Transcription and translation of CSF-1 in the dental follicle

The dental follicle, a loose connective tissue sac which surrounds the unerupted tooth, is required for eruption to occur. Injection of colony-stimulating factor-1 (CSF-1) will accelerate molar eruption in rats, as well as stimulate tooth eruption in osteopetrotic rats. Utilizing in situ hybridization and reverse- transcription polymerase chain-reaction techniques, we show here that CSF-1 mRNA is present in vivo in the dental follicle of the first mandibular molar of the rat. Analysis of the molars from day 0 through day 10 post-natally demonstrates that the maximal expression of CSF-1 mRNA is at day 3 post-natally. Immunostaining also reveals that the CSF-1 mRNA is translated, with immunostaining for the CSF-1 itself, being heavy in early post-natal days and absent by day 9 postnatally. In view of the fact that there is a maximal influx of mononuclear cells (monocytes) into the dental follicle at day 3 post-natally--an influx which increases the numbers of osteoclasts needed to form a tooth eruption pathway--it is probable that the maximal expression of CSF-1 mRNA by day 3 post-natally contributes to this monocyte influx. Thus, this study establishes a relationship among a molecule (CSF-1), cell (monocyte), and tissue (dental follicle) that appear to play a major role in tooth eruption.

The myosin catalytic domain does not rotate during the working power stroke

Electron paramagnetic resonance spectroscopy of a spin probe attached to cys-707 on myosin cross-bridges was used to monitor the orientation of the myosin catalytic domain at the beginning and end of the working power stroke in active muscle. Elevated concentrations of orthophosphate and decreased pH were used to shift the population of cross-bridges from force-producing states into low force, pre-power-stroke states. The spectrum of probes in active fibers was not changed by conditions that reduced tension by 70%, indicating that the orientation of the catalytic domain was the same at the beginning and end of the power stroke. Thus the data show that the catalytic domain remains rigidly oriented on the actin filament during the power stroke.

Structure of the gene for the catalytic subunit of human DNA polymerase delta (POLD1)

We have isolated genomic DNA clones covering the gene for human DNA polymerase delta catalytic subunit (POLD1) and its 5' flanking sequence. This gene is divided into 27 exons and is distributed over at least 32 kb of DNA. The exons and most of the introns are relatively small. The sizes of the exons range from 55 to 201 bp. Seven introns are smaller than 100 bp. Intron 1 is the largest intron, with a size of greater than 10 kb. All of the intron-exon junctions match well with the reported consensus sequences. Multiple copies of the Alu repetitive sequence and the variable number of tandem repeats were found in several introns. Transcription of POLD1 appears to initiate at multiple sites. The major start site was 53 nucleotides upstream of the ATG start codon. The sequence of the promoter and upstream DNA is G+C rich and does not contain a TATA sequence. Several potential transcription factor-binding sites, including the AP2-, CTF-, Ets1-, GCF-, MBF-1-, NF-E1-, and Sp1-binding sites, were found in this region. A 1.8-kb pol delta promoter DNA directed the expression of a luciferase reporter gene when transfected into HeLa cells.

Effects of purified Clostridium difficile toxin A on rabbit distal colon

BACKGROUND & AIMS

Antibiotic-associated pseudomembranous colitis in humans is caused by proliferation of Clostridium difficile, which elaborates an enterotoxin toxin A that causes epithelial damage and altered motility in rabbit small intestine. The aim of this study was to assess the effects of toxin A on rabbit distal colonic motility and to relate this to histological damage and inflammatory mediator production.

METHODS

Two hundred micrograms per milliliter of toxin A was placed in a distal colonic loop in anesthetized rabbits, and myoelectric activity was recorded for the following 7 hours. The colon was histologically evaluated and assayed for eicosanoid production. The effects of toxin A on longitudinal and circular muscle were also assessed in vitro.

