Increase in plasma interleukin-10 following hydrocortisone injection

In view of the fact that glucocorticoids have an immunosuppressive effect and the fact that interleukin-10 (IL-10) is inhibitory to T helper cell function, we have now investigated the effect of hydrocortisone on plasma IL-10 concentrations. Seven normal subjects were injected with 100 mg hydrocortisone intravenously between 8 and 9:00 a.m. Sequential blood samples were obtained prior to and 1, 2, 4, 8 and 24 h after the injection. Plasma IL-10 concentrations increased significantly and consistently following the injection in all subjects. The peak increase of IL-10 occurred at 4 h and the restoration to baseline by 8 h. The sequential values were (mean+/-SD): 3.0+/-1.3 pg/ml at 2 h, 9+/-4.2 pg/ml at 4 h, 3.7+/-1.8 pg/ml at 8 h and 3.7+/-1.4 pg/ml at 24 h. The magnitude of increase was 436% of the basal at peak effect. This effect of hydrocortisone (and possibly other glucocorticoids) may contribute to the immunosuppressive effect of this drug. IL-10 may also be potentially useful in the assessment of Cushing's Syndrome as a marker of end organ effect of glucocorticoids.

Effect of gender differences and estrogen replacement therapy on vascular reactivity

The incidence of cardiovascular disease is lower in premenopausal women compared with men; following menopause, the risk of mortality from cardiovascular disease increases in females. Postischemic dilatation of the brachial artery has been used previously as an index of endothelium-mediated vasodilation. Using this index, we examined a group of premenopausal and postmenopausal women, some of whom were on estrogen replacement therapy (ERT). All subjects were normotensive (blood pressure [BP] <140/90 mm Hg) and normoglycemic (blood glucose, <100 mg/dL). Fourteen healthy women (mean age, 27 +/- 0.8 years; mean total cholesterol, 174 +/- 6.7 mg/dL) and fourteen healthy men (mean age, 26 +/- 1.4 years; mean total cholesterol, 181 +/- 7.2 mg/dL) were investigated. Nineteen postmenopausal women were also examined; 11 were on ERT (mean age, 55 +/- 2.1 years; mean total cholesterol, 213 +/- 6.6 mg/dL) and eight were not on ERT (mean age, 60 +/- 3.6 years; mean total cholesterol, 222 +/- 14.4 mg/dL). Ischemia was induced by inflating a cuff over the forearm to a pressure of 40 mm Hg above systolic for 5 minutes. Doppler ultrasonography (Acuson [Mountain View, CA] 128XP/10c ultrasonograph with a 7.5-MHz linear array transducer) was used to measure the brachial artery diameter before inflation and 15 seconds and 45 to 60 seconds following cuff deflation. Flow-mediated dilatation (FMD%) and hyperemia were defined as the percentage increase over basal diameter and basal flow, respectively. Postischemic median dilatation in men was 4.20% (interquartile range, 2.13% to 5.56%) and 11.48% (interquartile range, 8.70% to 14.29%) in age-matched premenopausal women (P < .01). For women on ERT, the postischemic median dilatation was 8.11% (interquartile range, 6.01% to 11.60%), as compared with 2.82% (interquartile range, 1.32% to 3.28%) for women without ERT (P < .01). Premenopausal women showed significantly greater dilatation after ischemia than postmenopausal women without ERT (P < .0001). Hyperemia was similar in all groups. These findings show that postischemic vasodilation of the brachial artery is greater in premenopausal women versus age-matched men; it is decreased in postmenopausal women, and ERT restores it toward normal. The pathophysiology underlying the diminution in postischemic dilatation may be relevant to atherogenesis and coronary artery disease (CAD).

Age dependent alterations in photosystem II acceptor side in Cucumis sativus cotyledonary leaf thylakoids: analysis of binding characteristics of herbicide [14C]-atrazine

Senescence induced temporal changes in photosystems can be conveniently studied in cotyledonary leaves. We monitored the protein, chlorophyll and electron transport activities in Cucumis sativus cv Poinsette cotyledonary leaves and observed that by 20th day, there was a 50%, 41% and 30-33% decline in the chlorophyll, protein and photosystem II activity respectively when compared to 6th day cotyledonary leaves taken as control. We investigated the changes in photosystem II activity (O2 evolution) as a function of light intensity. The photosystem II functional antenna decreased by 27% and the functional photosystem II units decreased by 30% in 20-day old cotyledonary leaf thylakoids. The herbicide [14C]-atrazine binding assay to monitor specific binding of the herbicide to the acceptor side of photosystem II reaction centre protein, D1, showed an increase in the affinity for atrazine towards D1 protein and decrease in the QB binding sites in 20th day leaf thylakoids when compared to 6th day leaf thylakoids. The western blot analysis also suggested a decrease in steady state levels of D1 protein in 20th day cotyledonary leaf thylakoids as compared to 6th day sample which is in agreement with [14C]-atrazine binding assay and light saturation kinetics.

