Ernest Urban

Failure of Lactose and Glucose to Influence in vivo Intestinal Calcium Transport in Normal Rats

The influence of luminal lactose and glucose on intestinal calcium transport was examined in vivo in normal post-weaned young rats. In situ segments of duodenum, midgut and ileum were perfused through the lumen for 1 h with an isotonic solution of 3.4 mM calcium containing radioactive isotopic 45Ca as tracer and additionally either 10 mM lactose or 10 mM glucose, or, for the control group, no saccharide. Net absorption was measured by disappearance of luminal calcium; lumen-to-plasma flux was determined from the disappearance of tracer 45Ca. This concentration of luminal lactose or glucose had no direct effect on calcium transport under these steady state conditions.

Net movements of magnesium and calcium in the rat small intestine in vivo.

Summary Magnesium absorption by the rat small intestine in vivo was studied by a perfusion technique under controlled conditions. Net magnesium absorption per unit weight of intestinal segment was significantly greater in duodenum than in ileum. However, comparisons with previous studies of calcium transport using a similar experimental model show that magnesium was less well absorbed than calcium in the duodenum, while ileal absorption rates were almost equal. Tissue concentrations of magnesium were higher in the duodenum than in ileum, the reverse of the pattern for calcium. Neither tissue calcium nor the secretion of calcium into the intestinal lumen was influenced by transported magnesium. We thank Dr. Paul E. Leaverton for his advice and help with statistical analyses of the data.

Glucose transport by rat small intestine after extensive small-bowel resection

In rats 50 cm of proximal or distal small intestine were resected, preserving duodenum and terminal ileum. Glucose transport was studied 5–6 weeks later, using everted gut sacs from duodenum, ileum, and also from a midgut segment consisting of intestine located preresection at mid-small intestine. Sham-operated animals served as controls: The inner (serosal) fluid medium in sacs from duodenum and midgut gained glucose; ileal sac serosal medium lost glucose. Proximal resection resulted in significant growth of duodenal and midgut mucosa. Duodenal transport specific activity (transport per gram dry mucosa) decreased from control values, but mucosal growth compensated so sac transport capacity (transport per centimeter sac length) remained unchanged. Midgut transport specific activity remained unchanged, thus sac transport capacity directly mirrored increased mucosal mass. Ileal sac serosal medium now accumulated glucose; there was no mucosal growth. Transport specific activity and sac transport capacity of ileum increased in parallel. After distal resection there was no alteration of either duodenal and midgut mucosal masses or transport specific activities, hence sac transport capacities remained unchanged. Ileal sac serosal medium also accumulated glucose, but now both transport specific activity and mucosal mass increased. The resultant increased sac transport capacity was identical to that of ileum after proximal resection. In all sacs from control and resected animals uphill [14C]glucose concentration differences developed between medium and mucosa. Activity of the mucosal uptake process, assessed in terms of a ratio of mucosal intracellular fluid radioactivity to mucosal medium radioactivity, usually mirrored altered transport specific activity. This indicates that the increased undercoats tissue mass that accompanied increased mucosal mass did not critically affect transport. The most striking findings were: (1) decreased duodenal transport specific activity after proximal resection with mucosal growth compensating; and (2) identical adaptations of ileal segment transport capacities after either proximal or distal small-bowel resections, although mechanisms differed. The present study provides a base for further examinations of carrier-mediated hexose transport after extensive loss of small intestine.In rats 50 cm of proximal or distal small intestine were resected, preserving duodenum and terminal ileum. Glucose transport was studied 5–6 weeks later, using everted gut sacs from duodenum, ileum, and also from a midgut segment consisting of intestine located preresection at mid-small intestine. Sham-operated animals served as controls: The inner (serosal) fluid medium in sacs from duodenum and midgut gained glucose; ileal sac serosal medium lost glucose. Proximal resection resulted in significant growth of duodenal and midgut mucosa. Duodenal transport specific activity (transport per gram dry mucosa) decreased from control values, but mucosal growth compensated so sac transport capacity (transport per centimeter sac length) remained unchanged. Midgut transport specific activity remained unchanged, thus sac transport capacity directly mirrored increased mucosal mass. Ileal sac serosal medium now accumulated glucose; there was no mucosal growth. Transport specific activity and sac transport capacity of ileum increased in parallel. After distal resection there was no alteration of either duodenal and midgut mucosal masses or transport specific activities, hence sac transport capacities remained unchanged. Ileal sac serosal medium also accumulated glucose, but now both transport specific activity and mucosal mass increased. The resultant increased sac transport capacity was identical to that of ileum after proximal resection. In all sacs from control and resected animals uphill [14C]glucose concentration differences developed between medium and mucosa. Activity of the mucosal uptake process, assessed in terms of a ratio of mucosal intracellular fluid radioactivity to mucosal medium radioactivity, usually mirrored altered transport specific activity. This indicates that the increased undercoats tissue mass that accompanied increased mucosal mass did not critically affect transport. The most striking findings were: (1) decreased duodenal transport specific activity after proximal resection with mucosal growth compensating; and (2) identical adaptations of ileal segment transport capacities after either proximal or distal small-bowel resections, although mechanisms differed. The present study provides a base for further examinations of carrier-mediated hexose transport after extensive loss of small intestine.

