PH Microclimate

The absorption of short-chain weak acids in the rat intestine, as a function of pH, does not appear to conform to the pH partition hypothesis [44]. Similar anomalies were found with weak bases [77]. The apparent pKa values observed in the absorption-pH curve were shifted to higher values for acids and to lower values for bases, compared with the true pKa values. Such deviations could be explained by the effect of an acid layer on the apical side of cells, the so-called acid pH microclimate [44,70,73,76-84].

Shiau et al. [73] directly measured the microclimate pH, pHm, to be 5.2-6.7 in different sections of the intestine (very reproducible values in a given segment) covered with the normal mucus layer, as the luminal (bulk) pH, pHb, was maintained at 7.2. Good controls ruled out pH electrode artifacts. With the mucus layer washed off, pHm rose from 5.4 to 7.2. Values of pHb as low as 3 and as high as 10 remarkably did not affect values of pHm. Glucose did not affect pHm when the microclimate was established. However, when the mucus layer had been washed off and pHm was allowed to rise to pHb, the addition of 28 mM glucose caused the original low pHm to be reestablished after 5 min. Shiau et al. [73] hypothesized that the mucus layer was an ampholyte (of considerable pH buffer capacity) that created the pH acid microclimate.

Said et al. [78] measured pHm in rat intestine under in vitro and in vivo conditions. As pHb was kept constant at 7.4, pHm values varied within 6.4-6.3 (proximal to distal duodenum), 6.0-6.4 (proximal to distal jejunum), 6.6-6.9 (proximal to distal ileum), and were 6.9 in the colon. Serosal surface had normal pH. When glucose or sodium was removed from the bathing solutions, the pHm values began to rise. Metabolic inhibitors (1 mM iodoacetate or 2,4-dinitrophenol) also caused the pHm values to rise. Said et al. [78] hypothesized that a Na+/H+ antiporter mechanism, dependent on cellular metabolism, was responsible for the acid pH microclimate.

The tips of villi have the lowest pHm values, whereas the crypt regions have pHm > 8 values [70]. Most remarkable was that an alkaline microclimate (pHm 8) was observed in the human stomach, whose bulk pHb is generally about 1.7. In the stomach and duodenum, the near-neutral microclimate pH was attributed to the secretion of HCO33 from the epithelium [70].

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