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Biomedical subjects

M J Whitaker

Publications and source records attributed to M J Whitaker.

At least 19 recordsLinked to original sources
PubMed2002

Muscarinic signalling affects intracellular calcium concentration during the first cell cycle of sea urchin embryos.

The existence of a response to acetylcholine (ACh) and cholinomimetic drugs in sea urchin eggs and zygotes was investigated in two sea urchin species: Paracentrotus lividus and Lytechinus pictus. The calcium sensitive fluorescent probe, Fura-2 dextran, was employed to investigate the regulation of cytosolic free calcium concentration ([Ca(2+)](i)) by cholinomimetic drugs in unfertilised and fertilised eggs of both the sea urchin species. Exposure to cholinomimetic agonists/antagonists, either extracellularly or intracellularly, had no effect either on resting [Ca(2+)](i) levels in the unfertilised sea urchin egg, or on the transient [Ca(2+)](i) increase at fertilisation. However, following fertilisation, extracellular application of ACh receptors agonists, such as ACh and carbachol, predominantly muscarinic agonist, but not nicotine, induced a significant increase in [Ca(2+)](i), which was partially inhibited by atropine. As a consequence of exposure after fertilisation to the agonists of ACh receptors, chromatin structure was transiently affected. The hypothesis is proposed that muscarinic receptors may be involved in the (presumably Ca(2+)-dependent) modulation of the nuclear status during the first cell cycles.

Acetylcholine↗
PubMed2001

Growth factor release from tissue engineering scaffolds.

Synthetic scaffold materials are used in tissue engineering for a variety of applications, including physical supports for the creation of functional tissues, protective gels to aid in wound healing and to encapsulate cells for localized hormone-delivery therapies. In order to encourage successful tissue growth, these scaffold materials must incorporate vital growth factors that are released to control their development. A major challenge lies in the requirement for these growth factor delivery mechanisms to mimic the in-vivo release profiles of factors produced during natural tissue morphogenesis or repair. This review highlights some of the major strategies for creating scaffold constructs reported thus far, along with the approaches taken to incorporate growth factors within the materials and the benefits of combining tissue engineering and drug delivery expertise.

Bioartificial Organs↗
PubMed1999

Action of serotonin antagonists on cytoplasmic calcium levels in early embryos of sea urchin Lytechinus pictus.

Possible interaction of the serotonergic system with intracellular calcium mechanisms was investigated using techniques of ratio imaging measurement of intracellular Ca2+ and confocal microscopy in cleaving embryos of sea urchin Lytechinus pictus. Some serotonin antagonists specifically increase free intracellular Ca2+ and evoke transient regression of the first cleavage furrow, suggesting possible linkage of serotonergic and calcium mechanisms in the regulation of cellular events during cleavage divisions. These effects were more pronounced in the experiments with hydrophilic 5-HT-antagonists, quarternary ammonium salts that do not penetrate the cell membrane. Thus, it appears that 5-HT-receptors which mediate these effects are localised on the cell membrane, whereas previously studied receptors mediating the cytostatic action of lipophilic 5-HT-antagonists are localised intracellularly.

Animals↗
PubMed1998

Consensus guidelines for evaluating and treating patients with upper gastrointestinal symptoms in the primary care setting.

Upper gastrointestinal (UGI) disease represents a significant part of the primary care physician's workload and is ultimately responsible for a major portion of the costs relating to his or her drug prescribing. This paper outlines the scale of the problem and the possibility of a structured management plan for UGI disease. Although this type of strategy is not new, the International Gastro Primary Care Group (IGPCG) has approached it from a practical standpoint that addresses the concerns of general practitioners and the fears and anxieties of patients. In addition, the direct representation of this model with a cost-effective programme is the first opportunity to develop strategies, in primary care, for testing a protocol for the management of UGI disease. The approach is aimed at the primary care setting and is adaptable for use in different cultural groups and healthcare systems.

Anti-Inflammatory Agents, Non-Steroidal↗
PubMed1997

Nicotinic antagonists (piperidines and quinuclidines) reduce the susceptibility of early sea urchin embryos to agents evoking calcium shock.

1. Some nicotinic antagonists (piperidine and quinuclidine derivatives and bis-quaternary compounds) protect early embryos of the sea urchin Lytechinus pictus against a calcium shock evoked by ionomycin or a mixture of phorbol myristate acetate and nicotine. 2. Maximal protective potency was found for drugs that did not penetrate the plasma membrane. 3. Early sea urchin embryos have nicotinic acetylcholine receptors (nAChR) or nAChR-like structures localized on the cell surface that, apparently, take part in the control of Ca2+ influx.

