Absorption by the colon during prolonged infusions in conscious, unrestrained rats

The effect of drugs acting on cholinoceptors and mucosal chloride on luminal bicarbonate transport by rat caecum under in vitro conditions

1. The transport of HCO3- (Jsm) from a HCO3(-)-buffered serosal to an unbuffered mucosal saline solution has been studied in rat caecum in vitro. 2. Carbachol, bethanechol and acetylcholine (ACh) caused a concentration-dependent fall in Jsm with similar maximum effects. 1,1-Dimethyl-4-phenyl-piperazinium iodide (DMPP) also inhibited Jsm but the effect was less than with the other drugs. Maximum cholinoceptor inhibition was less than that obtained with anoxia. 3. Responses were blocked by atropine (10(-5) M) but hexamethonium (2 x 10(-4) M) significantly altered the response only to DMPP. 4. Physostigmine (10(-5) M) shifted the ACh response curve to the left but physostigmine itself caused inhibition of Jsm which was blocked by atropine. 5. Substitution of mucosal Cl- by NO3- reduced Jsm to a similar extent to maximum cholinoceptor effect and abolished responses to bethanecol. Anoxia further reduced Jsm in the presence of NO3-. 6. Mucosal SITS and DIDS (1 mM) reduced Jsm but this was less than the maximum inhibition seen with drugs acting on cholinoceptors or mucosal Cl- removal. Serosal DIDS caused a similar inhibition. 7. We conclude that cholinoceptor agonists inhibit but do not abolish luminal bicarbonate transport by an action on muscarinic receptors.

Adaptation of electrolyte transport in rat large intestine after proximal resection. II. Colon after 50% jejunoilectomy combined with cecectomy

Electrolyte transport was studied in rat colon adapting to 50% small intestinal resection with cecectomy. Four weeks after surgery, colonic gross surface area, dry and wet weight were increased compared to sham-operated controls. Net absorption in vivo of sodium, chloride and volume was stimulated per organ and per unit tissue mass, and net potassium secretion was diminished. The electrical potential difference, mucosal Na-K-ATPase specific activity and cAMP concentration remained unaffected. In vitro, measurements of unidirectional fluxes and electrical parameters across isolated mucosa indicated enhanced electrically neutral sodium chloride absorption in the proximal and possibly diminished bicarbonate secretion in the distal colon. Thus, removal of the cecum in addition to partial jejunoilectomy induces profound adaptive changes in the colon, involving not only mucosal growth but also a functional reaction of the individual epithelial cell.

Absorption of electrolytes and volatile fatty acids in the hind-gut of the rabbit. Circadian rhythm of hind-gut electrolytes and plasma aldosterone

Interrelationships between the amount of water, solute concentrations in the hind-gut, plasma aldosterone concentrations, and the excretory cycle in the rabbit were studied as well as the effects of hormones generally implicated in electrolyte regulation. It appeared that the levels of water, electrolytes and volatile fatty acids (VFA) in the large intestine varied cyclicly with the excretion pattern: during the soft-phase, water, sodium, chloride and VFA were absorbed and potassium secreted as in other mammals; during the hard-phase two opposite phenomena occurred: secretion in the ascendant colon and absorption in the rest of the colon. Absorption of water, Na, K, chloride and VFA was increased and by this means only material without nutritional value was lost. Plasma levels of aldosterone showed a circadian rhythm parallel to hind-gut changes, i.e. the higher values were found during the hard-phase. Exogenous aldosterone enhanced secretive and absorptive processes, which occurred normally when hard faeces were elaborated.

Hormonal regulation of electrolyte and water transport in the colon

The colon participates in water and electrolyte homeostasis by the absorption of sodium (Na) and water as well as by potassium (K) secretion. The primary step of colonic transport is the active Na transport via a transcellular route. Steroidal hormones considerably increase Na absorption by utilizing two mechanisms: (1) passive Na entry into the cells in enhanced by an increased membrane permeability; (2) active transport capacity is increased by a stimulation of ATPase synthesis. Mineralocorticoid versus glucocorticoid actions of steroids have not yet been clearly differentiated; parallel influences are possible. Active chloride (Cl) secretion is found in the colon under certain pathological conditions and is induced by a number of factors, e.g., hormones produced by pancreas tumors. Cellular events involve a rise of intracellular cAMP and calcium (Ca) concentrations, and altered Cl permeabilities. Functional changes of colonic epithelial cells caused by hormones assume a significant role in the etiology of diarrhea, as well as in compensatory processes by which an intestinal loss of electrolytes and water is prevented.

