RESECTION OF RABBIT ILEUM: EFFECT ON JEJUNAL STRUCTURE AND CARRIER-MEDIATED AND PASSIVE UPTAKE

¹ Department of Medicine, University of Alberta, Edmonton, Canada T6G 2G3

Adaptive changes occur in nutrient uptake following intestinal resection. A previously validated in vitro technique was used to measure the rate of uptake of solutes six weeks following surgical removal of the distal half of the small intestine in the rabbit. The rate of jejunal uptake of a homologous series of fatty alcohols, cholesterol, and six bile acids was unchanged in the resected animals. The uptake of short- and long-chain length fatty acids was also unchanged following resection, although there was a modest but significant increase in the uptake of decanoic and dodecanoic acid. The incremental change in free energy, Fw l, was increased in the jejunum of resected animals when assessed with the medium chain-length fatty acids, but was unchanged when estimated from the uptake of long chain-length fatty acids. The value of Fw l for bile acids was similar in the jejunum of control and resected animals. There was an increase in the maximal transport rate (J^m_d) for D-glucose, galactose and L-leucine, an increase in the Michaelis constant (K_m) for 3-O-methylglucose, and an increase in both the J^m_d and K_m for fructose. There was no change in the passive component of the uptake of these solutes following intestinal resection. In the resected animals there was no significant increase in the height, width, and surface area of the jejunal villi, the number of mucosal cells per villus, or in the mucosal surface area, although there was a significant increase in the weight of the scraped mucosa and in the height of the microvilli. The modifications in the active and passive kinetic constants were not related to the alterations in the microvillus surface area, and changes in the K_m or Fw l were not due to any variation between control and resected animals in the effective resistance of the intestinal unstirred water layer. These results indicate that, (1) ileal resection was not associated with changes in the effective resistance of the intestinal unstirred water layer; (2) the variable changes in the uptake of passively adsorbed solutes following resection suggests a functional heterogeneity of the villus towards passive permeation; (3) estimation of the incremental change in free energy of the intestine using a homologous series of saturated fatty acids may provide variable results depending upon the chain length of the probes; (4) the estimation of the permeability properties of the membrane based on the uptake of one group of probes cannot be generally applied to describe the permeability properties of the membrane towards all passively transported solutes; (5) it is likely that at least one major adaptive response to ileal resection was an increase in the number of active transport sites per unit area of mucosa, leading to an increase in the value of J^m_d; and (6) those signals responsible for changes in intestinal transport following resection may differ from those signals influencing alterations in morphology.