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J T Fleming

Publications and source records attributed to J T Fleming.

At least 37 records · Page 2Linked to original sources
PubMed1996

Alterations in skeletal muscle microcirculation of head-down tilted rats.

The head-down tilted whole body suspended (HDT/WBS) rat is recognized as a model that reproduces many of the responses seen during exposure to microgravity including an increase in systemic blood pressure. Functional alterations of microscopic blood vessels (arterioles) in skeletal muscle (cremaster muscle) were assessed for their role in the observed elevations of blood pressure associated with HDT/WBS. Arteriolar baseline diameters, vasoconstrictor responses to norepinephrine and vasodilation to nitroprusside were assessed in control rats, rats suspended for 7 or 14 days, and rats allowed to recover for 1 day after 7 days of HDT/WBS. Using in vivo videomicroscopy, neither baseline diameters nor ability to dilate were altered by HDT/WBS. Maximum vasoconstriction to norepinephrine was significantly greater in arterioles of hypertensive 14 day HDT/WBS rats. This study of the intact microvasculature of skeletal muscle reveals an elevated contractility of arterioles to norepinephrine in suspended rats, and suggests that an elevated peripheral resistance in skeletal muscle may contribute to the increase in blood pressures among animals subjected to HDT/WBS.

Animals↗
PubMed1995

A novel ferritin heavy chain messenger ribonucleic acid in the human brain.

In the aging human brain, the concentrations of iron and its major storage protein, ferritin, rise but the distribution of metal and protein remains non-uniform. More ferritin could be isolated from the brains of humans who died of Alzheimer's disease (AD) than from age- and sex-matched controls. Also, brain ferritin of rats chronically exposed to aluminum chloride in their drinking water contained more aluminum and iron. Based on these earlier observations, a more detailed study of human brain ferritin was initiated. The results showed that ferritin is a component of neuritic (senile) plaques in AD. Ferritin obtained from normal or AD brains is composed of 24 subunits (70% heavy (H) chain; 30% light (L) chain). With high performance liquid chromatography, the subunits resolved into a cluster of four H-chain peaks and one major L-chain peak. Western blot analysis confirmed the identity of H- and L-fractions. The techniques of molecular biology revealed the presence of an additional ferritin messenger ribonucleic acid (mRNA) species for the H subunit which was more abundant in the brain than in other human tissues. It contained the entire sequence of 919 nucleotides of H chain mRNA from liver but also an additional segment of 279 nucleotides in the 3'-untranslated region. The two mRNA seemed to arise by the use of an alternate polyadenylation site of the same primary transcript. Ribonuclease protection assays revealed that the concentrations of the longer mRNA in the normal hippocampus and the hippocampus of patients with AD brains were similar.

Aluminum↗
PubMed1995

Molecular cloning and functional co-expression of a Caenorhabditis elegans nicotinic acetylcholine receptor subunit (acr-2).

A number of putative nicotinic acetylcholine receptor subunit clones were isolated by screening a lambda library of Caenorhabditis elegans genomic DNA with a probe derived from the Drosophila melanogaster ard gene (a non-alpha nicotinic acetylcholine receptor subunit clone). Studies on one of these loci, acr-2, are described; acr-2 is located between sup-7 and unc-6 on the X chromosome. A full-length cDNA was isolated and sequenced. The cDNA encodes a putative non-alpha subunit of a nicotinic acetylcholine receptor that shows many of the conserved features of vertebrate and invertebrate non-alpha nicotinic acetylcholine receptor subunits. To investigate the functional expression of the subunit, the corresponding cRNA was produced, in vitro, and micro-injected into Xenopus oocytes. When expressed alone acr-2 shows no levamisole-gated channel activity. When co-expressed with a C. elegans alpha subunit (unc-38), which is itself unable to form functional homo-oligomers, acr-2 contributed to the formation of a functional channel. This is the first functional expression of a nematode nicotinic acetylcholine receptor and supports the interpretation that the differentiation between alpha and non-alpha subunits dates back to the earliest stages of the evolution of the metazoa.

Amino Acid Sequence↗
PubMed1994

Reduced renal microvascular reactivity to angiotensin II in diabetic rats.

