Ion Transport In Distal Colon Research Articles

The secretory KCa1.1 channel localises to crypts of distal mouse colon: functional and molecular evidence

The colonic epithelium absorbs and secretes electrolytes and water. Ion and water absorption occurs primarily in surface cells, whereas crypt cells perform secretion. Ion transport in distal colon is regulated by aldosterone, which stimulates both Na(+) absorption and K(+) secretion. The electrogenic Na(+) absorption is mediated by epithelial Na(+) channel (ENaC) in surface cells. Previously, we identified the large conductance Ca(2+)-activated K(+) channel, K(Ca)1.1 or big potassium (BK) channel, as the only relevant K(+) secretory pathway in mouse distal colon. The exact localisation of K(Ca)1.1 channels along the crypt axis is, however, still controversial. The aim of this project was to further define the localisation of the K(Ca)1.1 channel in mouse distal colonic epithelium. Through quantification of mRNA extracted from micro-dissected surface and crypt cells, we confirmed that Na(+)/K(+)/2Cl(-) (NKCC1) is expressed primarily in the crypts and γ-ENaC primarily in the surface cells. The K(Ca)1.1 α-subunit mRNA was like NKCC1, mainly expressed in the crypts. The crypt to surface expression pattern of the channels and transporters was not altered when plasma aldosterone was elevated. The mRNA levels for NKCC1, γ-ENaC and K(Ca)1.1 α-subunit were, however, under these circumstances substantially augmented (K(Ca)1.1 α-subunit, twofold; NKCC1, twofold and ENaC, tenfold). Functionally, we show that ENaC-mediated Na(+) absorption and BK channel-mediated K(+) secretion are two independent processes. These findings show that K(Ca)1.1-mediated K(+) secretion mainly occurs in the crypts of the murine distal colon. This is in agreement with the general model of ion secretion being preferentially located to the crypt and not surface enterocytes.

β-Adrenoceptors, but not dopamine receptors, mediate dopamine-induced ion transport in late distal colon of rats

Dopamine, an important modulator in the gastrointestinal system, induces concentration-dependent transepithelial ion transport in the distal colon of the rat, as shown by a decrease in the short-circuit current, and acts in a segmentally dependent manner. However, the receptor(s) that mediates dopamine-induced ion transport is unknown. We have investigated the receptor mechanisms underlying dopamine-induced colonic ion transport by means of short-circuit current recording, real-time polymerase chain reaction, and Western blotting analysis, plus gene transfection and enzyme-linked immunosorbance assay. mRNA transcripts of adrenoceptors (alpha, beta) and dopaminergic receptors (D(1) and D(2)) were detected in the rat late distal colonic mucosa, with beta(2) displaying the highest expression. A similar result was found in human colorectal mucosa (equivalent of late distal colon in rat). Pretreatment with a beta(1)-adrenoceptor antagonist (CGP-20712A) and a beta(2)-adrenoceptor antagonist (ICI 118,551) inhibited the dopamine-induced short-circuit current response by 52.59% and 92.51%, respectively. However, neither dopamine D(1) receptor antagonist SCH-23390 nor dopamine D(2) receptor antagonist sulpiride blocked the effect of dopamine. Protein expression of both beta(1)- and beta(2)-adrenoceptors was found in the mucosa of rat distal colon and human sigmoid colon and rectum. Dopamine significantly increased intracellular cAMP levels in COS-7 cells transfected with beta(1)- or beta(2)-adrenoceptors. Thus, beta-adrenoceptors (mainly beta(2)-adrenoceptors), but not dopamine receptors, mediate dopamine-induced ion transport in the late distal colon of the rat. This extends our knowledge of the late distal colon (rats) or colorectum (human) and provides further experimental evidence that might aid the prevention, diagnosis, and clinical therapy of human colorectal diseases.