Animals↗
PubMed1997

Controversy and consensus in the management of upper gastrointestinal disease in primary care. The International Gastro Primary Care Group.

Approximately 5% of all primary care consultations in the UK are for upper gastrointestinal (GI) diseases, the most common of which is dyspepsia, with a prevalence of between 25 and 50% in the western world. The exact definition of dyspepsia is elusive, which has resulted in confusion about diagnosis and treatment, highlighting the need for management guidelines. The International Gastro Primary Care Groups (IGPCG) has developed, by consensus, practical guidelines to help GPs manage patients with upper GI symptoms. After a detailed history is taken, alarm symptoms identified and organic disease excluded, the predominant symptom should be identified. This strategy, as outlined in the IGPCG upper GI disease management plan, can help the GP in the selection of the most appropriate treatment for each patient. This plan is flexible enough to be used in a wide variety of healthcare systems and will evolve as new evidence becomes available.

Anti-Inflammatory Agents, Non-Steroidal↗
PubMed1995

Calcium requirements during mitotic cdc2 kinase activation and cyclin degradation in Xenopus egg extracts.

Activation of p34cdc2 kinase is essential for entry into mitosis while subsequent deactivation and cyclin degradation are associated with exit. In Xenopus embryos, both of these phases are regulated by post-translation modifications and occur spontaneously on incubation of extracts prepared late in the first cell cycle. Even though high levels of calcium buffer were initially used to prepare these extracts, we found that free calcium levels in them remained in the observed physiological range (200-500 nM). Further addition of calcium buffers only slightly reduced free calcium levels, but inhibited histone H1 (cdc2A) kinase deactivation and cyclin degradation. Higher buffer concentrations slowed the kinase activation phase. Reducing the free buffer concentration by premixing with calcium reversed the effects of the buffer, indicating that the inhibitory effects arose from the calcium-chelating properties of the buffer rather than non-specific side effects. Furthermore, additions of calcium buffer at the end of the H1 kinase activation phase did not prevent deactivation. From these results, and the order of effectiveness of different calcium buffers in disrupting the H1 kinase cycle, we suggest that local transient increases in free calcium influence the rate of cdc2 kinase activation and are required to initiate the pathway leading to cyclin degradation and kinase inactivation in mitotic cell cycles.

Animals↗
PubMed1990

Second messengers at fertilization in sea-urchin eggs.

The activation of the sea-urchin egg at fertilization is triggered by two ionic changes in the egg cytoplasm. First there is a wave of increase in cytoplasmic free calcium concentration (Ca2+i) that travels across the egg within the first minute of fertilization. The calcium wave is followed by a sustained rise in intracellular pH (pHi). Fertilizing spermatozoa cause these two ionic changes by stimulating the hydrolysis of phosphatidylinositol 1,4,5-bisphosphate (PtdInsP2). PtdInsP2 hydrolysis produces inositol 1,4,5-trisphosphate, which releases calcium from intracellular stores, and diacylglycerol, which stimulates a plasma membrane sodium/hydrogen antiporter causing the rise in pHi. A positive feedback cycle of intracellular calcium release and InsP3 production appears to be part of the mechanism by which the calcium wave is propagated. It has been suggested that the spermatozoon triggers the calcium wave by acting on a cell surface receptor that couples to the egg phosphoinositidase C via a G-protein. We discuss evidence for an alternative hypothesis of egg activation in which a spermatozoon triggers the calcium wave by introducing a soluble activating factor into the egg cytoplasm.

Animals↗
PubMed1984

A multisample apparatus for kinetic evaluation of skin penetration in vitro: the influence of viability and metabolic status of the skin.

An apparatus is described for convenient in vitro quantitation of the rate, extent, and character of living skin penetration by chemicals. Evidence that viability markedly affects epidermal penetration was based upon observations utilizing benzo(a)pyrene. This compound displayed saturation kinetics in the extent of penetration, marked elevation in the rate, and extent of penetration following induction of epidermal mixed function oxidase and negligible penetration of skin previously frozen. These data are interpreted to indicate that for certain compounds, such as benzo(a)pyrene where cutaneous metabolism is expected, meaningful in vitro measures of skin penetration should take into consideration the biochemical viability and metabolic status of the tissue.

Animals↗
PubMed1983

Calcium-dependent exocytosis in an in vitro secretory granule plasma membrane preparation from sea urchin eggs and the effects of some inhibitors of cytoskeletal function.

