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J M Dietschy

Publications and source records attributed to J M Dietschy.

At least 145 records · Page 8Linked to original sources
PubMed1973

Determinants of intestinal mucosal uptake of short- and medium-chain fatty acids and alcohols.

Uptake rates across the jejunal brush border have been measured for water-soluble fatty acids and alcohols and analyzed to determine the relative roles of the unstirred water layer and the lipid cell membrane as determinants of the intestinal absorptive process. Initial studies involving measurement of time courses of electrical transients developed across the intestine exposed to poorly permeant solute molecules showed no anomalous discrimination of probe molecules of different size or charge. This finding suggests that the diffusion barrier in the intestine can be considered as an unstirred water layer. Next, uptake rates of fatty acid were found to be linear with respect to concentration of the test solute, demonstrated no competitive inhibition or contralateral stimulation, had low temperature dependency, and were insensitive to metabolic inhibition, indicating that uptake proceeds by passive diffusion. Passive permeability coefficients, *P, varied from 22 +/- 1.4 to 395 +/- 9.2 nmoles.min(-1).100 mg(-1).mm(-1) for the saturated fatty acids 2:0 through 12:0 and from 119 +/- 3.3 to 581 +/- 45.2 for the saturated alcohols 6:0 through 10:0. Vigorous stirring of the bulk buffer solution enhanced *P values in direct proportion to chain length while the presence of bile acid micelles depressed apparent permeability coefficients in proportion to fatty acid chain length. These results demonstrate that uptake of short-chain fatty acid monomers is rate limited by the lipid cell membrane but diffusion through the unstirred water layer becomes increasingly rate limiting as the chain length increases. It is also possible to conclude from these data that diffusion through the unstirred water layer becomes totally rate limiting for uptake of long-chain fatty acid monomers of physiological importance.

Animals↗
PubMed1972

Characterization of the kinetics of the passive and active transport mechanisms for bile acid absorption in the small intestine and colon of the rat.

Bile acid uptake occurs via passive diffusion in all regions of the intestine and via active absorption in the ileum. Determination of the passive permeability coefficient for ionized monomers ((*)P(-)) demonstrated that permeability decreased by a factor of 3.4, 6.8, and 8.1 for the addition of a hydroxyl, glycine, or taurine group, respectively, to the steroid nucleus. Removal of the negative charge increased permeation by a factor of 4.4; however, permeability coefficients for the protonated monomers showed the same relative decrease with addition of a hydroxyl group. The calculated incremental free energies of solution (deltaDeltaF(W-->1)) associated with these additions equaled + 757 (hydroxyl), + 1178 (glycine), and + 1291 (taurine) cal/mole. Passive permeability coefficients for the transverse colon showed the same relative relationships among the various bile acids. After making appropriate corrections for passive permeability across the ileum, apparent values for the maximal transport velocity ((*)V(max)) and Michaelis constant ((*)K(m)) of the active transport system were measured. (*)V(max) depended upon the number of hydroxyl groups on the steroid nucleus; values for the trihydroxy bile acids were high (1543-1906 pmoles/min per cm) while those for the dihydroxy (114-512 pmoles/min per cm) and monohydroxy (45-57 pmoles/min per cm) acids were lower. In contrast, (*)K(m) values were related to whether the bile acid was conjugated; unconjugated bile acids had values ranging from 0.37 to 0.49 mM, while values for the conjugated bile acids were approximately half as high (0.12-0.23 mM).

Animals↗
PubMed1972

Characterization of bile acid absorption across the unstirred water layer and brush border of the rat jejunum.

We have examined the rate-limiting steps involved in bile acid absorption across the unstirred water layer and lipid cell membrane of the jejunal mucosa. Uptake of the polar bile acid taurocholate is limited solely by the cell membrane since this compound permeates the unstirred water layer more rapidly than the lipid cell membrane and stirring does not enhance uptake. With less polar bile acids which permeate the cell membrane relatively more rapidly, however, the unstirred water layer does exert resistance to mucosal uptake of these compounds. That the unstirred water layer is even more rate limiting to uptake from micellar solutions is indicated by the facts that the rate of bile acid absorption from such solutions is lower than from corresponding monomer solutions, stirring markedly enhances uptake from micellar solutions while increases in viscosity of the incubation media depress uptake and expansion of the micelle size further depresses absorption rates. We also have examined the important question of whether the micelle crosses the brush border intact once it reaches the aqueous-lipid interface. The observations that the calculated permeation rate of the micelle should be extremely low, the rate of mucosal cell uptake plateaus at a constant value when the critical micelle concentration is reached at the aqueous-lipid interface, and the different components of a mixed micelle are taken up at different rates indicate that uptake of the intact micelle does not occur; rather, bile acid absorption must be explained in terms of monomers in equilibrium with the micelle. Finally, after correction of the permeability coefficients of the various bile acids for the unstirred layer resistance the incremental partial molar free energy of solution of the hydroxyl group in the brush border membrane was calculated to equal -6126 cal.mole(-1) indicating that passive diffusion of these compounds occurs through a very polar region of the cell membrane.

