Somatostatin enhances cAMP-dependent short-circuit current in human colon via somatostatin receptor subtype-2.

Somatostatin inhibits colonic ion secretion in animal models and cultured intestinal cell lines via somatostatin receptor subtype 2 and subtype 1, respectively. In a recent in vitro ion transport study of the human colon, somatostatin was shown to stimulate short-circuit current, a measure of electrogenic ion transport. In this study we have used the reverse-transcription polymerase chain reaction (RT-PCR) and measurements of changes in short-circuit current (Isc) in response to receptor subtype-specific analogs of somatostatin, to define the somatostatin receptor subtype responsible for the stimulation of short-circuit current in human colon. Somatostatin receptor subtypes 1, 2, and 5, but not 3 and 4, were detected in the human colonic epithelium. Measurements of short-circuit current showed somatostatin and octreotide (1 micromol/liter) increased the prostaglandin stimulated short-circuit current by 12.3+/-1 and 11.0+/-1 microA/cm2, respectively. Similarly, analogs selective for somatostatin receptor subtypes 2 and 5 (1 micromol/liter) produced an increase of short-circuit current of 11.7+/-1 and 13.2+/-1 microA/cm2, respectively. However, at a concentration (10 nmol/liter) near the EC50, the somatostatin receptor subtype 2 analog increased short-circuit current by 9+/-1 microA/cm2, whereas the receptor subtype 5 analog had no effect. There was no difference in receptor expression or effect of the peptides related to the anatomical site of tissue collection. In conclusion, human colonic mucosa expresses multiple somatostatin receptor subtypes, of which subtype 2 mediates the stimulatory effect of somatostatin on ion transport.

Somatostatin and octreotide stimulate short-circuit current in human colonic epithelium.

In vitro somatostatin is a potent inhibitor of intestinal ion secretion in animal models and cultured human cell lines, providing a rationale for its use in secretory diarrheas. However, the effects of somatostatin on ion transport in native human colonic epithelium have not been reported. In this study the effects of somatostatin and octreotide on the basal short-circuit current and the cAMP- and Ca2+-stimulated short-circuit current were studied in isolated human colonic mucosa mounted in Ussing chambers. Under basal conditions somatostatin and octreotide (1 micromol/liter) stimulated a small, bumetanide-sensitive increase in short-circuit current. Following stimulation of secretion with prostaglandin E2, somatostatin and octreotide further increased the short-circuit current in a dose dependent fashion (ED50 approximately 10 nmol/liter for both). This stimulation of short-circuit current was not affected by pretreatment of the tissue with basolateral tetrodotoxin (1 micromol/liter) or mucosal amiloride (10 micromol/liter). In contrast, somatostatin and octreotide had no effect when secretion was stimulated with 8-bromo-cAMP, and pretreatment of the tissue with somatostatin and octreotide (0.1 micromol/liter) did not alter the secretory response to carbachol. The absence of any inhibitory effect of somatostatin and octreotide on electrogenic secretion in the human colon may explain the variable results obtained when somatostatin or octreotide are used for the treatment of secretory diarrheas.

Omeprazole therapy decreases the need for dilatation of peptic oesophageal strictures.

BACKGROUND: Better control of gastric acid secretion with omeprazole appeared to decrease the need for dilatation of oesophageal strictures complicating gastro-oesophageal reflux disease in our hospital-based endoscopy service. AIM: To investigate whether the perceived decrease in the need for oesophageal dilatation could be documented from endoscopy records, and, if confirmed, whether this could be related to the treatment used. PATIENTS AND METHODS: Retrospective study of the records of 69 patients who had peptic oesophageal strictures dilated, followed by treatment with acid inhibition for at least 6 months. Mean duration of follow-up was 3.9 years during treatment with H2-receptor antagonists and 2.1 years while on omeprazole (258 and 78 patient-years, respectively). Re-dilatation rates were compared between those treated with H2-receptor antagonists or omeprazole. RESULTS: There has been a significant decrease in dilatations performed for gastro-oesophageal reflux induced strictures (P<0.001), while dilatation rates for other indications remained constant. Treatment with omeprazole not only decreased the need for further dilatations, but also prolonged the mean time between any further dilatations to 26.3 months compared to 9.3 months for those on an H2-receptor antagonist (P<0.0001). CONCLUSIONS: Following dilatation of peptic oesophageal strictures, treatment with omeprazole in place of an H2-blocker significantly decreases the need for repeat dilatation.