Sodium absorption with bacterial fatty acids and bile salts in the proximal and distal colon as a guide to colonic resection

Rates of sodium absorption in the presence of bacterial fatty acids and bile salts are unknown along the length of the colon. Such information may guide resection of colon with a view to leaving colon most optimal for ion absorption. Absorption of sodium and permeability of the colonic mucosa to 51Cr EDTA was measured in the proximal (PC) and distal colon (DC) instilled with NaCl (120 mM), n-butyrate (40 mM) and chenodeoxycholic acid (1 or 2 mM) in varying combinations. Sodium absorption in the PC was 123.6 +/- 19 (nmoles/min/cm2)(n = 7) with saline alone and was doubled (P less than .001) when 40 mM n-butyrate replaced chloride anions. Sodium absorption in the DC with saline alone was 106.9 +/- 18 without significant alteration by addition of n-butyrate. CDC (1 mM) diminished sodium absorption in the PC and DC. Diminished sodium absorption induced by bile salts was significantly reversed by n-butyrate in the PC but not DC. Permeability of 51Cr EDTA was greatest with 2 mM CDC in the distal colon. Mucosal function--sodium absorption with SCFAs, permeability changes, and protection by bacterial fatty acids from sodium losses due to bile salts--was superior in the PC compared with DC. Our results suggest that it may be preferable to preserve the proximal rather than distal colon in operations of the colon to secure optimal absorption of sodium.

Morphologic and functional adaptations of large bowel after small-bowel resection in the rat

Seventy percent small bowel was resected in rats. Two and four weeks later transport of sodium, chloride, and water was examined in cecum and more distal large bowel (colon) using a well-established in vivo luminal perfusion technique. Sham-operated and unoperated rats served as controls. In cecum mucosa grew 29% by two weeks after resection but transport remained unchanged. There were no further adaptive changes by four weeks after resection. Unexpectedly, cecum of all rats secreted water and electrolytes. The mechanism remains unclear. In colon there were no adaptive changes by two weeks after resection but by four weeks colon mucosa increased 14% and luminal absorption increased proportionately. Separate studies showed hexose transport could not be induced in cecum or colon, although we have previously demonstrated its induction in contiguous remnant ileum.

Anion transport in the colon

The principal anions transported by colonic epithelium are Cl-, HCO3- and organic anions (OA-), particularly acetate, butyrate and pyruvate, these last being formed by microbial degradation of carbohydrate. In the normal absorptive rat colon, Cl- is transported from lumen to plasma both by the transcellular and paracellular pathways. The transcellular route appears to depend on amiloride-insensitive coupling of Na+-Cl- at the mucosal (apical) membrane, the Na+ electrochemical gradient energizing Cl- uptake. Intraluminal [HCO3-] rises as Cl- as absorbed, and a mucosal Cl- -HCO3- exchange carrier has been postulated. In some species (and in distal colon of the rat when sodium-depleted), the putative Na+-Cl- carrier is absent so that Cl- absorption then depends largely on the paracellular electrochemical gradient. Absorption of OA- is independent of the transepithelial p.d., is associated with HCO3- secretion and is considerably reduced by acetazolamide. In the absence of Cl-, OA- supports Na+ absorption but does not depend on it continuing unchanged when the latter is blocked. Colonic epithelium can become secretory and an example of this state is congenital chloridorrhoea in which an elevated transepithelial p.d. is associated with excessive Cl- secretion. Here, it appears that the Na+-Cl- and Cl- -HCO3- carriers are lost and Cl- conductance of the mucosal membrane substantially increased. The transepithelial uphill movements of Cl- or HCO3- in the absorptive and secretory colon appear to depend on coupling to other ionic flows, and there seems to be no need to postulate active transport of these ions.