Calcium Transport by the Normal Rat Colon In vivo and In vitro

We studied calcium transport in vivo and in vitro in large bowel of normally fed young male rats. Solutions containing zero, 1.7, 3.4 and 5.0 mM calcium and isotopic 45Ca as tracer were perfused in vivo through the lumen of cecum and descending colon. In vitro flux studies were also carried out in cecum with 2.0 and 0.2 mM calcium solutions utilizing the technique of Ussing and Zerahn whereby both chemical and electrical gradients across planar sheets of cecal tissues were nulled. The combined findings demonstrated that (i) cecum was more permeable to calcium than descending colon indicating a proximal to distal absorption gradient in large bowel; (ii) the transport of calcium in both segments increased with increasing luminal calcium concentration, and (iii) calcium transport in both cecum and descending colon was passive only. Thus the large bowel appears to play only a minor role in calcium homeostasis in the normally fed rat.

Permeability of Polyethylene Glycol in Remnant Small Bowel after Massive Intestinal Resection

Abstract Studies were designed to determine if permeability of adapted (remnant) small bowel mucosa to polyethylene glycol (PEG) was altered after major intestinal resection. Rats underwent 50% small bowel resection with preservation of duodenum and terminal ileum. Sham-operated animals served as controls. Two and four weeks later we cannulated the portal vein and measured mucosal permeability to luminal [3H]PEG and [14C]PEG in isotonic Ringer solution in remnant proximal or distal in situ closed intestinal loops. A lumen-to-portal blood gradient of at least 1000/1 persisted throughout the one-hour experimental period in both resected and sham-operated animals. Thus the adapted remnant intestinal mucosa was highly impermeable to luminal radiotracer PEG. In separate experiments 2 and 4 weeks after 70% small bowel resection or sham operation, in vivo segments of proximal and distal small intestinal were perfused through the lumen for one hour with hypertonic (800 mOsm) mannitol or NaCl solution containing [3H]PEG. There was equal and almost total recovery of [3H]PEG at the end of the experimental period in resected and control animals. The combined data of all experiments indicate that radiotracer PEG may be confidently used as a luminal water phase marker in transport studies of remnant bowel following intestinal resection.

