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Biomedical subjects

J W Phillis

Publications and source records attributed to J W Phillis.

At least 37 records · Page 2Linked to original sources
PubMed2000

Mechanisms involved in coronary artery dilatation during respiratory acidosis in the isolated perfused rat heart.

A rat Langendorff heart preparation, perfused at constant pressure, was used to evaluate the role of K(ATP) channels in respiratory acidosis-induced coronary hyperemia. Prior administration of glibenclamide, an inhibitor of K(ATP) channels, reduced basal flow rates and eliminated the hyperemia associated with hypercapnia. These results implicate K(ATP) channels as a functional link in the respiratory acidosis-induced increase in coronary flow.

Acidosis, Respiratory↗
PubMed2000

Topical insulin and accumulation of excitotoxic and other amino acids in ischemic rat cerebral cortex.

Insulin plays a neuroprotectant role in the brain and spinal cord during ischemia. However, studies have shown insulin to increase the sensitivity of cultured cortical cells to glutamate toxicity. The present study looked at the relationship between topically administered insulin (1 mIU insulin/ml and 100 mIU insulin/ml) during a four-vessel model of global ischemia and the accumulation of amino acids, especially glutamate, from the ischemic rat cerebral cortex. The lower dose of insulin was found to attenuate the release of excitotoxic and other amino acids from the cortex in ischemia/reperfusion. This may occur because insulin increases glucose availability to glial cells resulting in maintenance of glycolysis and ionic pumps that can reduce glutamate release and maintain uptake during ischemia/reperfusion. The higher dose of insulin, which significantly increased the amount of aspartate, glutamate, taurine, and GABA during reperfusion, may act to stimulate the amount of glycogen stored in astrocytes, reducing the availability of glucose for metabolic purposes.

Administration, Topical↗
PubMed2000

Topical glucose and accumulation of excitotoxic and other amino acids in ischemic cerebral cortex.

Pre-ischemic hyperglycemia aggravates brain damage due to transient global ischemia as demonstrated by exacerbation of brain lesions. Lactacidosis and elevated glutamate levels have been implicated as mechanisms of the increased damage. Our objective was to determine the effects of different levels of glucose (0, 66.5, 450 mg/dL) in cortical superfusates on the ischemia/reperfusion-evoked release of amino acids from the rat cerebral cortex. Physiologic levels of glucose significantly reduced the amount of aspartate, glutamate and gamma-aminobutyric acid and the supra-physiologic levels of glucose reduced the amount of aspartate and phosphoethanolamine released from the cortex during ischemia/reperfusion in comparison with no glucose. The decrease in glutamate release may be due to increased availability of glucose for glycolysis with the subsequent formation of ATP and lactate, which has been shown to act as an energy source for neurons. The decreased levels may also reflect the continued energy-dependent uptake of glutamate by glial cells.

Animals↗
PubMed1999

Melittin enhances amino acid and free fatty acid release from the in vivo cerebral cortex.

The effects of the phospholipase activator melittin on amino acid and free fatty acid release from the rat cerebral cortex were monitored and compared with those of a secretory PLA(2), using a cortical cup technique with topical application of these agents in artificial cerebrospinal fluid. Melittin (10 microg/ml; 3.5 microM) elicited a rapid increase in the levels of superfusate amino acids; aspartate, glutamate, GABA, glycine, taurine, glutamine, phosphoethanolamine, alanine, serine and the free fatty acids arachidonic, linoleic, palmitic and oleic acid. PLA(2) (25 microg/ml) also enhanced amino acid efflux but its effects were significantly slower to develop than those of melittin. The results confirm previous indications of an ability of phospholipases to augment extracellular levels of several amino acids, including the excitotoxins glutamate and aspartate, and further implicate phospholipase activation as a significant contributor to cerebral ischemic injury. Melittin has the potential to be a useful tool with which to evaluate the role of phospholipases in ischemia injury.

Amino Acids↗
PubMed1999

Hyposmotically induced amino acid release from the rat cerebral cortex: role of phospholipases and protein kinases.

