Biomedical subjects

Y Xia

Publications and source records attributed to Y Xia.

At least 19 recordsLinked to original sources

PubMed1992

Dopamine transporter mRNA content in human substantia nigra decreases precipitously with age.

The dopamine transporter is the primary means of inactivating synaptic dopamine as well as a major site of action for psychostimulants (such as cocaine and amphetamine) and for neurotoxins that induce parkinsonism. In the present study, a human dopamine transporter partial cDNA clone obtained by polymerase chain reaction exhibited 87% and 89% identity at the nucleic acid and amino acid levels, respectively, with transmembrane domains 3-5 of the rat homolog. This clone was used to quantitate human dopamine transporter mRNA by nuclease protection assay. The postmortem content of dopamine transporter mRNA in the substantia nigrae of 18- to 57-yr-old subjects was relatively constant, while in subjects greater than 57 yr old, a precipitous (greater than 95%) decline in substantia nigra dopamine transporter mRNA was evident. In contrast, tyrosine hydroxylase mRNA in the same samples declined in a linear manner with increasing age. In situ hybridization experiments confirmed the profound loss of dopamine transporter gene expression in melanin-positive (presumptive dopamine) nigral neurons. These data may begin to shed light on compensatory changes occurring in human dopamine neurons during normal aging.

Adult↗

PubMed1992

Predicting protein secondary structure using neural net and statistical methods.

A comparison of neural network methods and Bayesian statistical methods is presented for prediction of the secondary structure of proteins given their primary sequence. The Bayesian method makes the unphysical assumption that the probability of an amino acid occurring in each position in the protein is independent of the amino acids occurring elsewhere. However, we find the predictive accuracy of the Bayesian method to be only minimally less than the accuracy of the most sophisticated methods used to date. We present the relationship of neural network methods to Bayesian statistical methods and show that, in principle, neural methods offer considerable power, although apparently they are not particularly useful for this problem. In the process, we derive a neural formalism in which the output neurons directly represent the conditional probabilities of structure class. The probabilistic formalism allows introduction of a new objective function, the mutual information, which translates the notion of correlation as a measure of predictive accuracy into a useful training measure. Although a similar accuracy to other approaches (utilizing a mean-square error) is achieved using this new measure, the accuracy on the training set is significantly and tantalizingly higher, even though the number of adjustable parameters remains the same. The mutual information measure predicts a greater fraction of helix and sheet structures correctly than the mean-square error measure, at the expense of coil accuracy, precisely as it was designed to do. By combining the two objective functions, we obtain a marginally improved accuracy of 64.4%, with Matthews coefficients C alpha, C beta and Ccoil of 0.40, 0.32 and 0.42, respectively. However, since all methods to date perform only slightly better than the Bayes algorithm, which entails the drastic assumption of independence of amino acids, one is forced to conclude that little progress has been made on this problem, despite the application of a variety of sophisticated algorithms such as neural networks, and that further advances will require a better understanding of the relevant biophysics.

Bayes Theorem↗

PubMed1992

The translocation (1;14)(p34;q11) in human T-cell leukemia: chromosome breakage 25 kilobase pairs downstream of the TAL1 protooncogene.

Nearly 30 percent of patients with T-cell acute lymphoblastic leukemia (T-ALL) exhibit a tumor-specific rearrangement of the TAL1 gene (also called TCL5 or SCL). These rearrangements are generated by either local DNA deletion or a (1;14)(p34;q11) chromosome translocation, and they typically result in structural alterations of the TAL1 transcription unit. In this report we present a molecular characterization of the t(1;14)(p34;q11) from a T-ALL patient. As a consequence of the translocation, TAL1 is transposed from its normal position on chromosome 1 into the T-cell receptor alpha/delta chain locus on chromosome 14. Unlike previous cases, the chromosome 1 breakpoint in this patient did not disrupt the continuity of the TAL1 transcription unit, but instead occurred approximately 25 kilobase pairs (kb) downstream of TAL1. This observation suggests that malignant alteration of TAL1 can be mediated by long-range cis-activating mechanisms that are triggered by DNA rearrangement at a distant site. Genes Chrom Cancer 4:211-216 (1992).

