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Biomedical subjects

C A Boyd

Publications and source records attributed to C A Boyd.

At least 91 records · Page 5Linked to original sources
PubMed1986

Cation transport in lung epithelial cells derived from fetal, newborn, and adult rabbits.

Recent studies done with fetal and adult sheep and with monolayers of cultured rat alveolar type II cells suggest that active transport of Na+ across the lung epithelium may contribute to liquid absorption from air spaces, an essential component of the normal switch from placental to pulmonary gas exchange at birth. The goals of this work were 1) to study the ontogeny of cation transport in lung epithelial cells derived from fetal, newborn, and adult rabbits and 2) to determine the influence of premature birth, air breathing, labor, and postnatal lung maturation on K+ uptake in these cells. We harvested granular pneumonocytes by tracheal instillation of proteolytic enzymes followed by centrifugation of the dispersed cells over a discontinuous density gradient of metrizamide. This procedure yielded 65-90% granular pneumonocytes, of which more than 80% excluded vital dye. Using freshly isolated cells, we measured uptake of 86Rb+, which mimics transmembrane movement of K+, in the presence or absence of 10(-4) M ouabain and in the presence or absence of 5 X 10(-4) M furosemide or bumetanide. In adult rabbit studies, 86Rb+ uptake was twice as fast in lung epithelial cells (98 +/- 7 nmol X 10(6) cells-1 X h-1) as it was in alveolar macrophages (51 +/- 6 nmol X 10(6) cells-1 X h-1). Ouabain inhibited 55-60% of the uptake by pneumonocytes, and "loop" diuretics inhibited an additional 15-20%. The rate of 86Rb+ uptake in fetal cells was less than 10% (6 +/- 1 nmol X 10(6) cells-1 X h-1) of the rate in adult cells; ouabain inhibited 80-85% of 86Rb+ uptake in fetal cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1984

Computed tomography of thoracic and lumbar spine injuries.

CT scans of 73 patients with acute thoracic/lumbar spine injuries from T3 to L5 were reviewed. Injuries were classified as burst fractures (48), fracture-dislocations (ten), wedge compression fractures (11), and seatbelt-type injuries (four). Thirty-one (42%) had motor deficits due to spinal cord or nerve root damage. Such neurologic deficits were present in all patients with fracture-dislocations, and 60% of those with burst fractures. Seven patients, four initially normal, developed progressive neurologic impairment early after injury. Burst fractures, one with dislocation, were the spinal injury associated with each progressive deficit. Burst fractures at T12 or L1 with 50% or more decrease of the mid-sagittal neural canal diameter had a significant risk of neurologic involvement, and of progressive deficit. CT scans demonstrate vertebral column damage well, and help identify those patients at risk of acute neurologic compromise.

Adolescent↗
PubMed1983

Cotransport systems in the brush border membrane of the human placenta.

The cotransport systems present in the brush border membrane of the human placental syncytiotrophoblast are reviewed. Attention is focused on the systems that are powered by the electrochemical gradient of Na+ (for example, for neutral amino acids but not for glucose), and on recently described systems in which K+ flux is coupled to C1- flux or to that for Na+ and C1-. These systems are similar to those found in other tissues and may be significant for net trans-placental transport and its control, as well as for the regulation of placental trophoblast volume.

Amino Acids↗
PubMed1982

A micro-electrode study of oligopeptide absorption by the small intestinal epithelium of Necturus maculosus.

1. The effects of some amino acids and oligopeptides on the electrical properties of the brush-border membrane of the small intestine of the mudpuppy, Necturus maculosus were studied in vitro using micro-electrodes. 2. A number of amino acids (glycine, L-proline and L-leucine) and small peptides (carnosine, glycyl-L-proline, L-leucyl-L-leucine, glycylglycine and glycylglycylglycine) depolarized the brush-border membrane. This was associated with a reduction of input resistance. Tetraglycine did not appreciably reduce the membrane potential. 3. Evidence is presented that the electrical effects caused by application of the peptides are not solely due to the effects of the products of peptide hydrolysis. Furthermore there appears to be more than a single system available to the peptides. 4. Stereospecificity is found for both amino acids and peptides but appears to be more marked for the peptides. 5. Peptide-induced depolarizations are less markedly reduced in the absence of external Na+ than are the depolarizations caused by the amino acids. 6. These results are discussed with respect to the mechanism and significance of amino acid and peptide transport in the intestinal tract.