In an evaluation of the contribution of swelling-induced amino acid release, through the regulatory volume decrease (RVD) process, to cerebral ischemic injury, studies of the role of phospholipases and protein kinases in the response to hyposmotic stress were undertaken using an in vivo rat cortical cup model. Hyposmotic stress induced significant releases of aspartate, glutamate, glycine, phosphoethanolamine, taurine and GABA from the rat cerebral cortex. Taurine release was most affected, exhibiting a greater than 9-fold increase during the hyposmotic stimulus. The phospholipase A2 (PLA2) inhibitors 4-bromophenacyl bromide (1 microM) and 7,7-dimethyleicosadienoic acid (5 microM) had no significant effects on hyposmotically induced amino acid release. AACOCF3 (50 microM), an inhibitor of cytosolic PLA2 decreased taurine release to 84% of DMSO controls. The release of the other amino acids was not affected. The phospholipase C inhibitor U73122 (5 microM) had no significant effects on amino acid release. The protein kinase C (PKC) inhibitor chelerythrine (5 microM) significantly reduced hyposmotically induced taurine release to 72% of saline controls but had no significant effects on the other amino acids. Stimulation of PKC with phorbol 12-myristate, 13-acetate (10 microM) did not significantly change taurine, glutamate, glycine or phosphethanolamine release. The releases of aspartate and GABA were enhanced 2 to 3 fold. Phorbol 12,13-didecanoate (10 microM), another potent stimulator of PKC, significantly increased taurine release to 122% of DMSO controls. The releases of aspartate, glutamate and glycine were enhanced 2.5 to 3.5 fold. Similarly, stimulation of protein kinase A with forskolin (100 microM) significantly increased taurine, aspartate, and glycine release 1.5- to 2-fold compared to DMSO controls. In summary, phospholipases may play a minor role in volume regulation. These studies also support the hypothesis that protein kinases play a modulatory role in the RVD response. The results show that although RVD may play a role, additional mechanisms, including phospholipase activation, must be involved in the ischemia-evoked release of excitotoxic amino acids.

Acetophenones↗
PubMed1999

Failure of kynurenic acid to inhibit amino acid release from the ischemic rat cerebral cortex.

The ability of a broad spectrum glutamatergic receptor antagonist, kynurenic acid (1 and 5 mM) to attenuate release of excitant and other amino acids from the ischemic cerebral cortex was examined in a four vessel occlusion rat model. Kynurenic acid, administered topically onto the cortex in artificial cerebrospinal fluid using bilateral cortical cups, failed to attenuate ischemia-evoked release of aspartate, glutamate, phosphoethanolamine, taurine and at 1 (but not 5) mM it depressed GABA release. There was no effect on basal, pre-ischemic, release. This result suggests that ischemia-evoked amino acid release is not a significant consequence of the activation of ionotropic receptors by synaptically released glutamate.

Amino Acids↗
PubMed1999

Lactate reduces amino acid release and fuels recovery of function in the ischemic brain.

Pre-ischemic hyperglycemia exacerbates the neuronal injury associated with cerebral ischemia/reperfusion, possibly because the accumulation of glycolytically derived lactate causes acidosis. Twenty minutes of four vessel occlusion caused significant increases in rat cerebral cortical superfusate levels of aspartate, glutamate, phosphoethanolamine, taurine, and GABA. Lactate (20 mM), applied topically, significantly reduced the ischemia/reperfusion evoked releases of glutamate and GABA, with further reductions following the higher dose (40 mM), which also significantly reduced the evoked release(s) of the other amino acids. EEG recovery was robust in the presence of 40 mM lactate. These results substantiate the ability of lactate to serve as a neuronal energy substrate but question the role of lactate accumulation in the enhancement of ischemic brain injury by hyperglycemia.

Amino Acids↗
PubMed1999

5-(N-Ethyl-N-isopropyl)-amiloride, an Na(+)-H(+) exchange inhibitor, protects gerbil hippocampal neurons from ischemic injury.

