Anaesthesia for vascular emergencies - a state of the art review

In this state-of-the-art review, we discuss the presenting symptoms and management strategies for vascular emergencies. Although vascular emergencies are best treated at a vascular surgical centre, patients may present to any emergency department and may require both immediate management and safe transport to a vascular centre. We describe the surgical and anaesthetic considerations for management of aortic dissection, aortic rupture, carotid endarterectomy, acute limb ischaemia and mesenteric ischaemia. Important issues to consider in aortic dissection are extent of the dissection and surgical need for bypasses in addition to endovascular repair. From an anaesthetist's perspective, aortic dissection requires infrastructure for massive transfusion, smooth management should an endovascular procedure require conversion to an open procedure, haemodynamic manipulation during stent deployment and prevention of spinal cord ischaemia. Principles in management of aortic rupture, whether open or endovascular treatment is chosen, include immediate transfer to a vascular care centre; minimising haemodynamic changes to reduce aortic shear stress; permissive hypotension in the pre-operative period; and initiation of massive transfusion protocol. Carotid endarterectomy for carotid stenosis is managed with general or regional techniques, and anaesthetists must be prepared to manage haemodynamic, neurological and airway issues peri-operatively. Acute limb ischaemia is a result of embolism, thrombosis, dissection or trauma, and may be treated with open repair or embolectomy, under either general or local anaesthesia. Due to hypercoagulability, there may be higher numbers of acutely ischaemic limbs among patients with COVID-19, which is important to consider in the current pandemic. Mesenteric ischaemia is a rare vascular emergency, but it is challenging to diagnose and associated with high morbidity and mortality. Several peri-operative issues are common to all vascular emergencies: acute renal injury; management of transfusion; need for heparinisation and reversal; and challenging postoperative care. Finally, the important development of endovascular techniques for repair in many vascular emergencies has improved care, and the availability of transoesophageal echocardiography has improved monitoring as well as aids in surgical placement of endovascular grafts and for post-procedural evaluation.

Penicillin induces alterations in glutamine metabolism in Streptococcus pneumoniae

Penicillin is a bactericidal antibiotic that inhibits the synthesis of the peptidoglycan by targeting penicillin-binding proteins. This study aimed to assess through transcriptional profiling the stress response of S. pneumoniae strains after exposure to lethal penicillin concentrations to understand further the mode of action of penicillin. Two experimental designs (time-course and dose-response) were used for monitoring the effect of penicillin on the transcriptional profile. The expression of some genes previously shown to be modulated by penicillin was altered, including ciaRH, pstS and clpL. Genes of the glnRA and glnPQ operons were among the most downregulated genes in the three strains. These genes are involved in glutamine synthesis and uptake and LC-MS work confirmed that penicillin treatment increases the intracellular glutamine concentrations. Glutamine conferred a protective role against penicillin when added to the culture medium. Glutamine synthetase encoded by glnA catalyses the transformation of glutamate and ammonium into glutamine and its chemical inhibition by the inhibitor L-methionine sulfoximine is shown to sensitize S. pneumoniae to penicillin, including penicillin-resistant clinical isolates. In summary, a combination of RNA-seq and metabolomics revealed that penicillin interferes with glutamine metabolism suggesting strategies that could eventually be exploited for combination therapy or for reversal of resistance.

Serotyping of Streptococcus pneumoniae based on capsular genes polymorphisms

Streptococcus pneumoniae serotype epidemiology is essential since serotype replacement is a concern when introducing new polysaccharide-conjugate vaccines. A novel PCR-based automated microarray assay was developed to assist in the tracking of the serotypes. Autolysin, pneumolysin and eight genes located in the capsular operon were amplified using multiplex PCR. This step was followed by a tagged fluorescent primer extension step targeting serotype-specific polymorphisms. The tagged primers were then hybridized to a microarray. Results were exported to an expert system to identify capsular serotypes. The assay was validated on 166 cultured S. pneumoniae samples from 63 different serotypes as determined by the Quellung method. We show that typing only 12 polymorphisms located in the capsular operon allows the identification at the serotype level of 22 serotypes and the assignation of 24 other serotypes to a subgroup of serotypes. Overall, 126 samples (75.9%) were correctly serotyped, 14 were assigned to a member of the same serogroup, 8 rare serotypes were erroneously serotyped, and 18 gave negative serotyping results. Most of the discrepancies involved rare serotypes or serotypes that are difficult to discriminate using a DNA-based approach, for example 6A and 6B. The assay was also tested on clinical specimens including 43 cerebrospinal fluid samples from patients with meningitis and 59 nasopharyngeal aspirates from bacterial pneumonia patients. Overall, 89% of specimens positive for pneumolysin were serotyped, demonstrating that this method does not require culture to serotype clinical specimens. The assay showed no cross-reactivity for 24 relevant bacterial species found in these types of samples. The limit of detection for serotyping and S. pneumoniae detection was 100 genome equivalent per reaction. This automated assay is amenable to clinical testing and does not require any culturing of the samples. The assay will be useful for the evaluation of serotype prevalence changes after new conjugate vaccines introduction.