Amino Acids↗
PubMed1982

Amino acid inhibition and stimulation of 2-aminoisobutyric acid exit from anuran small intestine.

1. Using the vascularly perfused frog small intestine, the exit of the non-metabolized amino acid 2-aminoisobutyric acid (AIB) from the pre-loaded epithelium into the blood has been studied in winter animals.2. Marked inhibition of the instantaneous rate constant for AIB exit into the vascular bed is observed when L-leucine, but not D-leucine, is added either to the intestinal lumen or to the vascular bed. The extent of the inhibition is related to the leucine concentration in an alinear fashion. The concentration of luminal L-leucine giving half maximal inhibition is 2.5 mM.3. The instantaneous rate constant for AIB exit is similarly decreased by 10 mM-L-tryptophan and by L-phenylalanine added to the intestinal lumen and to a lesser extent by L-asparagine, L-valine, L-glutamine, L-isoleucine, and L-norleucine.4. 10 mM-L-proline added to the lumen stimulates AIB exit from the pre-loaded epithelium into the blood. This stimulation is due to an increased rate constant for movement of AIB across the basolateral membrane.5. No inhibition is found when the dipeptide L-leucyl-L-leucine (10 mM) is added to the intestinal lumen in the presence of 10 mM-L-leucine. When added to the vascular compartment this dipeptide has no effect upon AIB exit from the epithelium.6. Possible mechanisms by which amino acids and peptides may influence AIB movement out of the epithelium into the blood are discussed and conclusions are drawn concerning AIB transport across the intestinal basolateral membrane of the intact epithelium.

Amino Acids↗
PubMed1981

Transamination and asymmetry in glutamate transport across the basolateral membrane of frog small intestine.

Transport of L-glutamate across the basolateral membrane of frog small-intestinal epithelium, unlike that of L-alanine, is highly asymmetric; thus the rate constant (K entry) describing the entry of glutamate into the epithelium from the vascular bed across this membrane is one order of magnitude greater than the rate constant (K exit) describing its exit. This asymmetry, which appears not to depend upon the Na gradient, may be important in maintaining a high intracellular concentration of glutamate relative to alanine thereby favouring the production of alanine from glutamate by transamination.

Alanine↗
PubMed1981

L-proline transport by brush border membrane vesicles prepared from human placenta.

1. Brush border microvillous plasma membranes were prepared from syncytiotrophoblast of human term placenta by a method of differential centrifugation. 2. Such plasma membranes form closed, osmotically active, right-side-out vesicles into which L-proline (188 microM) is shown to be transported in a time-dependent manner. There is no detectable metabolism of L-proline within the vesicles during 30 min of incubation. 3. Transient accumulation of L-proline to levels of up to three times its equilibrium value occurs in the presence of an inward gradient of sodium chloride. The proline and sodium are shown to have reached electrochemical equilibrium by 30 min, at which stage there is about 100 pmol L-proline mg protein-1. 4. This transient accumulation is abolished by the prior equilibration of the sodium chloride gradient, or by the replacement of sodium by inwardly directed gradients of other cations. Entry of the amino acid into the vesicles is also shown to be influenced by the permeability of the anion in the medium and by an imposed potassium diffusion potential. L-Proline transport across the brush border membrane of human placental syncytiotrophoblast is thus a sodium-dependent, electrogenic process. 5. Studies of the transport processes indicate saturation at higher concentrations of L-proline with a 'Km' of about 1 mM; Vmax averaged about 2 nmol mg protein-1 min-1 varied considerably between preparations. 6. L-Proline (188 microM) transport is inhibited competitively by the presence of many amino acids and by the dipeptide glycyl-L-proline. The Ki for inhibition by methyl AIB is 300 microM. 7. These findings are discussed in relation to the mechanism of transplacental amino acid transfer.

Amino Acids↗
PubMed1980

Intracellular study of ionic events underlying intestinal membrane transport of oligopeptides.

