Applicability of the dual isotopes δ15N and δ18O to identify nitrate in groundwater beneath irrigated cropland.

Identification of the nitrate sources that adversely impact groundwater quality is a necessary first step in the control of this major worldwide pollutant. The impact of nitrate leachate from urea-ammonium nitrate (UAN) (50% urea-N, 25% ammonium-N, 25% nitrate-N) fertilizer, whose use has increased dramatically in the last three decades largely because it can be applied through sprinkler irrigation systems to corn in all growth stages, is investigated. The dual isotopes δ15NNO3 and δ18ONO3 were measured in groundwater samples from 39 irrigation wells in two intensively sprinkler-irrigated, corn-growing areas of Nebraska with nitrate-contaminated (N > 10 mg/L) groundwater and documented UAN use to ascertain whether nitrified ammonia and nitrate fertilizers can be distinguished in the High Plains aquifer. The areas, which are highly vulnerable to nitrate leaching and differ only in the composition and thickness of their unsaturated zones, are uniquely suited to provide scientific evidence of the feasibility of identifying nitrate fertilizer leachate in groundwater and thereby add significantly to the small body of existing and inconclusive data. The dual isotope method (DIM) results indicate that the nitrate contamination in 38 wells is mostly nitrified ammonium fertilizer. Most importantly, nitrate fertilizer from UAN was not identified isotopically in groundwater beneath almost all fields with documented heavy UAN use. This could be a potentially valuable finding for fertilizer management or it could convey limitations on the appropriateness of the DIM for nitrate fertilizer source identification in groundwater. Slightly enriched δ15NNO3 values in a few wells coincide with the practice of wintering cattle on corn stubble, which reportedly occurred more frequently in one focus area. The absence of natural soil-N leachates and denitrification in groundwater enabled an apparently reliable identification of manure leachates in both areas.

The CF salt controversy: in vivo observations and therapeutic approaches.

There is controversy over whether abnormalities in the salt concentration or volume of airway surface liquid (ASL) initiate cystic fibrosis (CF) airway disease. In vivo studies of CF mouse nasal epithelia revealed an increase in goblet cell number that was associated with decreased ASL volume rather than abnormal [Cl(-)]. Aerosolization of osmolytes in vivo failed to raise ASL volume. In vitro studies revealed that osmolytes and pharmacological agents were effective in producing isotonic volume responses in human airway epithelia but were typically short acting and less effective in CF cultures with prolonged volume hyperabsorption and mucus accumulation. These data show that (1) therapies can be designed to normalize ASL volume, without producing deleterious compositional changes in ASL, and (2) therapeutic efficacy will likely depend on development of long-acting pharmacologic agents and/or an increased efficiency of osmolyte delivery.

Cholecystokinin decreases intestinal hexose absorption by a parallel reduction in SGLT1 abundance in the brush-border membrane.

The dual lumenaly and vascularly perfused small intestine was used to determine the mechanism by which cholecystokinin octapeptide (CCK-8) decreases the rate of glucose absorption. With CCK-8 in the vascular perfusate the rate of 3-O-methyl-D-glucose absorption decreased, whereas the rate of D-fructose absorption was unaffected. The substrate pool size within the tissue during steady-state transport, in the presence and absence of CCK-8, was estimated by compartmental analysis of the 3-O-methyl-D-glucose washout into the vascular bed. When CCK-8 was included in the vascular perfusate, the absorptive cell pool size decreased when compared with untreated tissue. Both the steady-state hexose absorption data and the washout studies indicated that the locus of action of CCK-8 was the SGLT1 transporter located in the brush-border membrane. The SGLT1 protein abundance in isolated brush-border membranes, as quantified by Western blotting, showed a decrease that paralleled the decrease in the steady-state transport rate induced by CCK-8. These results indicate that CCK-8 diminishes the rate of intestinal hexose absorption by decreasing SGLT1 protein abundance in the brush-border membrane of the rat jejunum and therefore provides evidence for acute enteric hormonal regulation of the rate of glucose absorption across the small intestine.

Inhibition of glucose absorption in the rat jejunum: a novel action of alpha-D-glucosidase inhibitors.

BACKGROUND & AIMS: alpha-D-Glucosidase inhibitors act primarily by decreasing disaccharide hydrolysis and thus reduce the amount of free monosaccharides available for absorption. A novel action of alpha-D-glucosidase inhibitors is presented, indicating a direct effect on free glucose absorption by the rat jejunum. METHODS: The jejunum was isolated and free hexose was measured using in vivo single-pass luminal perfusion and dual vascular and luminal single-pass in vitro perfusion. Xenopus oocytes were injected with RNA transcript encoding recombinant sodium-glucose cotransporter 1, and uptake of 3H-labeled 3-O-methyl-D-glucopyranose (3-O-MG) was assessed. RESULTS: Acarbose (0.1 mg/mL), added to the lumen, decreased D-glucose absorption by 20% in vivo. Addition of 0.1 or 1.0 mg/mL acarbose to the lumen in vitro decreased the appearance of 3-O-MG in the vascular effluent by 28% and 60%, respectively. Accumulation of D-glucose within the enterocytes was decreased significantly by 67% and 79% when acarbose (1 mg/mL) or phloridzin (2 mmol/L), respectively, were present in the luminal perfusate. In contrast, acarbose did not affect the transport rate of free D-fructose and did not inhibit 3-O-MG uptake in oocytes expressing sodium-glucose cotransporter 1. CONCLUSIONS: The findings indicate that alpha-D-glucosidase inhibitors act specifically on the entry of free glucose into the enterocyte, an additional means by which they can reduce postprandial hyperglycemia.