Echocardiographic evaluation of patients with a left ventricular device

Limited donor heart availability is primarily responsible for the renewal of interest in mechanical left ventricular assist devices (LVADs) as a bridge to transplantation. Donor availability is unlikely to increase significantly in the near future. Experience to date has shown that many patients can be maintained long enough to undergo transplantation, and LVADs may be acceptable as alternate therapy in some who may not be candidates for transplantation. However, criteria for noninvasive evaluation of patients on LVADs have not been developed. In a prospective study using serial echocardiography, we found that aortic valve opening, aortic forward flow, nonlaminar flow in the left ventricle, and mismatch of Doppler derived cardiac output at the pulmonic valve and device output are associated with device malfunction. Echocardiography was diagnostic in five of six patients with clinical instability unrelated to the device. These findings suggest that echocardiography is helpful in the routine evaluation of patients on LVADs.

The Combination of Insulin and Metformin in Treatment of Non-Insulin-Dependent Diabetes Mellitus

OBJECTIVE

To assess whether, in the treatment of non-insulin-dependent diabetes mellitus (NIDDM), (1) metformin in conjunction with insulin can safely cause a decrease in glycosylated hemoglobin (HbA1c) to 7% or less and (2) this combination therapy may result in weight loss and lower insulin dose in comparison with insulin treatment alone.

METHODS

Forty patients with NIDDM being treated with insulin on their first visit to the Diabetes Center were identified by retrospective review of medical records of all patients encountered during a 1-year period. These patients were classified into groups who were receiving insulin only (group 1) or insulin + metformin (group 2) at the most recent visit. Group 2 was subdivided into those with a body mass index of either =30 kg/m 2 (group 2A) or >30 kg/m 2 (group 2B). Blood glucose, HbA1c, insulin dose, and weights were analyzed from their initial and most recent visits.

RESULTS

HbA1c decreased from 10 +/- 2.7% to 7 +/- 1.1% (P<0.01) in group 1 and from 9.8 +/- 2.1% to 7.2 +/- 1.4% (P<0.01) in group 2. The magnitude of decrease in HbA1c, however, was not different between the two groups. Total insulin dose increased from 40 (33 to 50) U/day to 58 (41 to 67) U/day (P<0.05) in group 1 and from 63 (42 to 118) U/day to 67 (50 to 96) U/day in group 2 (not significantly different). The median increase in insulin dose was 8 U in group 1, whereas the median decrease was 3 U in group 2 (P<0.05). Similar decreases were noted in group 2A. The decrease in insulin dose was inversely related to the initial insulin dose per kilogram of body weight in group 2 (r = -0.5; P<0.01). Patients in group 1 had an increase in weight from 75.0 +/- 8.6 kg to 77.7 +/- 9.0 kg (P<0.01), whereas weight decreased from 100.4 +/- 24.2 kg to 98.5 +/- 22.3 kg in group 2 (P<0.05). A decrease in weight was seen even in group 2A. The increase in weight was 3 +/- 3.3 kg in group 1, whereas weight decreased by 1.9 +/- 3.9 kg in group 2 (P<0.01).

CONCLUSION

Insulin + metformin is safe and is as effective as insulin alone in improving glycemic control in obese and nonobese patients with NIDDM. This combination therapy, however, lowers insulin dose and promotes weight loss, which may be of importance in decreasing the cardiovascular risk factors in these subjects.

Ultraviolet-B effects on Spirulina platensis cells: modification of chromophore-protein interaction and energy transfer characteristics of phycobilisomes

Exposure of ultraviolet-B (280-320 nm, 1.9 mW m-2 s-1) radiation of intact Spirulina platensis for 9 h caused specific loss of the 85.5 KDa anchor protein of phycobilisomes, the major light-harvesting antenna complex of photosystem II. Associated with the loss of 85.5 KDa protein, the UV-B irradiation also caused photobleaching of phycobilins and alteration in the chromophore protein interactions, as evidenced from the visible circular dichroic measurements, and it also affected the energy transfer process within the phycobilisomes, as inferred from the low-temperature, 77 K, fluorescence spectral analysis. Our results, thus, clearly demonstrate for the first time that the phycobilisomes effectively act as targets for UV-B induced damage of photosynthetic apparatus in cyanobacteria.