A phase Ib study of oxaliplatin in combination with fluorouracil (5FU) and leucovorin (LV) in pediatric patients (pts) with advanced solid tumors

9548

Background: Platinums have significant activity in a wide variety of pediatric tumors, and oxaliplatin exhibits synergy with 5FU and LV in adults with colorectal cancer in FOLFOX regimens. The primary objectives of this study are to determine the maximum tolerated dose (MTD) of the modified FOLFOX6 regimen in pediatric tumors. Secondary objectives include safety, PK pharmacokinetic (PK) and PET scan efficacy analyses. Methods: Pts age <21 years with advanced cancers and adequate organ function are eligible. Patients are stratified by tumor type (CNS or non-CNS) to better delineate the potential neurotoxicity in patients with altered neurologic function. Pts received oxaliplatin starting at 85 mg/m2 and LV 400 mg/m2 over 2 hours on day 1 followed by a 5FU bolus of 400 mg/m2 then 2,400 mg/m2 continuous infusion over 46 hours, every 2 weeks (3 courses = 1 cycle), with integrated PK sampling in a limited dose escalation design. Standard 3+3 dose escalation, definitions of dose limiting toxicity (DLT), and dose modification for toxicity are implemented. The MTD is expanded to 15 patients to confirm and further characterize tolerability and toxicity. Results: To date, 15 very heavily pre-treated patients (7 M, 8 F) have received 25 cycles (range 1–4, median 2) of treatment at 2 dose levels. One of 8 pts at dose level 2 (oxaliplatin 100 mg/m2) developed DLT (delay in repeat treatment > 14 days due to grade 3 platelets). Treatment has been well tolerated. The most frequently reported related grade 3–4 adverse events (AEs) are reversible leukocytes (29%), neutropenia (43%), platelets (35%) and lymphopenia (21%). 31 of 90 courses (34%) have been delayed for neutropenia and thrombocytopenia. Anti-tumor activity to date includes a confirmed partial response lasting 15 weeks in a patient with osteosarcoma, and prolonged stable disease in 5 other pts with brain tumors (2), hepatoblastoma (2) and sarcoma (1). Conclusions: The modified FOLFOX6 regimen has significant but reversible myelosuppression in heavily pre-treated pediatric patients, but is tolerable and has promising activity in several tumor types. Cohort expansion continues at 100 mg/m2 of oxaliplatin. PET scan and pharmacokinetic analyses will be presented.

No significant financial relationships to disclose.

Phase I pharmacokinetic (PK) and pharmacodynamic (PD) study of 17-Allylamino-17-demethoxygeldanamycin (17AAG) in children with solid tumors

