A new correlation-based measure of spike timing reliability.

We introduce a new correlation-based measure of spike timing reliability. Unlike other measures, it does not require the definition of a posteriori "events". It relies on only one parameter, which relates to the timescale of spike timing precision. We test the measure on surrogate data sets with varying amounts of spike time jitter, and missing or additional spikes, and compare it with a widely used histogram-based measure. The measure is efficient and faithful in characterizing spike timing reliability and produces smaller errors in the reliability estimate than the histogram-based measure based on the same number of trials.

Emergency upper gastrointestinal bleeding. Management and outcomes in specialty private practice.

Severe upper gastrointestinal (UGI) bleeding is a common medical emergency associated with significant morbidity and mortality. Recent studies from selected academic medical centers show that emergency UGI endoscopy with therapeutic intervention prevents recurrent hemorrhage, reduces complications, and limits costs. We determined prospective outcomes for patients presenting to 11 hospitals in Minneapolis and treated by 17 gastroenterologists from an independent single-specialty group. All 291 patients with severe UGI bleeding seen from July 1994 to January 1995 were enrolled and treated according to a guideline that the gastroenterologists had previously agreed upon. Chart review after hospital discharge showed that therapeutic endoscopy resulted in substantial reductions in the risk of recurrent bleeding compared with recent historic controls; the reductions were comparable to those seen in randomized studies from academic centers. Low risk of recurrent bleeding was associated with fewer blood transfusions and fewer days in hospital and in ICU. We conclude that 1) committed specialists can develop and adhere to treatment plans that optimize patient benefit and limit costs, and 2) therapeutic endoscopy performed by gastroenterologists in community hospitals may be as effective as endoscopy performed by academicians with a special interest in UGI bleeding.

Influence of hyperalimentation on rat hepatic microsomal fluidity and function.

Total parenteral nutrition with an amino acid-glucose solution has previously been shown to decrease rat hepatic drug metabolism compared with drug metabolic activity observed in rats receiving the same solution enterally and chow-fed animals. Because changes in membrane fluidity and lipid composition are reported to influence activity of a number of liver enzymes, effects of parenteral and enteral nutrition on hepatic microsomal membrane fluidity and lipid composition were assessed and compared with hepatic mixed-function oxidase activity. Both parenteral and enteral hyperalimentation produced a significant decrease in microsomal membrane fluidity (fluorescence anisotropy = 0.155 +/- 0.003 in both experimental groups versus 0.129 +/- 0.003 for microsomes from chow-fed animals). However, meperidine demethylase activity was significantly decreased compared with chow-fed experiments only in hepatic microsomes from parenterally hyperalimented animals, whereas ethoxyresorufin deethylase activity was significantly reduced only in the enteral-nutrition group. Inclusion of lipid in the parenterally administered hyperalimentation solution normalized microsomal membrane fluidity and lipid profile to those of chow-fed animals but did not increase hepatic meperidine demethylation. Both parenteral and enteral nutrition produce significant changes in physical state and lipid composition of rat hepatic microsomal membranes, but these changes are not responsible for the altered hepatic drug metabolism observed during hyperalimentation.

Modulation of hepatic ferrochelatase activity by dietary manipulation of mitochondrial phospholipid fatty acyl groups.

Ferrochelatase is an enzyme bound to the inner mitochondrial membrane, which is important in heme biosynthesis. Activity of purified ferrochelatase is affected by the presence of certain fatty acids. In the present study, we examined whether the activity of ferrochelatase is altered by dietary manipulation of the composition of mitochondrial membrane phospholipid fatty acyl groups. Rats were fed diets containing triolein, safflower or menhaden oil as 5% (w/w) of the diet. After 3 weeks, the animals were killed and liver mitochondria were isolated. Phospholipid fatty acid composition and ferrochelatase activity were assayed in the isolated mitochondria. Marked differences were seen. The proportion of oleic acid was highest in the triolein oil-fed group, that of linoleic and arachidonic acid was highest in the safflower oil-fed group and the proportion of eicosapentaenoic acid was highest in the menhaden oil-fed group. Ferrochelatase activity was greatest in the triolein oil-fed group and lowest in the menhaden oil-fed group regardless of whether the mitochondria were intact, sonicated or sonicated and treated with Tween 20. Mixing of mitochondria from menhaden oil-fed rats with triolein oil resulted in a significant increase in ferrochelatase activity. Membrane fluidity and activities of the mitochondrial membrane enzymes succinic dehydrogenase and cytochrome oxidase did not differ among the groups. We conclude that dietary manipulation of mitochondrial membrane phospholipid fatty acyl group composition can directly modulate hepatic ferrochelatase activity. This has potential application in the treatment of protoporphyria, the genetic disorder in which ferrochelatase activity is deficient.

