Prehospital cooling to improve successful targeted temperature management after cardiac arrest: A randomized controlled trial.

RATIONALE: Targeted temperature management (TTM) improves survival with good neurological outcome after out-of-hospital cardiac arrest (OHCA), but is delivered inconsistently and often with delay. OBJECTIVE: To determine if prehospital cooling by paramedics leads to higher rates of 'successful TTM', defined as achieving a target temperature of 32-34°C within 6h of hospital arrival. METHODS: Pragmatic RCT comparing prehospital cooling (surface ice packs, cold saline infusion, wristband reminders) initiated 5min after return of spontaneous circulation (ROSC) versus usual resuscitation and transport. The primary outcome was rate of 'successful TTM'; secondary outcomes were rates of applying TTM in hospital, survival with good neurological outcome, pulmonary edema in emergency department, and re-arrest during transport. RESULTS: 585 patients were randomized to receive prehospital cooling (n=279) or control (n=306). Prehospital cooling did not increase rates of 'successful TTM' (30% vs 25%; RR, 1.17; 95% confidence interval [CI] 0.91-1.52; p=0.22), but increased rates of applying TTM in hospital (68% vs 56%; RR, 1.21; 95%CI 1.07-1.37; p=0.003). Survival with good neurological outcome (29% vs 26%; RR, 1.13, 95%CI 0.87-1.47; p=0.37) was similar. Prehospital cooling was not associated with re-arrest during transport (7.5% vs 8.2%; RR, 0.94; 95%CI 0.54-1.63; p=0.83) but was associated with decreased incidence of pulmonary edema in emergency department (12% vs 18%; RR, 0.66; 95%CI 0.44-0.99; p=0.04). CONCLUSIONS: Prehospital cooling initiated 5min after ROSC did not increase rates of achieving a target temperature of 32-34°C within 6h of hospital arrival but was safe and increased application of TTM in hospital.

Long-term outcomes and healthcare utilization following critical illness--a population-based study.

BACKGROUND: The purpose of this study was to examine hospital mortality, long-term mortality, and health service utilization among critically ill patients. We also determined whether these outcomes differed according to demographic and clinical characteristics. METHODS: We conducted a retrospective cohort study of adults (age ≥ 18 years) who survived admission to an intensive care unit (ICU) in Ontario, Canada, between 1 April 2002 and 31 March 2012, excluding isolated admissions to step-down or intermediate ICUs, coronary care ICUs, or cardiac surgery ICUs. Adults (age ≥ 18 years) who survived an acute hospitalization that did not include an ICU stay formed the comparator group. The primary outcome was mortality following hospital discharge. Secondary outcomes were healthcare utilization, including emergency room admissions and hospital readmissions during follow-up. RESULTS: Over the study interval, 500,124 patients were admitted to ICUs and 420,187 (84%) survived to hospital discharge. Median follow-up for survivors was 5.3 (interquartile range 2.5, 8.2) years. Patients admitted to an ICU were more likely to subsequently visit the emergency department, be readmitted to the hospital and ICU, receive home care support, require rehabilitation, and be admitted for long-term care. Those requiring more resources within the ICU required more resources after discharge. One-third of patients admitted to the ICU died during long-term follow-up, with overall probabilities of death of 11% and 29% at 1 year and 5 years, respectively. In the adjusted analysis, there was an increasing hazard of death with increasing age, reaching a hazard ratio of 18.08 (95 % confidence interval 16.60-19.68) for those ≥ 85 years of age compared with those aged 18-24 years. CONCLUSIONS: Healthcare utilization after hospital discharge was higher among ICU patients, and also among those requiring more healthcare resources during their ICU admission, than among all hospitalized patients as a group. One-third of ICU patients died within the 5 years following discharge, and age was the most influential determinant of outcome. These findings should help target post-ICU discharge services for high-risk groups and better inform goals-of-care discussions for elderly critically ill patients.

Accuracy of healthcare worker recall and medical record review for identifying infectious exposures to hospitalized patients.

