of cold inactivation were significantly higher for PPDK extracted from Mississippi plants. The activity of PEPC of Mississippi extracts was slightly, but significantly reduced by a 60 min treatment at 0°C.     

Salinity tolerance of Kosteletzkya virginica. II. Root growth, lipid content, ion and water relations

Abstract. Kosteletzkya virginica (L.) Presl., a dicot halophyte native to brackish tidal marshes, was grown on nutrient solution containing 0. 85, 170 or 255 mol m ³ NaCl, and the effects of external salinity on root growth, ion and water levels, and lipid content were examined in successive harvests. Root growth paralleled shoot growth trends, with some enhancement observed at 85 mol m ³ NaCl and a reduction noted at the higher salinities. Root Na⁺ content increased with increasing external NaCl, but remained constant with time for each treatment. K⁺ content, although lower in salt-grown plants after 14 d salinization, subsequently increased to levels comparable to unsalinized plants. A strong K⁺ affinity was reflected in the increased K⁺/Na⁺ selectivity of salt-grown plants and by their low Na⁺/K⁺ ratios. Cl levels rose in salinized plants and values were double or more those for Na⁺, indicating the possibility of a sodium-excluding mechanism in roots. Root phospholipids and sterols, principal membrane constituents, were maintained or elevated and the free sterol/phospholipids ratio increased in salinized K. virginica plants, suggesting retention of overall membrane structure and decreased permeability. This response, considered in light of root calcium maintenance and high potassium levels, suggests that salinity-induced changes in membrane lipid composition may be important in preventing K⁺ leakage from cells.     

Phospholipase Cγ1 in Bovine Rod Outer Segments: Immunolocalization and Light-Dependent Binding to Membranes

Abstract: We have investigated the isozymes of a phosphoinositide-specific phospholipase C (PLC) in bovine retina using several monoclonal antisera to PLCβ₁, γ₁, and δ₁. Immunoblot analysis showed that all three isozymes were present in the retina. Immunocytochemical localization in frozen bovine retina sections showed that PLCγ₁ was present in the photoreceptor cell layer, outer plexiform cell layer, inner plexiform cell layer, and ganglion cell layer. Immunoreaction within the photoreceptor cell layer was dependent on dark/light adaptation state of retinas. Immunoblot analysis of rod outer segments (ROS) with monoclonal or polyclonal antibodies to PLCγ₁ showed the presence of an immunoreactive band of 140 kDa. ROS prepared from retinas light-adapted in vitro had more PLCγ₁ on immunoblots than ROS from dark-adapted retinas. PLC enzyme activity in ROS from light-adapted retinas was 69 and 46% higher than ROS from dark-adapted retinas, when assayed in the presence and absence of ATP, respectively. This increase in enzyme activity was observed at [Ca²⁺]_free between 0.32 and 100 µ M . These results demonstrate the presence of PLCγ₁ in bovine ROS and show that ROS prepared from light-adapted retinas are enriched in this isozyme, suggesting that light may promote the binding of this isozyme to bleached ROS membranes.     

Influence of aluminium on phosphate and calcium uptake in beech (Fagus sylvatica) grown in nutrient solution and soil solution

The effects of AICI₃ on uptake of Ca²⁺ and phosphate in roots of intact beech ( Fagus sylvatica L. provenance Maramures) plants were studied in nutrient solution and soil solution. Aluminium reduced the concentrations of Ca, Mg and P in plants and increased that of K. In short term experiments, uptake of Ca²⁺(⁴⁵Ca) was reduced by exposure of the roots to Al. The effect of aluminium on Ca²⁺(⁴⁵Ca) uptake was immediate and primarily of a competitive nature, preventing Ca²⁺ from being adsorbed. Uptake of ³²P-phosphate increased with increasing Al concentration up to 0.1 m M and then decreased at higher Al concentrations. The effect of Al on ³²P-phosphate uptake was most pronounced during the first hours of exposure. Growth of plants for 15 days in soil solution, collected from the upper A horizon of a beech forest soil, had no effect on uptake of Ca²⁺(⁴⁵Ca) and ³²P-phosphate, probably because of a low concentration of labile bound monomeric Al and binding of Al to organic compounds. Soil solution from the deeper B horizon reduced Ca²⁺(⁴⁵Ca) uptake and increased ³²P-phosphate uptake in a manner similar to that with Altreatment in nutrient solution. It is concluded that in soil solution from the deeper regions of the soil, mineral uptake by roots was affected by Al.     

In vitro susceptibility of Erwinia amylovora (Burrill) Winslow et al. to geraniol and citronellol

The in vitro antimicrobial activity of geraniol and citronellol towards seven strains of Erwinia amylovora , the causal agent of 'fire blight'of Rosaceous plants, was assessed in tube cultures. All of the strains tested at 1 × 10⁵ cfu/ml were inhibited for 24 h by geraniol in the range 600–1500 mg/1, whereas its minimum bactericidal concentration was 800–1700 mg/1. Citronellol was less effective, being bactericidal for only two of seven strains. RIF-NY, isolated from apple orchards, was relatively resistant to geraniol; 1700 mg/1 of the chemical only reduced the growth of an inoculum of 1 × 10⁷ cfu/ml. In general, such terpenoids commenced exerting a bactericidal effect 6 h after addition to the suspensions, even if geraniol added at 1700 mg/1 to 1 × 10³ cfu/ml of five strains, commenced its bactericidal activity earlier than 6 h.     