Membrane-membrane interactions associated with rapid transfer of liposomal bilirubin to microsomal UDP-glucuronyltransferase. Relevance for hepatocellular transport and biotransformation of hydrophobic substrates.

Bilirubin may be transported within intracellular membranes of the hepatocyte and may undergo membrane-membrane transfer to gain access to the conjugating enzyme UDP-glucuronyltransferase in the endoplasmic reticulum. We have demonstrated previously that the lipid composition of liposomal membranes incorporating bilirubin substrate influences the rate of transfer and glucuronidation of bilirubin by hepatic microsomes. To examine the mechanism(s) of substrate transfer, we incorporated radiolabelled bilirubin into small unilamellar model membranes of egg phosphatidylcholine or natural phospholipids in the proportions present in native hepatic microsomes. The rate at which bilirubin was transferred to rat liver microsomes and glucuronidated was then examined in the presence of various endogenous compounds that promote membrane fusion. For bilirubin substrate in membranes of egg phosphatidylcholine, the addition of Ca2+ (2 mM) increased the microsomal glucuronidation rate, whereas retinol enhanced microsomal conjugation rates for bilirubin in membranes of both lipid compositions. When the transfer of [3H]bilirubin from dual-labelled liposomes to microsomes was enhanced by Ca2+ or retinol, there was no associated increase in [14C]phospholipid transfer. Thus it appears likely that bilirubin is transferred to the endoplasmic reticulum by rapid cytosolic diffusion or membrane-membrane collisions, rather than by membrane fusion; this process may be modulated by changes in the lipid microenvironment of the substrate or the effective intracellular concentrations of Ca2+ or retinol. The observation that polymyxin B induced concomitant membrane-membrane transfer of [3H]bilirubin and [14C]phospholipid suggests that under certain circumstances membrane fusion or aggregation may promote the movement of lipophilic substrates in hepatocytes.

Hepatic microsomal glucuronidation of bilirubin is modulated by the lipid microenvironment of membrane-bound substrate.

Hepatocyte intracellular membranes may facilitate the directed movement of bilirubin and other hydrophobic substrates to the active site of UDP-glucuronyltransferase in the endoplasmic reticulum. We postulated that the lipid composition and physical properties of membranes that transport substrate may modulate bilirubin glucuronidation. To examine this hypothesis, we incorporated [14C]bilirubin substrate into the membrane bilayer of small unilamellar liposomes composed of native phospholipid purified from rat hepatic microsomes. The initial velocity of bilirubin glucuronide formation in rat liver microsomes, measured by radiochemical assay, was considerably more rapid than for bilirubin in liposomes of egg phosphatidylcholine (p less than 0.001). Moreover, the ratio of bilirubin diglucuronide to monoglucuronides synthesized was markedly increased (p less than 0.01), approaching that observed in normal rat bile. Although the rates of bilirubin glucuronidation did not correlate with fluidity of the liposomal membrane core region, specific phospholipid head groups were associated with an increase, and cholesterol a decrease, in rates of glucuronidation. Movement of [3H]bilirubin from dual-labeled liposomes to microsomes occurred without concomitant [14C]phospholipid transfer. Thus, the lipid composition of membranes incorporating bilirubin appears to modulate the rate of glucuronidation and the relative rates of bilirubin mono- and diglucuronide formation. Phospholipid head groups on the surface of the bilayer, not the hydrocarbon core regions, may be implicated in the rapid process of membrane transport, which is likely to involve membrane-membrane collisions or diffusion of free substrate rather than membrane fusion.