Base Sequence↗

PubMed1992

"One-shot" velocity microscopy: NMR imaging of motion using a single phase-encoding step.

The use of the pulsed gradient spin-echo sequence in NMR microscopy enables the measurement of molecular translational motion and simultaneous construction of velocity and self-diffusion images, a technique that has been termed dynamic NMR microscopy. In this method the PGSE contrast gradient is stepped in a fourth dimension (q space) and so is inherently inefficient. Provided that one is prepared to sacrifice some of the additional information provided by the multiple PGSE gradient approach, it is possible to construct a velocity image alone by means of a single PGSE phase-encoding step. We illustrate applications of this method in which a signal from the stationary spins is nulled by the use of both gradient phase cycling and a final "z-storage" rf pulse. The limits to velocity resolution are around 10 microns s-1 in free water but can be considerably smaller for molecules with a low self-diffusion coefficient. We demonstrate this method in a study of water capillary flow at 12 microns transverse pixel resolution, extending the velocity range by employing a four-quadrant analysis method. This method is also used to measure vascular transport in a living plant and find a flow rate of around 45 microns s-1.

Diffusion↗

PubMed1992

Ontogeny and distribution of GABAA receptors in rat brainstem and rostral brain regions.

Previous studies from our laboratory and others have shown that there are major age-related differences in brainstem neuronal function. Since GABAA receptors are major targets for GABA-mediated inhibitory modulation and play a key role in regulating cardiorespiratory function, especially during O2 deprivation, we examined differences in GABAA receptor density and distribution during postnatal development. Using quantitative receptor autoradiography, the present study was performed to examine the postnatal expression of GABAA receptors in the rat brainstem and rostral brain areas at five ages, i.e. postnatal day 1 (P1), P5, P10, P21 and P120. Ten-micrometer brain sections at different brain levels were labelled with [3H]muscimol in Tris-citrate buffer. We found that (i) GABAA receptors appeared very early in almost all the brainstem as well as rostral areas; (ii) at P1, the brainstem had a higher GABAA receptor binding density than rostral areas and its density peaked at P5 or P10; and (iii) receptor densities of the cerebellum and rostral brain areas such as cortex, thalamus and dentate gyrus increased with age, especially between P10 and P21, but most other subcortical areas like caudate-putamen and hippocampal CA1 area did not increase remarkably after birth. We conclude that: (i) GABAA receptors exist in most brain areas at birth; (ii) there are several patterns of postnatal development of GABAA receptors in the CNS with dramatic differences between the brainstem and cortex; (iii) brainstem functions rely more on GABAA receptors in early postnatal life than at more mature stages. We speculate that GABAA receptors develop earlier in phylogenetically older structures (such as brainstem) than in newer brain regions (such as cortex).

Aging↗

PubMed1992

Purpose-designed probes and their applications for dynamic NMR microscopy in an electromagnet.

The electromagnet provides a favorable environment for certain applications of NMR microscopy. These include plant imaging experiments and measurements of slow molecular diffusion, where high magnetic field gradients for the pulsed gradient spin echo (PGSE) technique are required. In this paper, two probes designed specifically for these two applications are described. In the first case, the open space within the probe has been maximized in order to incorporate environmental support systems for the plant, while in the second the smallest possible PGSE gradient coil former has been used to maximize the gradient strength. Examples are given of Dynamic NMR Microscopy experiments on a castor bean stem and on poly(ethylene oxide)/water solutions under shear thinning conditions.

Magnetic Resonance Spectroscopy↗

PubMed1992

Mapping of the gene for the cardiac sarcolemmal Na(+)-Ca2+ exchanger to human chromosome 2p21-p23.