Effect of cholecystokinin and related peptides on jejunal transepithelial hexose transport in the Sprague-Dawley rat.

An in situ dual vascular and luminal perfusion technique was used to study the effect of cholecystokinin octapeptide (CCK-8) on the transport of hexoses by the jejunum of the Sprague-Dawley rat from the lumen to the vascular bed. The lumen of the jejunum was perfused with hexoses in oxygenated Krebs buffer, while the superior mesenteric artery was infused with Krebs buffer containing Ficoll 70 as a plasma expander. CCK-8 (0.8-8 pM) in the vascular infusate selectively reduced hexose transport in a dose-dependent manner by 20-47%, although having no effect on L-glucose or L-leucine absorption. Vascular tetrodotoxin did not block CCK-8 inhibition, whereas a specific CCK-A receptor antagonist, lorglumide, did. The CCK-B receptor agonist cholecystokinin tetrapeptide had a small effect on hexose absorption, whereas somatostatin-14 and -28 had no effect. These results suggest that cholecystokinin can decrease intestinal absorption of hexoses in the small intestine, acting via CCK-A-type receptors.

Calcium mobilization and isometric tension in bovine tracheal smooth muscle: effects of salbutamol and histamine.

We determined if decreases in relative free intracellular calcium concentration ([Ca2+]i) caused by salbutamol, a selective beta2-adrenoreceptor agonist, were paralleled by calcium egression from the cytosol in bovine trachealis muscle strips. [Ca2+]i, or tissue-surface extracellular calcium changes (Ts[Ca2+]ext), were monitored using Fluo-3 acetoxymethylester or Fluo-3 pentaammonium salt simultaneously with isometric tension. Salbutamol (1 microM) decreased histamine-induced isometric tension from an average peak tension of 128.5 +/- 18.4 to -4.9 +/- 0.3 mN/mm2, and reduced the associated sustained increases in [Ca2+]i from 100% at peak to 20.4 +/- 7.6%. Both histamine-induced elevation in [Ca2+]i and isometric tension were reversed completely by forskolin (1 microM). In muscle strip at active resting tension, salbutamol caused a decrease (49.6 +/- 12.1%) in [Ca2+]i. Following precontraction with histamine, salbutamol caused an immediate and sustained increase in Ts[Ca2+]ext which was not seen in a Na(+)-free solution. Finally, propranolol (10 microM) blocked both increases in Ts[Ca2+]ext and muscle relaxation caused by salbutamol. These findings indicate that in bovine trachealis muscle, the effect of salbutamol to decrease [Ca2+]i and isometric tension is via a beta2-adrenoceptor, and the changes in [Ca2+]i are by an increase in calcium egression via the Na(+)/Ca2+ exchanger, and reuptake by myoplasmic stores.

Characterization of platelet-activating factor binding to human airway epithelial cells: modulation by fatty acids and ion-channel blockers.

Radioligand-binding studies were performed in primary cultured human airway epithelial cells with [3H]PAF to determine whether these cells express platelet-activating factor (PAF) receptors. Scatchard analysis of PAF binding data revealed a single class of PAF binding sites with Kd 1.8 +/- 0.2 nM and Bmax. 21.0 +/- 2.1 fmol/10(6) cells (13,000 receptors/cell). PAF binding increased the intracellular free Ca2+ concentration ([Ca2+]i), indicating functional PAF receptors. Palmitate (C16:0), linoleic acid (C18:2 omega 6) or eicosapentaenoic acid (C20:5 omega 3) was incubated with the cells to test the effect on PAF binding. Incorporation of each fatty acid into cellular phospholipid occurred. [3H]PAF (1 nM) binding decreased in cells supplemented with C20:5 omega 3, but increased in the cells supplemented with C16:0. Scatchard analysis revealed that the inhibition of PAF binding by supplementation with C20:5 omega 3 was due to a decrease in both affinity and number of PAF receptors. PAF-stimulated increase in [Ca2+]i was also decreased by 60% in cells supplemented with C20:5 omega 3. Verapamil, a Ca(2+)-channel blocker, and amiloride, a Na(+)-channel blocker, inhibited specific binding of [3H]PAF to the cells, with IC50 4-5 microM and 0.2 mM respectively. Diphenylamine-2-carboxylate (DPC), a Cl(-)-channel blocker, dramatically increased PAF binding to the cell in a dose-dependent manner. Scatchard analysis revealed that verapamil and amiloride decreased both binding affinity and number of PAF receptors, whereas DPC increased PAF binding sites without affecting binding affinity. These results demonstrate that human airway epithelial cells have a functional receptor for PAF and that PAF receptor binding can be modulated by exogenous fatty acids and by ion-channel blockers.