Effects of ultraviolet-B radiation on phycobilisomes of Synechococcus PCC 7942: alterations in conformation and energy transfer characteristics

Phycobilisomes (PBS), the major light harvesting antenna of the cyanobacterium Synechococcus contain phycocyanin (PC) and allophycocyanin (APC) as major pigment-protein complexes. PBS also absorb ultraviolet-B (280-320 nm) radiation. Exposure of Synechococcus PBS to low dose of UV-B (approximately 0.28 mw.cm-2) for 90 min induced change in absorption, emission and excitation characteristics of PBS and these changes got enhanced after 3 h of exposure. Room temperature excitation and emission spectra clearly indicated uncoupling of energy transfer from PC to APC on exposure to UV-B. Also, the 77K emission spectra suggested that F682 emission originating from APC decreased by 42% after 3 h of exposure. Circular dichroism (CD) spectra of UV-B exposed PBS indicated changes (14% decrease) in the alpha-helical content after 90 min treatment. SDS-PAGE analysis indicated degradation of a 75 kDa polypeptide (which appear to be a linker polypeptide) on UV-B treatment. The degradation of this polypeptide seems to induce changes in pigment-protein interaction and decoupling of energy transfer within the PBS. Our results for the first time clearly indicate that the PBS of Synechococcus are targets for UV-B damage.

Action of K-crown ether on photosystem II electron transport: characterization of the site of action

We have investigated the inhibitory effect of K-crown (18-crown-6 potassium picrate) on photosystem II (PSII)-enriched membrane fragments and O2-evolving core complexes. K-crown at 2-4 microM inhibits about half the control level of O2-evolution activity in both types of PSII samples. Oxygen-evolution studies demonstrated that the ether works by inactivating the centres and not by interfering with antenna function or energy transfer to the reaction centre. K-crown does not disrupt binding of the extrinsic proteins associated with O2 evolution nor complex with bound Ca2+ or Cl- cofactors, but rather it directly inhibits electron transfer after the tetrameric Mn cluster. Fluorescence studies on active and Tris-treated samples showed that K-crown does not prevent artificial donors from transferring electrons to PSII but like DCMU inhibits on the acceptor side after QA, the primary quinone acceptor. However, the ether is a leaky inhibitor and may also act as a weak donor when the Mn cluster is not present. Oxygen-production experiments using silicomolybdate as an artificial acceptor (which accepts from both pheophytin and QB in PSII membranes) demonstrated that the inhibition is at or near the DCMU site.

Targeted mutagenesis of acyl-lipid desaturases in Synechocystis: evidence for the important roles of polyunsaturated membrane lipids in growth, respiration and photosynthesis

Acyl-lipid desaturases introduce double bonds (unsaturated bonds) at specifically defined positions in fatty acids that are esterified to the glycerol backbone of membrane glycerolipids. The desA, desB and desD genes of Synechocystis sp. PCC 6803 encode acyl-lipid desaturases that introduce double bonds at the delta12, omega3 and delta6 positions of C18 fatty acids respectively. The mutation of each of these genes by insertion of an antibiotic resistance gene cartridge completely eliminated the corresponding desaturation reaction. This system allowed us to manipulate the number of unsaturated bonds in membrane glycerolipids in this organism in a step-wise manner. Comparisons of the variously mutated cells revealed that the replacement of all polyunsaturated fatty acids by a monounsaturated fatty acid suppressed growth of the cells at low temperature and, moreover, it decreased the tolerance of the cells to photoinhibition of photosynthesis at low temperature by suppressing recovery of the photosystem II protein complex from photoinhibitory damage. However, the replacement of tri- and tetraunsaturated fatty acids by a diunsaturated fatty acid did not have such effects. These findings indicate that polyunsaturated fatty acids are important in protecting the photosynthetic machinery from photoinhibition at low temperatures.