OBJECTIVE: Renal hyperfiltration in early diabetes is often correlated with increased renal blood flow, reflecting dilation of resistance arterioles. This loss of arteriolar tone has been associated with an impaired reactivity to angiotensin II (Ang II). This study determined if outer cortical arterioles (preglomerular and/or postglomerular) of diabetic rats exhibit a diminished reactivity to Ang II and established if renal vascular prostaglandins account for the diminished responsiveness. METHODS: The constriction of renal microvessels to Ang II (applied to the kidney bath) was quantitated in hydronephrotic kidneys of diabetic rats (7-10 days after streptozotocin treatment) and nondiabetic rats by in vivo videomicroscopy. RESULTS: Interlobular, afferent, and efferent arterioles of diabetic rats were found to be less reactive to Ang II than arterioles of nondiabetic rats. Indomethacin, added to the bath to inhibit renal vascular prostaglandin synthesis, enhanced the interlobular and efferent arteriolar reactivity to the peptide among diabetic rats. Yet, after indomethacin treatment, the afferent and efferent arterioles of diabetic rats were still less reactive than control arterioles to Ang II. CONCLUSIONS: We conclude that the blunted reactivity of afferent and efferent arterioles to Ang II among diabetic hydronephotic kidneys cannot be fully explained by the influence of renal vascular-derived dilator prostaglandins.

Angiotensin II↗
PubMed1993

Acetylcholine receptor molecules of the nematode Caenorhabditis elegans.

Receptors for acetylcholine are present in nematodes. Studies using physiological and biochemical methods have revealed the existence of nicotinic acetylcholine receptors with a novel pharmacology. Caenorhabditis elegans provides a particularly suitable organism with which to investigate such receptors using molecular genetic approaches. Mutants resistant to the cholinergic agonist (and anthelmintic drug) levamisole have permitted the isolation of a number of genes, including structural subunits of the nicotinic acetylcholine receptor. The only known viable mutants of nicotinic receptors are those of Caenorhabditis elegans. This organism offers the prospect of studying the developmental and regulatory effects of the loss of a single component of the receptor. Using Caenorhabditis elegans it is possible to select interesting phenotypic mutations by in vivo mutagenesis before determining the causative lesion. Resistance genes other than those encoding structural subunits are of particular interest, as they will encode additional polypeptides closely associated with nicotinic receptor function. Such proteins are often difficult or impossible to identify using conventional biochemical approaches, whereas genetic selection should permit their identification.

Amino Acid Sequence↗
PubMed1993

Juxtamedullary afferent and efferent arterioles constrict to renal nerve stimulation.

Sympathetic neural control of afferent and efferent arterioles of inner cortical (juxtamedullary) glomeruli has not been established, in part, because of difficulty accessing these vessels, normally located deep below the kidney surface. In this study we utilized the rat hydronephrotic kidney model to visualize the renal microcirculation and to quantitate the responses of juxtamedullary arterioles to brief (30 sec) renal nerve stimulated (RNS). Juxtamedullary afferent and efferent arterioles constricted in a frequency-dependent fashion to RNS, achieving a maximal constriction of 35% to 8 Hz stimulation. In these same kidneys, outer cortical afferent arterioles also constricted to RNS but outer cortical efferent arterioles did not. Microinjection of norepinephrine (NE) around single outer cortical efferent arterioles (to avoid the constriction of preglomerular vessels) constricted the efferent arterioles. However, the afferent arterioles of the same glomeruli were considerably more responsive to microinjected NE. Thus, the lack of constriction of outer cortical efferent arterioles to RNS may relate, in part, to their low sensitivity to NE, the primary neurotransmitter. These direct observations indicate that the juxtamedullary efferent arterioles are responsive to renal nerve stimulation whereas the outer cortical efferent vessels are not. These results, which should be cautiously extrapolated to normal filtering kidneys, indicate that glomerular hemodynamic changes evoked by the sympathetic nervous system are different for outer cortical and inner cortical glomeruli.

Animals↗
PubMed1993

Cadmium-induced arteriolar constriction in skeletal muscle microcirculation.