The effect of the selective Na(+)/H(+) antiporter inhibitor 5-(N-ethyl-N-isopropyl)-amiloride (EIPA) on cerebral ischemia/reperfusion injury was evaluated in the Mongolian gerbil. Ischemia was induced in unanaesthetized gerbils by a 5-min period of bilateral common carotid artery occlusion followed by reperfusion for 6 days. Two groups of gerbils were injected intraperitoneally with either dimethyl sulfoxide (DMSO; 10 microl) or EIPA (5 mg/kg in 10 microl DMSO) 30 min prior to ischemia. The increase in locomotor activity in the EIPA-treated group was significantly less than that of the control group at both 24 h and 6-day post-ischemia. The extent of CA1 pyramidal neuron loss was significantly reduced in the EIPA-treated group in comparison with that of DMSO treated controls. These results suggest that EIPA can protect cerebral neurons from ischemia/reperfusion injury and implicates acidosis and Na(+)/H(+) exchange as a causative factor in such injury.

Amiloride↗
PubMed1999

Effect of hyperglycemia on extracellular levels of amino acids and free fatty acids in the ischemic/reperfused rat cerebral cortex.

This study analyzed the effects of pre-existing hyperglycemia on the extracellular levels of glutamate, other amino acids and free fatty acids, including arachidonic acid, in the ischemic/reperfused rat cerebral cortex, using a cortical cup technique. Forebrain cerebral ischemia (20 min) was induced by four vessel occlusion. Glucose (3.4 g/kg) was administered 30 min prior to ischemia. Glucose administration had no effect on basal levels of superfusate amino acids and reduced basal levels of linoleic and oleic acids. Cerebral ischemia elicited increased superfusate levels of aspartate, glutamate, phosphoethanolamine, taurine, gamma-aminobutyric acid (GABA) and arachidonic acid when compared with basal levels. Reperfusion caused a further increase in phosphoethanolamine and arachidonic acid levels and transient increases in linoleic, oleic and palmitic acids. Hyperglycemia resulted in significantly reduced levels of glutamate, phosphoethanolamine, GABA and arachidonic, myristic, palmitic, linoleic and oleic acids during ischemia/reperfusion in comparison with the saline-injected ischemic controls. The results indicate that ischemia/reperfusion-evoked increases in the extracellular levels of glutamate, certain other amino acids and free fatty acids are attenuated by prior systemic glucose administration.

Amino Acids↗
PubMed1999

Effects of hyperosmolarity and ion substitutions on amino acid efflux from the ischemic rat cerebral cortex.

The contributions of sodium and chloride ions and of osmotic stresses to the ischemia-evoked efflux of excitotoxic and other amino acids were explored using a rat four vessel occlusion model. Replacement of Na+ with choline or N-methyl-D-glucamine (NMDG) and of Cl- with sulfate or gluconate was used to evaluate the contribution that these ions make to amino acid efflux. The contribution of ischemia-evoked swelling to amino acid release was studied by applying mannitol or sucrose to minimize the cell volume increases and the compensatory regulatory volume decrease evoked efflux of amino acids. Aliquots of artificial cerebrospinal fluid (aCSF), appropriately adjusted for ion replacement or 150 mM mannitol or sucrose, were pipetted into cortical cups 35 min prior to ischemia and perfusate samples were obtained prior to, during and following ischemia (20 min) and reperfusion (40 min). Replacement of Na+ by NMDG depressed basal (normoxic) efflux of most amino acids, with choline substitution having little effect. During ischemia NMDG substitution increased glutamate and GABA efflux and choline enhanced the release of aspartate, glutamate, GABA and taurine. A reduction in extracellular Na+ would facilitate reversal of Na+-dependent transporters with extrusion of amino acids. Another possible explanation for the elevated release is that the absence of Na+ would inhibit the Ca2+/Na+ counter transport system, with a deleterious accumulation of intracellular Ca2+. Chloride replacement with sulfate or gluconate enhanced the efflux of aspartate, glutamate, GABA and taurine during ischemia. Removal of Cl- would depolarize cells, and block the Cl--dependent action of inhibitory amino acid transmitters, with both actions enhancing the ischemic injury and, consequently, amino acid release. Exposure to hyperosmotic mannitol (150 mM) aCSF enhanced ischemia-evoked release of some amino acids (taurine, GABA) and decreased that of aspartate and phosphoethanolamine. Sucrose aCSF enhanced the ischemia-evoked release of most amino acids. A potential explanation for these observations is that both agents may be able to rapidly penetrate the plasma membranes of ischemic neurons, actually contributing to the release of other osmolytes. The unanticipated nature of many of the observations made during these manipulations of the aCSF serves to accentuate the complex nature of the mechanisms responsible for the ischemia-evoked amino acid efflux into the interstitial spaces.