RESULTS

Beginning 1 hour after instillation, toxin A caused a significant increase in the number of spike bursts without altering slow wave frequency; this was associated with an increase in mucosal neutrophils and increased production of prostaglandin E2 and leukotrienes B4 and C4/D4/E4. Seven hours after administration of toxin A, mediator levels and myoelectric activity remained increased but significant mucosal damage was now also present. Toxin A did not affect longitudinal or circular muscle in vitro.

CONCLUSIONS

C. difficile toxin A caused a significant neutrophil infiltration and an increased myoelectric activity before producing mucosal damage. The myoelectric effect may be indirect, resulting from the production of motility-altering arachidonic acid metabolites.

The cardiovirulent phenotype of coxsackievirus B3 is determined at a single site in the genomic 5' nontranslated region

We report the construction of chimeric coxsackievirus B3 (CVB3) strains in which sequences of an infectious cDNA copy of a noncardiovirulent CVB3 genome were replaced by the homologous sequences from a cardiovirulent CVB3 genome to identify which of 10 predicted genetic sites determine cardiovirulence. Cardiovirulent phenotype expression was consistently linked to nucleotide 234 (U in cardiovirulent CVB3 and C in avirulent CVB3) in the 5' nontranslated region. Reconstructions of the parental noncardiovirulent CVB3 genome from chimeras restored the noncardiovirulent phenotype when tested in mice. Inoculation of severe combined immunodeficient (scid) mice with the noncardiovirulent CVB3 strain resulted in massive cardiomyocyte necrosis in all animals. Sequence analysis of viral genomes isolated from twelve scid mouse hearts showed that only nucleotide position 234 was different (a C-->U transition) from that in the input parental noncardiovirulent CVB3 genome. Higher-order RNA structures predicted by two different algorithms did not demonstrate an obvious local effect caused by the C-->U change at nucleotide 234. Initial studies of parental and chimeric CVB3 replication in primary cultures of fetal murine heart fibroblasts and in adult murine cardiac myocytes demonstrated that viral RNA transcriptional efficiency is approximately 10-fold lower for noncardiovirulent CVB3 than for cardiovirulent CVB3. CVB3 did not shut off protein synthesis in murine cardiac fibroblasts, nor were levels of viral protein synthesis significantly different as a function of viral phenotype. Taken together, these data support a significant role for determination of the CVB3 cardiovirulence phenotype by nucleotide 234 in the 5' nontranslated region, possibly via a transcriptional mechanism.

[Relationship between serum micronutrients and precancerous gastric lesions]

Serum micronutrient levels and their relationship to precancerous gastric lesions were studied in 600 subjects aged 35-64 years living in high-risk area of gastric cancer in Linqu County, Shandong Province. Serum micronutrient levels in local residents were 0.54 micrograms/ml, 0.29 micrograms/ml, 3.14 micrograms/ml, 9.62 micrograms/ml, 30.2 micrograms/L, 924 micrograms/L, 1 016 micrograms/L, and 42.0 micrograms/L for vitamin A, beta-carotene, vitamin C, vitamin E, selenium, zinc, copper and ferritin, respectively. Serum levels of beta-carotene, vitamin C and ferritin, and ratio of serum levels of zinc and copper correlated inversely to severity of pathological changes in gastric mucous membrane. With increase of serum level of beta-carotene or vitamin C, odds ratios (OR) of intestinal dysplasia and metaplasia lowered to 0.8, 0.6 and 0.9, 0.5, respectively, and with increase of those of both beta-carotene and vitamin C, their OR lowered further to 0.16, with patients of chronically atrophic gastritis as controls. It indicated maybe beta-carotene and vitamin C played a strong contributing role in protecting from development of precancerous gastric lesions.