Cadmium, an environmental pollutant, is known to induce hypertension in animal models, in part via an increase in peripheral vascular resistance. Since prior studies have investigated the vascular effects of cadmium using large, nonresistance arteries, we directly assessed cadmium's action on resistance size arterioles in skeletal muscle using the intact rat cremaster muscle preparation. Cadmium evoked a concentration-dependent constriction of the large arterioles (120 to 50 microns in diameter) but elicited no change in the diameter of smaller arterioles (30 to 15 microns). Blockade of alpha-adrenergic receptors did not diminish the constrictor response of the larger arterioles to cadmium, but bathing the cremaster muscles with a solution containing low calcium attenuated the arteriolar constriction to cadmium. Calcium repletion caused the arterioles to constrict further. These observations provide the first direct evidence that cadmium constricts resistance arterioles in skeletal muscle. The cadmium constriction: (1) is selective for the large arterioles, (2) is not mediated by alpha-adrenergic receptors, and (3) is influenced by the extracellular level of calcium. We conclude that arteriolar constriction in skeletal muscle tissue may play a role in the hypertensive actions of cadmium.

Animals↗
PubMed1992

Skeletal muscle arteriolar constriction to ANG II: evaluation of a myogenic component.

This study addressed the hypothesis that an increase in blood pressure contributes to the overall constrictive response of skeletal muscle arterioles to angiotensin II (ANG II). Diameters of second-order arterioles (2A) and third-order arterioles (3A) in the rat cremaster muscle were quantitated after intravenous administration of ANG II. Hindquarter blood pressure was either allowed to increase or was maintained at normal levels. Constriction of 3A to bolus injection of ANG II was the same whether hindquarter pressure increased or not. However, the total vascular constrictive response of the cremaster muscle (based on 2A blood flow) and of the entire hindquarter (based on iliac arterial blood flow) to bolus ANG II was greater when hindquarter pressure was held constant. During slow infusion of ANG II, 3A constriction was unaffected by an abrupt decrease or increase in hindquarter pressure. However, an abrupt reduction of hindquarter pressure caused a significant decline in hindquarter vascular resistance. Thus an increase in blood pressure, whether rapid or gradual, does not influence 3A constriction to ANG II. However, in the entire hindquarter, a rapid rise in blood pressure opposes constriction to ANG II, whereas a gradual pressure rise evokes a mechanism that enhances constrictive response to the peptide.

Angiotensin II↗
PubMed1992

Selective preglomerular constriction to nerve stimulation in rat hydronephrotic kidneys.

The purpose of this study was to quantitate the constrictor responses of pre- vs. postglomerular microvessels during brief increases in renal nerve activity by directly observing the intact microcirculation. To validate the use of the rat hydronephrotic kidney model for this purpose, the vascular reactivity of hydronephrotic kidneys, and normal kidneys was assessed to adrenergic and neural stimulation by quantitating whole kidney blood flow velocity changes elicited by intravenous administration of norepinephrine, electrical stimulation of the posterior hypothalamus, and direct renal nerve stimulation. Hydronephrotic and nonhydronephrotic kidneys responded comparably to norepinephrine and posterior hypothalamic stimulation; however, the hydronephrotic kidneys were less responsive than normal kidneys to direct renal nerve stimulation. In the microcirculatory experiments, stimulation of the splanchnic nerve (2-8 Hz) induced a frequency-dependent constriction of interlobular arteries and afferent arterioles. Preglomerular vessel diameters decreased by 42-53% during the 8-Hz stimulation, whereas the efferent arteriolar diameters did not significantly change. Thus, constriction of preglomerular vessels mediates, in large part, the changes in renal hemodynamics evoked by increases in renal sympathetic nerve activity.

Animals↗
PubMed1991

Isolation, characterization, and localization of human genomic DNA encoding the beta 1 subunit of the GABAA receptor (GABRB1).