The cardiac sarcolemmal Na(+)-Ca2+ exchanger is the primary mechanism for extrusion of calcium from the cardiac myocyte and therefore is important in regulating cardiac contractility. As part of an effort to determine whether the exchanger is associated with any genetic disorders of the heart or blood pressure, we have assigned the exchanger gene (designated NCX1) to human chromosome 2p21-p23 by analysis of a panel of mouse-human somatic cell hybrids and by in situ hybridization.

Animals↗

PubMed1992

Quantitation of rat dopamine transporter mRNA: effects of cocaine treatment and withdrawal.

Dopamine transporter mRNA levels in the rat substantia nigra were quantified using a sensitive nuclease protection assay with a highly homologous human dopamine transporter cDNA clone. The same probe was also used to visualize dopamine transporter mRNA in the substantia nigra by in situ hybridization. Repeated cocaine administration (15 mg/kg, twice a day for 6.5 days) resulted in a greater than 40% decrease in nigral dopamine transporter mRNA levels. In contrast, dopamine transporter mRNA levels were unchanged after either acute treatment (4 h before death) or repeated cocaine treatment followed by a 72-h withdrawal period. Thus, blockade of the dopamine transporter by repeated cocaine administration may result in the down-regulation of dopamine transporter gene expression in dopamine neurons.

Animals↗

PubMed1992

[3H]-MK 912 binding delineates two alpha 2-adrenoceptor subtypes in rat CNS one of which is identical with the cloned pA2d alpha 2-adrenoceptor.

1. Simultaneous computer modelling of control and guanfacine-masked [3H]-MK 912 saturation curves as well as guanfacine competition curves revealed that the drugs bound to two alpha 2-adrenoceptor subtypes in the rat cerebral cortex with very different selectivities. These alpha 2-adrenoceptor subtypes were designated alpha 2A and alpha 2C. The Kd value of [3H]-MK 912 for the alpha 2A-subtype was 1.77 nM and for the alpha 2C-subtype 0.075 nM; the receptor sites showing capacities 296 and 33 fmol mg-1 protein, respectively. The Kds of guanfacine were 19.9 and 344 nM, respectively. 2. Binding constants of 26 compounds for the two rat cerebral cortex alpha 2-adrenoceptor subtypes were determined by simultaneous computer modelling of control and guanfacine-masked drug competition curves as well as plain guanfacine competition curves using [3H]-MK912 as labelled ligand (i.e. a '3-curve assay'). Of the tested drugs WB4101, corynanthine, rauwolscine, yohimbine, ARC 239 and prazosin were found to be clearly alpha 2C-selective with selectivities ranging from 16 to 30 fold whereas guanfacine, oxymetazoline, BRL 44408 and BRL 41992 were found to be alpha 2A-selective with selectivities ranging from 9 to 22 fold. 3. The Kds of compounds obtained for the cerebral cortex alpha 2C-adrenoceptors showed an almost 1:1 correlation with the corresponding Kds for alpha 2-adrenoceptors expressed by the pA2d-gene (the rat 'alpha 2-C4' adrenoceptor) in CHO-cells. The cerebral cortex alpha 2A-adrenoceptors did not correlate well with the pA2d alpha 2-adrenoceptor Kds. 4. In the rat spinal cord [3H]-MK 912 bound to alpha 2A- and alpha 2C-adrenoceptor sites with similar affinities as in the cerebral cortex and with densities 172 and 7.4 fmol mg-1 protein, respectively. Drug affinities for some compounds showing major selectivity for alpha 2A- and alpha 2C-adrenoceptors were fully compatible with the notion that the spinal cord sites were alpha 2A- and alpha 2C-adrenoceptors.

Adrenergic alpha-Antagonists↗

PubMed1992

Role of ATP-sensitive K+ channels during anoxia: major differences between rat (newborn and adult) and turtle neurons.

