In vivo alkaline secretion by mammalian esophagus

Alkaline secretion (AS) was sought in vivo in the rabbit and opossum esophagus, using a recirculated unbuffered saline solution and pH stat technique. The rabbit and opossum were used because both have stratified squamous epithelium; however, only the opossum has, in addition, an extensive network of submucosal glands. Mean basal AS in the rabbit was 0.02 +/- 0.005 muEq/h . cm2, with one-third of the animals having no AS. Basal AS was present in all opossums with a mean value 20-fold greater than in rabbits (0.39 +/- 0.03 muEq/h . cm2). Basal AS in both species was found by titration methodology to be comprised almost entirely of secreted bicarbonate ions. In the rabbit, AS could not be stimulated by luminal HCl, i.v. carbachol, or i.v. NaHCO3. In contrast, luminal HCl stimulated AS in opossums three- to fourfold above basal values, and neither the acid-stimulated nor basal AS in the opossum was blocked by i.v. atropine. Acid clearance by AS was monitored in both species in vivo. In the rabbit, pH increased from 2.0 to 3.6 in 1 h, with a steady rate of rise after equilibration of 0.1 pH units/10 min. In the opossum, pH increased from 2.0 to 6.4 in 1 h, with a steady rate of rise five times faster than rabbits (0.5 pH units/10 min). These results indicate that the mammalian esophagus can secrete a bicarbonate-containing fluid with the capacity to clear acid from the esophageal lumen. The major source of this secretion is not the squamous epithelium, but probably the submucosal glands. Because the human esophagus has both submucosal glands and a mechanism for acid clearance consistent with the presence of AS, the findings in opossums may have relevance to protection against reflux disease in humans.