Animals↗
PubMed1972

Determination of unidirectional uptake rates for lipids across the intestinal brush border.

An in vitro method is presented which measures valid, unidirectional uptake rates for lipids across the intestinal brush border. This method combines analysis by a newly devised, double isotope counting system for solubilized tissue with the use of a nonabsorbable marker to correct gross uptake determinations for contamination by adherent mucosal fluid. Of seven markers, only [(3)H]inulin measured adherent mucosal fluid volumes as much as 20% greater than the other markers. Diffusion of the nonabsorbable marker, as well as of the compound being studied, into the unstirred layer made the time course of uptake critically important. The time lag for diffusion of marker invalidates the use of 1-min incubation periods; however, a linear time course of uptake that intercepts essentially at zero was found for taurocholate and octanoate for periods of from 2 to 5 min. Working within this critical time period with jejunum, it was shown that tissue dry weight was an appropriate measure of the amount of tissue and that uptake rates for taurocholic, octanoic, and lauric acids were linear with respect to concentration. Tissue binding of compounds was not significant. The results demonstrate that careful use of the described method yields accurate measurement of unidirectional uptake rates of lipids across the brush border that are of critical importance in defining the characteristics of membrane penetration and the rate-limiting steps in fat and sterol absorption.

Animals↗
PubMed1971

Unstirred water layers in intestine: rate determinant of fatty acid absorption from micellar solutions.

Bile acid and fatty acid uptake from micellar solutions by intestinal cells fails to reflect the incremental free energy changes expected for permeation that is rate limited by cell membranes. However, altering the size of the diffulsing particle or the thickness of tle unstirred water layer does change uptake. These observations show that the unstirred water layer is rate limiting for intestinal absorption of lipids from micellar solutions.

Animals↗
PubMed1971

Cholesterol synthesis in the intestine of man: regional differences and control mechanisms.

Cholesterol in the circulating serum pool is derived either from absorption of dietary cholesterol or from endogenous synthesis principally in the liver and gastrointestinal tract. While the control of intestinal cholesterogenesis has been elucidated in several lower animal species, no data currently are available in the case of man. In the present study using tissue specimens obtained by suction biopsy in 29 normal subjects, we have shown the rate of cholesterogenesis is low in the stomach (25 +/-6 mmumoles/g per 2 hr) and rectum (40 +/-8 mmumoles/g per 2 hr); in the small bowel the rate progressively decreases in the proximal duodenum (90 +/-16 mmumoles/g per 2 hr); distal duodenum (80 +/-11 mmumoles/g per 2 hr); and distal jejunum (35 +/-5 mmumoles/g per 2 hr); but abruptly increases in the distal ileum (280 +/-33 mmumoles/g per 2 hr). Indirect evidence is provided that the intestinal crypt epithelium is the main site of this sterol synthesis. Fasting for 48 hr suppressed the rate of cholesterogenesis in the distal duodenum from a control value of 80 +/-11 mmumoles/g per 2 hr to 40 +/-8 mmumoles/g per 2 hr while cholesterol feeding for 7 days did not alter the rate of cholesterol synthesis (75 +/-12 mmumoles/g per 2 hr). This resistance to cholesterol feeding also was present in the distal ileum where control and cholesterol-fed subjects had comparable rates of cholesterogenesis (280 and 261 mmumoles/g per 2 hr, respectively). Interruption of the enterohepatic circulation, in contrast, resulted in greatly enhanced sterol synthesis with a mean rate of 259 +/-29 mmumoles/g per 2 hr being found in the duodenum of four patients with biliary obstruction as compared with the rate of 80 +/-11 mmumoles/g per 2 hr in control subjects. These studies indicate that the mechanisms of control of cholesterol synthesis by the human intestine are similar to those described for the intestine of lower animals; this also appears to be true for the human liver. Thus, the marked differences in over-all cholesterol metabolism between various lower mammalian species and man cannot be explained by fundamental differences in control mechanisms; rather, these differences must reflect variations in some other parameter of cholesterol metabolism.

Acetates↗