Threshold levels of NaCl upholding solute-coupled water transport in the cecum of germfree and conventional rats.

Abstract

It has been suggested that certain characteristics of germfree rodents, namely, cecal enlargement, semi–liquid contents of the lower bowel, and constant mild diarrhea (1) develop on the basis of abnormal composition of intestinal contents and on consecutive water–absorption inhibition. Thus, in cecal contents of germfree rats, the following have been observed in comparison to conventional controls: (a) marked concentration of muco–proteins (2–4), and (b) very low levels of bicarbonate and chloride ions (5–7). (c) The levels of cations, referring, mainly to sodium and potassium, were found unchanged or only slightly reduced (5–7). On feeding chloride–yielding resin to germfree rats, considerable improvement of water absorption could be obtained (8). In addition, (d) total osmotic pressure was found to be isotonic (9) and (e) colloid osmotic pressure pronouncedly hypertonic (10) in comparison to these values in blood plasma. It is well known that in conditions of isotonicity on either side of the intestinal membrane, solute transfer and water transfer are closely linked (11–14). Under such a condition, the absorption of water appears to be secondary to the inward pumping of solutes, mainly of sodium chloride. Below a certain critical level of sodium ion in the gut contents, this form of water absorption comes to a stop. Thus, it appears that the excess of macromolecular compounds in the germfree lower gut, originating from secretions and from mucosal desquamation, is responsible for binding cations and replacing diffusible anions, which in turn “starves” the mucosa in the process of solute coupled water absorption. In conventional animals, the intestinal microflora seems to be responsible for the degradation of these colloids. It was found that the lower bowel of germfree rats lacks certain mucinases, while such enzymes are present in intestinal contents of conventional controls (15).