Uptake of chloroform by skin during short exposures to contaminated water

Uptake of chloroform into hairless rat stratum corneum from dilute aqueous solutions was studied using tape-stripping to determine amounts deposited in the skin under various environmental exposure scenarios. The length of exposure of sedated animals to the chloroform-containing medium, the frequency and duration of tape-stripping, and the number of tape-strips per location were varied to map the stratum corneum substantivity of chloroform. Eight minutes immersion of the rat within a well-stirred solution at 36 degrees C was found to be adequate time for the gradient to be established fully across the stratum corneum. Penetration was progressively deeper as the exposure time increased. Substantial evaporative loss of chloroform from the aqueous medium of application seem to be responsible for lower cumulative amounts taken up when the same solution was held on the rat's skin within a stainless steel template of fixed area. Of the total uptake (29 mg) from a dilute stirred solution of chloroform (0.44 mg/ml) at 36 degrees C, about 95% was systematically absorbed after a 30 min exposure as determined by residuals (measurement of bath concentrations).

Muscle cross-bridges bound to actin are disordered in the presence of 2,3-butanedione monoxime

Electron paramagnetic resonance spectroscopy was used to monitor the orientation of muscle cross-bridges attached to actin in a low force and high stiffness state that may occur before force generation in the actomyosin cycle of interactions. 2,3-butanedione monoxime (BDM) has been shown to act as an uncompetitive inhibitor of the myosin ATPase that stabilizes a myosin.ADP.P(i) complex. Such a complex is thought to attach to actin at the beginning of the powerstroke. Addition of 25 mM BDM decreases tension by 90%, although stiffness remains high, 40-50% of control, showing that cross-bridges are attached to actin but generate little or no force. Active cross-bridge orientation was monitored via electron paramagnetic resonance spectroscopy of a maleimide spin probe rigidly attached to cys-707 (SH-1) on the myosin head. A new labeling procedure was used that showed improved specificity of labeling. In 25 mM BDM, the probes have an almost isotropic angular distribution, indicating that cross-bridges are highly disordered. We conclude that in the pre-powerstroke state stabilized by BDM, cross-bridges are attached to actin, generating little force, with a large portion of the catalytic domain of the myosin heads disordered.

Immunolocalization of interleukin-1 alpha in rat mandibular molars and its enhancement after in vivo injection of epidermal growth factor

Immunolocalization of interleukin-1 alpha in the first mandibular molars of rats from day 0-12 postnatally showed that the protein was localized in the epithelial stellate reticulum adjacent to the dental follicle. Staining of the stellate reticulum was most prominent in the early days postnatally and was absent by postnatal day 11. Injection of epidermal growth factor into rats at day 0 greatly increased the intensity of the staining for interleukin-1 alpha in the stellate reticulum. Epidermal growth factor (EGF) enhanced the gene expression of interleukin-1 alpha in stellate reticulum cells in vitro, and this study suggests there is enhanced translation of interleukin-1 alpha messenger RNA in the stellate reticulum following EGF injection. In turn, the interleukin-1 alpha may exert its effect on the dental follicle cells adjacent to the stellate reticulum because EGF also enhanced expression of the interleukin-1 receptor type I messenger RNA in cultured dental follicle cells as well as enhancing its expression in vivo. In view of the fact that injection of EGF will stimulate precocious eruption of teeth, its stimulus of interleukin-1 alpha synthesis in the stellate reticulum may be the mechanism by which EGF initiates a cascade of molecular events to signal the onset of tooth eruption.

[Affection of growth of zinc deficiency growing rats]

Fifty male Sprague-Dawley rats with an average live mass of 42 +/- 4g were divided into 5 groups of 10 animals each. An artificial semi-systhesis diet was given to the animals. Group 1 received a control diet (32.2 mg Zn/kg DM), and group 2 a zinc deficient diet (3.4 mg Zn/kg DM) ad libitum. The animals of group 3 were pair fed to group 2 with a diet (84.9 mg Zn/KgDM). Diets of group 4 and 5 contained zinc (4.4 and 5.5 kg Zn/micrograms DM), respectively. The two groups also pair fed to group 2. The food intake and live mass gain were observed every day. After 26 days of experiment, the animals were decapitated; the blood samples were collected for the analysis of serum zinc, zinc binding capacity, and alkaline phosphatase.

