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Biomedical subjects

D Allan

Publications and source records attributed to D Allan.

At least 37 records · Page 2Linked to original sources
PubMed1997

Apoptosis in haemopoietic progenitor cells exposed to extremely low-frequency magnetic fields.

Epidemiological studies have indicated a modestly increased risk for the development of acute myeloid leukaemia in children who live close to high-voltage power-lines. Recent evidence has suggested that a common property shared by a number of known and suspected tumour promoters is their ability to block the process of apoptosis. Therefore, one possible mechanistic explanation for the apparent leukaemogenic effect of weak, low-frequency magnetic fields, such as emitted by power-lines and electrical appliances, would be their expression of tumour-promoting activity by interfering with the regulation of apoptosis in multipotent haemopoietic progenitor cells. In order to test this hypothesis, we have employed the well-characterized multipotential haemopoietic progenitor cell line FDCP-mix(A4). These cells are non-leukaemic and undergo apoptosis when deprived of appropriate growth factors such as Interleukin-3. We have tested a series of different regimes of weak, low-frequency magnetic fields: nulled fields, Ca2+-ion cyclotron resonance conditions at 50 Hz, and vertical 50 Hz fields of 6 microT(RMS), 1 mT(RMS) and 2 mT(RMS), exposing the cells for 2 hours, 24 hours, 4 days or 7 days under various culture conditions. We have not seen any significant alteration in apoptosis induced by any of the exposure regimes tested. We therefore conclude that the regulation of viability and apoptosis in FDCP-mix(A4) cells is not disturbed by weak magnetic fields of the magnitude and type indicated.

Animals↗
PubMed1996

Phorbol ester-sensitive phospholipase D is mainly localized in the endoplasmic reticulum of BHK cells.

The localization of phorbol ester-sensitive phospholipase D (PLD) in baby hamster kidney cells has been investigated by determining the subcellular distribution of the phosphatidylbutanol produced when the cells are incubated with phorbol 12-myristate 13-acetate and n-butanol. Results derived by isolation of plasma membrane vesicles from intact cells or by subcellular fractionation on a sucrose density gradient suggest the PLD is specific for phosphatidylcholine and its primary site of action is not the plasma membrane but the endoplasmic reticulum.

Acetic Acid↗
PubMed1996

Ethical boundaries in genetic testing.

The scarcity of resources that can be allocated to genetic testing will ultimately limit the number of diseases subjected to molecular analysis. Medical student David Allan, who claimed first prize in CMAJ's 1995 Logie Medical Ethics Essay Contest, looks at the ethical principles that should guide decisions about genetic testing, and the importance of communicating these principles to patients and their families.

Age of Onset↗
PubMed1996

Exposure to extremely low frequency magnetic fields has no effect on growth rate or clonogenic potential of multipotential haemopoietic progenitor cells.

Recent reports indicate an increased risk of acute myeloid leukaemia in children exposed to extremely low frequency magnetic fields (ELFMFs) emitted by high voltage power lines, suggesting that ELFMFs may act as weak tumour promoters. We have investigated possible interactions of weak ELFMFs with primitive haemopoietic cells in vitro using the multipotential progenitor cell line FDCP-mix(A4). We have determined the proliferative activity and clonogenic potential of cells under both optimal and sub-optimal growth conditions and exposed to either ambient laboratory ELFMFs or three other ELFMF regimes representative of those produced by high voltage power lines: nulled fields, Ca2+-ion cyclotron resonance conditions at 50 Hz, and vertical 50 Hz fields of 6 muT(RMS). Using exposures of 1, 4, 7 and 21 days, we found no significant alteration of growth rate, cell-cycle state or clonogenic efficiency indicating that neither the proliferation nor self-renewal of multipotential FDCP-mix(A4) cells was perturbed.

Acute Disease↗
PubMed1994

Phospholipid asymmetry in plasma membrane vesicles derived from BHK cells.