Egg cortical granules remain attached to the egg plasma membrane when the egg is ruptured. We present evidence that demonstrates that, when the cytoplasmic face of the egg plasma membrane is exposed to micromolar calcium concentrations, an exocytosis of the cortical granules occurs which corresponds to the cortical granule exocytosis seen when the egg is fertilized. The calcium sensitivity of the preparation is decreased by an increase in magnesium concentration and increased by a decrease in magnesium concentration. Exocytosis is inhibited by trifluoperazine (half inhibition at 6 microM), a drug that inhibits the action of the calcium-dependent regulatory protein calmodulin. Colchicine, vinblastine, nocodazole, cytochalasin B, phalloidin, N-ethylmaleimide-modified myosin subfragment 1, and antibody to actin are without effect on this in vitro exocytosis at concentrations that far exceed those required to disrupt microtubules and microfilaments. Conditions are such that penetration to the exocytotic site is optimal. It is unlikely, therefore, that either actin or tubulin participate intimately in exocytosis. Our data also exclude on quantitative grounds several other mechanisms postulated to account for the fusion of the secretory granule with the plasma membrane.

Actins↗
PubMed1983

Evidence in support of the hypothesis of an electrically mediated fast block to polyspermy in sea urchin eggs.

Action currents were recorded extracellularly in sea urchin (Echinus esculentus and Psammechinus miliaris) eggs at fertilization. The action current was shown to correspond to the rising phase of a regenerative action potential by simultaneous intracellular recording. The uniform occurrence of such an action current at fertilization indicates that the resting potential of the unimpaled egg must be more negative than -50 mV, the action potential threshold, and offers confirmation of the hypothesis that the fast block to polyspermy in the sea urchin egg is electrical in nature.

Action Potentials↗
PubMed1981

The relation between the increase in reduced nicotinamide nucleotides and the initiation of DNA synthesis in sea urchin eggs.

Previous work has suggested that the activation of the sea urchin egg at fertilization is the result of a transient increase in intracellular free calcium and an increase in intracellular pH. We have investigated the absence of nuclear activation in incompletely activated eggs and have found a correlation between nuclear activation and the levels of total reduced nicotinamide nucleotides (NAD[P]H). Eggs activated with ammonia show a similar correlation: besides its action as a weak base in raising intracellular pH (which we conclude is insufficient to stimulate or maintain nuclear activation as judged by nuclear envelope breakdown or DNA synthesis), ammonia increases NAD(P)H. This increase is associated with the stimulation of 6-3H-thymidine incorporation into egg DNA. Removing ammonia decreased NAD(P)H, and tritiated thymidine incorporation ceases. We conclude that a critical level of NAD(P)H is essential to nuclear activation and that the increase of NAD(P)H at fertilization must be included with the increase in calcium and pH as a causal agent in development.

Ammonium Chloride↗
PubMed1980

Negative chemical ionization studied of human and food chain contamination with xenobiotic chemicals.

Negative chemical ionization mass spectrometry with a mixture of isobutane, methylene chloride, and oxygen as the reagent gas has been used to explore contamination of environmental substrates with xenobiotic chemicals. The substrates in question, fish tissue, human seminal plasma, and human adipose tissue, were cleaned up by one of the following three cleanup procedures: (1) continuous liquid-liquid extraction steam distillation; (2) gel-permeation chromatography; and (3) adsorption on activated carbon followed by elution with toluene. The third procedure was used only for the examination of planar polychlorinated aromatic hydrocarbons in environmental samples. Using these techniques, we have found evidence for contamination of fish samples with polychloronaphthalenes, polychlorostyrenes, polychlorobiphenyls, polychlorodibenzofurans, and polychlorodibenzodioxins among other chemicals. The polychlorodibenzodioxins appeared only in the spectra of extracts of fish obtained from the Tittabawassee River at Midland Michigan. The polychlorodibenzofuran ions appeared in NCI mass spectra of fish that were significantly contaminated (above 2 ppm) with polychlorobiphenyls. Toxic substances occurring in human seminal plasma included pentachlorophenol, hexachlorobenzene, DDT metabolites, and polychlorobiphenyls. We have investigated toxic substances in human seminal plasma because of the apparent decrease in sperm density in U.S. males over the last 30 years. Results of screening human adipose tissue for contamination with xenobiotic chemicals have been largely coincident with result of the EPA human monitoring program. Polychlorobiphenyls, DDT metabolites, nonachlor, and chlordane have appeared in most samples examined. Detection limits for all of these chemicals were of the order of 1 ppb.

Adipose Tissue↗