OBJECTIVE: To determine the validity of using healthcare worker (HCW) recall of patient interactions and medical record review for contact tracing in a critical care setting. DESIGN: Trained observers recorded the interactions of nurses, respiratory therapists, and service assistants with study patients in a medical-surgical intensive care unit. These observers' records were used as the reference standard to test the criterion validity of using HCW recall data or medical record review data to identify exposure characteristics. We assessed the effects of previous quarantine of the HCW (because of possible exposure) and the availability of patients' medical records for use as memory aids on the accuracy of HCW recall. SETTING: A 10-bed medical-surgical intensive care unit at Mount Sinai Hospital in Toronto, Ontario. PATIENTS: Thirty-six HCWs observed caring for 16 patients, for a total of 55 healthcare worker shifts. RESULTS: Recall accuracy was better among HCWs who were provided with patient medical records as memory aids (P<.01). However, HCWs tended to overestimate exposures when they used patient medical records as memory aids. For 6 of 26 procedures or care activities, this tendency to overestimate was statistically significant (P<.05). Most HCWs with true exposures were identified by means of this technique, despite the overestimations. Documentation of the activities of the 4 service assistants could not be found in any of the patients' medical records. Similarly, the interactions between 6 (19%) of 32 other patient-HCW pairs were not recorded in patients' medical records. CONCLUSIONS: Data collected from follow-up interviews with HCWs in which they are provided with patient medical records as memory aids should be adequate for contact tracing and for determining exposure histories. Neither follow-up interviews nor medical record review alone provide sufficient data for these purposes.

Non-invasive Providencia alcalifaciens strains fail to attach to HEp-2 cells.

We investigated the adherence properties of six P. alcalifaciens strains with previously characterized differential invasive capabilities in HEp-2 cells. Highly invasive strains were found to attach to HEp-2 cell monolayers within 2 h post-infection and in large numbers on the eukaryotic cell surfaces within 3 h post-infection. In contrast, weakly or non-invasive P. alcalifaciens strains were non-adherent to HEp-2 cells even at 3 h post-infection. Highly invasive isolates were found to weakly bind F-actin using the fluorescent actin staining assay although these strains were negative for Escherichia coli attachment and effacing gene (eaeA) of enteropathogenic E. coli (EPEC). These results suggest that the strain variation in the ability of P. alcalifaciens to invade HEp-2 cells previously noted by several investigators may be linked to expression of key adhesin(s) on the cell surface of invasive isolates.

Gingival plasma cell granuloma.

Plasma cell granulomas (pseudotumors) are rare benign, tumor-like proliferations composed chiefly of plasma cells that manifest primarily in the lungs, but may occur in various anatomic locations. We report this case of a 54-year-old male who presented with an unusual maxillary anterior gingival overgrowth treated by excisional biopsy. Histological examination revealed a dense inflammatory cell infiltrate containing mainly plasma cells. Immunohistochemistry for kappa and lambda light chains showed a polyclonal staining pattern confirming a diagnosis of plasma cell granuloma. Intraoral plasma cell granuloma is exceedingly rare, although case reports documenting such lesions have been reported. This case highlights the need to biopsy unusual lesions to rule out potential neoplasms.

Nonproliferating bystander CD4+ T cells lacking activation markers support HIV replication during immune activation.

HIV replicates primarily in lymphoid tissue and immune activation is a major stimulus in vivo. To determine the cells responsible for HIV replication during Ag-driven T cell activation, we used a novel in vitro model employing dendritic cell presentation of superantigen to CD4(+) T cells. Dendritic cells and CD4(+) T cells are the major constituents of the paracortical region of lymphoid organs, the main site of Ag-specific activation and HIV replication. Unexpectedly, replication occurred in nonproliferating bystander CD4(+) T cells that lacked activation markers. In contrast, activated Ag-specific cells were relatively protected from infection, which was associated with CCR5 and CXC chemokine receptor 4 down-regulation. The finding that HIV replication is not restricted to highly activated Ag-specific CD4(+) T cells has implications for therapy, efforts to eradicate viral reservoirs, immune control of HIV, and Ag-specific immune defects.

HIV gag mRNA transfection of dendritic cells (DC) delivers encoded antigen to MHC class I and II molecules, causes DC maturation, and induces a potent human in vitro primary immune response.

Dendritic cells (DC) are the major APCs involved in naive T cell activation making them prime targets of vaccine research. We observed that mRNA was efficiently transfected, resulting in superior translation in DC compared with other professional APCs. A single stimulation of T cells by HIV gag-encoded mRNA-transfected DC in vitro resulted in primary CD4(+) and CD8(+) T cell immune responses at frequencies of Ag-specific cells (5-12.5%) similar to primary immune responses observed in vivo in murine models. Additionally, mRNA transfection also delivered a maturation signal to DC. Our results demonstrated that mRNA-mediated delivery of encoded Ag to DC induced potent primary T cell responses in vitro. mRNA transfection of DC, which mediated efficient delivery of antigenic peptides to MHC class I and II molecules, as well as delivering a maturation signal to DC, has the potential to be a potent and effective anti-HIV T cell-activating vaccine.