The transbilayer distribution of phospholipids in plasma membrane vesicles derived from BHK cells by treatment with iodoacetamide or fluoride and merocyanine 540 has been examined by exposing the vesicles to bee venom phospholipase A2 (PLA2) or to Bacillus cereus sphingomyelinase. The results show that almost all of the phosphatidylserine (PS) is on the inner lipid leaflet and most of the sphingomyelin is on the outer lipid leaflet. In contrast, about 50% of the phosphatidylcholine (PC) and 30-40% of the phosphatidylethanolamine (PE) is rapidly degraded by PLA2 and thus appears to be present on the surface of the vesicles. The pools of PC and PE which are accessible only slowly to PLA2 are degraded with halftimes of about 5 h and 2 h, respectively, and it is suggested that this rate reflects the rate of transbilayer migration of these lipids. We conclude that the profound energy depletion caused by treatment with iodoacetamide or fluoride does not alter the asymmetric distribution of PS across the plasma membrane but does have a marked effect on the transbilayer distribution of PE. Residual cells after treatment with fluoride and MC540 were also exposed to PLA2. The results were broadly in agreement with those obtained with vesicles, suggesting that the vesicles were representative of the BHK cell plasma membrane in terms of phospholipid asymmetry. Fluoride or MC540 added separately caused little vesicle release but did lead to significant loss of phospholipid asymmetry. When centrifuged on a sucrose density gradient, vesicles were separated into two major fractions accounting for about two thirds and about 20%, respectively, of total phospholipid but no significant differences were seen in the transbilayer phospholipid asymmetry of the two fractions.

Animals↗
PubMed1994

Synthesis of surface sphingomyelin in the plasma membrane recycling pathway of BHK cells.

Sphingomyelin, which has been degraded at the BHK cell surface by exogenous sphingomyelinase, is converted back into sphingomyelin with kinetics similar to those of plasma membrane recycling. Resynthesis of sphingomyelin under these conditions proceeds at a rate about 4-fold higher than normal biosynthesis of sphingomyelin. Neither resynthesis of sphingomyelin nor its return to the surface is inhibited by brefeldin A (BFA), which is a potent blocker of vesicular transport through the Golgi but has no effect on plasma membrane recycling. However, resynthesis of plasma membrane sphingomyelin is greatly decreased in cells undergoing mitosis or energy depletion, where endocytosis is inhibited. We conclude that the main site of surface sphingomyelin synthesis in BHK cells could be in recycling endosomes and not in the Golgi apparatus as proposed previously. We also suggest a model pathway by which cholesterol may reach the plasma membrane via recycling endosomes.

Animals↗
PubMed1994

A personal computer-based visual evoked potential stimulus and recording system.

A system for recording electroretinograms and visual evoked cortical potentials has been constructed with the use of a personal computer and a digital signal processing card. The system is based on widely available commercial hardware. It has been designed to be capable of performing routine visual electrophysiology as well as allowing the development of novel visual stimuli and signal detection techniques. The system enables both transient and steady-state stimulation rates. Pattern stimuli can be presented in pattern-reversal, pattern-onset, pattern-offset or motion-onset modes. In addition to conventional signal averaging, the digital signal processing card can also provide on-line Fourier analysis and is facilitating the development of adaptive filtering techniques for the detection of steady-state visual evoked cortical potentials. This versatile system is in regular clinical use for the measurement of electroretinograms and visual evoked cortical potentials.

Electrophysiology↗
PubMed1994

Is plasma membrane lipid composition defined in the exocytic or the endocytic pathway?

Compared with intracellular membranes, the plasma membrane is rich in cholesterol and sphingomyelin. How does this distinct composition arise? Here David Allan and Karl-Josef Kallen take a critical view of the belief that these lipids arrive at the plasma membrane via vesicular traffic from the Golgi complex and propose instead that they may be accreted in the endocytic recycling pathway.

Journal Article↗
PubMed1994

Responses to converting enzyme and renin inhibition. Role of angiotensin II in humans.

