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Biomedical subjects

M Dunlop

Publications and source records attributed to M Dunlop.

At least 55 records · Page 3Linked to original sources
PubMed1989

A phospholipase D-like mechanism in pancreatic islet cells: stimulation by calcium ionophore, phorbol ester and sodium fluoride.

In neonatal rat islet cells prelabelled with [14C-methyl] choline, the phorbol ester 12-O-tetradecanoylphorbol-13-acetate rapidly activated a phospholipase D-like mechanism as suggested by the accumulation in cells and medium of choline (but not of phosphorylcholine or glycerophosphorylcholine, markers for phospholipase C and phospholipase A2 action on phosphatidylcholine). This finding was confirmed by a rise in phosphatidic acid (but not diglyceride or arachidonic acid) in fatty acid-labelled cells. Phospholipase D was also activated by ionomycin or sodium fluoride; however, this was accompanied by parallel increases in diglyceride, monoacylglycerol and arachidonic acid in the absence of phosphorylcholine generation, suggesting that these agents also activated a phospholipase C-diglyceride lipase pathway acting on non-choline-containing phosphoglycerides (presumably phosphoinositides). In conjunction with our recent demonstration of insulinotropic effects of phosphatidic acid (M. Dunlop and R. Larkins, Diabetes, in press), our findings suggest for the first time a possible role for phospholipase D activation in the stimulation of insulin release and may imply a novel site of action for phorbol esters in the regulation of exocytosis.

Animals↗
PubMed1989

Serum glucose and insulin levels in normotensive (WKY) and spontaneously hypertensive (SH) rats during and after the cessation of continuous (10 day) clonidine infusion.

The concentrations of glucose were elevated whereas those of insulin were decreased in the sera of normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SH) rats on day 10 of a continuous subcutaneous infusion of clonidine (10 micrograms kg-1 h-1 for 10 days). Fifteen to 18 h after cessation of infusion, the glucose levels of both strains had fallen to below those of the respective controls whereas the insulin levels remained suppressed. As such the hypoglycaemia may be related to the general increase in metabolic requirements associated with the post-infusion 'withdrawal syndrome'.

Animals↗
PubMed1988

Evidence that muscarinic receptors in islet cells are not coupled functionally to adenylate cyclase through the inhibitory guanine nucleotide binding protein (Ni).

The effect of muscarinic agonist on adenylate cyclase was investigated in neonatal islet cells and in a clonal pituitary cell line (GH4C1) following labelling of the intracellular ATP pool with [2,8 3H]adenine. In islet cells carbamylcholine was without effect on basal or glucagon-stimulated adenylate cyclase activity, measured as 3H cyclic AMP production, but inhibited 3H cyclic AMP production in the clonal pituitary cells. The involvement of the inhibitory guanine nucleotide binding protein of adenylate cyclase (Ni) was investigated by the use of the Bordetella pertussis exotoxin, islet activating protein (IAP). Pre-treatment of islet cells with IAP was without effect on adenylate cyclase following carbamylcholine but in the clonal pituitary line abolished the inhibition of 3H cyclic AMP production. It is concluded that in the islet cell, in contrast to the clonal pituitary cell, muscarinic receptors are not effectively coupled through Ni to inhibit adenylate cyclase.

Adenylate Cyclase Toxin↗
PubMed1987

Acute changes in myo-inositol uptake and 22Na+ flux in murine neuroblastoma cells (N1E-115) following insulin.

myo-Inositol uptake was investigated in a murine neuroblastoma clone (N1E-115) to determine the effect of altered Na+,K+-ATPase activity. The Na+ ionophore monensin, and veratridine, an alkaloid affecting voltage-dependent Na+ entry, increased acute 22Na+ uptake and 22Na+ efflux from pre-loaded cells, concomitant with enhanced myo-inositol uptake. This effect was also seen following insulin. Insulin-stimulated myo-inositol uptake was inhibited by amiloride, ouabain and pyrithiamine. Amiloride inhibition suggests that activation of Na+/H+ exchange preceding Na+,K+-ATPase activation is involved in insulin stimulation of myo-inositol uptake. Pyrithiamine inhibition is an indication of prior activation of the Na+,K+-ATPase alpha + catalytic subunit by insulin. The results provide evidence that insulin contributes to the maintenance of Na+,K+-ATPase in neuronal tissue.

Animals↗
PubMed1987

Effect of phorbol and glucose on insulin secretion from the human fetal pancreas.