Studies on electron donation to photosystem I sites by exogenous donors in Spirulina thylakoids

The kinetic parameters of different sites of electron donation to photosystem I (PS I) were evaluated in Spirulina platensis thylakoids. Reduced 2,6-dichlorophenolindophenol (DCIPH2) exhibited two sites of electron donation, with apparent K(m) values of 8 and 40 microM each. The corresponding value for reduced N-tetramethyl-p-phenylenediamine (TMPDH2) and diaminodurene (DADH2) which donate electrons at a single site to PS I were 103 and 48 microM, respectively. The electron donation by these three exogenous donors were differentially inhibited by KCN (70 mM) affecting the apparent K(m) and Rmax values to varying extent. This cyanide inhibition of PS I catalyzed electron transport suggests the presence of plastocyanin in the photosynthetic electron transport chain of Spirulina platensis.

Thermoluminescence investigations on the site of action of o-phthalaldehyde in photosynthetic electron transport

Glow curves from spinach leaf discs infiltrated with o-phthalaldehyde (OPA) show significant similarity to those obtained by DCMU treatment which is known to block the electron flow from QA, the stable acceptor of Photosystem II (PS II). In both the cases, the thermoluminescence (TL) peak II (Q band) was intensified significantly, whereas peaks III and IV (B band) were suppressed. Total TL yield of the glow curve remained constant even when the leaf discs were infiltrated with high concentrations of OPA (4 mM) or with DCMU (100 μM), indicating that even at these high concentrations no significant change in the number of species undergoing charge recombination in PS II occurred. However, studies with thylakoids revealed significant differences in the action of OPA and DCMU on PS II. Although OPA, at a certain concentration and time of incubation, reduced the B band intensity by about 50-70%, and completely abolished the detectable oxygen evolution, it still retained the TL flash yield pattern, and, thus, S state turnover. OPA is known to inhibit the oxidoreductase activity of in vitro Cyt b6/f (Bhagwat et al. (1993) Arch Biochem Biophys 304: 38-44). However, in the OPA treated thylakoids the extent of inhibition of O2 evolution was not reduced even in the presence of oxidized tetramethyl-p-phenylenediamine which accepts electrons from plastoquinol and feeds then directly to Photosystem I. This suggests that OPA inhibition is at a site prior to plastoquinone pool in the electron transport chain, in agreement with it being between QA and QB. However, an unusual feature of OPA inhibition is that even though all oxygen evolution was completely suppressed, a significant fraction of PS II centers were functional and turned over with the same periodicity of four in the absence of any added electron donor, an observation which appears to be similar to that reported by Wydrzynski (Wydrzynski et al. (1985) Biochim Biophys Acta 809: 125-136) with lauroylcholine chloride, a lipid analogue compound. The detailed chemistry of OPA inhibition remains to be studied. Since we dedicate this paper to William A. Arnold, discoverer of delayed light and TL in photosynthesis, we have also included in the Introduction, a brief history of how TL work was initiated at BARC (Bombay, India).

Prolonged incubation with low concentrations of mercury alters energy transfer and chlorophyll (Chl) a protein complexes in Synechococcus 6301: changes in Chl a absorption and emission characteristics and loss of the F695 emission band

Synechococcus PCC 6301 cells grown in the presence of low sublethal levels of (about 2 microM) mercury induced alterations in chlorophyll (Chl) a absorption without significant alterations in phycocyanin. Chl a fluorescence emission in Hg(2+)-raised cells showed a large (about 18 nm) blue shift in the peak emission. No major spectral changes in phycobilisome (PBsome) emission characteristic were noticed, indicating major structural alterations in Chl-protein complexes by incubation with Hg2+ ions. Low temperature (77K) emission spectra of cells grown in the presence of Hg2+ showed a loss of the characteristic Chl a emission band at 695 nm (F695), which is known to be linked to photosystem II photochemistry and to originate from the Chl a of core antenna polypeptide CP 47 of photosystem II. The SDS-PAGE polypeptide profile of thylakoids indicates a loss of a polypeptide(s) with a molecular mass between 40 and 60 kDa by Hg2+ incubation of cells. Our results suggest that prolonged incubation of Synechococcus 6301 cells with low concentrations of Hg2+ affects the Chl a spectral properties and the structure of Chl-protein complexes.