We compared the renal vascular responses to angiotensin converting enzyme inhibition and renin inhibition to assess the influence of angiotensin II (Ang II). We examined the renal and endocrine responses to the renin inhibitor enalkiren, to captopril, and to placebo in nine healthy and nine hypertensive men on a 10-mmol sodium diet. Ang II was infused to assess effects of the agents on renal and adrenal responsiveness to Ang II. Plasma Ang II concentration was suppressed similarly with enalkiren and captopril--an identical level of blockade was achieved. Although renal plasma flow was stable during placebo, a substantial rise was seen with both enalkiren (+133 +/- 26 mL/min per 1.73 m2) and captopril (+99.4 +/- 22.6). There was remarkable intrasubject concordance between the renal plasma flow responses to renin inhibition and converting enzyme inhibition (r = .90, P < .004). The vasodilator response to both agents correlated inversely with the fall in renal plasma flow induced by Ang II alone (r = -.66, P < .05). Both agents significantly enhanced the renal vascular response to Ang II (P = .01), and, furthermore, the renal vasodilator response to captopril predicted the potentiation of the renal plasma flow response to Ang II after either agent (enalkiren: r = .91, P < .001; captopril: r = .56, P < .05). Concordance of the maximal renal plasma flow response to the two agents appeared in the hypertensive men as well. Our results indicate that the acute renal response to captopril largely reflects a reduction in Ang II formation.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1994

Familial nephropathic systemic amyloidosis caused by apolipoprotein AI variant Arg26.

A point mutation in the apolipoprotein AI (apoAI) gene causing autosomal dominant non-neuropathic systemic amyloidosis is described in a previously unreported Canadian family of British origin with five affected individuals in three generations. Amyloid deposits in the renal biopsy from the proband, a 31-year-old female presenting with hypertension and renal failure, stained immunospecifically with antiserum to apoAI. The plasma of all family members with amyloidosis contained both wild-type apoAI and a variant bearing one additional positive charge. Sequencing of the apoAI gene demonstrated that the proband was a heterozygote for a single base substitution in exon 3, changing codon 26 from GGC(Gly) to CGC(Arg). Concordance of the mutant allele with the presence of variant plasma apoAI and clinical features of amyloidosis was demonstrated. This is the third family in which this amyloidotic mutation has been described, but the distribution of amyloid deposits and their clinical effects are clearly determined by other genetic and/or environmental factors.

Adult↗
PubMed1993

Plasma membrane vesicles from BHK and HL60 cells treated with merocyanine 540 and iodoacetamide.

Treatment of BHK or HL60 cell lines with merocyanine 540 in the presence of the sulphydryl blocker iodoacetamide caused budding of the cell surface to release vesicles about 50-100 nm in diameter which accounted for up to 25% of the total surface membrane lipid. Smaller amounts of vesicular material were released in the presence of fluoride and merocyanine 540. The vesicles had a membrane lipid composition which was characteristic of other purified plasma membranes, with large amounts of sphingomyelin, phosphatidylserine and cholesterol and low proportions of phosphatidylinositol, phosphatidylcholine, triacylglycerol and cholesterol ester. This procedure for the isolation of vesicles should be a general method for the purification of plasma membrane components from a wide range of different cell types.

Animals↗
PubMed1993

Monensin inhibits synthesis of plasma membrane sphingomyelin by blocking transport of ceramide through the Golgi: evidence for two sites of sphingomyelin synthesis in BHK cells.

The monovalent cationophore monensin, which is known to interfere with vesicular transport through the Golgi apparatus, inhibits synthesis of sphingomyelin in BHK cells by up to 40%. The monensin-sensitive component of sphingomyelin synthesis appears to be the pool which normally reaches the cell surface since treatment of cells with exogenous sphingomyelinase causes an almost identical loss of sphingomyelin. Monensin causes increases in ceramide and glucosylceramide labelling which together are equivalent to the decrease in sphingomyelin labelling. Monensin also increases synthesis of cholesterol ester, probably due to the decreased delivery of sphingomyelin to the plasma membrane. However, monensin has no effect on resynthesis of plasma membrane sphingomyelin which has been degraded by extracellular sphingomyelinase. The results support the idea that synthesis of sphingomyelin destined for the plasma membrane does not occur in the cis- or medial-Golgi but depends on vesicular transport of ceramide to a second synthesis site which is distal to the medial-Golgi.

Animals↗
PubMed1993

Effects of brefeldin A on sphingomyelin transport and lipid synthesis in BHK21 cells.