Animals↗
PubMed1999

Role of nitric oxide in rat coronary flow regulation during respiratory and metabolic acidosis.

A rat Langendorff heart preparation, perfused at constant pressure, was used to evaluate the role of nitric oxide in the increases in coronary vessel flow during hypercapnic and metabolic acidosis. Prior administration of the nitric oxide synthase (NOS) inhibitor N6-nitro-L-arginine methyl ester (100 microM) significantly reduced the basal, resting, rate of coronary flow but did not attenuate the increases in flow during brief (2-min) periods of perfusion with acidotic solutions. These results suggest that nitric oxide is not a significant contributor to rat heart coronary flow regulation during respiratory or metabolic acidosis.

Acidosis↗
PubMed1999

Further evidence for the role of adenosine in hypercapnia/acidosis-evoked coronary flow regulation.

Experiments were performed on isolated, nonworking rat hearts perfused at constant pressure according to the Langendorff technique to evaluate the role of adenosine in hypercapnia-evoked coronary vasodilation. Hypercapnia/acidosis resulted in increases in heart rate and coronary flow rates in conjunction with a decrease in ventricular contractile tensions. The adenosine deaminase inhibitor erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA, 10 microM) reduced the heart rate and enhanced CO2-evoked increases in coronary vascular flow. 5-Iodotubercidin (1 microM), an inhibitor of adenosine kinase, caused a reduction in heart rate and enhanced coronary flow rates during hypercapnic perfusion. Adenosine deaminase (1 U/ml) significantly attenuated CO2-evoked increases in coronary vascular flow. These results extend those of previous investigations implicating adenosine in the regulation of coronary flow during conditions of respiratory or metabolic acidosis.

Acidosis↗
PubMed1999

Effects of anion channel blockers on hyposmotically induced amino acid release from the in vivo rat cerebral cortex.

A cortical cup model with continuous perfusion of artificial cerebrospinal fluid (containing 134 mM NaCl) was used to investigate the effects of anion channel blockers on the hyposmotically-induced release of amino acids from the in vivo rat cerebral cortex. The hyposmotic stimulus (25 mM NaCl) evoked a release of taurine, glutamate, aspartate, glycine, phosphoethanolamine and GABA. Topically applied anion channel blockers 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (1 mM); 4-acetamido-4'-isothiocyanatostilbene-2,2-disulfonic acid (2 mM); 5-nitro-2-(3-phenylpropylamino) benzoic acid (350 microM); niflumic acid (500 microM); tamoxifen (20 microM) and arachidonic acid (0.5 microM) all significantly reduced the hyposmotically-induced release of taurine. The releases of glutamate, aspartate, glycine, phosphoethanolamine and GABA were variably susceptible to inhibition by these compounds. These results demonstrate that osmoregulatory processes in cortical cells, in vivo, involve amino acids, with taurine playing a dominant role. The efflux of taurine and, to a lesser extent, the other amino acids may be mediated by anion channels.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1999

Activation of cPLA2, PKC, and ERKs in the rat cerebral cortex during ischemia/reperfusion.

Release of the excitotoxic amino acid, glutamate, into the extracellular space during ischemia/reperfusion contributes to neuronal injury and death. To gain insights into the signal transduction pathways involved in glutamate release we examined the time course of changes in enzyme levels and activities of cPLA2, PKC and ERKs in the rat cerebral cortex after four vessel (4VO) ischemia followed by reperfusion. Measurement both by enzymatic assay and Western blot analysis showed significant increases in the activity and protein levels of cPLA2 during 10-20 min of ischemia. Activity remained elevated at 10 min and 20 min of reperfusion, whereas cPLA levels had returned to base line levels after 20 min of reperfusion. PKC activity increased significantly in the particulate, but not in the cytosolic, fractions both during ischemia and reperfusion. Increases in PKCgamma levels were recorded in the particulate fraction during ischemia and reperfusion, and in the cytosolic fraction during ischemia. Western blot analysis with a phosphospecific antibody for characterization of MAPK (ERKs) activation revealed significantly increased phosphorylation of ERK1 and ERK2 in the particulate fraction, of ERK2 in the cytosolic fraction, during ischemia and of both enzymes in the particulate and cytosolic fractions after 10 min of reperfusion. The relevance of the results to glutamate release is discussed.