Effect of chronic metabolic acidosis on net electrolyte transport in rat colon

Rats fed NH4Cl (5 meq.100 g body wt-1.day-1) for one week developed chronic metabolic acidosis and had an arterial blood pH and plasma HCO3- concentration of 7.27 +2- 0.02 and 16.2 +/- 0.8 meq/l, respectively; control animals had values of 7.36 +/- 0.01 and 22.4 +/- 0.5 meq/l, respectively. Net electrolyte transport was measured in proximal and distal colonic segments by in situ perfusion. In proximal colon, chronic metabolic acidosis increased HCO3- absorption from 3.3 +/- 0.8 to 6.4 +/- 0.6 mu eq.min-1.g-1 but did not alter Na+ absorption. In distal colon, although Na+ transport was unaffected, chronic acidosis reduced HCO3- secretion from -6.9 +/- 0.8 to -4.4 +/- 0.7 mu eq.min-1.g-1 and increased voltage from -18.9 +/- 2.0 to -51.1 +/- 4.2 mV. To evaluate the dependence of these effects on altered arterial pH and HCO3- concentration, NaHCO3 was infused intravenously, raising pH and HCO3- concentration to 7.53 +/- 0.04 and 23.9 +/- 1.7 meq/l, respectively. Although acute correction of chronic metabolic acidosis reduced HCO3- absorption in proximal colon, it did not affect HCO3- secretion or voltage in the distal segment, suggesting that proximal and distal colon respond differently to chronic metabolic acidosis. These results also suggest that chronic metabolic acidosis alters the mechanisms of ion transport in distal colon.

Effect of disodium azodisalicylate on electrolyte transport in rabbit ileum and colon in vitro: Comparison with sulfasalazine and 5-aminosalicylic acid

Azodisalicylate, used to treat ulcerative colitis, causes diarrhea in up to 12.5% of patients. We compared the in vitro effects of azodisalicylate, sulfasalazine, and 5-aminosalicylic acid on rabbit intestinal electrolyte transport. Distal ileal mucosae mounted in Ussing chambers were exposed to varying concentrations of the drugs. Mucosal addition of azodisalicylate (>5 mM) caused the greatest aniondependent increase in short-circuit current of 83 μA/cm2 (ED50 = 0.3 mM). Isotope flux measurements suggest that azodisalicylate may stimulate predominantly electrogenic HCO3 secretion and induces net NaCl secretion. In contrast, serosal addition of azodisalicylate and sulfasalazine (>5 mM) decreased short-circuit current, and 5-aminosalicylic acid had no effect. Azodisalicylate had no effect on ion transport in distal colon. The effects of azodisalicylate in ileum were not inhibited with piroxicam (an inhibitor of cyclooxygenase). Mucosal cyclic adenosine monophosphate and cyclic guanosine monophosphate levels were unchanged after ileal exposure to azodisalicylate. Azodisalicylate appears to be a mechanistically unusual secretagogue, possibly explaining the increased incidence of diarrhea seen in patients taking the drug.

Steroids alter ion transport and absorptive capacity in proximal and distal colon

Steroids are potent absorbagogues, increasing Na and fluid absorption in a variety of epithelia. This study characterizes the in vitro effects of pharmacological doses of gluco- and mineralocorticoids on transport parameters of rabbit proximal and distal colon. Treatment with methylprednisolone (MP, 40 mg im for 2 days) and desoxycortone acetate (DOCA, 12.5 mg im for 3 days) resulted in a significant increase in short-circuit current (Isc) in distal colon, suggesting an increase in basal Na absorption. Amiloride (10(-4) M) caused a significantly negative Isc in MP-treated tissue, demonstrating a steroid-induced, amiloride-insensitive electrogenic ion transport in distal colon. The effect of two absorbagogues, impermeant anions (SO4-Ringer) and amphotericin, were compared in control and steroid-treated distal colon. In controls, both absorbagogues increased Isc. Impermeant anions caused a rise in Isc in both MP and DOCA tissues, suggesting that the high rate of basal Na absorption had not caused a saturation of the Na pump. The steroid-treated colons, however, did not consistently respond to amphotericin. Amiloride inhibited the entire Isc in MP-treated distal colon that had been exposed to amphotericin; this suggested that amphotericin had not exerted its characteristic effect on the apical membrane of steroid-treated colon. In proximal colon, steroids did not alter basal rates of transport; however, epinephrine-induced Na-Cl absorption was significantly greater in MP-treated vs control (P less than 0.005). Steroids increase the absorptive capacity of both proximal and distal colon for Na, while increasing basal Na absorption only in the distal colon.(ABSTRACT TRUNCATED AT 250 WORDS)