Sodium Transport from Blood to Brain: Inhibition by Furosemide and Amiloride

Abstract: Brain sodium uptake in vivo was studied using a modified intracarotid bolus injection technique in which the uptake of ²²Na ⁺ was compared with that of the relatively impermeable molecule, [³H]l-glucose. At a Na ⁺ concentration of 1.4 m M , Na ⁺ uptake was 1.74 ± 0.07 times greater than l -glucose uptake. This decreased to 1.34 ± 0.04 at 140 m M Na ⁺, indicating saturable Na ⁺ uptake. Relative Na ⁺ extraction was not affected by pH but was inhibited by amiloride ( K _i= 3 ± 10⁻⁷ M ) and by 1 m M furosemide. The effects of these two inhibitors were additive. Brain uptake of ⁸⁶Rb ⁺, a K ⁺ analogue, was measured to study interaction of K ⁺ with Na ⁺ transport systems. Relative ⁸⁶Rb ⁺ extraction was also inhibited by amiloride; however, it was not inhibited by furosemide. The results suggest the presence of two distinct transport systems that allow Na ⁺ to cross the luminal membrane of the brain capillary endothelial cell. These transport systems could play an important role in the movement of Na ⁺ from blood to brain.     

Uptake and distribution of calcium, magnesium and potassium in cucumber of different age

Uptake and distribution of Ca⁺, Mg²⁺ and K²⁺ were investigated in plants of cucumber ( Cucumis sativus L. var. Cila) which had been cultivated for 12, 19, 32, or 53 days in complete nutrient solution with 1.0 m M Ca²⁺, 2.0 m M Mg²⁺ and 2.0 m M K⁺. The ⁺ concentration was about the same in roots and shoots, while the Ca²⁺ and Mg²⁺ concentrations were low in roots compared to shoots. The K⁺ concentration decreased with increasing leaf age, while the Ca²⁺ and Mg²⁺ concentrations increased, except in older plants with flowers and fruits, where an increased concentration was found in the youngest leaves. This is discussed in connection with increased indoleacetic acid (IAA) synthesis in the shoot. Excision of leaves at different levels from 21-day-old plants, followed by uptake for 24 h from the nutrient solution on days 22 and 23, resulted in no immediate reduction in Ca²⁺ (⁴⁵Ca) uptake. Transport of Ca²⁺ increased to leaves above and below the excision point and total Ca²⁺ uptake remained at the same level as for the intact plant. It is suggested that regulation of Ca²⁺ uptake is primarily achieved in the root while the distribution in the shoot is regulated by the accessability of negative binding sites.     

Mechanisms of cobalt uptake in plants: 60CO uptake and distribution in Chara

The mechanism of cobalt uptake was investigated using cells of the giant alga Chara corallina in which it is possible to resolve separately uptake by the cell wall and actual influx across the cell membrane. The absorption of ⁶⁰Co by Chara cells appeared to saturate within 2 h, but this was mainly due to rapid uptake into the cell wall which accounted for 87–92% of the total activity. Even after prolonged desorption most of the cell‐associated ⁶⁰Co was found on the cell wall. The intracellular distribution of absorbed ⁶⁰Co was investigated by fractionating the cell into cytoplasm and vacuole. It was shown that ⁶⁰Co influx to the vacuole occurs simultaneously with influx to the cytoplasm. The transported species appears to be Co²⁺ rather than the less charged Co(OH)⁺ or Co(OH)₂. ⁶⁰Co influx is pH dependent (optimum pH 7–9), and is sensitive to some other divalent metals. Influx from solutions containing 1 µ M ⁶⁰Co was inhibited by 5 µ M Cd²⁺, Cu²⁺, and Zn²⁺, but Mn²⁺ and Ni²⁺ had no significant effect. The sensitivity of Co uptake to N ‐ethyl maleimide (NEM) and cysteine suggests that the transport system involves direct binding of CO²⁺ to ‐SH groups.     

Melatonin Effect on [3H]Glutamate Uptake and Release in the Golden Hamster Retina

Abstract: The effect of melatonin on [³H]glutamate uptake and release in the golden hamster retina was studied. In retinas excised in the middle of the dark phase, i.e., at 2400 h, melatonin (0.1 and 10 n M ) significantly increased [³H]glutamate uptake, and this effect persisted in a Ca²⁺-free medium. On the other hand, melatonin significantly increased [³H]glutamate release in retinas excised at 2400 h, but this effect was Ca²⁺ sensitive. Melatonin significantly increased ⁴⁵Ca²⁺ uptake by a crude synaptosomal fraction from retinas of hamsters killed at 2400 h. In retinas excised at 1200 h, melatonin had no effect on [³H]glutamate uptake, [³H]glutamate release, or ⁴⁵Ca²⁺ uptake at any concentration tested. Cyclic GMP analogues, i.e., 8-bromoguanosine 3',5'-cyclic monophosphate and 2'- O -dibutyrylguanosine 3',5'-cyclic monophosphate, significantly increased [³H]glutamate uptake, [³H]glutamate release, and ⁴⁵Ca²⁺ uptake by tissue removed at 1200 and 2400 h, suggesting that the effects of melatonin could correlate with a previously described effect of melatonin on cyclic GMP levels in the golden hamster retina. Taking into account the key role of glutamate in visual mechanisms, the results suggest the participation of melatonin in retinal physiology.