Characterization of PGE2 receptors in isolated rabbit colonic crypt cells

The physiological effects of prostaglandins (PG) are mediated through their interactions with specific receptors on effector cells. In this study the properties of PGE2 receptors in the rabbit distal colon were examined. We report the presence of specific, saturable, and high-affinity binding sites of PGE2 of the EP2 subtype in isolated colonic crypts. Scatchard analysis revealed the presence of two binding sites with dissociation constants of 0.3 and 10.8 nM and corresponding maximum number of receptors of 15 and 134 fmol/10(6) cells. From competition experiments in the presence of guanosine 5'-O-(3-thiotriphosphate), PGE2 binding was decreased, suggesting that the receptor is coupled to a G protein. No PGE2 binding sites were detected in surface cells. Levels of adenosine 3',5'-cyclic monophosphate (cAMP) were measured in isolated epithelial cells after being exposed to different concentrations of PGE2. cAMP levels were significantly increased only in the crypt cells when exposed to PGE2. These data provide the first demonstration for the existence of PGE2 receptors on colonic crypt cells, which when activated lead to increased levels of cAMP.

Detection of residual leukemic cells in patients with acute promyelocytic leukemia by the fluorescence in situ hybridization method: potential for predicting relapse

The translocation between chromosomes 15 and 17, t(15;17)(q22-24;q11-21), is present in the bone marrow cells of most patients with acute promyelocytic leukemia (APL). Although conventional cytogenetic methods are useful for diagnosing this disease, difficulties are experienced in detecting residual disease among those patients who have achieved remission. In this study, we used the fluorescence in situ hybridization (FISH) method to attempt to detect residual leukemic cells in 10 APL patients in clinical remission. The duration of remission ranged from 2 to 93 months at the time of study. Multiple bone marrow samples were analyzed by FISH in most patients. In 6 patients, no cell with t(15;17) was found. These patients remain in complete remission at present (approximately 25 to 33 months since first studied by FISH). In 4 patients, low frequencies of cells with t(15;17) were observed in at least one bone marrow sample examined. All of these patients relapsed within 1 to 14 months. No cell with t(15;17) was identified by the conventional G-banding method in any sample. The FISH results correlated well with that of a two-round nested reverse transcription polymerase chain reaction assay that was performed on the same samples. Thus, our study suggests that FISH is potentially a useful tool for detecting residual APL cells and for identifying patients at high risk of relapse.

Effect of zinc on strength and fatigue resistance of amalgam

OBJECTIVES

This study was conducted to determine the effect of zinc in amalgam on the static mechanical properties and resistance to fatigue-crack propagation of amalgams.

METHODS

Fatigue, creep, compressive and flexure tests were performed on high-Cu Dispersalloy (Johnson and Johnson Dental Products Co., East Windsor, NJ, USA) and low-Cu Velvalloy (S.S. White Dental Products Int., Philadelphia, PA, USA) in both Zn-containing and Zn-free formulations. Linear Elastic Fracture Mechanics principles were used to characterize the fatigue behavior (crack lengths were monitored).

RESULTS

The incorporation of Zn into these amalgams significantly improved their fatigue and creep resistance, while the effect of Zn on the static compressive and flexure strengths was not consistent. Zn significantly increased (p < 0.05) the resistance to fatigue crack propagation during Stage II crack growth for both amalgams, and increased the variations in crack velocity for a given stress intensity difference, without visibly altering the path or nature of the fatigue cracks. Possible influences on fatigue behavior were the mixed microstructure (particles and matrix), the nature of the crack tip, and creep.

SIGNIFICANCE

The superior resistance to tensile fatigue crack propagation of amalgams containing small amounts of Zn (approximately 1 wt%) in vitro compared with amalgams with no Zn correlated with the superior resistance to marginal breakdown in vivo of Zn-containing amalgams. The range of stress intensities over which stable cracks could propagate was small, while the large variations observed in fatigue crack growth rates for individual materials invalidate predicting a unique fatigue life from the empirical equations obtained.