Impaired intestinal electrolyte transport in rats infested with the common parasite Syphacia muris.

An incidentally discovered infestation with the nematode Syphacia muris of cecum and colon in spontaneously hypertensive (SHR) and normotensive control (WKY) rats was investigated over a two-year period. Infestation rates in WKY were higher than in SHR, while clinical signs as well as histological changes of colonic tissues were absent in both strains. In vivo net water absorption (microliter/hr/cm2) in control worm-free SHR turned into secretion in infested rats, ie, from 74.2 +/- 23.2 to -7.5 +/- 35.0 (P less than 0.001); this corresponded with a decrease in net absorption (mumol/hr/cm2) of Na from 18.5 +/- 2.4 to 9.3 +/- 4.3 (P less than 0.001) and of Cl from 14.0 +/- 3.2 to 3.2 +/- 5.7 (P less than 0.001). In WKY, net water absorption decreased from 112.2 +/- 23.2 to 48.0 +/- 25.1 (P less than 0.001) and Na and Cl absorption from 22.3 +/- 3.1 to 16.0 +/- 4.2 (P less than 0.005) and from 19.4 +/- 2.7 to 10.9 +/- 4.7 (P less than 0.005), respectively. Antihelminthic treatment with 0.007% pyrvinium pamoate in the ration (four weeks on, six months off) eradicated Syphacia muris in both rat strains. Body weight gain of young rats on normal and pyrvinium pamoate-substituted diet studied over 18 months was similar, indicating a good tolerance of the treatment. It is concluded that results obtained during comparative intestinal transport studies between SHR and WKY may not only be impaired but also significantly distorted by Syphacia muris infestation as SHR appear to be more susceptible to effects induced by this common parasite than WKY.

Only pharmacological doses of atrial natriuretic peptide affect intestinal ion transport in non-volume expanded rats.

Pharmacological doses of atrial natriuretic peptide were infused into rats to study its effect on intestinal transport. Saline control or two concentrations of rat alpha atrial natriuretic peptide (0.06 or 1.0 nmol/min/kg) were administered intravenously (1 ml) over one hour. Jejunal net transport of water and electrolytes was measured with a plasma-like luminal electrolyte solution using a 'closed loop' technique. Distal colonic potential difference and arterial blood pressure were monitored continuously. Blood samples for analysis of plasma atrial natriuretic peptide concentrations were taken at the end of the experiments. Plasma concentrations were increased (mean (SD) (2.1 (0.5) and 24.0 (1.1) nmol/l respectively) compared with the controls (0.023 (0.016) nmol/l). Blood pressure dropped by 30% (p less than 0.05) in both groups of rats receiving atrial natriuretic peptide but remained unchanged when control saline was infused. Jejunal net absorption was reduced (p less than 0.01) only in animals receiving the higher concentration of peptide (H2O from 173 (33) to 64 (69) microliters.h-1.cm-2, Na from 25.7 (5.3) to 10.9 (8.9) mumol.h-1.cm-2). Distal colonic potential difference was not affected by atrial natriuretic peptide. In conclusion massive doses of atrial natriuretic peptide are required to produce any change in intestinal salt and water transport in normal, non-volume expanded rats; these effects could be a non-specific or 'toxic' response.

Atrial natriuretic peptide and water and electrolyte transport in the human jejunum.

The effects of atrial natriuretic peptide were investigated on water and electrolyte transport in the human jejunum. Six healthy male volunteers (aged 21-33 years) were studied using a triple lumen perfusion technique. A plasma like electrolyte solution containing polyethylene glycol (5 milligrams) as a non-absorbable marker was perfused into the jejunum at 10 ml/min, and net water and electrolyte transport and transepithelial potential difference were measured. Subjects were studied single blind on two occasions with either intravenous atrial natriuretic peptide (6 pmol/min/kg for 90 minutes) or placebo (saline), both after controlled sodium intake over three days. Plasma atrial natriuretic peptide concentrations rose from (mean (SD)) 10.3 (3.6) pmol/l to a peak of 96.0 (61.8) pmol/l. Jejunal net water and electrolyte fluxes and potential difference were identical in both the atrial natriuretic peptide and the control studies. Compared with placebo atrial natriuretic peptide induced a significantly greater diuresis (peak 10.2 (6.0) v 1.8 (1.0) ml/min, p less than 0.05) and natriuresis (peak 1069 (351) v 376 (208) mumol/min, p less than 0.01) and haemoconcentration (haematocrit 0.405 (0.040) v 0.368 (0.018), p less than 0.01). There was no difference in blood pressure, pulse rate, plasma electrolytes, and plasma osmolality between the two studies. There was no evidence to suggest an effect of atrial natriuretic peptide on jejunal water and electrolyte transport in healthy human subjects.