Purification and characterization of an ATPase inhibitor protein from buffalo mitochondria

A heat stable, 12kDa protein was purified to homogeneity from buffalo heart mitochondria. It suppressed hydrolytic activity of membrane bound mitochondrial ATPase and its functional activity was Mg++ and ATP dependent. Maximal inhibition was achieved at slightly acidic pH. Its ability to inhibit ATP hydrolysis was significantly diminished at alkaline pH and high ionic strength and the purified protein had a tendency to aggregate under such conditions. Circular dichroism (CD) studies revealed that the protein undergoes reversible changes in secondary structure from a predominantly alpha-helical form at alkaline pH to a beta-sheet structure at slightly acidic pH. These distinctly different conformations could be correlated with functionally 'inactive' and 'active' forms.

pH dependent conformational changes modulate functional activity of the mitochondrial ATPase inhibitor protein

A 12kDa, heat stable protein (IF1) inhibiting hydrolytic activity of submitochondrial particles was purified to electrophoretic homogeneity from buffalo heart mitochondria. Specific activity of the purified fraction was > 5000 units/mg. Maximal inhibition was observed at pH 6.0 and was Mg++ and ATP dependent. Circular dichroism studies showed that the inhibitor peptide undergoes a dramatic, reversible conformational change in response to pH which correlates well with its ability to inhibit ATP hydrolysis catalyzed by inhibitor depleted submitochondrial particles. It is shown for the first time that IF1 with a predominantly beta-sheet component is more efficient at suppressing ATPase activity.

Cobalt chloride induced stimulation of Photosystem II electron transport in Synechocystis sp. PCC 6803 cells

Synechocystis sp. PCC 6803 when grown in the presence of sublethal (μM) levels of cobalt chloride shows an enhancement of Photosystem II (PS II) catalyzed Hill reaction. This stimulation seems to be induced by cobalt ions as other metal ions inhibit para-benzoquinone catalyzed Hill reaction. At saturating white light intensity, this enhancement is two times over that of the control cells on unit chlorophyll basis. Analysis of the PS II electron transport rate at varying intensities of white, blue or yellow light suggests an increased maximal rates but no change in the quantum yield or effective antenna size of CoCl2-grown cells. There were no structural and functional changes in the phycobilisome as judged by the absence of changes in the phycocyanin/allophycocyanin ratio, fluorescence emission spectra, second derivative absorption spectra at 77 K and SDS-PAGE analysis of isolated phycobilisomes. The 77 K fluorescence emission spectra of the cells showed a decrease in the ratio of Photosystem I emission (F725) to Photosystem II emission (F685) in CoCl2-grown cells compared to the control cells. These observations indicate three possibilities: (1) there is an increase in the number of Photosystem II units; (2) a faster turnover of Photosystem II centers; or (3) an alteration in energy redistribution between PS II and PS I in CoCl2-grown cells which causes stimulation of Photosystem II electron transport rate.

Characterization of aliphatic amine 1,4-diazobicyclo (2,2,2) octane-induced stimulation of beet spinach thylakoid electron transport

Electron transport activity of beet spinach thylakoids was enhanced in the presence of aliphatic amine, DABCO (1,4-diazobicyclo (2,2,2) octane), a hydrophilic proton trapping agent. The extent of stimulation was pH-dependent and similar to the effect of the uncoupler ammonium chloride on electron transport. The stimulation of whole-chain (H2O-->MV) electron transport activity was observed only at high (rate-saturating) light intensity. The light-induced proton uptake coupled to electron transport of thylakoid was also arrested by DABCO, suggesting that DABCO uncouples thylakoid phosphorylation by proton trapping.

Analysis of dark-relaxation kinetics of variable fluorescence in intact leaves

The dark-relaxation kinetics of variable fluorescence, Fv, in intact green leaves of Pisum stativum L. and Dolichos lablab L. were analyzed using modulated fluorometers. Fast (t1/2 = 1 s) and slow (t1/2 = 7-8 s) phases in fv dark-decay kinetics were observed; the rate and the relative contribution of each phase in total relaxation depended upon the fluence rate of the actinic light and the point in the induction curve at which the actinic light was switched off. The rate of the slow phase was accelerated markedly by illumination with far-red light; the slow phase was abolished by methyl viologen. The halftime of the fast phase of Fv dark decay decreased from 250 ms in dark-adapted leaves to 12-15 ms upon adaptation to red light which is absorbed by PSII. The analysis of the effect of far-red light, which is absorbed mainly by PSI, on Fv dark decay indicates that the slow phase develops when a fraction of QA (-) (the primary stable electron acceptor of PSII) cannot transfer electrons to PSI because of limitation on the availability of P700(+) (the primary electron donor of PSI). After prolonged illumination of dark-adapted leaves in red (PSII-absorbed) light, a transient. Fv rise appears which is prevented by far-red (PSI-absorbed) light. This transient fv rise reflects the accumulation of QA (-) in the dark. The observation of this transient Fv rise even in the presence of the uncoupler carbonylcyanide m-chlorophenyl hydrazone (CCCP) indicates that a mechanism other than ATP-driven back-transfer of electrons to QA may be responsible for the phenomenon. It is suggested that the fast phase in Fv dark-decay kinetics represents the reoxidation of QA (-) by the electron-transport chain to PSI, whereas the slow phase is likely to be related to the interaction of QA (-) with the donor side of PSII.