It has been reported previously that 12-0-tetradecanoylphorbol-13-acetate is capable of stimulating the release of insulin from adult and neonatal pancreatic tissue. The data from this study show that this agent at a concentration of 1.3 uM, in the presence of 2.8 mM glucose, was unable to cause significant secretion of insulin from cultured human fetal pancreatic explants. By contrast 20 mM glucose was able to cause a small but significant immediate increase in secretion of insulin, but was unable to maintain this response beyond ten minutes. When the two agents were combined, a synergistic effect was seen throughout the entire 50 minute period of stimulation. The reason for this synergism is unclear since, whilst both secretagogues were able to cause a rise in the levels of diacylglycerol, together no extra effect was observed.

Arachidonic Acid↗
PubMed1987

Plasma prolactin concentrations in non-pregnant mares at different times of the year and in relation to events in the cycle.

Plasma prolactin concentrations were measured in mares using an homologous radioimmunoassay. An annual rhythm in plasma prolactin was found, with concentrations higher during the summer than during the winter. In addition to this seasonal pattern, occasional high concentrations of prolactin were seen when concentrations were otherwise basal. Blood samples taken from mares during an oestrous cycle in October-November showed that prolactin values were basal for most of the cycle, with a marked rise in prolactin shortly before the onset of oestrus. This prolactin peak was associated with an increase in the size of the largest follicle, and with a peak of PGFM in some mares, but did not appear to be related to the LH surge. In oestrous cycles in March and May-June, there was a wide variation in the baseline of prolactin secretion, in accordance with the seasonal pattern already mentioned. However, the peak of prolactin seen around oestrus in October-November was less obvious in March and May-June. Post-partum mares showed a high but irregular profile of prolactin concentrations with no clear-cut pattern in relation to the oestrous cycle.

Animals↗
PubMed1986

Time-course of altered islet phospholipids and of calcium binding and ionophoretic properties of islet lipids following glucose stimulation.

The time-course of alteration in islet cell phospholipid content following D-glucose exposure in islet cells and in islet cell membranes was related to the ability of lipids extracted from both cultured pancreatic islet cells and from plasma membranes isolated from the islet cells to translocate calcium in two model membrane systems. The first model system (bulk-phase system) detected lipid species with the ability to bind calcium, irrespective of their ability to enhance calcium transport across cell membranes. The second system (multilamellar membrane system) detected lipid species with the ability to both bind calcium and to enhance calcium transport across cell membranes (true ionophores). Pre-exposure to high D-glucose concentration led to a rapid (within 1 min) fall in membrane phosphoinositides. This was partially blocked by mannoheptulose. A concurrent fall in calcium binding activity of lipids from the plasma membrane was observed. In the whole islet cell fraction, D-glucose induced a marked increase in Ca2+ ionophoretic activity. Unlike the fall in membrane polyphosphoinositides and membrane Ca2+ binding activity, these changes were dependent on the presence of added extracellular calcium. L-Glucose was without effect on membrane phosphoinositide content. It is concluded that altered membrane and intracellular phospholipids may contribute to the increased availability of intracellular Ca2+ following D-glucose stimulation by virtue of their Ca2+ binding and ionophoretic properties.

Animals↗
PubMed1985

Presence of membrane-associated phosphatidate phosphohydrolase activity in cultured islets and its stimulation by glucose.

The cellular location at which exogenous phosphatidic acid is hydrolysed in cultured neonatal rat islets was examined. Phosphatidate phosphohydrolase activity could be demonstrated in both whole cell sonicates and isolated plasma membranes. In the whole cell fraction phosphatidic acid hydrolysis to diacylglycerol was stimulated 43% by the presence of Mg2+. The activity present in isolated membranes was totally dependent on the presence of Mg2+ and was increased in plasma membranes from glucose-stimulated islets. Following exposure of islets to low glucose concentrations, raising the Ca2+ concentration from 150 nM to 40 microM in the presence of Mg2+ did not affect the formation of diacylglycerol in whole cell fractions or plasma membranes. These results indicate the presence within the islet of membrane-bound phosphatidate phosphohydrolase activity and demonstrate its activation by glucose.

Animals↗
PubMed1984

Effects of inhibitors of eicosanoid synthesis on insulin release by neonatal pancreatic islets.

