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Biomedical subjects

J E Stephens

Publications and source records attributed to J E Stephens.

9 recordsLinked to original sources
PubMed2005

Glucagon-like peptide-2 protects against TPN-induced intestinal hexose malabsorption in enterally refed piglets.

Premature infants receiving chronic total parenteral nutrition (TPN) due to feeding intolerance develop intestinal atrophy and reduced nutrient absorption. Although providing the intestinal trophic hormone glucagon-like peptide-2 (GLP-2) during chronic TPN improves intestinal growth and morphology, it is uncertain whether GLP-2 enhances absorptive function. We placed catheters in the carotid artery, jugular and portal veins, duodenum, and a portal vein flow probe in piglets before providing either enteral formula (ENT), TPN or a coinfusion of TPN plus GLP-2 for 6 days. On postoperative day 7, all piglets were fed enterally and digestive functions were evaluated in vivo using dual infusion of enteral ((13)C) and intravenous ((2)H) glucose, in vitro by measuring mucosal lactase activity and rates of apical glucose transport, and by assessing the abundances of sodium glucose transporter-1 (SGLT-1) and glucose transporter-2 (GLUT2). Both ENT and GLP-2 pigs had larger intestine weights, longer villi, and higher lactose digestive capacity and in vivo net glucose and galactose absorption compared with TPN alone. These endpoints were similar in ENT and GLP-2 pigs except for a lower intestinal weight and net glucose absorption in GLP-2 compared with ENT pigs. The enhanced hexose absorption in GLP-2 compared with TPN pigs corresponded with higher lactose digestive and apical glucose transport capacities, increased abundance of SGLT-1, but not GLUT-2, and lower intestinal metabolism of [(13)C]glucose to [(13)C]lactate. Our findings indicate that GLP-2 treatment during chronic TPN maintains intestinal structure and lactose digestive and hexose absorptive capacities, reduces intestinal hexose metabolism, and may facilitate the transition to enteral feeding in TPN-fed infants.

Algorithms↗
PubMed2004

Phase transitions in K2Cr2O7 and structural redeterminations of phase II.

Crystals of phase II K2Cr2O7, potassium dichromate, space group P1 , grown from aqueous solution undergo a first-order transition to phase I, space group reportedly P21/n, at a phase-transition temperature, TPT, of 544 (2) K on first heating; the corresponding transition on cooling is at 502 (2) K. The endotherm on subsequent heatings occurs reproducibly at TPT = 531 (2) K. Mass loss between ca 531 and 544 K, identified as included water, is rapid and continues more slowly to higher temperatures for a total loss of ca 0.20%. The higher TPT on first heating is associated with the increasing pressure of superheated water occupying inclusion defects. The latent diagonal glide plane in phase II allows the structure of phase I to be inferred. The triclinic structure at 296 K has been independently redetermined. Normal probability analysis shows high consistency between the resulting and previous atomic coordinates, but with uncertainties reduced by a factor of ca 2. The earlier uncertainties are systematically underestimated by a comparable factor. The structure of phase IIb, space group A2/a on transposing axes, was determined at ca 300 K by Krivovichev et al. [Acta Cryst. (2000), C56, 629-630]. The first-order transition between phases I and II arises from the ca 60 degrees relative rotation of terminal O atoms in each tetrahedron as the n glide plane is gained or lost. A transition between phases IIb and I, also of first order, is likely but not between phases II and IIb. An intermediate phase may exist between phases IIb and I.

Journal Article↗
PubMed1991

Monitoring of specific antibodies to human immunodeficiency virus structural proteins: clinical significance.

Levels of antibodies to six major structural proteins of human immunodeficiency virus type 1 (gp120, gp41, p66, p31, p24, and p17) were assessed in serial samples from 22 persons with severe hemophilia (16 asymptomatic and 6 who developed acquired immunodeficiency syndrome [AIDS] or AIDS-related complex) with an automated dot blot assay using purified recombinant antigens. High and sustained levels of antibody to gp120, gp41, and p31 were found in all patients irrespective of their clinical condition for 4 to 6 years after seroconversion. In contrast, immune response to p66 and p17 was significantly lower in symptomatic patients. Over time, the levels of these two antibodies, as well as anti-p24, decreased and tended to become undetectable. Abnormal immune response and low levels of antibody to p66 and p17 are early indications of rapid clinical progression.

Adult↗
PubMed1989

High-yield trapping of EGF-induced receptor dimers by chemical cross-linking.