9022

Background: 17AAG is a benzoquinone ansamycin that binds to heat shock protein 90 (Hsp90) and alters levels of cancer-associated proteins that are regulated by Hsp90. 17AAG has been well-tolerated in adults, but has not previously been administered to children. Methods: A Phase I study of 17AAG was initiated to define the maximally tolerated dose and toxicity profile of this drug in children. PK and PD were also studied. Cohorts of 3–6 patients with recurrent or refractory solid tumors were treated every 21 days with escalating doses of 17AAG twice weekly for two weeks. Plasma PK of 17AAG and its major metabolite, 17AG, were measured on day 1 by HPLC. Changes in levels of the inducible isoform of Hsp70 were assessed by Western blot using peripheral blood mononuclear cells (PBMCs) obtained 24 h after the 17AAG infusion. Actin was measured for comparison. Because 17AAG is a substrate for CYP3A4/5 and MDR1, pharmacogenetic analyses have been undertaken to determine if genotypes including CYP3A4*1B, CYP3A5*3, and MDR1 G2677T/A and C3435T influence 17AAG disposition. Results: 12 pts (median age 11 years, range 5–18) with neuroblastoma (5), osteosarcoma (4), Ewing’s family tumors (2), and desmoplastic small round cell tumor (1) have been treated with 17AAG. An MTD has yet to be defined though one dose limiting toxicity (Grade 3 hypoxia) was observed at Dose Level 4 (360 mg/m2). The AUC of 17AAG increased with dose, with a linear relationship between end of infusion 17AAG plasma concentration and AUC. The AUC of 17AAG increased with dose, with a linear relationship between end of infusion 17AAG plasma concentration and AUC. Clearance ranged between 12.5 and 29.6 l/hr/m2 (median, 21.6 l/h/m2) and did not change with increasing doses. Post-treatment increases in Hsp70 in PBMCs have been observed in pts treated with 17AAG doses at or above 150 mg/m2. Declines in Akt and IGF1R in PBMCs have been seen in some but not all pts following treatment. Conclusions: 17AAG is well tolerated in children at dose levels studied to date. 17AAG dose escalation continues and at the time of the meeting, updated data will be reported.

No significant financial relationships to disclose.

T-cell activation leads to poor activation of the HIV-1 clade E long terminal repeat and weak association of nuclear factor-kappaB and NFAT with its enhancer region

The enhancer region in the human immunodeficiency virus type 1 (HIV-1) 5'-long terminal repeat (LTR) is very important for viral transcription. This promoter sequence binds both nuclear factor-kappaB and NFAT, two important modulators of HIV-1 gene expression. Previous studies have indicated that the enhancer regions of the different HIV-1 clade LTRs differ in their number of NF-kappaB-binding sites. In this study, we have compared the activation potential of the different HIV-1 clade and HIV-2 LTRs and assessed their interaction with NFAT and NF-kappaB. In T-cell lines and primary CD4(+) T-cells, the results showed that the HIV-1 clade E LTR (with a single NF-kappaB-binding site) was the weakest LTR regardless of the tested activators, whereas the HIV-2 LTR was the most responsive LTR. The clade E enhancer region was also demonstrated to be the weakest enhancer region in transfection experiments with luciferase reporter-based vectors. Electrophoretic mobility shift assays with extracts from activated CD4(+) T-cells indicated that, although NF-kappaB and NFAT bound all enhancers, HIV-1 clade E and HIV-2 LTR enhancers were poor binding targets for these two factors. Weak NFAT binding to clade E enhancers was also confirmed using NFAT1-expressing 293T cells in competition experiments. We have also shown the absence of interaction of NF-kappaB or NFAT with the third NF-kappaB repeat present in clade C. However, the clade C enhancer bound NFAT more efficiently than all other enhancer regions tested. Our results hence demonstrate for the first time that differences in the binding of NF-kappaB and NFAT to the enhancer regions could be responsible for some of the observed variation in HIV-1 clade LTR activation, whereas HIV-2 LTR activation seems mostly independent of these interactions.

A +2138InsCAGACC polymorphism of the melanocortin receptor 3 gene is associated in human with fat level and partitioning in interaction with body corpulence

BACKGROUND

The melanocortin system includes five receptors (MC1R to MC5R), and mouse and human MC4R has been shown to be involved in the regulation of feeding, and mouse MC3R in body composition. To verify a possible similar effect of MC3R in humans, we analyzed one insertion and one single nucleotide polymorphism by restriction fragment length polymorphisms (RFLP), and a microsatellite (D20S32e) in relation to body composition and glucose metabolism.

METHODS

Eight hundred twelve subjects of the Québec Family Study (QFS) cohort were analyzed for body composition, food intake, and energy metabolism phenotypes. Southern Blot with the complete MC3R cDNA was used to detect a new +2138InsCAGACC variant by Pst1 restriction. PCR-RFLP with BsaJ1 was used to type amino acid polymorphism V81I arising from a G241A nucleotide change. PCR and automatic DNA sequencers were used for the analysis of the TG dinucleotide repeat D20S32e located between -1933/-1892 of MC3R. In a covariance analysis among genotypes, phenotypes were adjusted for age and sex as covariates. Food intake and energy metabolism phenotypes were also adjusted for body mass index (BMI), and leptin and abdominal fat, as assessed by a computed tomography scan, for fatness using six skinfold thicknesses.