In the pancreatic islet, eicosanoids may arise from both cyclooxygenase- and lipoxygenase-dependent metabolism of arachidonic acid. The inclusion of inhibitors of selective steps in these pathways indicated that in cultured neonatal rat islets, arachidonic acid may be metabolised through both pathways, concurrent with insulin release stimulated by D-glucose, D-glyceraldehyde and 2-ketoisocaproate. The effects of the inhibitors suggested that the products of the lipoxygenase pathway were necessary for the stimulatory effects of nutrients to be observed. In contrast to glucose, where insulin release was stimulated in the presence of inhibitors of cyclooxygenase, the stimulatory action of D-glyceraldehyde, 2-ketoisocaproate and melittin was only minimally affected by these inhibitors, although it was inhibited by lipoxygenase inhibition. These findings support a major stimulatory role for products of the lipoxygenase pathway of arachidonic acid metabolism in nutrient-induced secretion, and a negative or modulatory role of cyclooxygenase pathway products on glucose-stimulated insulin release in the neonatal islet.

Animals↗
PubMed1984

Trans activation of plasmid-borne promoters by adenovirus and several herpes group viruses.

This paper describes experiments to test the ability of a number of viruses of the Herpes group, and also Adenovirus-2 and SV40, to activate transcription from the Herpes simplex virus-1 glycoprotein D and the rabbit beta-globin promoters. Plasmids containing these genes were transfected into HeLa cells which were then infected with various viruses. Transcriptional activation in trans of the plasmid-borne promoters was monitored by quantitative S1 nuclease analysis of total cytoplasmic RNA isolated after infection. The results showed that Herpes simplex viruses 1 and 2, Pseudorabies virus, Variella Zoster virus, Human Cytomegalovirus, Equine herpes virus-1 and Adenovirus-2 activate transcription from both promoters tested. In contrast, SV40 did not activate transcription in trans in this assay. The possible mechanisms of this activation are discussed.

Adenoviruses, Human↗
PubMed1984

Lipid associated calcium ionophores in islet cell plasma membrane following glucose stimulation.

The effect of glucose exposure on lipid associated calcium ionophoretic activity was measured in cultured neonatal rat pancreatic islet cells using two model systems. The first measured the ability of a lipid extract of islet cells to facilitate calcium transfer from an aqueous to organic phase and thus detected lipids which transfer calcium in the manner of authentic ionophores or which chelate the ion. In this system glucose stimulation was followed by an increase in total cell ionophoretic activity and a decrease in the activity associated with the plasma membrane. The second system measured the transfer of calcium across an artificial phospholipid membrane and detected authentic ionophoretic activity. In this model an increase in total ionophoretic activity was again seen following glucose but there was no change in the ionophoretic activity of a plasma membrane extract. The results indicate that the lipid modifications which accompany glucose-induced insulin release may alter cellular calcium stores by decreasing lipid bound calcium at the plasma membrane and increasing the capacity for calcium ionophoresis at intracellular sites.

Animals↗
PubMed1982

The effect of maternal caffeine ingestion on pancreatic function in the neonatal rat.

Pancreatic function was investigated in neonatal suckling offspring of caffeine-ingesting dams, with or without maternal sucrose supplementation, throughout pregnancy and lactation. In offspring of rats ingesting caffeine without sucrose supplementation, there was initial hyperinsulinaemia, followed by a progressive fall of plasma insulin to subnormal levels. This fall in plasma insulin coincided with depletion of pancreatic insulin stores. Both the fall in plasma insulin and depletion of pancreatic insulin stores were prevented by sucrose supplementation of caffeine-ingesting dams. Offspring of dams fed sucrose alone and control offspring also maintained pancreatic insulin stores and circulating insulin levels over the first 14 days of postnatal life. Pancreases from offspring of caffeine-exposed animals tested in vitro showed enhanced sensitivity of the insulin release process to glucose. This was reflected in the glucose concentration required to elicit half-maximal insulin release (2.4 +/- 0.2 mmol/l for caffeine offspring, 2.3 +/- 0.2 mmol/l for caffeine with sucrose, 3.8 +/- 0.3 mmol/l for sucrose and 4.1 +/- 0.3 mmol/l for control offspring, mean +/- SEM). In contrast, offspring of sucrose-supplemented (with or without caffeine) dams showed increased sensitivity of the proinsulin biosynthetic process to glucose, whereas offspring of dams ingesting caffeine alone showed no significant enhancement of the biosynthetic process compared with control offspring. Thus enhanced sensitivity of the insulin secretory process to glucose without a change in the sensitivity of the biosynthetic process in the offspring of the caffeine ingesting (non-sucrose supplemented) dams could explain the progressive depletion of pancreatic insulin stores and eventual hypoinsulinaemia seen in this group.

Animals↗