Effects of a 100% perfluorooctylbromide emulsion on ischemia/reperfusion injury following cardioplegia

Protective effects of a perfluorooctylbromide emulsion on myocardial ischemia and reperfusion (MI/R) injury were evaluated in a modified Langendorff rat heart preparation. Isolated rat hearts were equilibrated in Krebs-Henseleit solution (KH) for 35 minutes and perfused with either cardioplegic solution (CPS) or a 100% perfluorooctylbromide (PFOB) emulsion in CPS for 3 minutes. Hearts were then bathed in the emulsion or CPS. Both groups were subjected to 30 minutes of ischemia. Following 30 minutes of ischemia and 30 minutes of reperfusion with KH solution, hearts subjected to the 100% PFOB emulsion showed improved recovery of left ventricular function. Tissue activities of the antioxidant enzymes glutathione peroxidase, superoxide dismutase, and catalase were not affected by the emulsion in this model. Activity of lactate dehydrogenase (LDH) in the bathing medium was elevated at the end of the experimental period in both control and PFOB-treated hearts. The PFOB emulsion reduced the decline in ATP and GSH levels produced by cardioplegia and subsequent reperfusion. No differences were noted in oxidized glutathione (GSSG) levels. These data suggest that the PFOB emulsion provides some protection for the myocardium against injury associated with cardioplegia.

Separation of pure populations of epithelial cells from rabbit distal colon

A simple method using divalent chelators is described for the isolation of viable populations of surface and crypt cells from rabbit distal colon. Histological studies were performed to monitor colonocyte dissociation and determine contamination by nonepithelial cells. Cell viability was assessed by trypan blue exclusion assay and by 22Na uptake measurements. Electron microscopy was used to determine the integrity of the isolated cells. Alkaline phosphatase and [3H]thymidine uptake were measured to assess the purity of the different cell fractions. Combined fractions 4 and 5 contained the highest percentage of pure surface cells, while fractions 10, 11, and 12 were predominantly crypts. Alkaline phosphatase activity was 13 +/- 3-fold higher in the surface cells than in the crypt cells, while [3H]thymidine uptake was 8 +/- 4-fold higher in the crypt cells than in the surface cells. Amiloride-sensitive and -insensitive 22Na uptake was the same in the surface cells directly after isolation and after 3 h in culture. In this study we demonstrate a method for the preparation of highly enriched fractions of rabbit colon surface and crypt cells that remain viable and functional in short-term culture.

[The anti-HCV assay in viral hepatitis and hepatoma and the relationship between HCV infection and blood transfusion]

Two hundred cases of various kinds of viral hepatitis and hepatocellular carcinoma were tested for serum anti-HCV. The positive rates of anti-HVC in patients with severe hepatitis and patients with cirrhosis were 42.86% and 46.15%, respectively. They were significantly higher than those in patients with other kinds of hepatitis (P < 0.05). The positive rate of anti-HCV was 67.5% in patients with posttransfusion hepatitis, 20.47% in healthy blood donors. In posttransfusion hepatitis B it was only 2.5%. Our results demonstrated that blood transfusion played an important role in transmitting HCV. Our findings also indicated that dual infection of HBV and HCV was important in the course of chronic hepatitis, cirrhosis and severe hepatitis. 50% of the anti-HCV positive patients with chronic hepatitis had slightly elevated serum alanine aminotransferase level. This showed that liver damage caused by HCV may be a chronic course.