Evaluation of a new fine needle technique in routine percutaneous liver biopsy.

A new fine needle aspiration biopsy technique (Vacu-Cut, 19.5 G) was compared to the 17 G Menghini needle in 20 consecutive patients scheduled for routine percutaneous liver biopsy (10 at random with each needle). The Menghini system was superior in producing sufficient material for histological assessment with 90% success on first pass biopsy (100% with 12 passes in 11 patients) as compared to 60% with the Vacu-Cut needle (90% with 16 passes in 10 patients). The latter was easier to use. Ability to reach a histological diagnosis was 100% with both needles. No major complications occurred: only one Vacu-Cut patient (10%) developed pain at the puncture site as compared to seven patients (65%) in the Menghini group, irrespective of number of passes. The ease of use and lower incidence of pain may favour the Vacu-Cut needle in selected ill patients.

Intestinal ion transport in rats with spontaneous arterial hypertension.

1. Ion balance, intestinal ion transport in vivo with luminal Ringer, and direct voltage clamping in vivo with luminal Ringer and sodium-free choline-Ringer were studied in young (40 days old) and adult (120 days old) spontaneously hypertensive rats (SHR) and age-matched normotensive controls (Wistar-Kyoto rats, WKY). 2. Faecal sodium output was significantly higher in SHR compared with WKY in both young (+67%) and adult (+43%) rats. 3. Small-intestinal sodium absorption was equal in young SHR and WKY, but significantly greater net sodium absorption was found in the ileum of adult SHR. In contrast, net sodium absorption was reduced from the colon of both young and adult SHR. 4. In adult SHR, the colonic transepithelial short-circuit current (Isc) and the transepithelial potential difference (PD) were significantly higher, whereas the transepithelial membrane resistance (Rm) was significantly lower than in WKY. There was an identical drop in Isc in both strains when luminal sodium was replaced by choline. These data cannot be explained by increased electrogenic cation (sodium) absorption in the SHR, but would favour chloride secretion. 5. It is suggested that in SHR membrane electrolyte transport abnormalities may also be present in the epithelial cells of the small and large intestine, as have been demonstrated already in blood cells by several investigators. The SHR may become an interesting experimental animal model for the study of generalized ion transport disorders.

Ion transport in rat proximal colon in vivo.

Active Na+ absorption by the rat proximal colon in vivo is for the most part electrically silent. The rheogenic Na+ flux makes up only 8%. To elucidate the underlying transport pathways, the following experimental approaches were used: ion substitution experiments such as choline for Na+, cyclamate for Cl-, variation of luminal pH; administration of known inhibitors; and determination of changes in luminal CO2 tension and pH. The transcolonic ion fluxes as well as the electrical parameters potential difference, specific electrical resistance, and short-circuit current were monitored. Na+ transport was drastically reduced in the absence of luminal Cl-, and vice versa Cl- absorption was blocked at zero Na+. NaCl absorption was blocked by amiloride (10(-3) M) and 4-acetamido-4'-isothiocyanostilbene-2, 2'-disulfonic acid and was lowered by acetazolamide. Colonic NaCl absorption was not influenced by luminal furosemide. Na+ absorption increased with alkalinization of the luminal fluid. Tris instead of HCO-3 buffer at constant pH favored Cl- uptake. The results may easily be explained by the operation of a Na+-H+ antiport functionally coupled to a Cl(-)-HCO-3 antiport. These transport processes are supposed to be present in the columnar cells of the colonic epithelium. There is good evidence for the association of K+ secretion with rheogenic Cl- secretion by the crypt cells.

Determination of rheogenic ion transport in rat proximal colon in vivo.

A direct clamping technique is demonstrated, which allows monitoring of rapid changes of the short-circuit current (Isc) and the specific transepithelial resistance (Rm) as well as measurement of ion fluxes under short-circuit conditions in vivo. Due to the cylindrical symmetry of the colon the intraluminal electrode was devised as a centrally fixed silver rod, by which radial current injection was achieved. The geometrical arrangement of the electrodes guaranteed zero potential difference (PD) along the whole axis of the colon segment. The Isc was determined to 3.3 +/- 0.7 mueq h-1 cm-2 and Rm equal to 121 +/- 5 omega cm2. These data obtained by direct short-circuiting agree well with our earlier Rm and Isc data based on cable analysis, where the Isc was calculated from the open-circuit PD and Rm. This is considered as evidence for the reliability of the two independent in vivo techniques. Their validity was confirmed by the expected effects of drugs acting on rheogenic ion transport. Both the indirect (via Rm) as well as the direct Isc determination may be used alternatively as required; one may serve to match the other. For larger tubular structures like the rat colon the direct clamping should be preferred as the standard procedure for the Isc determination in vivo.