1. It is well known that anoxia induces an increase in extracellular K+. The underlying mechanisms for the increase, however, are not well understood. In the present study, we performed electrophysiological, pharmacological and receptor autoradiographic experiments in an attempt to examine K+ ionic homeostasis during anoxia. Ion-selective microelectrodes were employed to measure intracellular and extracellular K+ activity from hypoglossal neurons in brain slices. 2. During 3-4 min anoxia, adult hypoglossal neurons lose a large amount of their intracellular K+ and this contributes in a major way to the 8-fold increase in extracellular K+. 3. Loss of intracellular K+ from hypoglossal neurons is, to a great extent, due to activation of certain specific K+ channels. Glibenclamide, a potential sulphonylurea ligand and a specific blocker of ATP-sensitive K+ (KATP) channels, has no effect on K+ homeostasis during oxygenated states, but almost halves the anoxia-induced increase in extracellular K+ in the adult rat. 4. [3H]glibenclamide autoradiography shows that the hypoglossal nucleus in the adult rat has high sulphonylurea receptor density, a finding that is consistent with our electrophysiological observation. 5. Since we have previously shown that newborn mammals and reptiles are more resistant to O2 deprivation than adult mammals, we performed comparative studies among adult rat, newborn rat and adult turtle. In sharp contrast to the adult rat, extracellular K+ activity in newborn rat and adult turtle brain increases little (10 to 100 times less than the adult rat) and glibenclamide has a small and insignificant effect on K+ efflux in the newborn rat and none in the turtle. Glibenclamide receptor binding sites are much lower in the newborn rat than in the adult rat central nervous system (CNS) and barely detectable in the turtle brain. 6. These results support the hypothesis that in the adult rat, K+ is lost during anoxia from neurons through sulphonylurea receptor or KATP channels in a major way. Generally, however, KATP channels are poorly expressed in the newborn rat and adult turtle CNS and have little role to play during O2 deprivation.

Animals↗

PubMed1992

CFTR channels in immortalized human airway cells.

The cystic fibrosis (CF) gene codes for CF transmembrane regulator (CFTR), a small-conductance linear Cl- channel, but numerous studies have identified a larger conductance, rectifying Cl- channel as the adenosine 3',5'-cyclic monophosphate (cAMP)-regulated channel that is defective in airway cells. We examined Cl- conductance in a bronchial epithelial cell line that expresses CFTR, 16HBE14o-, (CFTR+) and in an airway cell line that does not, 9HTEo-/S, (CFTR-). Ionomycin or hypotonic Ringer increased iodide efflux from both cell lines; however, forskolin increased iodide efflux or whole cell Cl- currents only in CFTR+ cells. Forskolin-stimulated whole cell currents were linear, voltage independent, and blocked by iodide. Cell-attached and outside-out patches from confluent CFTR+ but not CFTR- cells revealed 6-pS channels having linear current-voltage relations, permselectivity Cl > I (partial block by external iodide), and little or no inhibition by 5-nitro-2-(3-phenylpropylamino)-benzoate. The number of active channels per patch increased from 0.6 to 3.0 after forskolin. Channels closed after excision with tau = 4 s, but activity could be prolonged with ATP or protein kinase A plus ATP. Channels were modeled with one open and four closed states and show apparent cooperativity in gating. Rectifying Cl- channels previously implicated in CF were not seen in cell-attached recordings from either cell line but were abundant in excised patches from both cell lines. Thus CFTR channels are the pathway for cAMP-mediated Cl- conductance in these human airway cells, Ca2+ and swelling-induced channels do not require CFTR, and CFTR-cells display a CF phenotype.

Anions↗

PubMed1992

Oxidative and glycolytic pathways in rat (newborn and adult) and turtle brain: role during anoxia.