Effects of insulin on protein kinase-C (PKC) in HIRC-B cells: specific activation of PKC epsilon and its resistance to phorbol ester-induced down-regulation

We evaluated the role of protein kinase-C (PKC) during insulin action in HIRC-B cells. Insulin provoked rapid increases in 1) diacylglycerol; 2) translocation of PKC epsilon, but not PKC alpha, PKC delta, or PKC zeta, from the cytosol to the membrane fraction; 3) membrane PKC enzyme activity; and 4) phosphorylation of immunoprecipitable 80-kilodalton (kDa) myristylated alanine-rich C-kinase substrate (MARCKS) protein and heat-stable 80-kDa protein (also probably MARCKS). Phorbol esters stimulated the translocation of PKC alpha and PKC delta as well as PKC epsilon, but not PKC zeta. The effects of phorbol esters on 80-kDa MARCKS phosphorylation were approximately 4 times as strong as those of insulin. Treatment of HIRC-B cells with phorbol esters for 20-24 h resulted in complete loss of immunoreactive PKC alpha and PKC delta in cytosol and membrane fractions, but substantial amounts of PKC epsilon were persistently translocated to the membrane fraction of down-regulated cells. This persistently translocated, residual PKC epsilon in down-regulated cells was associated with increased basal hexose uptake, but this was not due to PKC activation, as it was not inhibited by the PKC inhibitor, RO 31-8220. Acute insulin treatment, on the other hand, increased hexose uptake in down-regulated cells, and this insulin-stimulated uptake was inhibited by RO 31-8220 in down-regulated cells as well as in nondown-regulated cells. Insulin also stimulated the phosphorylation of the heat-stable 80-kDa protein in down-regulated cells, suggesting that the residual PKC epsilon in these cells can be activated by insulin.

Effects of alloimmune injury on contraction and relaxation in cultured myocytes and intact cardiac allografts

OBJECTIVES

This study was performed to determine the mechanisms by which allosensitized lymphocytes cause contractile dysfunction in cultured ventricular myocytes and to compare the effects on isolated myocytes with those observed in an intact heart preparation during allograft rejection.

BACKGROUND

Allograft rejection may be associated with reversible abnormalities of both systolic and diastolic function. The immunologic mechanisms that cause ventricular dysfunction are poorly understood.

METHODS

Vascularized heterotopic abdominal heart transplantation was performed in mice. Contractile function of excised allografts undergoing rejection was assessed using a Langendorff perfusion apparatus and a strain gauge. Spontaneously beating monolayers of cultured ventricular myocytes from donor strain fetal mice were exposed to the allosensitized cytotoxic T lymphocytes, and the effects on myocyte motion, intracellular calcium transients, relaxation half-time, membrane potential and myocyte lysis (chromium-51 release) were measured.

RESULTS

In intact hearts, histologically mild rejection without myocyte necrosis was associated with decreased systolic function without slowing of relaxation. In cultured fetal myocytes, sensitized lymphocytes induced a progressive decrease in the amplitudes of myocyte motion and calcium transients, with cessation of beating within 40 min. Also, the diastolic membrane potential and amplitude of the action potential decreased. Relaxation half-time, as estimated by measurement of cell motion, was unchanged. The effect was allospecific and was reversible with early removal of lymphocytes from the myocyte monolayer. Pretreatment of lymphocytes with the degranulation inhibitor 4,4'-diisothiocyano-2,2'-disulfonic acid stilbene blocked both the negative inotropic effect and myocyte lysis.

CONCLUSIONS

We conclude that impaired relaxation is not a prominent feature of contractile dysfunction caused directly in myocytes by alloimmune injury from cytotoxic lymphocytes. Allosensitized lymphocytes can cause reversible systolic dysfunction in myocytes by means of a direct cell-cell interaction. This effect may be in part responsible for the reversible systolic dysfunction associated with allograft rejection.