Intestinal Brush Border Alkaline Phosphatase in the Rat after Proximal Small Bowel Resection

Summary In rats 50 cm of proximal intestine were resected with preservation of duodenum and ileum. Mucosal brush border alkaline phosphatase was measured 6-7 weeks later in segments of duodenum, ileum, and a midgut segment located prere-section at mid small intestine. Sham-operated animals served as controls. After resection there was significant mucosal growth in duodenum and midgut. In duodenum brush border alkaline phosphatase specific activity (activity per gram of wet mucosa) decreased, but increased mucosal mass compensated so total enzyme activity (activity per centimeter of intestinal length) did not change. In midgut both enzyme specific activity and total activity increased maximally while in ileum neither enzyme specific activity nor total activity changed. These studies show that brush border alkaline phosphatase undergoes postresection changes but alterations vary with the location of the intestinal segment studied. Comparisons with an earlier study of calcium transport suggest that duodenal brush border alkaline phosphatase may play a role in postresection transport adaptation. The author would like to thank Dr. John Ghidoni for the preparation of a number of electron micrographs of the brush border preparations, Marshal Pena for his technical assistance, and Betty Medina for her secretarial skills.

Permeability of adolescent rat intestine to pancreatic ribonuclease.

There are indications that exocrine pancreatic enzymes may undergo an enteric recirculation. Ribonuclease (RNase) is present in considerable quantities in pancreatic exocrine secretions. Therefore we studied the intestinal transport of pancreatic RNase using everted gut sacs from duodenum, midgut, and ileum of young rats. Gut sacs were incubated aerobically at 37 degrees C, 22 degrees C, and 0 degrees C, and anaerobically at 37 degrees C. Measurable amounts of RNase crossed the intestinal barrier, but neither duodenum, midgut nor ileum absorbed RNase preferentially. The experimental data favor simple diffusion of RNase across rat small bowel. However, the amounts of pancreatic RNase absorbed were small. Thus, while our data do not negate the concept of an enteropancreatic recirculation in the rat, only a minor quantity of luminal, pancreatic RNase could recirculate.

Extensive small bowel resection in the rat: lack of effect on serum proteins.

Rats were subjected to serial venesections 3 days prior to and then sequentially for 4 weeks after surgical removal of either the proximal or distal half of the small bowel. Total serum protein concentration and protein electrophoretic patterns were determined on these sera. Both sham-operated and unoperated animals served as controls. The data showed differences in serum proteins in the first week after operation between the three groups of operated rats and the unoperated controls. However, there were no significant differences in serum proteins between the sham-operated and the two types of resected animals. This indicates that the combined effects of laparotomy, anesthesia, pre- and postoperative fasting outweighed any additional early effects of loss of 50% small bowel on serum proteins. The study also demonstrates the necessity of having both unoperated and sham-operated control groups in designing experiments involving surgical extirpation of intestine.

Serum proteins in the rat after extensive proximal or distal small bowel resection.

Rats underwent surgical removal of either the proximal or distal half of the small bowel. 5-6 weeks later, total protein concentrations and electrophoretic patterns were determined in sera from the two groups of resected animals. The data compared to similar measurements from sham-operated and unoperated control groups, showed no differences between resected or control animals. This indicates re-establishment of overall nutritional adequacy by this time interval after substantial loss of either proximal or distal small intestine.

In vivo absorption of pancreatic ribonuclease from the small intestine of young and mature rats.

It has been postulated that exocrine pancreatic enzymes may undergo an enteropancreatic recirculation as a mechanism for the conservation of digestive enzymes. Exocrine pancreatic secretions contain considerable amounts of ribonuclease (RNase), Therefore we examined the absorption of RNase from the small bowel of 4-week and 10-week-old rats by measuring concentration changes of the substrate in portal blood after instilling RNase into in vivo, in situ intestinal loops. Unphysiologically high intestinal luminal RNase concentrations were required for sufficient RNase to cross the mucosal barrier and raise portal vein blood RNase levels. A gradient of 1,000:1 existed between intestinal lumen and portal vein blood RNase concentrations in the 4-week-old rats. The gradient was 10 times greater in the mature animals. No significant differences in luminal absorption of RNase from duodenum or ileum were detected. The data indicate that RNase penetrates the intestinal mucosa in vivo but that only small amounts are absorbed. Therefore if an enteropancreatic recirculation of exocrine pancreatic RNase exists in the rat, only neglible quantities of RNase are included in such a recirculation.