Using enzyme histochemistry and in vitro electrophysiological recordings in brain slices, we studied 1) the relative activity of cytochrome c oxidase (Cytox) and hexokinase (HK) and 2) cellular function by examining ionic homeostasis across cell membranes in the turtle and newborn (5 days old) and adult rat central nervous system. We found that Cytox was higher in the rostral than in the caudal brain regions of the adult rat and that the activity in the newborn is at least as high as in the adult rat. In contrast, adult turtles had very low Cytox activity throughout the central nervous system. Compared with that in the adult rat, HK activity in the newborn was generally lower in the rostral brain and cerebellum but similar or higher in the brain stem and spinal cord. In the turtle, HK activity was higher in the cerebellum, brain stem, and ventral horn of the spinal cord than in those in the rat. During anoxia, extracellular K+ increased by approximately 10-fold (from 3.2 to approximately 32 mM) in the adult brain stem but only by 2.6 mM in newborn rats. After glycolysis was blocked with iodoacetic acid (10-20 mM), extracellular K+ increased remarkably in both adult and newborn rats to approximately 35 mM. In contrast, the turtle brain tissue showed a slight and insignificant increase in extracellular K+ during complete anoxia or with iodoacetic acid; there was a modest increase in K+ when anoxia and iodoacetate were administered together. We conclude that 1) the newborn rat brain must rely either on higher glycolytic capacity or on a reduction of metabolic rate during O2 deprivation and 2) the turtle brain can subsist on nonglucose fuels or on fuels not requiring the citric acid cycle and the electron transfer chain.

Aging↗

PubMed1991

TAL2, a helix-loop-helix gene activated by the (7;9)(q34;q32) translocation in human T-cell leukemia.

Tumor-specific alteration of the TAL1 gene occurs in almost 25% of patients with T-cell acute lymphoblastic leukemia (T-ALL). We now report the identification of TAL2, a distinct gene that was isolated on the basis of its sequence homology with TAL1. The TAL2 gene is located 33 kilobase pairs from the chromosome 9 breakpoint of t(7;9)(q34;q32), a recurring translocation specifically associated with T-ALL. As a consequence of t(7;9)(q34;q32), TAL2 is juxtaposed with sequences from the T-cell receptor beta-chain gene on chromosome 7. TAL2 sequences are actively transcribed in SUP-T3, a T-ALL cell line that harbors the t(7;9)(q34;q32). The TAL2 gene product includes a helix-loop-helix protein dimerization and DNA binding domain that is especially homologous to those encoded by the TAL1 and LYL1 protooncogenes. Hence, TAL2, TAL1, and LYL1 constitute a discrete subgroup of helix-loop-helix proteins, each of which can potentially contribute to the development of T-ALL.

Amino Acid Sequence↗

PubMed1991

Major differences in CNS sulfonylurea receptor distribution between the rat (newborn, adult) and turtle.

Our previous results have shown that KATP channels play an important role in K+ efflux and extracellular K+ accumulation in the rat brain, and this role was quantitatively more important in the adult than in the newborn brain. The purpose of this study was to localize by autoradiographic techniques the binding sites of glibenclamide, a potent sulfonylurea ligand that targets KATP channels, in the adult and newborn rat central nervous system (CNS). Since the adult turtle is resistant to anoxia, we also compared the rat to the turtle brain sulfonylurea receptor distribution. In all three animal groups (newborn rat, adult rat, adult turtle), specific glibenclamide binding was saturable. Scatchard plots were curvilinear in the rat, thus suggesting that glibenclamide binds to two types of sites, i.e., high and low affinity sites. Scatchard analysis on turtle brain tissue showed evidence of one binding site only. We also found that the distribution of glibenclamide binding sites was heterogeneous in the adult rat CNS with a higher density in rostral than in caudal regions. The highest binding densities were seen in the cortex, hippocampus, cerebellum, substantia nigra, and a few thalamic nuclei; intermediate densities were observed in the basal ganglia, septum, thalamus, and the hypoglossal nucleus. There was a low density in most areas of the hypothalamus, midbrain, brainstem, and spinal cord. Compared with the adult rat, the newborn had a very homogeneous distribution of binding sites and densities were very low throughout the CNS; the level of binding density was even lower in some regions undetectable in the adult turtle. Our results indicate that (1) there are high and low affinity sulfonylurea receptors in the rat CNS, (2) there is a striking heterogeneity in the distribution and density of sulfonylurea receptors in the adult rat CNS and this is in sharp contrast to the homogeneous distribution and low density in both newborn rat and adult turtle; (3) sulfonylurea receptors increase in number postnatally in the rat since binding density increases and the Kd in the newborn rat is similar to that in the adult rat. We speculate that KATP channels and sulfonylurea receptors are poorly developed in the turtle and develop mostly after birth in the rat, reaching highest density in adulthood.