Reduced mercury vapor release from dental amalgams prepared with binary Hg-in liquid alloys

For the past ten years, the amounts of mercury vapor released from dental amalgams and the possibility of side-effects caused by these amounts have been discussed. Although no adverse health effects have been substantiated from these minute amounts, besides rare cases of allergies, this release should be reduced. The aim of this study was to reduce the mercury evaporation from a high-copper amalgam during setting by triturating the alloy powder with binary Hg-In liquid alloys having various indium concentrations set at 0, 5, 10, 20, 30, 40, and 50 wt%. Specimens (n = 4) were made for each amalgam according to ANSI/ADA Specification #1 and placed into a measuring chamber kept at 36.5 +/- 0.5 degrees C, through which a continuous air flow of 0.75 L/min was pumped. The total mercury release was measured during the setting period, according to the method described by Ferracane et al. (1992). Four additional specimens were prepared from a modified 14% Indisperse amalgam powder for the purpose of comparison. The results showed that the release of mercury vapor decreased with increasing indium concentrations, and that the amalgams made with the Hg-In liquid alloy with 10% In or more released significantly less mercury than the modified 14% Indisperse. The method of mixing indium in mercury prior to trituration appeared to be another effective method for reducing mercury evaporation during setting. The reduced mercury release may be explained by a reduction of mercury concentration in the structure, a reduction of vapor pressure for the mercury in the matrix phases, or more efficient formation of a surface oxide layer.

Familial predisposition to precancerous gastric lesions in a high-risk area of China

Aggregation of intestinal metaplasia and dysplasia was investigated among families participating in a population-based gastroscopic screening survey in an area of China with one of the world's highest rates of stomach cancer. The prevalence of gastric dysplasia was significantly increased among those with dysplasia among siblings or spouses, but not parents. The odds of dysplasia were nearly doubled if an eldest brother or a spouse was affected. Sibling and spousal associations for intestinal metaplasia were much less pronounced and not statistically significant. The specificity of the findings suggests that familial risk of advanced precancerous lesions (dysplasia) is influenced not only by genetic factors, but also by environmental factors operating in childhood and early adult life.

Time after excision and temperature alter ex vivo tissue relaxation time measurements

Previously unreported effects of tissue storage were recently observed in the authors' experimental magnetic resonance (MR) studies. To evaluate the effect of elapsed time after excision and storage temperature on tissue relaxation time measurements, tissue samples from the liver, pancreas, kidney, testis, spleen, and brain were obtained in rats. T1 and T2 were first measured within 5 minutes of excision, and between subsequent measurements, tubes were kept in a water bath at 40 degrees C, at room temperature (28 degrees C), or in an ice bath (4 degrees C). Cellular and organellar integrity was assessed with electron microscopy and correlated with the MR findings. At 40 degrees C (20-MHz spectrometer), the T1 of liver decreased from 280 msec +/- 8 to 212 msec +/- 10 during the first 60 minutes; the T1 of pancreas decreased from 276 msec +/- 3 to 208 msec +/- 2. Other tissues showed less than a 5% decrease in T1. T2 changes were smaller than T1 changes in all tissues. Electron microscopy of pancreatic acinar cells showed postmortem changes in mitochondria evolving over the first 60 minutes after death. Manganese loading experiments implicated mitochondrial manganese stores in the observed enhanced postmortem decrease in T1. This study calls into question reported relaxation time data for liver and pancreas. MR studies of excised tissues must account for time and temperature to prevent systematic experimental errors.

Jackknife estimators of variance for parameter estimates from estimating equations with applications to clustered survival data

An estimate of a parameter vector beta is often obtained by setting a "score" vector equal to zero and solving for beta. Estimating equations of this type include maximum likelihood, quasi-likelihood (McCullagh, 1983, Annals of Statistics 11, 59-67), and generalized estimating equations (Liang and Zeger, 1986, Biometrika 73, 13-22). White (1982, Econometrica 50, 1-26) proposed a variance estimator for beta that is robust to model misspecification. We show that a "one-step" jackknife estimator of variance is asymptotically equivalent to the variance estimator proposed by White. The one-step variance estimator may be preferred when the appropriate computer packages are not available to compute White's estimator directly. This jackknife estimator is very useful in our example with clustered survival data.