It is now clear that after a protein meal much of the alpha-amino nitrogen which is absorbed from the small intestinal tract is transported into the intestinal epithelium across the brush border membrane of the enterocyte in the form of oligopeptides rather than free amino acids. We report here the results of electrophysiological experiments using intracellular microelectrodes which allow us to add important information concerning the mechanism of membrane transport which must underlie oligopeptide absorption, in particular with respect to the role of Na+ in this peptide transport. Such experiments not only reveal the energetics involved in the major pathway for protein absorption from the gastrointestinal tract but may also be important for understanding the mechanism of action of small peptides in quite different cells, such as the Na+-sensitive binding of the neurohormone oligopeptide enkephalin to the enkephalin receptors in the central nervous system.

Animals↗
PubMed1979

Movements of monosaccharides between blood and tissues of vascularly perfused small intestine.

1. A method involving the analysis of pulse transients of the vascular concentrations of test sugar and extracellular marker has been used to study the movements of non-metabolized sugars between the cells and vascular fluid of the vascularly perfused small intestine of R. ridibunda. Reasons are given for supposing that the method does properly measure the net entry of sugars into a cellular compartment that includes at least the epithelium. 2. It is found that while the glucose analogues, 3-O-methyl-D-glucose (3MG) and 2-deoxy-D-glucose (2)DG) are able to enter a cellular compartment when they are added to the vascular bed, alpha-methyl-D-glucoside (alpha MG) is able to enter the compartment only at a very slow rate from the vascular bed. In contrast, 3MG and alaph MG are well absorbed from the lumen whereas the inward permeability of 2DG across the lumen face of the epithelium is very low and, unlike 3MG and alpha MG, is not influenced by the presence in the lumen of other transported sugars. 3. The presence of phlorizin in the intestinal lumen increases the flux of alpha MG and of 3MG in the direction vascular bed--bulk phase of lumen. Reasons are given for supposing that the movement of sugars from the vascular bed into the lumen may involve a cellular route but occurs, at least in part, through a paracellular, extracellular route. 4. The exit of monosaccharides that have been loaded previously into the epithelium either from the lumen or from the vascular bed has been investigated. 3MG washes out rapidly into the vascular bed and the exit is stimulated by the addition of D-glucose to the intestinal lumen and by the addition of 2DG, but not alpha MG, to the arterial infusate. In contrast, alpha MG, loaded into the epithelium from the lumen, washes out of the cells only slowly into the vascular bed, so that even with high rates of vascular perfusion alpha MG accumulates within the tissue. The sustained accumulation of alpha MG implies that not only is the permeability for exit into the blood restricted for this sugar, but also the permeability is low across the brush border in an outward direction, cell to lumen ('lobster pot effect'). The wash-out of 3MG into the vascular effluent is sufficently rapid that only in the absence of vascular perfusion is 3MG accumulated within the tissue. 5. The contrasting properties of the monosaccharide transport systems accessible from the intestinal lumen and the vascular bed respectively are discussed in relation to the problems of epithelial transport of monosaccharide.

Animals↗
PubMed1979

Studies on amino acid inhibition of monosaccharide exit from anuran small intestinal epithelium.

1. The effect of the addition of amino acids to the intestinal lumen upon the movement of the monosaccharide alpha-methyl-D-glucopyranoside (alpha MG) from the preloaded epithelium into the blood and into the lumen of the vascularly perfused frog small intestine has been studied. 2. The neutral hydrophobic amino acids tryptophan, leucine, phenylalanine, tyrosine, isoleucine, valine, norleucine and cycloleucine all rapidly inhibit the exit of alpha MG out of the epithelium into the vascular bed. They stimulate backflux of the sugar from the epithelium into the lumen to a very much smaller extent. 3. L-Leucine is a more effective inhibitor of alpha MG exit into the blood than is D-leucine. Near-maximal inhibition of alpha MG exit is seen with 10 mM-L-leucine in the intestinal lumen. 4. The addition of leucine (10 mM) to the lumen of the intestine preloaded with alpha MG approximately halves the rate constant for alpha MG washout into the blood from 12.6 +/- 1.7 (4) x 10(-3) to 5.5 +/- 1.4 (4) x 10(-3) min-1, without appreciably altering the pool of monosaccharide in the tissue. The inhibitory effect of L-leucine upon alpha MG exit into the blood is not abolished by the presence of phlorizin (5 x 10(-5) M) in the intestinal lumen. 5. The complex pattern of inhibition of alpha MG transfer from the lumen to the blood observed upon the addition of L-leucine to the lumen is consistent with the finding that the amino acid inhibits the exit of the monosaccharide out of the epithelium into the blood in addition to any inhibitory effect upon sugar entry across the brush border. 6. It is suggested that alpha MG may be a substrate for a proposed transport system for neutral hydrophobic amino acids which, it is suggested, is present in the basolateral membrane of the epithelial cell.