The binding of epidermal growth factor (EGF) to its plasma membrane receptor results in the stimulation of a tyrosyl residue-specific protein kinase, which has been shown to be part of the receptor. The mechanism by which EGF binding give rise to the stimulation of kinase activity is not understood in detail; however, a number of recent studies have implicated receptor dimerization or oligomerization in this process. We prepared Triton X-100 extracts of A431 cells in which the concentration of EGF receptors was on the order of 10(-7) M. When samples of the extracts were incubated with or without EGF and then treated with the high-yield cross-linking reagent bis(sulfosuccinimidyl)suberate (BS3), covalent receptor dimers could be detected in high yield in samples that had been treated with both EGF and BS3, whereas only monomeric receptor was detected in untreated samples or in samples that had been treated with either EGF or BS3. The yield of receptor dimers trapped by cross-linking correlated with the stimulation of autophosphorylation by EGF and with the concentration of EGF present. EGF-induced receptor dimers were also efficiently cross-linked in highly purified receptor preparations, suggesting that EGF-induced dimerization is a process intrinsic to the receptor, requiring no additional accessory proteins.

Cross-Linking Reagents↗
PubMed1982

Chloride transport by intact rat liver and cultured rat hepatocytes.

Chloride is the predominant inorganic anion in bile, and it has been proposed that active chloride transport, possibly via a sodium-coupled mechanism, may contribute to that portion of canalicular bile formation not directly related to bile acid transport (bile acid-dependent bile formation or BAIBF). We have therefore examined the anion specificity of BAIBF using the isolated perfused rat liver and have studied sodium-chloride flux coupling and the sodium dependence of intracellular chloride content using 22Na and 36Cl transport by cultured rat hepatocytes. BAIBF by the isolated rat liver was unaltered by replacement of chloride with nitrate or benzenesulfonate but was significantly reduced by replacement of chloride with sulfate or thiocyanate. In cultured hepatocytes, sodium entry rate was reduced when chloride in the incubation medium was replaced by cyclamate, benzenesulfonate, or sulfate and mannitol but was unaffected when chloride was replaced by nitrate, gluconate, or thiocyanate. Conversely, chloride entry rate was decreased when sodium was replaced with choline but was unaffected when sodium was replaced by lithium or when ouabain was added to the medium. Thus no consistent evidence of sodium-chloride flux coupling was observed. Steady-state exchangeable intracellular chloride in the cultured hepatocytes was unaffected by ouabain or by replacement of sodium with choline and was increased when sodium was replaced by lithium. These findings indicate that basal BAIBF exhibits no specific chloride requirement. Although they do not exclude the possible existence in rat liver of sodium-coupled chloride transport, they provide no evidence that such a mechanism accounts for a major portion either of chloride transport by individual rat hepatocytes or of basal BAIBF by intact rat liver.

Animals↗
PubMed1982

Bile acid transport in cultured rat hepatocytes.

The mechanisms of bile acid uptake have been studied with primary monolayer cultures of rat hepatocytes. Hepatocytes were incubated with taurocholic acid (TC), glycocholic acid (GC), cholic acid (CA), glycochenodeoxycholic acid (GCDC), chenodeoxycholic acid (CDCA), deoxycholic acid (DOCA), lithocholic acid (LCA), or cholylglycylhistamine (CCH), a neutral bile acid derivative for 10 s to 60 min in medium containing sodium chloride, sodium chloride with 1 mM ouabain, or choline chloride. Cells were washed free of radioactive tracer, cell-associated radioactivity was quantitated, and bile acid uptake rates, kinetic parameters of uptake, and steady-state bile acid content were calculated. Two mechanisms for bile acid uptake were identified. Uptake of TC, GC, CA, and GCDC occurred predominantly via a sodium-dependent, ouabain-suppressible saturable mechanism, presumably sodium-coupled transport. Estimates of apparent Km and Vmax for these bile acids were TC, 33 micro M and 0.36 nmol . min-1 . mg prot-1; GC, 18 micro M and 0.22 nmol . min-1 . mg prot-1; CA, 13 micro M and 0.10 nmol . min-1 . mg prot; and GCDC, 6 micro M and 0.21 nmol . min-1 . mg prot, respectively. Uptake via this sodium-coupled mechanism exhibited considerable substrate selectivity. It was enhanced by increased ring hydroxylation and amino acid conjugation and decreased by further conjugation with a neutral histamine group (CGH). In contrast, uptake of CDCA, DOCA, LCA, and CGH occurred primarily via a nonsaturable sodium-independent mechanism, possibly simple diffusion. This mechanism accounted for only a small portion of uptake of TC, GC, CA, and GCDC at low bile acid concentrations. Nonsaturable bile acid uptake rates appeared to correlate with decane-buffer partition coefficients and to be related to bile acid structure.