ATP-Binding Cassette Transporters↗

PubMed1991

Ontogeny and distribution of opioid receptors in the rat brainstem.

The distribution and postnatal ontogeny of opioid receptors have been investigated using in vitro quantitative receptor autoradiography. Rats were studied at postnatal day 1 (P1), P5, P10, P21 and P120 (adult). Opioid receptor sites for (D-Ala2,N-MePhe4,Gly-ol5)-enkephalin (DAMGO) binding were labelled with 4 nM of 3H-DAMGO; (D-Ala2,D-Leu5)-enkephalin (DADLE) binding sites were labelled with 4 nM of 3H-DADLE in the presence of 1 microM unlabelled mu-agonist (N-MePhe3,D-Pro4)-morphiceptin (PL107). We found that both binding sites have strikingly different distributional patterns. [3H]DADLE binding sites were rather homogeneous, whereas the distribution of [3H]DAMGO binding was very heterogeneous with the highest density in the nucleus of the solitary tract (NTS), ambiguus nucleus, dorsal motor nucleus of the vagus and the parabrachial areas. [3H]DAMGO binding density was 2- to 40-fold higher than [3H]DADLE binding sites in most brainstem nuclei. [3H]DAMGO binding sites appeared in most brainstem nuclei at birth, with a high density in cardiorespiratory-related nuclei, whereas [3H]DADLE binding sites were too scarce to be quantitated at P1. Both binding sites increased with age, but the developing patterns depended on the nucleus and the type of binding site. In most areas, the densities of both binding sites reached a maximum between P10 and P21 and then decreased to an adult level, but in some nuclei (e.g. the caudal part of the NTS and dorsal raphe nucleus), [3H]DAMGO binding sites kept increasing until adulthood. In contrast with the brainstem, cortical areas had a lower binding density in the newborn and reached peak levels later than brainstem regions (post P21). We conclude that (1) since [3H]DAMGO binding sites mainly reflect mu-receptors and [3H]DADLE binding sites delta-receptors (in the presence of PL017), the brainstem is essentially a mu-receptor region through delta-receptors are present; (2) both opioid receptors are present at birth but delta-receptors are very scarce in the newborn; (3) both receptors increase with age, but the time course depended on various nuclei and receptor types; (4) cardiorespiratory-related nuclei have high density of mu-receptors at all ages; and (5) opioid receptors develop earlier in the brainstem than in the cortex.

Animals↗

PubMed1991

Determination of levels of cyclic AMP in the myenteric plexus of guinea-pig small intestine.

Enzymatically dissociated ganglia from the myenteric plexus of the guinea-pig small intestine were used to investigate changes in levels of cyclic 3',5'-adenosine monophosphate (cAMP) in response to stimulation of adenylate cyclase by forskolin and inhibition of phosphodiesterase by 3-isobutyl-1-methylxanthine (IBMX). A linear relation with a positive correlation coefficient greater than 0.98 was found between: (1) amount of cAMP and number of ganglia; (2) amount of protein and number of ganglia; (3) amount of DNA and amount of protein; (4) amount of DNA and number of ganglia. Basal levels of cAMP were 2.25 +/- 0.21 fmol per ganglion for 900 ganglia. Forskolin stimulated a dose-dependent increase in cAMP over a concentration range of 0.05 to 50 microM, with a level of 18.6 +/- 4.9 fmol/ganglion at 50 microM forskolin. The inactive forskolin analog 1,9-dideoxyforskolin did not elevate cAMP. Addition of IBMX to the incubation medium stimulated a dose-dependent increase in cAMP over a concentration range of 0.1-1000 microM, with a level of 17.58 +/- 3.38 fmol/ganglion at 1000 microM IBMX. Application of 1 mM IBMX strongly potentiated the stimulating action of forskolin on cAMP levels. Our results derived from direct determination of cAMP changes in small intestinal myenteric ganglia are consistent with existing electrophysiological evidence for second messenger function of cAMP in slow synaptic modulation of excitability in AH/Type 2 neurons of the enteric nervous system.