Proline enhances primary photochemical activities in isolated thylakoid membranes of Brassica juncea by arresting photoinhibitory damage

The presence of L-proline in the reaction mixture enhances the photosystem II (H2O----DCPIP) and whole chain (H2O----MV) catalysed electron transport activities of thylakoids isolated from the cotyledonary leaves of Brassica juncea seedlings raised in the absence and the presence of NaCl. The extent of stimulation in activities was higher in the thylakoids of NaCl raised plants than the controls. The extent of proline mediated stimulation was seen even in the presence of uncoupler NH4Cl suggesting that this stimulation is not due to uncoupling. However, photosystem I (DCPIPH2----MV) catalysed photoreaction remained almost insensitive to proline. The presence of proline in the incubation medium brought about a significant reduction in the time dependent loss in photochemical activity of thylakoids exposed to strong light suggesting that proline prevents photoinhibitory loss in chloroplast activity. Also, proline brought about a considerable reduction in the production of lipid peroxidation linked maiondialdehyde during strong illumination. We suggest that proline protects the components involved in water oxidation capacity by reducing the production of free radicals and/or scavenging the free radicals and thereby reducing thylakoid lipid peroxidation.

N,N-diethylhydroxylamine: a new electron donor to photosystem II

Diethylhydroxylamine, when added to beet spinach thylakoid membranes in the reaction mixture enhanced both photosystem II mediated dichlorophenolindophenol photoreduction and whole chain electron transport supported by methyl viologen. Diethylhydroxylamine supports dichlorophenolindophenol photoreduction when oxygen evolving complex is inactivated by hydroxylamine washings. All the electron transport assays were found to be highly sensitive to diuron, indicating that diethylhydroxylamine donates electrons to the photosystem II before the herbicide binding site. The stimulation of the photochemical activity by diethylhydroxylamine is not solely due to its action as an uncoupler. It was also observed that the action of diethylhydroxylamine was not altered by preincubations of thylakoids in light in the presence of diethylhydroxylamine. Also, thylakoid membranes did not lose their benzoquinone Hill activity by the pre-incubations with diethylhydroxylamine either in light or in dark. Thus, unlike the photosystem II electron donor, hydroxylamine, diethylhydroxylamine was found to donate electrons without the inactivations of oxygen evolving complex. It is suggested that diethylhydroxylamine is a useful electron donor to the photosystem II.

Analysis of chlorophyll a fluoresence changes in weak light in heat treated Amaranthus chloroplasts

After preheating of Amaranthus chloroplasts at elevated temperatures (up to 45°C), the chlorophyll a fluorescence level under low excitation light rises as compared to control (unheated) as observed earlier in other chloroplasts (Schreiber U and Armond PA (1978) Biochim Biophys Acta 502: 138-151). This elevation of heat induced fluorescence yield is quenched by addition of 0.1 mM potassium ferricyanide, suggesting that with mild heat stress the primary electron acceptor of photosystem II is more easily reduced than the unheated samples. Furthermore, the level of fluorescence attained after illumination of dithionite-treated samples is independent of preheating (up to 45°C). Thus, these experiments indicate that the heat induced rise of fluorescence level at low light can not be due to changes in the elevation in the true constant F0 level, that must by definition, be independent of the concentration of QA. It is supposed that the increase in the fluorescence level by weak modulated light is either partly associated with dark reduction of QA due to exposure of chloroplasts to elevated temperature or due to temperature induced fluorescence rise in the so called inactive photosystem II centre where QA are not connected to plastoquinone pool. In the presence of dichlorophenyldimethylurea the fluorescence level triggered by weak modulated light increases at alkaline pH, both in control and heat stressed chloroplasts. This result suggests that the alkaline pH accelerates electron donation from secondary electron donor of photosystem II to QA both in control and heat stressed samples. Thus the increase in fluorescence level probed by weak modulated light due to preheating is not solely linked to increase in true F0 level, but largely associated with the shift in the redox state of QA, the primary stable electron acceptor of photosystem II.