Vagus nerve stimulation therapy for epilepsy in older adults.

The authors assessed the efficacy, safety, and tolerability of vagus nerve stimulation (VNS) for refractory epilepsy in 45 adults 50 years of age and older. They determined seizure frequency, adverse effects, and quality of life. At 3 months, 12 patients had a >50% decrease in seizure frequency; at 1 year, 21 of 31 studied individuals had a >50% seizure decrease. Side effects were mild and transient. Quality of life scores improved significantly with time.

Bartonella henselae neuroretinitis in cat scratch disease. Diagnosis, management, and sequelae.

OBJECTIVE: This study aimed to report the long-term outcomes of patients treated with an antibiotic drug combination for Bartonella henselae neuroretinitis. DESIGN: The study design was a retrospective case series. PARTICIPANTS: Seven consecutive patients with neuroretinitis and cat scratch disease participated. INTERVENTIONS: Patients underwent medical and ophthalmic evaluations. Blood cultures were obtained, and B. henselae antibody titers were measured. Tuberculosis, Lyme, toxoplasmosis, syphilis, and sarcoidosis were excluded. Patients received oral doxycycline 100 mg and rifampin 300 mg twice daily for 4 to 6 weeks and were observed for an average of 16 months (range, 10-24 months). Formal electrophysiologic testing was performed in three patients after resolution of neuroretinitis. MAIN OUTCOME MEASURES: The changes in ocular inflammation and visual function associated with treatment were recorded. Follow-up examinations and electrophysiologic testing documented sequelae. RESULTS: Patients presented following cat exposure with fever, malaise, and blurred vision. Decreased visual acuity (ranging from 20/40 to counting fingers) frequently was associated with dyschromatopsia and afferent pupillary defects. Ophthalmoscopic analysis showed signs of neuroretinitis, including nerve fiber layer hemorrhages, cotton-wool spots, multiple discrete lesions in the deep retina, and stellate macular exudates. B. henselae infection was confirmed with positive blood cultures or elevated immunofluorescent antibody titers or both. Therapy appeared to promote resolution of neuroretinitis, restoration of visual acuity, and clearance of bacteremia. After 1 to 2 years, two eyes had residual disc pallor, afferent pupillary defects, retinal pigmentary changes, and mildly decreased visual acuity. Electrophysiologic studies showed that when compared to the fellow eye, affected eyes had subnormal contrast sensitivity, abnormal color vision, and abnormal visually evoked potentials. Conversely, electroretinograms were normal in all subjects. CONCLUSIONS: B. henselae is a cause of neuroretinitis in cat scratch disease. Compared to historic cases, doxycycline and rifampin appeared to shorten the course of disease and hasten visual recovery. Long-term prognosis is good, but some individuals may acquire a mild postinfectious optic neuropathy.

Hypoenergetic nutrition support in hospitalized obese patients: a simplified method for clinical application.

Nutrition support in obese hospitalized patients is controversial, with some practitioners advocating restricted energy or hypoenergetic feedings when patients are being actively treated for another disease. To eliminate the need for indirect calorimetry, this randomized, double-blind, prospective study was undertaken to determine whether obese hospitalized patients given a hypoenergetic parenteral regimen administered to provide 2 g protein x kg ideal body wt (IBW)(-1) x d(-1), could achieve nitrogen balance comparable with that of control subjects given isonitrogenous normoenergetic formula. Thirty obese hospitalized patients with an average body mass index (BMI; in kg/m2) of 35 were randomly assigned to the hypoenergetic [energy (kJ):nitrogen (g) = 314:1; energy (kcal):nitrogen (g) = 75:1; n = 16] or control [energy (kJ):nitrogen (g) = 628:1; energy (kcal):nitrogen (g) = 150:1; n = 14] formulas. The initial formula volume administered provided 2 g protein x kg IBW(-1) x d(-1). Nitrogen balance was determined on day 0 and weekly. The total daily energy intake [per kg actual body weight (ABW)] was 57 +/- 12 kJ (hypoenergetic) compared with 94 +/- 21 kJ (control), P < 0.001, and the nonprotein energy intake was 36 +/- 10 kJ (hypoenergetic) compared with 73 +/- 17 kJ (control), P < 0.001. Protein intake was the same per ABW, 2.0 +/- 0.2 and 2.0 +/- 0.1 g kg IBW(-1) x d(-1), NS, for the hypoenergetic and control formulas, respectively. Mean net nitrogen balance was not significantly different between the groups, even after patients were subgrouped by illness, nor was the percentage of patients achieving positive nitrogen balance. Duration of treatment averaged 10.5 +/- 2.6 d. Weight change did not differ significantly between groups. These data indicate that patients receiving hypoenergetic feedings providing 2 g protein x kg IBW(-1) x d(-1) achieved nitrogen balance comparable with patients given conventional total parenteral nutrition regimens, even when critically ill.