Animals↗
PubMed1998

5-(N-Ethyl-N-isopropyl)-amiloride inhibits amino acid release from the ischemic rat cerebral cortex: role of Na+-H+ exchange.

The effect of the selective Na+/H+ antiporter inhibitor 5-(N-ethyl-N-isopropyl)-amiloride (EIPA) on amino acid release from the ischemic/reperfused rat cerebral cortex was investigated using a cortical cup technique. EIPA (25 microM in artificial cerebrospinal fluid), applied topically, inhibited the ischemia-reperfusion evoked efflux of aspartate, glutamate, gamma-aminobutyric acid, taurine and phosphoethanolamine. Reductions in the ischemia-evoked releases of these amino acids suggest that ischemia precipitates acidosis, Na+/H+ exchange and cell swelling with amino acid release as the cells mount a regulatory volume decrease response. EIPA, by blocking Na+/H+ exchange, would reduce cell swelling and the resulting amino acid release.

Amiloride↗
PubMed1998

Inhibitory action of CGS 21680 on cerebral cortical neurons is antagonized by bicuculline and picrotoxin-is GABA involved?

The possibility of an involvement of endogenously released GABA in the inhibitory actions of A1 and A2a adenosine receptor agonists on rat cerebral cortical neurons discharges was examined using the GABAA antagonists bicuculline and picrotoxin. The A1 agonist N6-cyclopentyladenosine (CPA), the A2a agonist CGS 21680 and the non-selective receptor agonist, adenosine, depressed neuronal firing. Applications of bicuculline or picrotoxin enhanced the spontaneous firing rate of cortical neurons, indicating the presence of ongoing GABA-ergic inhibition. Antagonism of GABAA receptors blocked the depressant effects of CGS 21680 on neuronal firing; was without effect on CPA-evoked inhibitions and tended to reduce the duration of adenosine-evoked inhibitions. These results suggest that the depressant effects of A2a receptor activation are due to an increase in GABA-ergic inhibition, likely as a consequence of increased GABA release. GABA does not appear to be involved in adenosine A1 receptor-mediated inhibition of neuronal firing.

Action Potentials↗
PubMed1998

The role of adenosine in rat coronary flow regulation during respiratory and metabolic acidosis.

The role of adenosine in rat coronary flow regulation during acidosis was evaluated in isolated, perfused, Langendorff rat heart preparations exposed to brief periods of hypercapnic or metabolic acidosis. Acidosis resulted in increases in coronary flow rate, in conjunction with decreases in ventricular contractile tensions. Heart rates were non-significantly increased. Two non-selective adenosine antagonists, caffeine and 8-phenyltheophylline, markedly attenuated the increases in coronary flow during hypercapnic acidosis without affecting the decline in contractile tension or the heart rate. ZM 241385 (4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a]triazin-5-ylami no]ethyl)phenol), a selective adenosine A2A receptor antagonist, also blocked hypercapnic acidosis-evoked coronary flow rate increases. The adenosine A1 selective antagonist, 8-cyclopentyl-1,3-dipropylxanthine, did not affect flow rate increases during hypercapnic acidosis. SCH 58261 (5-amino-7-(2-phenyl ethyl)-2-(2-furyl)pyrazolo-[4,3-e]-1,2,4-triazolo[1,5-c] pyrimidine, a selective adenosine A2A receptor antagonist, blocked the increases in coronary flow rate evoked by metabolic acidosis. An adenosine transport inhibitor, dipyridamole, doubled coronary flow rates during hypercapnic acidosis. When taken in conjunction with previous reports that acidosis enhances adenosine release from cardiac preparations, these results suggest that adenosine is a significant contributor to acidosis-evoked increases in coronary flow.

Acidosis↗