Regional differences in rat large intestinal crypt function in relation todehydrating capacity in vivo

1. Rat descending colon absorbed fluid against a large hydraulic resistance , imposed by 10% agarose (w/v) gel plugs inserted in the lumen, by raising the tonicity of the absorbate from the gel to 880 +/- 54 mosmol kg(-1); the tonicity of the absorbate from 2.5% gels was 352 +/- 38 mosmol kg(-1). The hypertonic absorbate generated an osmotic pressure which created a fluid t ension in the crypt lumen. This was monitored as a suction tension in colon ic luminal gels of 45.3 +/- 3 cmH(2)O with 2.5% gels and 725 +/- 145 cmH(2) O with 10% gels. The caecum was unable to absorb fluid against a significan t hydraulic resistance. 2. Fluorescein isothiocyanate-labelled dextran (FITC dextran; molecular mas s 10 000 Da) accumulated within descending colonic crypt lumens by concentr ation polarization. Maximal accumulation at a depth of 20-40 mu m below the mucosal surface was 5.68 +/- 0.2-fold above control levels. Caecal crypts accumulated dextran to a maximum of 1.8 +/- 0.17-fold above control levels. 3. The relationship between crypt luminal tension and suction tension of th e distal colon was also demonstrated using paraffin, which occluded the cry pt lumens with microscopic droplets and completely inhibited fluid absorpti on from high resistance luminal gels. 4. Reduction in dietary Na+ intake raised plasma aldosterone and the capaci ty of the distal colon to dehydrate against a high luminal hydraulic resist ance. The caecum did not respond in this way to varied Na+ intake.