Hepatic microsomal glucuronidation of bilirubin in unilamellar liposomal membranes. Implications for intracellular transport of lipophilic substrates.

It has been assumed that following hepatic uptake, bilirubin is bound exclusively to cytosolic proteins prior to conjugation by microsomal UDP-glucuronyl-transferase. Since bilirubin partitions into lipid rather than the aqueous phase at neutral pH, we postulated that bilirubin reaches the sites of glucuronidation by rapid diffusion within membranes. To examine this hypothesis, [14C]bilirubin was incorporated into the membrane bilayer of small unilamellar liposomes of egg phosphatidylcholine. Radiochemical assay of this membrane-bound substrate in a physiologic concentration, using native rat liver microsomes, demonstrated immediate formation of bilirubin glucuronides at a more rapid initial velocity than for bilirubin bound to the high-affinity sites of purified cytosolic binding proteins, i.e. glutathione S-transferases (p less than 0.025) or native liver cytosol (p less than 0.05). Kinetic analysis suggested that the mechanisms of substrate transfer from liposomal membranes and from purified glutathione S-transferases to microsomal UDP-glucuronyltransferase were similar. The exchange of 3H- and 14C-labeled bilirubin substrate between binding proteins and liposomal membranes was then investigated using Sepharose 4B chromatography. As the concentration of bilirubin was increased relative to that of protein, net transfer of substrate from the protein to the membrane pool was observed. These findings indicate that bilirubin is efficiently transported by membrane-membrane transfer to hepatic microsomes, where it undergoes rapid conjugation. Bilirubin entering hepatocytes may partition between membrane and cytosolic protein pools, but as intracellular bilirubin concentration increases, the membrane pool is likely to provide a greater proportion of the substrate for glucuronidation.

The immunological generation of a platelet-activating factor and a platet-lytic factor in the rat.

Antigen challenge of the rat peritoneal cavity which had been prepared with IgGa-rich antiserum generated activities which released [14C]-serotonin from pre-labelled human platelets. After adsorption of these activities onto Amberlite XAD-8 and elution in 80% ethanol, two factors of differing polarity were resolved by chromatography on diethylaminoethyl cellulose in organic solvents. The activity eluting in the 7:1 chloroform:methanol solvent contained a platelet-lytic factor (PLF) assessed by the parallel release of lactic acid dehydrogenase and [14C]-serotonin; the cytotoxicity of this fraction was confirmed by phase-contrast microscopy examination which demonstrated fragmentation of the exposed platelets. The activity eluting in the 1:1 methanol: aqueous 1.0 M ammonium carbonate solvent was a platelet-activating factor (PAF) as defined by release of [14C]-serotonin without lactic acid dehydrogenase. Both the lytic and the activating principles were separable from slow reacting substance of anaphylaxis and polymorphonuclear leucocyte chemotactic activity, and each presented a single activity peak of differing mobility when chromatographed on silica gel H plates. Human eosinophil phospholipase D inactivated the lytic factor by more than 85% in 2 h at 37 degrees without affecting the activity of the activating factor. The release of [14C]-serotonin induced by the PAF was not affected by the absence of calcium from the medium or by elevations in the platelet concentrations of cyclic AMP or cyclic GMP that resulted from pre-incubation of platelets with prostaglandin D2 or sodium ascorbate, respectively.

Chemotactic deactivation of human eosinophils by the eosinophil chemotactic factor of anaphylaxis (38527).

Purified human eosinophils demonstrate diminished chemotactic responsiveness (deactivation) after incubation with the eosinophil chemotactic factor of anaphylaxis (ECF-A). The deactivation is rapid and selective in that ECF-A deactivated human eosinophils more markedly than neutrophilic or mononuclear leukocytes. The eosinophil can also be deactivated by C5a which is eosinophilotactic, and there is cross deactivation between C5a and ECF-A. Deactivation may be an important physiologic control mechanism enabling the eosinophil to remain at sites of ECF-A release in order to manifest its regulatory functions in immediate hypersensitivity reactions.