RESULTS

An association between the +2138InsCAGACC MC3R polymorphism was observed with fat mass (FM), percent body fat (%FAT), and total abdominal fat (ATF). Homozygote subjects for the +2138 insertion variant allele in normal weight (BMI < 25 kg/m(2)) and overweight (25 < or = BMI < 30 kg/m(2)) subjects showed a similar level of fatness despite the overall difference in BMI. In normal weight, homozygotes for the insertion allele showed higher mean values than heterozygotes and homozygotes for wild-type allele without insertion (%FAT: 24.0 +/- 1.1 versus 19.3 +/- 0.9 and 20.5 +/- 0.8, p = 0.0005; FM: 15.7 +/- 0.9 kg versus 11.7 +/- 0.7 kg and 12.6 +/- 0.6 kg, p = 0.0003). In contrast, overweight subjects homozygote for the variant allele showed lower mean values (%FAT: 27.0 +/- 1.2 versus 31.4 +/- 0.8 and 30.9 +/- 0.7, p = 0.002; FM: 18.3 +/- 1.0 kg versus 22.8 +/- 0.8 kg and 22.0 +/- 0.6 kg, p = 0.0001). This resulted in a similar level of body fat between both BMI groups for subjects homozygote for the insertion allele versus wild-type allele carriers (%FAT: +/-2-3% versus +/-10-12%; FM: +/-2 kg versus +/-9-11 kg). In obese subjects (BMI > or = 30 kg/m(2) ), a lower level of ATF was seen (-15%, p = 0.002). Other polymorphisms and phenotypes tested showed no association.

CONCLUSION

A new 12138InsCAGACC MC3R polymorphism is associated with the level of adiposity and with body fat partitioning in interaction with corpulence in humans.

Electrophysiological and behavioral output of the rat basal ganglia after intrastriatal infusion of d-amphetamine: lack of support for the basal ganglia model

Dopamine, by acting upon D1 and D2 dopamine receptors located on striatonigral and striatopallidal neurons, respectively, has been postulated to inhibit output from the substantia nigra pars reticulata (SNpr) and internal pallidal segment (GPi). The inhibition of the SNpr/GPi should, in turn, disinhibit the thalamus to facilitate movement. The present study tests this prediction in intact (unlesioned) rats by attempting to correlate changes in the single unit activities of SNpr neurons with motor (i.e. behavioral) responses in the 20-30 min after infusions of d-amphetamine into the striatum. Unilateral injections of amphetamine (20 microg/microl) into either the dorsal-rostral, central, or ventral-lateral striatum failed to appreciably alter behavior and, in parallel electrophysiological studies, failed to consistently or significantly alter the activities of SNpr neurons in either chloral hydrate-anesthetized rats or awake locally anesthetized rats. However, when amphetamine was infused bilaterally into the ventral-lateral striatum (VLS; 20 microg/microl per side), a robust behavioral activation ensued (increased locomotor activity, oral movements, and sniffing) with an onset ranging from immediate to 20 min post-infusion and persisting for at least 40 min. In parallel studies, bilateral amphetamine infusions into VLS also caused changes in the firing frequency of a majority of SNpr neurons. However, the changes in firing were extremely variable and, contrary to expectation, the net population response of SNpr neurons was an increase in firing which corresponded in time with the period of peak behavioral activation. These results show that (i) bilateral but not unilateral activation of striatal dopamine receptors is needed to elicit behavioral and electrophysiological output from the basal ganglia, and (ii) motor activation is apparently not signaled by a generalized inhibition of SNpr firing, as is predicted by the basal ganglia model.

Intratester and intertester reliability of the Cybex electronic digital inclinometer (EDI-320) for measurement of active neck flexion and extension in healthy subjects