Genomic DNA that encodes the beta 1 subunit of the human gamma-aminobutyric acidA (GABAA) receptor was cloned and mapped. Exons and flanking introns (greater than 14 kb) were sequenced to determine the structural organization of the gene. The gene was localized on human chromosome 4, in bands p12-13. The beta 1 subunit is encoded by a relatively large gene (greater than 65 kb) on nine exons. In contrast to other conserved regions of the subunit polypeptide, the proposed channel-forming domain (M2) is derived from more than one exon. The organization of exons was compared with that of the genes that code for subunits of nicotinic acetylcholine receptors. There is no evidence for conservation of gene structure between these two members of the proposed gene superfamily. However, intron-exon junctions were found to be conserved precisely between subtypes of GABAA receptor subunits.

Amino Acid Sequence↗
PubMed1990

Control of the renal microvasculature by vasoactive peptides.

In recent years, numerous techniques have been developed to study renal microcirculation. These technical advances have provided new insight regarding the specificity of action of vasoconstrictor peptides (angiotensin II, arginine vasopressin, endothelin) and vasodilator peptides (bradykinin, atrial natriuretic peptide) at discrete sites within the renal vascular bed. Differential signal transduction mechanisms, particularly those related to calcium regulation, appear to mediate the renal vascular actions of these compounds, both in a segment-specific and agonist-specific manner. These observations substantiate the concept that regulation of intrarenal and intraglomerular dynamics is accomplished by selective changes in pre- and postglomerular resistance induced by different endogenous peptides. This microvascular selectivity allows precise regulation of glomerular and peritubular capillary function, and ultimately exerts great influence on the volume and composition of the final urine.

Angiotensin II↗
PubMed1990

Concanavalin A is not a ferritin.

Contrary to recent claims, in vitro evidence has been obtained to establish that Concanavalin A (Con A) is not a ferritin. Four techniques including immunoprecipitation, gel filtration, sucrose density gradient ultracentrifugation and CsCl centrifugation were employed. None of them showed that Con A is a ferritin.

Centrifugation, Density Gradient↗
PubMed1989

Visualization of renal autoregulation in the split hydronephrotic kidney of rats.

The autoregulatory ability of the renal vascular system in the split hydronephrotic kidney was quantitated with intravital microscopy. The luminal diameters of the arcuate and interlobular arteries as well as the afferent and efferent arterioles were measured. Glomerular blood flow was determined by the dual slit technique. In the first series of experiments, the renal perfusion pressure was reduced by graded clamping of the abdominal aorta. Pressure reduction from 118 mm Hg to 95 mm Hg induced dilation of all preglomerular vessels except for the distal afferent arteriole; there was no change in the efferent arteriole and the blood flow was maintained. Further pressure reductions to 71 and 43 mm Hg caused additional dilations of the preglomerular vessels, a marginal reduction in diameter of proximal efferent arterioles and flow reductions by 15% and 41%, respectively. In the second series, systemic blood pressure was increased by continuous i.v. infusions of norepinephrine (NE). NE constricted pre- and postglomerular vessels except for the distal afferent arteriole; glomerular flow was decreased. Reduction of renal perfusion pressure during NE infusion to the preinfusion value did not diminish glomerular blood flow, but reduced the constrictor response to NE in preglomerular vessels. In a third series of experiments we examined the effect of atrial natriuretic factor (ANF) on renal autoregulation. Addition of ANF (10(-9) to 10(-7) M) to the renal bath induced a dose-dependent dilation of all preglomerular vessels and a constriction of the efferent arteriole. Pressure reduction from 120 to 95 mm Hg resulted in a further preglomerular vasodilation. These experiments demonstrate that autoregulation is mediated primarily by diameter changes in all preglomerular vessels excluding the distal segment of the afferent arteriole. Further, these data suggest that ANF induced dilation of preglomerular vessels does not impair the myogenic response of these vessels.

Acetylcholine↗
PubMed1988

The influence of extracellular calcium on microvascular tone in the rat cremaster muscle.