Apolipoprotein A-I conformation in reconstituted discoidal lipoproteins varying in phospholipid and cholesterol content.

The effects of the size and cholesterol content on the conformation of apolipoprotein A-I (apoA-I) have been studied in reconstituted discoidal lipoproteins containing two apoA-I per particle (Lp2A-I). The immunoreactivity of a series of 13 epitopes distributed along the apoA-I sequence has been evaluated in Lp2A-I with a phospholipid/apoA-I molar ratio ranging from 31 to 156 and in Lp2A-I with constant phospholipids but varying in cholesterol content from 0 to 22 molecules. The results are compatible with a three domain structure in apoA-I in which the central domain is located between residues 99 and 143 and postulated to be a hinged domain that responds differentially to changes in phospholipid and cholesterol contents. Increasing the phospholipid content results in significant changes of epitope immunoreactivity throughout the N-terminal and central domains of apoA-I with fewer modifications in the C-terminal domain. In contrast, increasing Lp2A-I of two central epitopes, A11 (residues 99-132) and 5F6 (residues 118-148), and an extreme N-terminal epitope, 4H1 (residues 2-8). Interestingly, the effects of increasing cholesterol or phospholipids on these epitopes are opposite. This suggests a specific effect of cholesterol on the central domain tertiary structure between residues 99 and 143. Competition binding assays among pairs of antibodies binding to apoA-I on Lp2A-I are best explained by invoking inter- as well as intramolecular competitions. The specificity of the intermolecular competitions suggests an N to C termini arrangement of the two apoA-I molecules around the disc. Increasing the phospholipid content of Lp2A-I mainly increases the competitions between 3G10 and antibodies binding to most adjacent epitopes. Simultaneously as Lp2A-I enlarges, several of these antibodies also enhance the binding of 3G10. This has been interpreted as evidence of a structural rearrangement of apoA-I as a result of the size increase where the alpha-helix (residues 99-121) that contains the 3G10 epitope is increasingly interacting with lipids resulting in the enhanced expression of this epitope. The increasing interactions of apoA-I helices with lipids in the enlarging disc are compatible with previous reports of a greater apoA-I stability in the large discs. By contrast, cholesterol has limited but specific effects on antibody competitions and decreases the interaction of the N-terminal domain with the domain containing 3G10, either by direct cholesterol protein interaction or by modification of the lipid phase packing.

Trimethoprim-sulfamethoxazole therapy for ocular toxoplasmosis.

BACKGROUND: Toxoplasmosis is a leading cause of retinochoroiditis. Conventional multidrug therapy using sulfadiazine, pyrimethamine, and folinic acid is increasingly difficult to procure and administer safely. METHODS: To evaluate the efficacy of trimethoprim-sulfamethoxazole, a fixed-combination antibiotic, patients with active toxoplasmosis were treated with trimethoprim-sulfamethoxazole (Bactrim DS) with or without adjunctive clindamycin and prednisone for 4 to 6 weeks. RESULTS: All patients in this study (n = 16) had resolution of active retinochoroiditis and had improved vision, with an average gain of 5.2 lines of vision. Two patients developed a drug allergy. CONCLUSION: Trimethoprim-sulfamethoxazole appears to be a safe and effective substitute for sulfadiazine, pyrimethamine, and folinic acid (Leucovorin) in treating ocular toxoplasmosis.

Morphological, immunological and biochemical characterization of purified transverse tubule membranes isolated from rabbit skeletal muscle.