Animals↗
PubMed1982

Effects of ion substitution on bile acid-dependent and -independent bile formation by rat liver.

To characterize the transport mechanisms responsible for formation of canalicular bile, we have examined the effects of ion substitution on bile acid-dependent and bile acid-independent bile formation by the isolated perfused rat liver. Complete replacement of perfusate sodium with choline and lithium abolished taurocholate-induced choleresis and reduced biliary taurocholate output by greater than 70%. Partial replacement of perfusate sodium (25 of 128 mM) by choline reduced bile acid-independent bile formation by 30% and replacement of the remaining sodium (103 mM) by choline reduced bile acid-independent bile formation by an additional 64%. In contrast, replacement of the remaining sodium (103 mM) by lithium reduced bile acid-independent bile formation by only an additional 20%, while complete replacement of sodium (128 mM) by lithium reduced bile formation by only 17%, and lithium replaced sodium as the predominant biliary cation. Replacement of perfusate bicarbonate by Tricine, a zwitterionic amino acid buffer, decreased bile acid-independent bile formation by greater than or equal to 50% and decreased biliary bicarbonate output by approximately 60%, regardless of the accompanying cation. In separate experiments, replacement of sodium by lithium essentially abolished Na,K-ATPase activity measured either as ouabain-suppressible ATP hydrolysis in rat liver or kidney homogenates, or as ouabain-suppressible 86Rb uptake by cultured rat hepatocytes. These studies indicate that bile acid(taurocholate)-dependent bile formation by rat liver exhibits a specific requirement for sodium, a finding probably attributable to the role(s) of sodium in hepatic sodium-coupled taurocholate uptake and/or in maintenance of Na,K-ATPase activity. The surprising finding that bile acid-independent bile formation was substantially unaltered by complete replacement of sodium with the permeant cation lithium does not appear to be explained by Na,K-ATPase-mediated lithium transport. Although alternative interpretations exist, this observation is consistent with the hypothesis that much of basal bile acid-independent bile formation is attributable to an ion pump other than Na,K-ATPase, which directly or indirectly mediates bicarbonate transport.

Animals↗
PubMed1981

Transport of sodium, chloride, and taurocholate by cultured rat hepatocytes.

Transport of sodium, chloride, and taurocholate was studied in primary cultures of adult rat hepatocytes incubated in a balanced electrolyte solution containing 150 mM NaCl, various concentrations of taurocholate, and (22)Na, (36)Cl, [(3)H]taurocholate, and 3-O-[(3)H]methyl-D-glucose. Lithium chloride, choline chloride, or Na(2)SO(4) and mannitol were substituted isotonically for NaCl in selected studies. The steady-state intracellular concentrations of exchangeable sodium and chloride averaged 6.5 mM and 30.1 mM, respectively. Ouabain reversibly increased intracellular sodium concentration. Chloride entry rate was about double that of sodium. Unlike sodium entry, chloride entry rate increased nonlinearly with increasing extracellular concentration. Taurocholate entry exhibited both saturable and nonsaturable components; the former accounting for virtually all taurocholate uptake at concentrations comparable to those found in vivo. Taurocholate was actively concentrated by the cultured cells, with the steady-state intracellular-to-extracellular concentration ratio decreasing from over 50 to about 1 as extracellular taurocholate concentration was increased from 10 muM to 4 mM. Both the saturable uptake component and concentrative taurocholate transport were virtually abolished by substitution of choline or lithium for sodium or by addition of ouabain. Taurocholate entry rate first increased in a sigmoid fashion and then decreased as extracellular sodium concentration was increased from 0 to 150 mM. Sodium entry rate increased in the presence of added taurocholate with an average of one sodium ion accompanying each taurocholate molecule into the cell. These findings indicate that sodium and chloride differ strikingly in their mechanism and rate of entry into cultured rat hepatocytes and in their intracellular concentration. Moreover, hepatocytes concentrate taurocholate by a sodium-coupled mechanism with an apparently equimolar transport stoichiometry.

Animals↗