Relationship of the Acid Micro-Climate in Rat and Human Intestine to Malabsorption

Hogben et al. (1959) have proposed the existence of a region at pH 5 on the surface of the intestine when the bathing medium has neutral pH, to explain discrepancies in transfer rates of weak electrolytes as predicted by the theory of non-ionic diffusion. The present experiments demonstrate for the first time the existence of such a layer by direct physical means. pH-sensitive microelectrodes were made with Corning 015 glass constructed in the form of McInnes-Dole ‘surface’ electrodes. Sensitive glass was made into thin, slightly concave membranes of 30pm depth and sealed on to inert glass capillary tubes (outer diam. Zmm, inner diam. 1.5mm). Such electrodes were filled with 0.1 M-HCl and connected via 1.5mm chloride-treated silver wire to a Keithley 605C electrometer of sufficiently high input impedance. The circuit was completed by a 3~-KCl/agar salt bridge via a calomel half-cell to the electrometer. The electrodes, which had a tip resistance of 1.4f0.34~10~~~, gave a linear response over the pH range 4-9 of 96% of the ideal Nernstian response (approximately 59.5mV/pH unit) and gave 95 % of their response within the first 30s. Rat gut was removed, during ether anaesthesia, from unstarved male rats (200-25Og). The excised jejunum was everted (Wiseman, 1961) before mounting on a plastic cannula consisting of two 3cm-long pieces (external diam. 2.5mm, internal diam. 1.5mm) perforated with 20 0.5mm holes. Everted sacs had a transmural potential difference 5.9f0.6 (mean+s.E.M.), five observations/mV and gave a final serosal/mucosal glucose concentration ratio of 1.66f0.23 (6), indicating active glucose transport and therefore viable preparations. pH-microelectrodes, mounted on a Leitz micromanipulator, were brought into incubation flasks containing 5cm-long cannulated everted sacs, bathed in 30ml of KrebsHenseleit (1932) bicarbonate buffer at 37”C, gassed with 02+COz (95: 5). Later experiments involved flat strips of intestine of 1 cm2 area, pinned with dissecting pins on to the cork floor of small containers holding 5ml of buffer. Both types of preparations gave similar results. The flat-strip technique was adopted for human biopsy samples, which were incubated in Krebs-phosphate buffer gassed with O2 (100%). Change of buffer had no effect on the results. Before the experiment, the electrodes were calibrated in pH 4, 7 and 9 buffers, and the mounted microelectrode was introduced into the bathing medium. After the pH of the bathing medium was measured, the electrode was racked down by eye on to the surface of the tissue and the pH recorded after 1 min. This was recorded as the ‘micro-climate pH and repeated at the end of 60min incubation. Human biopsy samples required 5min to reach stable values. Fig. 1 shows the results obtained from samples down the length of the rat gut after division of the entire length into five consecutive regions. There is a striking difference (PiO.001) in pH seen between the pH of the medium of 7.29 and all regions of the intestine in the presence and absence of 10mM-glucose. In the absence of glucose, recorded micro-climate pH values were slightly acid at pH6.6. A consistent small