A microsomal fraction consisting of membranes of transverse tubule origin has been purified by a modification of the calcium-loading procedure initially described by Rosemblatt et al. (J Biol Chem 256:8140-8, 1981). Enzymatic analysis of this fraction shows an enrichment of the vesicles in the Mg++ ATPase (basal) activity characteristic of the T-tubules and an absent or very low Ca++-dependent-ATPase activity. Stereological analysis of freeze fracture replica of the membranes in the purified fraction indicates that they have a very low density of particles in their P faces and lack the structural manifestation of the caveolae typical of the sarcolemma. Immunological analysis performed with monoclonal antibodies prepared against purified T-tubule and sarcoplasmic reticulum membranes define some T-tubule specific antigens and confirm the morphological and biochemical data regarding the origin and purity of the T-tubule preparation.

Trypsin digestion of junctional sarcoplasmic reticulum vesicles.

A putative constituent of the junctional processes, connecting the terminal cisternae of sarcoplasmic reticulum and the transverse tubules of skeletal muscle fibers, is a greater than or equal to 350,000-dalton (Da) protein that displays ryanodine binding and Ca2+ channel properties. Ryanodine modulation of Ca2+ fluxes suggests that the ryanodine receptor and calcium channel are integral parts of one functional unit corresponding to the greater than or equal to 350,000-Da protein [Inui, M., Saito, E., & Fleischer, S. (1987) J. Biol. Chem. 262, 1740-1747; Campbell, K. P., Knudson, C. M., Imagawa, T., Leung, A. L., Sutko, J. L., Kahl, S. D., Raab, C. R., & Madson, L. (1987) J. Biol. Chem. 262, 6460-6463]. We subjected vesicular fragments of junctional-cisternal membrane to stepwise trypsin digestion. The greater than or equal to 350,000-Da protein is selectively cleaved in the early stage of digestion, with consequent disappearance of the corresponding band in electrophoretic gels. The Ca2+-ATPase is cleaved at a later stage, while calsequestrin is not digested under the same experimental conditions. While the Ca2+-ATPase yields two complementary fragments that are relatively resistant to further digestion, the greater than or equal to 350,000-Da protein yields fragments that are rapidly broken down to small peptides. Under conditions producing extensive digestion of the greater than or equal to 350,000-Da protein, the junctional processes are still visualized by electron microscopy, with no discernible alterations of their ultrastructure. The functional properties of the Ca2+ release channel are also maintained following trypsin digestion, including blockage by Mg2+ and ruthenium red and activation by Ca2+ and nucleotides.(ABSTRACT TRUNCATED AT 250 WORDS)

Oligovanadate binding to sarcoplasmic reticulum ATPase. Evidence for substrate analogue behavior.

Solutions of vanadate were controlled through concentration and pH adjustment to give specific compositions of mono- and oligovanadates. By monitoring the EPR spectrum of iodoacetamide spin-labeled ATPase, it is shown that decavanadate and the oligovanadate species present at neutral pH exhibit behavior typical of a substrate analogue. This is seen in terms of Ca2+ binding site affinity (microM), outward Ca2+ site orientation, and conformational effects on the enzyme normally associated with enzyme activation. In contrast, monovanadates exhibit behavior identical to that observed with Pi, with one exception: the vanadoenzyme is stable to Ca2+ in the concentration range of high affinity binding at the vanadate concentrations used here (200 microM). It is further demonstrated that Ca2+ binding in the 100 microM range directly induces enzyme devanadation of the monovanadate enzyme complex through Ca2+ binding to internal sites. Extensive array formation of dimeric ATPase units is found only with decavanadate in the absence of Ca2+, and then stoichiometric amounts are sufficient. Electron micrographs of dimeric arrays show evidence of increased penetration into the lipid bilayer, including freeze-fracture replicas which show evidence of corresponding "pits" in the inner leaflet of the bilayer. In turn, EPR spectra provide a means of following vanadate binding to the ATPase per se, as well as monitoring Ca2+-induced changes in the vanadoenzyme conformation, as only binding to specific sites on the enzyme affect the EPR spectrum.

Calcium-ryanodine receptor complex. Solubilization and partial characterization from skeletal muscle junctional sarcoplasmic reticulum vesicles.