Amino Acids↗
PubMed1979

Separation of the microvillous (maternal) from the basal (fetal) plasma membrane of human term placenta: methods and physiological significance of marker enzyme distribution.

Plasma membranes from normal, full-term human placental trophoblast have been isolated by a new procedure. The method depends upon isopycnic zonal centrifugation using linear sucrose/Ficoll density gradients. Enrichment of plasma membrane marker enzymes with respect to trophoblast homogenate is found in two distinct peaks (designated B and D) of the fractionated effluent recovered from the rotor. Fraction B is enriched with membrane-bound alkaline phosphatase and 5'-nucleotidase, but not with (Na+, K+)-ATPase of F(-)-stimulated adenylate cyclase. It is suggested that this material is derived from the maternal-facing microvillous plasma membrane. Fraction D, enriched with (Na+, K+)-ATPase, F(-)-stimulated adenylate cyclase and, to a smaller extent, with 5'-nucleotidase and alkaline phosphatase is, by exclusion, proposed to be derived from the fetal-facing basal plasma membrane. Both plasma membrane fractions are shown to be free of appreciable contamination, using specific markers for endoplasmic reticulum, mitochondria, nuclei and lysosomes. The separation of the two membrane fractions is shown to depend both upon these membranes forming closed vesicles during homogenization and upon the buoyant densities of such vesicles differing in such a way that microvillous plasma membranes band at a lower density than basal plasma membranes. No separation of the membranes is achieved in gradients in which the vesicles are collapsed.

Alkaline Phosphatase↗
PubMed1978

Effects of vascular perfusion on the accumulation, distribution and transfer of 3-O-methyl-D-glucose within and across the small intestine.

1. Factors affecting the transfer of the non-metabolized, ;actively transported' sugar, 3-O-methyl-D-glucose (3MG) across the small intestinal epithelium have been examined in vascularly perfused anuran intestine. Transfer has been studied during absorption in the steady state, and also during the period of transition from one steady state to another.2. During the steady state, the rate of absorption of 3MG from the intestinal lumen is equal to the rate of appearance in the portal venous effluent; this rate of transfer is to a small, but significant, extent directly related to the rate of arterial perfusion. With phlorizin in the intestinal lumen, transfer across the epithelium is reduced to very low rates which are independent of the rate of vascular perfusion.3. The apparent size of the tissue pool(s) of 3MG that have to be loaded to achieve the steady state rate of transfer are less than those that unload into the vascular bed after 3MG is removed from the intestinal lumen. This ;up-down asymmetry' is abolished when phlorizin is present in the intestinal lumen during the unloading phase.4. When 3MG is abruptly removed from the intestinal lumen after the tissue has been previously loaded with the sugar, the rate of washout into the vascular bed can be described by the sum of two exponential terms. The two terms differ in that the rate constant of the earlier ;fast' term is sensitive to the rate of vascular perfusion, while the later, ;slow', rate constant is insensitive to flow rate. The total quantity of 3MG that can be unloaded from the tissue into the portal venous effluent is decreased when phlorizin is present in the intestinal lumen during the unloading phase.5. Absorption from the lumen of the anuran intestine continues while the mesenteric circulation is interrupted. An estimate of the concentration of 3MG during the period of vascular stoppage can be made from the quantity recovered in the portal venous effluent when vascular perfusion is reinstituted (;vascular stop-flow'). The extent of the accumulation depends upon the duration of the vascular stoppage and the presence of Na ions in the intestinal lumen is essential for accumulation to occur.6. The findings are discussed in relation to the transfer of 3MG between various possible compartments in the tissue during absorption. Evidence is presented that a re-uptake of previously absorbed 3MG may occur across the brush border membrane. Such a recycling of 3MG across the epithelium implies that the apparent unidirectional fluxes measured across the epithelium between the bulk phase of the lumen and the blood may underestimate the size of fluxes across the epithelium at the cellular level.

Animals↗