Extracellular accumulation of Na7+ detected by video-enhanced microscopic imaging of the impenneant fluorescent probe SBFI[1,2] confirms the view that colonic crypts produce a hypertonic absorbate ca 1000 mOsm·kg−1, thereby generating a large osmotic pressure across the crypt wall. This creates a high fluid tension within the crypt lumen, sufficient to dehydrate faeces [3,4]. When bathed in Tyrode the SBFI·Na+ fluorescence indicates a [Na+) ca 750 mM within the interstitial space of metabolizing rat descending colon. There is no evidence of interstitial Na+ accumulation in octanol (2 mM) or in rabbit colon incubated with 1.0 mM ouabain and no evidence of Na+ secretion via the crypt lumen during absorption [5].