The Ca2+-ryanodine receptor complex is solubilized in functional form on treating sarcoplasmic reticulum (SR) vesicles from rabbit fast skeletal muscle with 3-[(3-cholamidopropyl)dimethylammonio]-1-propane-sulfonate (CHAPS) (1 mg/mg protein) and 1 M NaCl at pH 7.1 by shaking for 30 min at 5 degrees C. The heavy membrane preparations obtained from pyrophosphate homogenates frequently exhibit junctional feet and appear to be derived primarily from the terminal cisternae of the SR. The characteristics of [3H]ryanodine binding are similar for the soluble receptor and the heavy SR vesicles with respect to dependence on Ca2+, pharmacological specificity for inhibition by six ryanoids and ruthenium red, and lack of sensitivity to voltage-dependent Ca2+-channel blockers, inositol 1,4,5-trisphosphate, or doxorubicin. In contrast, the cation sensitivity is decreased on receptor solubilization. The soluble receptor is modulated by cyclic nucleotides and rapidly denatured at 50 degrees C. Saturation experiments reveal a single class of receptors (Kd = 9.6 nM), whereas kinetic measurements yield a calculated association constant of 5.5 X 10(6) min-1 M-1 and a dissociation constant of 5.7 X 10(-4) min-1, suggesting that the [3H]ryanodine receptor complex ages with time to a state which is recalcitrant to dissociation. Sepharose chromatography shows that the receptor complex consists primarily of two protein fractions, one of apparent Mr 150,000-300,000 and a second, the [3H]ryanodine binding component, of approximately Mr 1.2 X 10(6). Preliminary analysis of the soluble receptor preparation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals subunits of Mr greater than 200,000 and major bands of calsequestrin and Ca2+-transport ATPase. These findings indicate that [3H]ryanodine binds to the Ca2+-induced open state of the channel involved in the release of contractile Ca2+.

High-affinity and low-affinity vanadate binding to sarcoplasmic reticulum Ca2+-ATPase labeled with fluorescein isothiocyanate.

Conditions were found that allowed both the fluorescence detection of vanadate binding to the Ca2+-ATPase of skeletal muscle sarcoplasmic reticulum and the vanadate-induced formation of two-dimensional arrays of the enzyme. The fluorescence intensity of fluorescein isothiocyanate-labeled Ca2+-ATPase increased with high-affinity vanadate binding (Ka = 10(6) M-1) as reported by Pick and Karlish (Pick, U. and Karlish, S.D. (1982) J. Biol. Chem. 257, 6120-6126). The Ca2+ and Mg2+ dependencies for high-affinity vanadate binding were similar but not identical to those for orthophosphate. In addition, it was found that there is low-affinity (Ka = 380 M-1) vanadate binding, which causes a 25% decrease in fluorescence. The Ca2+ and Mg2+ dependencies of the low-affinity vanadate binding were different from those of orthophosphate or high-affinity vanadate binding. The covalent attachment of fluorescein isothiocyanate (FITC) in the ATP site of the Ca2+-ATPase did not affect the formation of two-dimensional arrays, as detected by negatively stained electron micrographs. Vanadate concentrations high enough to saturate the low-affinity binding caused two-dimensional arrays as reported by Dux and Martonosi (Dux, L. and Martonosi, A. (1983) J. Biol. Chem. 258, 2599-2603). In addition, freeze-fracture replicas of quick-frozen specimens showed rows of indentations in the inner leaflet of the bilayer that corresponds to the arrays seen on the outer leaflet. This appearance of indentations suggests that low-affinity vanadate binding causes a transmembrane movement of the Ca2+-ATPase. By contrast, high-affinity vanadate binding was shown to cause neither array formation nor the appearance of indentations.

Skeletal muscle myosin subfragment-1 induces bundle formation by actin filaments.

As is well known, the light scattering intensity of F-actin solutions increases immediately upon formation of the rigor complex with subfragment-1 (S-1). We have found that after the initial rise in scattering, there is a further gradual increase in scattering (we call it "super-opalescence"). Fluorescence and electron microscopic observations of acto-S-1 solutions showed that super-opalescence results from formation of actin filament bundles once S-1 binds to F-actin. The actin bundles possessed transverse stripes with a periodicity of about 350 A, which suggested that in the bundles actin filaments are arranged in parallel register. The rate of the initial process of bundle formation (i.e. side-by-side dimerization) could be approximately estimated by measuring the initial rate of super-opalescence (V0). V0 had a maximum (V0m) at a molar ratio of S-1 to actin of 1;6-1;7, regardless of the actin concentration, pH (6-8.5), Mg2+ concentration (up to 5 mM), or ionic strength (up to 0.3 M KC1). Lower pH, higher Mg2+ concentration, and higher ionic strength increased V0m; V0 was proportional to the square of the actin concentration, regardless of the solution conditions.