This study examined the intratester and intertester reliability of the electronic digital goniometer EDI-320 for the measurement of active neck flexion and extension in healthy subjects. In the context of evidence-based practice, the EDI-320 instrument has the potential to improve patient assessment, provide a clearer picture of patient progress, and confirm the effectiveness of physiotherapy interventions. However, the psychometric properties of the EDI-320 have not yet been documented for cervical spine range of motion. Forty-four individuals with no known history of cervical disorder within the three months prior to the testing, voluntarily consented to participate in this study. Repeated measurements with the EDI-320 were taken by two trained testers (TH1 and TH2) and data were recorded by two separate observers. Subjects performed a standardized warm-up. Testers were required to repeat palpation of bony landmarks prior to each trial. Measurements were taken at the end-range of active cervical flexion and extension for each subject. Both testers measured each subject twice. The intraclass correlation coefficients (ICC) were derived from one-way ANOVA for intratester reliability and a two-way ANOVA for intertester reliability. Paired t -tests were then applied to verify for systematic error. Moderate intratester reliability was found for both testers for flexion (TH1: ICC=0.77; 95% CI: 0.62-0.87; TH2: ICC=0.77; 95% CI: 0.58-0.87). As for extension, high intratester reliability was found for TH1 (ICC=0.79; 95% CI: 0.65-0.88) and moderate for TH2: (ICC=0.83; 95% CI: 0.63-0.92). Intertester reliability results showed a moderate reliability for both flexion and extension (ICC=0.66; 95% CI: 0.24-0.84) on the first trial. On the second trial, reliability was moderate for flexion (ICC=0.73; 95% CI: 0.53-0.85) and high for extension (ICC=0.80; 95% CI: 0.64-0.89). The t -test analysis revealed the inclusion of systematic error by Tester 2 for intratester reliability. This error was also found for all but one of the intertester reliability calculations. This study has shown that the EDI-320 is a moderately reliable instrument for quantifying cervical flexion and extension range of motion. The presence of systematic error in the study highlights the importance of following standardized procedures and suggests that the EDI-320 could be more reliable than reported in this study. Further psychometric studies investigating the validity of the EDI and reliability with subjects affected by cervical pathology is warranted.

Development of a very sensitive luminescence assay for the measurement of paclitaxel and related taxanes

A rapid and very sensitive enzyme immunoassay was developed for the measurement of paclitaxel and related taxanes in crude extracts of Taxus sp., in human serum and in culture medium of paclitaxel-producing microorganisms such as Erwinia taxi. For the ELISA, paclitaxel was chemically modified by the introduction of an amine to enable coupling with biotin. The presence of paclitaxel or related taxanes competitively inhibited the binding of paclitaxel-biotin to anti-taxane monoclonal antibody. This method detected paclitaxel in concentrations as low as 33 pM; the affinity of the antibody was higher for paclitaxel than for cephalomanine, baccatin and DAB. The sensitivity of this assay makes it useful for estimating the paclitaxel and taxanes content of Taxus sp. extracts, monitoring the paclitaxel serum level of paclitaxel treated patients and in other biological fluids.

Expression of the GALalpha(1-3)GAL epitope on pig islets

Expression of Galalpha(1-3)Gal on endothelium has been implicated in the rejection of porcine xenografts. The aim of this study was to determine whether expression of Galalpha(1-3)Gal on pig islets varies between pigs aged 5, 12 and 24 weeks, and to investigate whether it is expressed on islets isolated by collagenase digestion or islets maintained in tissue culture. Samples of pancreas were obtained from pigs aged 5, 12 and 24 weeks. Islets were isolated by manual collagenase digestion and density gradient separation. Samples were taken immediately after isolation or after maintenance in tissue culture. Pancreas and islet samples were processed, sectioned and stained with the lectin BS1-B4 (which binds to Galalpha(1-3)Gal residues), and anti-insulin antibody using a double staining technique. There was no significant difference in the staining patterns to sections of pancreas obtained from 5, 12 and 24 week old pigs. Vascular endothelium, connective tissue and the luminal surface of duct epithelial cells stained with BS1-B4 in all sections; endocrine and exocrine cells did not stain. Preliminary experiments showed that lectin staining to isolated islets was inconsistent between preparations, but expression did not appear to differ significantly between ages: lectin staining of some beta-cells was evident in the majority of freshly isolated preparations, but was not detectable on beta-cells following tissue culture. In conclusion, expression of Galalpha(1-3)Gal did not differ significantly in pancreata from 5, 12 and 24 week old pigs. Preliminary experiments showed that Galalpha(1-3)Gal was expressed by beta-cells immediately following isolation, but not after maintenance in culture.

