pH-microclimate at the luminal surface of the intestinal mucosa of guinea pig and rat

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Abstract
Segments of guinea pig jejunum, proximal and distal colon and of rat jejunum were superfused either in vivo or in vitro with different electrolyte solutions. The pH in the bulk phase solution and at the surface of the epithelium was measured with two different types of glass pH-microelectrodes, a pointed tip (Hinke-type) and a flat membrane electrode (Dubuisson-type); both types of electrodes gave the same results. The existence of a pH-microclimate at the surface of the mucosa was demonstrated under both in vivo and in vitro conditions. In vivo the pH-microclimate was stable and virtually independent of changes in the luminal bulk phase pH. When the bulk phase pH of the guinea pig colon was changed between pH 5 and pH 8.6, the mean pH in the microclimate was 7.08±0.15 (n=163) in the proximal colon and 6.91±0.14 (n=75) in the distal colon. In the guinea pig jejunum pH in the microclimate was 7.37±0.21 (n=10) while the luminal pH was 7.27±0.10. Under in vitro conditions, the pH in the microclimate was more acidic (guinea pig jejunum ΔpH 0.93, rat jejunum ΔpH 0.40, guinea pig proximal colon ΔpH 0.22). Addition of glucose (10 mM) or short-chain fatty acids (80–90 mM) to the luminal solution or replacement of sodium by lithium did not influence the pH in the microclimate significantly. Also the addition of acetazolamide, amiloride, SITS or sodium deoxycholate to the luminal solution, did not affect the pH in the microclimate in vivo. A temporary interruption of the blood supply caused acidification of the pH in the microclimate. Restoring the blood flow reversed the effect, and the pH returned to the original values within a few minutes. The originally proposed acid-microclimate could not be confirmed under in vivo experimental conditions either in the jejunum or in the colon by direct measurement with pH-sensitive glass-microelectrodes.