No abstract

Whole-cell recordings were performed at isolated crypts from the distal colon of the rat. Enterocytes in intact crypts, patched from the basolateral side, exhibited a gradient in the resting zero-current potential. Along the axis of the crypt, the highest potentials were measured in the ground region, the lowest in the surface region. The cholinergic agonist, carbachol, induced a hyperpolarization and an increase of the outward current in both the middle and the ground cells of intact crypts. This effect could be prevented by Ba2+ or by the intracellular Ca2+ antagonist, 8-(N, N-diethylamino)-octyl-3,4,5-trimethoxy-benzoate hydrochloride (TMB-8). Its action, however, was not dependent on the presence of external Ca2+. Both ground cells and the cells in the middle part of the crypt responded to forskolin, an activator of the adenylate cyclase, with a depolarization. In the middle part of the crypt, the depolarization induced by forskolin was associated with an increase of the outward current. It could be blocked by the Cl- channel blocker, 5-nitro-2-(3-phenylpropylamino)-benzoate, indicating an increase of Cl- conductance. In contrast, the forskolin-induced depolarization in the ground part of the crypt was associated with a decrease of the outward current. This effect could be prevented by Ba2+, indicating a decrease of a potassium conductance. The changes in outward current could be prevented by the presence of an inhibitor of protein kinase A in the pipette solution. In conclusion, these results suggest that carbachol, an agonist acting on the Ca2+ pathway, indirectly causes Cl- secretion by an increase of the driving force, i.e. the membrane potential. Only the activation of cyclic AMP synthesis by forskolin is able to increase Cl- conductance in the rat colon. The latter response seems to be dependent on the state of differentiation of the enterocytes.

SUMMARY1. Electrolyte transport across two preparations of mucosa from rat colon descendens was compared to determine what influence the submucosal plexus has on electrolyte transport. One preparation consisted of the mucosa, muscularis mucosae, and the submucosal tissue and is referred to as the mucosa-submucosa preparation. The second preparation obtained by further blunt dissection ofthe mucosa-submucosa preparation consisted of only the mucosa and the circular muscle layer of muscularis mucosae and is referred to as the mucosa preparation.2. Histological studies showed that the submucosal tissue and the longitudinal layer of muscularis mucosae could be removed leaving only the mucosa and the circular layer of muscularis mucosae. The extensive neuronal network of the submucosa was shown when the submucosal tissue and longitudinal muscle layer of muscularis mucosae, which were removed, were stained histochemically for acetylcholinesterase activity.3. Both the mucosa-submucosa and mucosa preparations absorbed Na+ and Clwhen short-circuited. However, Na+ and Cl-absorption were significantly higher in the mucosa preparation. The increase in Na+ and Cl-transport in the mucosa preparation was accompanied with a decrease in the short-circuit current (I,,), the open-circuit potential difference (p.d.) and the transmural tissue conductance (Gt) when compared to the mucosa-submucosa preparation.4. Tetrodotoxin (TTX), a neurotoxin which blocks specifically the propagation of action potentials in excitable tissues, dose-dependently decreased ISC and p.d. in the mucosa-submucosa preparation when added to the serosal solution. The half-maximal effective concentration ofTTX was 5 nm and maximal effective concentration 100 nM. TTX (1 ,UM) had no effect on ISC or p.d. when added to the mucosal solution. The decrease in ISC and p.d. caused by TTX in the mucosa-submucosa preparation was accompanied with an increase in Na+ and Cl-absorption. TTX caused only a small decrease in ISC and p.d. in uhe mucosa preparation. However, there was no measurable change in Na+ and Cl-transport in the mucosa preparation.5. The results suggest that spontaneously active neurones from the submucosal plexus have an inhibitory influence on the mucosa. Physical removal of the submucosal plexus or pharmacological blockade of the neurones within the mucosa-H. ANDRES AND OTHERS submucosa preparation by TTX led to enhanced absorption, suggesting that the set point of the mucosa for electrolyte transport is at or near a maximal absorptive state. Regulation or modulation of the mucosa may therefore occur by mechanisms that lower this set point, causing an inhibition of absorption of electrolytes.

Two iron-binding proteins were isolated from rat intestinal mucosa. From determination of their molecular weights, their electrophoretic and iron-binding properties it was established that one was a mucosal ferritin and the other a mucosal transferrin.The mucosal ferritin is compared in its molecular weight, isoelectric point, amino acid composition and tryptic peptide pattern with the ferritins of rat spleen and liver. All three ferritins are distinctly different from one another. In addition the iron content of mucosal ferritin was found to be much lower than that of liver and spleen ferritins.Mucosal transferrin was separated into two components by isoelectric focusing, as was plasma transferrin. The plasma and mucosal transferrins differ in their isoelectric points and in their amino acid compositions. Differences were also found in vitro in the iron-binding of mucosal transferrin as compared with plasma transferrin. The role of these mucosal proteins in the absorption of iron is briefly discussed.

Exposure to a hypotonic medium caused an increase in the diameter of isolated crypts from the rat colon. The increase in cell volume was only transient and lasted about 7 min. Despite of the continuous presence of the hypotonic medium, cell volume decreased again. This regulatory volume decrease (RVD) was inhibited by 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB), a Cl- channel blocker, and by Ba2+, a K+ channel blocker. Cell-attached patch-clamp recordings revealed that the RVD was associated with the activation of previously silent basolateral channels. These channels were identified after excision of the patch as Cl- channels (28 pS) and as K+ channels (45-60 pS). The RVD was dependent on the presence of external Ca2+. The phospholipase A2 inhibitor, quinacrine, and the lipoxygenase blocker, nordihydroguaiaretic acid, inhibited RVD, while indomethacin had no effect. In Ussing chamber experiments an exposure to hypotonic media caused an initial, transient increase in tissue conductance (Gt), followed by a prolonged decrease in short-circuit current (Isc) and the potential difference (V). The height of the electrical response was dependent on the decrease in the osmolarity in a range from 20 mosmol l-1 to 90 mosmol l-1. The increase in Gt was blocked by NPPB and Ba2+, whereas the decrease in Isc or V was inhibited by NPPB but enhanced by Ba2+.(ABSTRACT TRUNCATED AT 250 WORDS)