No abstract

Adenosine 3',5'-cyclic monophosphate (cAMP)-dependent Cl- secretion provides the ionic basis for secretory diarrhea. We quantified the spatial distribution of this process by measuring local ion conductance in crypts and surface epithelium or villi of rat late distal colon and ileum. By use of an improved voltage-scanning technique, the tissue was clamped to a 30-Hz sine-wave current and the electrical field above the respective structures was sensed by a stepping glass microelectrode. Under control conditions, crypts and surface epithelium contributed 61 and 39%, respectively, to the total ion conductance of distal colon. Theophylline (10 mM) increased crypt conductance (Gc) by 64% from 2.5 +/- 0.2 to 4.1 +/- 0.3 mS/cm2 and surface epithelium conductance (Gs) by 69% from 1.6 +/- 0.1 to 2.7 +/- 0.1 mS/cm2. These changes in local conductances were completely Cl- dependent, since theophylline had no effect when Cl- was replaced by gluconate. Similar results were obtained when Cl- secretion was elicited by prostaglandin E1 (1 microM) or by dibutyryl-cAMP (DBcAMP, 1 mM). After stimulation, the Cl- channel blocker 5-nitro-2-(3-phenyl-propylamino)benzoic acid (1 mM) decreased both Gc and Gs. In rat ileum, theophylline plus DBcAMP caused an increase in total conductance of 19% only because of its large paracellular conductance. The ratio of scanning signals above villi and intervillous spaces was unaffected, indicating that Cl- conductance is induced in both crypts and villi. We conclude that in distal large intestine cAMP-dependent Cl- secretion is not confined to crypts but is evenly performed also by surface cells. A similar distribution exists in small intestine.

There is no quantitative assignment of large intestinal electrogenic Na+ absorption to surface epithelium and crypts so far. We determined the spatial distribution of electrogenic Na+ absorption to crypts and surface epithelium of rat late distal colon using a modified voltage-scanning technique. Voltage deflections resulting from external 30-Hz current were sensed by an extracellular microelectrode stepping at 0.7 Hz above crypt openings or surface epithelium. Local conductances were calculated applying a planar model of electrical field distribution to surface epithelium and a electrostatic disk source model to the crypts. These models were confirmed by methodological experiments where the electrode position was varied in vertical and horizontal direction. Electrogenic Na+ absorption was detected by blocking apical Na+ channels by mucosal 0.1 mM amiloride. Under control conditions surface epithelium contributed 44% (2.0 +/- 0.2 mS/cm2) and crypts 56% (2.6 +/- 0.2 mS/cm2) to the total conductance of 4.6 +/- 0.4 mS/cm2. Electrogenic Na+ absorption was induced by 6 h in vitro incubation in a medium containing 3 nM aldosterone. This caused a short-circuit current (ISC) of 12.1 +/- 0.8 mumol.h-1.cm-2, which was paralleled by a 2.5-fold increase in surface epithelial conductance to 5.1 +/- 0.4 mS/cm2, whereas crypt conductance was not significantly altered (3.0 +/- 0.2 mS/cm2). Amiloride reversed ISC to -0.8 +/- 0.1 mumol.-1.cm-2 and decreased surface epithelium conductance to 2.3 +/- 0.3 mS/cm2 but again had no significant effect on crypt conductance (2.5 +/- 0.3 mS/cm2). Sham incubation (no hormones added) for 6 h neither induced electrogenic transport nor altered local epithelial conductances.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic diagnosis of HPS and subsequent prenatal indomethacin therapy seems to have a beneficial effect on the natural course of HPS, especially progression of polyhydramnios; therefore, extreme prematurity could be prevented. Also, postnatally the early diagnosis allows the effective water and electrolyte substitution before severe volume depletion.

Aldosterone‐ and adrenaline‐induced K+ secretion were investigated in rat late distal colon using conductance scanning and Ussing chamber techniques. K+ secretion was unmasked by the K+ channel blocker tetraethylammonium (TEA). Electrogenic Na+ absorption was inhibited by amiloride. Rb+ net fluxes consistently measured about 80 % of K+ secretion estimated using change in short‐circuit current (ΔISC) measurements. Partial block of K+ absorption by mucosal ouabain did not change TEA‐sensitive K+ secretion. Thus, K+ absorption and K+ secretion are not coupled. Additivity of Rb+ fluxes as well as ΔISC caused by 3 nM aldosterone (6 h in vitro incubation) and, subsequently, adrenaline suggested additivity of aldosterone‐induced and cAMP‐mediated K+ secretion in the presence of amiloride. Conductance scanning under control conditions revealed a small TEA‐sensitive K+ conductivity in surface epithelium (0.3 ± 0.2 mS cm−2) but not in crypts, as well as a small basal K+ secretion in surface epithelium (ΔISC= 0.3 μmol h−1 cm−2), which increased during sham incubation. Aldosterone (3 nM, 6 h in vitro incubation) resulted, after correction for the basal K+ secretion, in a K+ secretion of ΔISC= 0.9 μmol h−1 cm−2. Aldosterone induced a TEA‐sensitive conductivity of 1.1 ± 0.3 mS cm−2 in surface epithelium, but not in crypts. Adrenaline (5 μm) caused, in fresh tissue, a K+ secretion of ΔISC= 1.2 μmol h−1 cm−2 and equal conductivity changes in crypts (0.7 ± 0.2 mS cm−2) and surface epithelium (0.7 ± 0.1 mS cm−2). We conclude that K+ secretion induced by aldosterone in physiological concentration is restricted to surface epithelium, whereas cAMP‐mediated K+ secretion is located equally in crypts and surface epithelium.