1. Addition of brefeldin A (BFA) to BHK cells incubated for 4 h with [3H]acetate led to a 3-4-fold increase in incorporation of label into sphingomyelin, monoglucosylceramide and cholesterol ester compared with untreated controls. There was a similar increase in incorporation of [3H]choline into sphingomyelin. The level of cholesterol ester increased 3-fold when BFA was added to cells labelled to equilibrium with [3H]acetate, but no statistically significant changes in the levels of other lipids were seen. 2. BFA appeared to act by diverting incorporation of acetate into sphingolipids and cholesterol ester at the expense of phosphatidylcholine (decreased by up to 15%), cholesterol (decreased by 30-40%) and triacylglycerol (decreased by 35-50%). 3. Forskolin (100 microM) prevented the changes in labelling induced by 0.25 micrograms of BFA/ml, but in the presence of 1 micrograms of BFA/ml it had no effect on sphingomyelin and triacylglycerol labelling and only partly blocked the effects of BFA on labelling of cholesterol and cholesterol ester. 4. None of the labelled sphingomyelin was degraded in BFA-treated cells which were subsequently exposed to an extracellular sphingomyelinase, showing that all the newly synthesized sphingomyelin remained inside the cells. Determinations of phospholipid phosphorus in unlabelled cells confirmed that, in the presence of BFA, no newly synthesized sphingomyelin was able to reach the cell surface, supporting the idea that sphingomyelin normally depends on vesicular transport for its passage to the plasma membrane. 5. The results are consistent with the hypothesis that cholesterol synthesis and esterification processes in BHK cells are sensitive to the plasma-membrane deficit of sphingomyelin caused by BFA.

Acetates↗
PubMed1992

Two separate pools of sphingomyelin in BHK cells.

In BHK cells labelled to equilibrium with [3H]choline and treated with sphingomyelinase the surface pool of sphingomyelin is degraded very rapidly (half-time 10 min) but the internal pool of sphingomyelin which accounts for about 30% of the total is only degraded slowly (half-time about 80 h) showing that the internal pool does not normally reach the surface. In [3H]choline incorporation experiments the internal pool begins to accumulate radioactivity at about the same time as phosphatidylcholine (30 min) but label does not enter the surface pool of sphingomyelin for a further 90 min. The internal and external pools reach the same specific activity only after about 20 h. Pulse-chase analysis with [3H]choline shows that radioactivity in each pool of sphingomyelin continues to increase when the specific radioactivity of phosphatidylcholine is decreasing, consistent with both pools being synthesised from a phosphatidylcholine precursor. The results suggest that sphingomyelin in BHK cells is present not only in the plasma membrane but also in a more rapidly labelling pool which does not mix with the surface pool.

Animals↗
PubMed1992

Isolation of plasma membrane exovesicles from BHK cells using merocyanine 540.

Treatment of cultured BHK cells with merocyanine 540 caused the non-lytic release of vesicular material having the phospholipid composition characteristic of plasma membrane. The protein composition of the vesicles closely resembled that of the soluble fraction of the cell, as expected for exovesicles budding from the cell surface. Vesicles prepared from cells surface-iodinated with 125I contained no obvious iodinated membrane polypeptides, suggesting that no major proteins in the plasma membrane of the BHK cell are free to diffuse with lipids. The procedure described should represent a general method, applicable to a wide range of cell types, for isolating plasma membrane vesicles.

Cell Membrane↗
PubMed1992

Flash and pattern reversal visual evoked responses in normal and demented elderly.

P1 and P2 components of visual evoked responses (VERs) to flash stimuli were compared between patients diagnosed as suffering from Senile Dementia of the Alzheimer Type (AD) and elderly controls. Additionally the P100 component of the VER to pattern reversal stimuli at high and low contrast and using large and small check patterns was compared. Significant differences between moderate AD patients and the normal elderly were found for the P2 component of the flash stimulus and for the P100 component of the high spatial frequency pattern reversal stimuli at low contrast. These findings suggest a deficit in primary visual processing and a selective deficit in secondary visual processing in moderate cases of dementia. This indicates that visual processing capacities of AD patients decline as the disease progresses in line with the continual degeneration of neuronal populations involved with vision.

Aged↗