In vivo responses of arterioles and venules to changes in bath calcium concentrations were observed in the cremaster muscle of male Sprague-Dawley rats. Small arterioles (2A, 3A) initially exposed to a solution containing calcium (2.55 mM) significantly dilated in response to a 0-calcium bath. Reexposure to calcium (greater than 0.65 mM) caused 2A and 3A arterioles to constrict to diameters similar to the initial control values. In contrast, large arterioles (1A) and all venules (1V, 2V, 3V) were unresponsive to exposure to a 0-calcium solution or to reexposure to calcium (0.65-5.10 mM). Treatment with mefenamic acid (10 micrograms/ml), a prostaglandin synthesis inhibitor, produced marked constriction of arterioles but not of venules, suggesting the involvement of endogenous vasodilator prostaglandins in the regulation of resting diameters of arterioles. In the presence of mefenamic acid, 1A arterioles dilated when exposed to a 0-calcium solution and constricted back to control diameters following reintroduction of calcium into the bath. These data demonstrate heterogeneity in the responsiveness of cremasteric microvessels to changes in extracellular calcium. The small arterioles were most responsive to calcium. The lack of response by the largest arterioles appears to be due to the dilator influences of endogenous prostaglandins.

Animals↗
PubMed1987

Passage of microspheres through vessels of normal and split hydronephrotic rat kidneys.

Microspheres (15 micron) were injected intracardially in rats. By means of in vivo microscopy of the split hydronephrotic kidney microcirculation, the passage of microspheres through renal blood vessels was analyzed. Additionally, three sets of experiments were conducted, one consisting of rats with normal kidneys, one of rats with hydronephrotic kidneys, and one of rats with hydronephrotic kidneys under vasodilation. In vivo microscopic as well as histological studies show that the passage of 15-micron microspheres is dependent on the postinjection interval. Microspheres are capable of locally dilating preglomerular vessels and moving towards the glomeruli. Therefore, estimates of preglomerular vessel diameters with microsphere experiments ought to be controlled by intravital microscopy.

Animals↗
PubMed1987

Endogenous prostaglandins selectively mask large arteriole constriction to angiotensin II.

Television microscopy was used to observe the responses of in vivo arterioles and venules of the rat cremaster muscle to the topical application of angiotensin II (10(-8) and 10(-6) M). Neither the first- (A1) or second-order arterioles (A2) nor the first- (V1) or second-order venules (V2) constricted significantly to angiotensin II. However, after the inhibition of local prostaglandin synthesis with either mefenamic acid or indomethacin, both A1 and A2, but not the venules, gave a significant constrictor response to angiotensin II (10(-6) M). Arterioles and venules, which were preconstricted with norepinephrine, dilated to their initial baseline diameters after angiotensin II (10(-6) M), a response not observed when the microvessels were pretreated with either an angiotensin antagonist or a prostaglandin synthesis inhibitor. These observations indicate that endogenous prostaglandins exert a significant dilator influence on the larger arterioles, that this dilator influence appears to oppose the constrictor effect of angiotensin II, and that angiotensin II acts on specific receptors to induce synthesis and/or release of dilator prostaglandins in large arterioles. However, prostaglandins cannot account for the absence of a venular constriction to angiotensin.

Angiotensin II↗
PubMed1987

Calcium antagonists preferentially dilate preglomerular vessels of hydronephrotic kidney.

The hydronephrotic kidney of Inactin-anesthetized female Wistar rats was exteriorized in a controlled bath to directly observe preglomerular and postglomerular vessels via television microscopy. Nitrendipine, added to the bath in a concentration that did not alter blood pressure, induced a concentration-dependent dilation of preglomerular vessels. The arcuate artery maximally dilated by 29 +/- 4%, the interlobular artery by 24 +/- 5%, the afferent arteriole near the interlobular artery by 60 +/- 9%, and near the glomerulus by 28 +/- 13%. In contrast the efferent arteriole near the glomerulus dilated by only 11 +/- 6% and near the welling point by 7 +/- 9%. Similarly, diltiazem significantly dilated preglomerular vessels but not efferent arterioles. Acetylcholine significantly dilated all preglomerular vessels and dilated the afferent arterioles near the glomerulus (by 51 +/- 8%) to a greater extent than the calcium blockers. Acetylcholine also significantly dilated the efferent arterioles (near the glomerulus by 26 +/- 5% and near the welling point by 12 +/- 3%). These data suggest that the tone of the preglomerular vessels of the hydronephrotic kidney is more dependent on the entry of extracellular calcium through calcium antagonist-sensitive channels (i.e., potential dependent) than is the tone of the afferent arterioles near the glomerulus and the efferent arterioles.

Acetylcholine↗