CD28 expression in T cell aging and human longevity

Functional decrements of the immune system have a major contribution to aging and age-related diseases. Here, we further characterize the decline in proportion of CD28-positive T cells previously identified in centenarians. Cohorts of 97 centenarians, 40 subjects aged 70-90 (ELD group), and 40 young adults (under age 40) were phenotyped for T cell surface expression of CD28, CD4, and CD8 antigens. The significant decline in T cells expressing CD28 (p < 10(-4) for comparisons between adults and either ELD or centenarians) affects preferentially the CD8+ subset of T cells. This decline accounts largely for the age-related diminution of T cell responsiveness to mitogenic signals. CD28 expression is modulated in T cell cultures in a growth-related fashion and this modulation is dampened in cultures from centenarians. We propose that the decrease in CD28 expression reflects a compensatory adaptation of the immune system during aging in the face of chronic stimulation.

[Cellular senescence and survival of T lymphocytes]

In order to study cellular senescence in T lymphocytes and its link with aging, we have undertaken long-term cultures from adult individuals (aged from 20 to 40) and centenarians. The proliferative advantage of CD4+ over CD8+ T cells is reversed after the second stimulation. Periodically stimulated cultures remained exponentially growing during nearly 200 days, whereas 2 of them that were continued for 300 days stopped proliferating. However, once this phase of senescence is reached, the cells do not die out. Six other cultures remained viable for 34 months without proliferation but with conservation of the cell number. Three of these cultures have clonal karyotypic abnormalities: trisomy 2 and telomeric fusions.

Decline in CD28+ T cells in centenarians and in long-term T cell cultures: a possible cause for both in vivo and in vitro immunosenescence

The dramatic decline in immune function with age, especially in T cell proliferative activity, has been documented extensively in experimental animal models and in clinical studies of the elderly. A similar proliferative decline is also seen in long-term T lymphocyte cultures used to study in vitro cellular senescence. We have compared the peripheral blood T lymphocytes of centenarians and younger controls for the cell surface expression of CD28, a costimulatory molecule that is required for optimal activation and proliferation following engagement of the T cell receptor. Our analysis shows a significant decrease (p < 0.001) in the percentage of T cells expressing CD28 in the elderly cohort, with values ranging from 44% to 90%, as compared to the mean control value of 91%. The decline in the percentage of CD28+ T cells correlates with a reduction in the CD4/CD8 ratio (r2 = 0.695, p < 0.0001). Concommitantly, experiments using an in vitro T cell culture system showed a progressive loss of CD28 expression with culture "age." The concordance of proliferative decline and loss of CD28 in the centenarians and in the in vitro cultures suggest that a Hayflick phenomenon may operate in vivo leading to immunosenescence.

Heat-stress stimulation of oxygen uptake by Photosystem I involves the reduction of superoxide radicals by specific electron donors

A Photosystem I submembrane fraction isolated from spinach was used to study the mechanism of heat-stress stimulation of oxygen uptake by the photosystem. Various artificial electron donors were shown to generate electron transport reactions with various degrees of thermally induced stimulation. A strong stimulation was observed with durohydroquinone as electron donor with a maximal effect at 50 °C. The degree of stimulation obtained was independent from the redox potential of the electron donors and from their oxidation site because the enzyme superoxide dismutase fully inhibited the stimulation. Instead, it is proposed that thermal stress causes the release of membrane bound superoxide dismutase from the thylakoids thus allowing the reduced form of electron donors with specific properties to reduce O2 (-) radicals to H2O2 besides the usual disproportionation of O2 (-) into O2 and H2O2.

Heat-stress stimulation of electron flow in a photosystem I submembrane fraction

Oxygen uptake using methyl viologen as the terminal electron acceptor was recorded in digitonin-derived photosystem I submembrane fractions incubated at either 25 or 50 degrees C. A two- to four-fold heat-stress stimulation of electron flow was detected at 50 degrees C when reduced 2,6-dichlorophenol-indophenol was used as the primary electron donor. However, no stimulation was seen with N,N,N',N'-tetramethylphenylenediamine as the donor. The stimulation was enhanced by specific cations (Mg2+, Na+, K+), but not by Mn2 or Ca2+. The enhancement obtained with Mg2+ could be eliminated by incubating for a prolonged period. It is proposed that the observed heat-stress stimulation is due to a conformational change at the level of the cytochrome b6-f complex. This change increased the affinity of the protein complex for 2,6-dichlorophenol-indophenol at its oxidation sites. The involvement of a conformational modification is demonstrated by the absence of heat-stress stimulation in submembrane fractions immobilized in an albumin-glutaraldehyde cross-linked matrix.