Quantitative assessment of luminal stirring in the perfused small intestine of the rat.

We measured the resistance (RL) to CO absorption that resulted from poor luminal stirring in the constantly perfused rat jejunum. RL or calculated unstirred layer thickness was greater for 30-cm than 10-cm long segments, indicating lack of a uniform thickness of unstirred layer. The possibility that laminar flow existed in the gut was first tested by calculating expected CO absorptions from fluid moving with laminar flow. These values agreed closely with observed absorption rates. Laminar flow also was supported by the observation that CO absorption was independent of perfusate viscosity. Lastly, after sudden addition of phenolsulfonphthalein (PSP) to the perfusate, PSP outflow concentration was similar in tygon tubing (which has laminar flow) and a gut segment of comparable dimension. We conclude that flow in the perfused gut is laminar and that this laminar flow has many implications for studies carried out with the constant-perfusion technique.

H2 excretion after ingestion of complex carbohydrates.

Malabsorption of fermentable material in a variety of foods was assessed by measurement of breath H2 excretion. Breath H2 increased well above that observed in fasting subjects after ingestion of 100 g of carbohydrate in oats, whole wheat, potatoes, corn, and baked beans. Rice caused only a minimal increase in H2 excretion and hamburger was associated with no increase. We estimated the malabsorption of fermentable material by comparing the H2 excretion for 9 h after ingestion of various complex carbohydrates with that after 10 g of lactulose. The mean malabsorption of fermented material after 100-g carbohydrate meals was 20 g for baked beans; 7-10 g for wheat, oats, potatoes, and corn; and 0.9 g for rice. Whole oats or whole wheat resulted in 2-5 times more H2 than did the refined flours. As purified fiber appeared to be a poor substrate for H2 production by fecal homogenates, we conclude that most complex carbohydrates, with the exception of rice, contain a good deal of fermentable material that escapes small bowel absorption and it seems likely that this fermentable material is malabsorbed starch.

Role of the unstirred layer in protecting the murine gastric mucosa from bile salt.

Bile salts disrupt a functional gastric mucosal barrier which normally minimizes back-diffusion of H+ into mucosa. Our previous studies have shown that ionized bile salts disrupt the barrier to H+ by dissolving membrane lipids. The presence of an unstirred water layer on the surface of the gastric mucosa could protect against bile salt injury either by creating a concentration gradient of bile salt from lumen to mucosal surface or by slowing diffusion of lipid-laden mixed micelles away from the mucosal surface. In the present study we investigated this possibility in the anesthetized rat. Measurements of H+ back-diffusion and Na+ forward-diffusion across the gastric mucosa were made before and after exposure to a bile salt solution that was either unmixed or mixed by continuous withdrawal and injection. Using carbon monoxide diffusion, we observed this method of mixing to decrease the unstirred layer thickness from 880 to 448 micron (p less than 0.02). Mixing increased mean H+ back-diffusion induced by a 10 mM mixture of six conjugated bile salts from -2.58 to -4.11 microEq/min (p less than 0.01) and increased mean forward-diffusion of Na+ from 1.81 to 3.27 microEq/min (p less than 0.01). Mixing also increased efflux of mucosal phospholipid (32.7 to 52.2 nmol/min, p less than 0.05) and of cholesterol (4.89 to 8.87 nmol/min, p less than 0.05) into the bile salt solution. Addition of saturation amounts of lecithin and cholesterol to the bile salt solution completely prevented disruption of the barrier whether the solution was mixed or not. Mixing also increased mucosal uptake of bile salt from 74.6 to 221.3 nmol/min (p less than 0.01) when no lipids were added. In the presence of lecithin and cholesterol, mixing increased absorption of bile salt from 63.5 to 165.6 (p less than 0.02). These findings further support the hypothesis that bile salts disrupt the gastric mucosal barrier by dissolution of mucosal membrane lipids, and provide evidence that the unstirred water layer helps protect the gastric mucosa from bile salt injury.

Use of carbon monoxide to measure luminal stirring in the rat gut.

We used carbon monoxide (CO) as a probe to quantitatively measure intestinal unstirred water layers in vivo. CO has several features that make it uniquely well suited to measure the unstirred layer in that its tight binding to hemoglobin makes uptake diffusion limited, and its relatively high lipid solubility renders membrane resistance negligible relative to the water barriers of the unstirred layer and epithelial cell. The unique application of CO was the measurement of the absorption rate of CO both from the gas phase as well as a solute dissolved in saline. Several lines of evidence showed that a gut stripped free of saline and then filled with gas contained a negligible unstirred layer. Thus, absorption of CO from the gas phase measured resistance of just the epithelial cell. Subtraction of this value from the resistance of CO absorption from saline provided a direct measure of unstirred layer resistance. Studies in the rat showed for a 3-min absorption period that the conventionally calculated apparent unstirred layer for the jejunum was 411 micron and for the colon was 240 micron. However, this conventionally calculated unstirred layer resistance did not truly depict the situation in the rat gut, since there was a continuing depletion of CO from outer surfaces of luminal contents throughout the experiment period. This produced a continually increasing diffusion barrier with time. Calculation of expected absorption rate from unstirred cylinders with the dimensions of the rat gut indicated that there was virtually no stirring in the small intestine and minimal stirring in the colon. The technique described in this paper appears to be simpler and to require fewer assumptions for validity than other techniques previously used to measure unstirred layers in vivo.