1-Methyl-3-isobutylxanthine↗

PubMed1991

Cystic fibrosis, the CFTR, and rectifying Cl- channels.

The human genetic disease cystic fibrosis is caused by a single defective gene on chromosome 7 that codes for a 1480 amino acid protein called the cystic fibrosis transmembrane conductance regulator (CFTR). The defect causes a profound reduction of Cl- permeability in several tissues, which in turn impairs salt absorption and fluid secretion. A 25-80 pS, rectifying Cl- channel has been targeted as the exclusive or primary channel affected in CF. However, we have found no evidence for significant activation or spontaneous activity of this channel in cell-attached patches of normal lymphoblasts or dog tracheal cells. However, in dog tracheal cells, we find lower conductance, linear Cl- channels that are spontaneously active in unstimulated cells and may show increased activity in stimulated cells. Attempts to correlate the expression of mRNA for the CFTR protein in various types of cells with the presence of the rectifying Cl- channel show a lack of correlation: i.e., depolarization-activated rectifying Cl- channesl have been found in excised, inside-out patches from all cell types that we have examined to date, but the CFTR mRNA has so far only been detected in a subset of epithelial cells.

Animals↗

PubMed1991

Active absorption of conjugated bile acids in vivo. Kinetic parameters and molecular specificity of the ileal transport system in the rat.

Active transport of conjugated bile acids by the distal ileum is required for efficient enterohepatic cycling of bile acids. Experiments were performed in the rat to obtain accurate values for Tmax and Michaelis constant (Km) of the absorptive area of the rat ileum and to define the structural specificity of the transport system. The distal fifth (20 cm) of the small intestine from an anesthetized animal with a biliary fistula was perfused using solutions of 10 taurine-conjugated bile acids; a flow rate was used that was sufficiently high such that unstirred water layer effects were negligible and the intraluminal concentration remained unchanged throughout the perfused segment. The absorption rate was equated with the rate of hepatic bile acid secretion. Values of Tmax (mumol/min.kg) were markedly influenced by bile acid structure: cholyltaurine, 12.9; ursocholyltaurine, 9.6; ursodeoxycholyl taurine, 5.0; and lagodeoxycholyl-(3 alpha,12 beta-dihydroxy-cholanoic acid)-taurine, 1.2. Decreasing the length of the side chain of ursodeoxycholate conjugates from 8 to 6 carbon atoms was associated with a modest increase in Tmax values from 5.0 to 9.1 mumols/min.kg. Values of Km correlated with Tmax values and ranged from 0.5 to 5 mmol/L, being highest for those bile acids that were best transported. The Tmax for cholyltaurine transport was not reached when the intraluminal concentration was as high as its critical micellization concentration, precluding the definition of its Tmax; however, for ursocholyltaurine, with a critical micellization concentration of 40 mmol/L, saturation of transport was clearly shown. Kinetic parameters could not be obtained for two common dihydroxy conjugates (chenodeoxycholyltaurine and deoxycholyltaurine) because at a transport rate of 2 mumols/min.kg systemic toxicity and death occurred. These studies define the maximal transport capacity of the rat ileum for taurine-conjugated bile acids; they indicate that the ileal transport system in the rat is of low affinity and high capacity for taurine conjugates of hydrophilic bile acids, and they show that both nuclear substituents and side chain length influence the transport rate of taurine-conjugated bile acids.

Animals↗