Alkaline secretion by frog gastric glands measured with pH microelectrodes in the gland lumen

Abstract

1. In the present work we have measured the pH of the secreted fluid within the gland lumen of isolated but intact gastric mucosa of Rana esculenta. Tissues were mounted in a double chamber allowing continuous perfusion of the mucosal and serosal compartment, and the measurements were made with double-barrelled pH glass microelectrodes inserted into the glands from the serosal surface under microscopic inspection. 2. During inhibition of H+ secretion by cimetidine (100 microM) the luminal gland pH (pHgl) averaged 7.60 +/- 0.05 pH units (mean +/- s.e.m.; n = 35), a value significantly higher than bath solution pH (7.45 +/- 0.02; P < 0.001) and also higher than intracellular pH of oxyntopeptic cells (pHi), which averaged 7.53 +/- 0.06 (n = 18). 3. Stimulation of acid secretion with histamine (500 microM) reversibly decreased pHgl to values which could be as low as 2.5. Together with electrophysiological criteria this response was routinely used to verify the proper location of the microelectrode tip within the gland lumen. 4. Stimulation with carbachol (100 microM) or pentagastrin (50 microM) in the presence of cimetidine rapidly and reversibly increased pHgl by 0.10 +/- 0.01 pH units (n = 24; P < 0.001) and 0.09 +/- 0.02 pH units (n = 6; P < 0.05), respectively. 5. The observation that gastric gland fluid is more alkaline than the bath solutions and that carbachol or pentagastrin further alkalinize it strongly suggests that oxyntopeptic cells participate in gastric alkaline secretion at least under cholinergic stimulation.

Figure 1. Micropuncture of an oxyntopeptic cell and of the gastric gland lumen with a double-barrelled pH microelectrode

Upper trace (left scale): transepithelial potential difference (V_t) in millivolts (mucosal surface negative). Middle trace (right scale): cell membrane potential (V_s) and gland lumen potential (V_gl) in millivolts. Lower trace (left scale): cell pH (pH_i) and pH of gland lumen (pH_gl) in pH units. The superimposed voltage pulses indicate response to transepithelial constant current pulses (50 μA cm⁻², 1 s duration) used for transepithelial resistance (R_t) measurements. Note that V_gl equals V_t after the microelectrode tip advanced from cell to gland lumen and that the gland lumen precipitously acidifies after changing serosal perfusion from cimetidine (100 μm) to histamine (500 μm) containing solution.

Figure 2. Micropuncture of gastric gland lumen

The reversible effect of stimulation with histamine (500 μm) was followed until pH_gl reached a stable value below pH 5. Details as in Fig. 1.

Figure 3. Time course of the gland fluid pH change in response to stimulation with histamine (500 μm)

Data are expressed as means ± s.e.m. of 8 individual recordings in 8 mucosae.

Figure 4. Effect of stimulation with carbachol (100 μm in the presence of 100 μm cimetidine) followed by the effect of stimulation with histamine (500 μm) on pH_gl in the same gland

Note that carbachol promptly and reversibly alkalinized while histamine acidified the gland lumen. Details as in Fig. 1.

Figure 5. Effect of stimulation with pentagastrin (50 μm)

Note the initial gland fluid alkalinization followed by a slow decline towards control values. Details as in Fig. 1.