Determination of short-circuit current in the in vivo perfused rat colon.

Current pulses (I) were injected into the lumen of proximal colonic segments in vivo, and the corresponding voltage deflections (delta PD) superimposed on the transcolonic PD were recorded. From the exponential decay of delta PD along the colon axis, the electrical length constant (lambda) was determined. Based on cable analysis the input resistance (= delta PD x = 0/I) and lambda made it possible to calculate the specific resistance (Rm) of the colonic epithelium as 128 +/- 16 omega X cm2. As Rm proved to be an ohmic resistor, the extrapolation from open-circuit PD (8-12 mV, lumen negative) to zero PD was feasible and made the calculation of short-circuit current (= PD/Rm) equal to 70 +/- 16 microA/cm2. In the presence of amiloride short-circuit current decreased to about 50%, whereas with theophylline it increased by about 30%. Substitution of luminal Na+ with choline or Cl- with cyclamate was associated with a marked increase of Rm. The rheogenic component of net Na+ transport was estimated to be only 8%. Electroneutral Na+ absorption functionally coupled with Cl- absorption displayed the characteristic feature of ion transport in the rat proximal colon.

Interrelationships of ion transport in rat submaxillary duct epithelium.

The transport of Na+, K+, Cl-, and HCO3(-) across the epithelium of the rat submaxillary salivary duct is postulated to be due to the coupling of the basolateral Na+-K+-ATPase with various ion transport systems in the luminal and basolateral membranes. Na+ reabsorption depends on the presence of a rheogenic (Na+ conductance) and an electroneutral (Na+:H exchange) pathway, both of which are sensitive to amiloride. K+ secretion is postulated to be mediated by a K+: H+ antiport, coupling between Na+ reabsorption and K+ secretion, thus depending on local H+ ion concentration. The ratio between electroneutral Na+ influx and K+ efflux, therefore, determines the rate of HCO3(-) secretion. In the absence of Na+ influx, although K+ efflux falls, HCO3(-) secretion rises to a value equal to that of K+ secretion. The maintenance of K+ secretion in the absence of luminal Na+ requires an additional Na+-entry step across the basolateral membrane, also postulated to be due to Na+:H+ exchange.

[Inhibition of the exchange of Na+ for K+ and and H+ by triamterene (in epithelia)(author's transl)]. / Hemmung des Austausches von Na+ gegen K+ und H+ durch Triamteren (in Epithelien)

The salivary duct epithelium, which actively transports Na+, K+ and H+/HCO3/- similarly to renal distal tubules, was used as a model tissue to study the mechanism of action of triamterene (Jatropus, Dyrenium) on electrolyte transport. Triamterene was only effective when administered from the luminal side of the duct, not from the interstitial side. 10-4 M triamterene completely blocked Na+-reabsorption. At the same time K+ secretion dropped to half of control, whereas HCO-/3 accumulated in the duct lumen following reduced H+ secretion. These changes in electrolyte transport are caused by an inhibition of Na+-entry by triamterene as suggested by measurements of ion permeability of the cell membrane. Triamterene has no specific effect on the membrane-bound ATPase. Since Na+-entry is functionally coupled with exit of K+ and H+ from cell to lumen, impairment of Na+-entry by triamterene necessarily causes reduction of K+ and H+ secretion into lumen.

On the mechanism of action of triamterene: effects on transport of Na+, K+, and H+/HCO3- -ions.

The rat salivary duct epithelium, which actively transports Na+, K+, and H+/HCO3- in a manner similar to renal distal tubules, was used as a model tissue to study the mechanism of action of triamterene on electrolyte transport. 10(-4) M triamterene completely blocked Na+ resorption and lowered net K+ secretion to half that of controls, whereas HCO3- accumlated in the lumen, probably due to a decrease in H+ secretion. The rates of K+ and H+/HCO3- transport in the presence of triamterene did not differ from those determined after omission of Na+ from the luminal fluid. This was considered to be evidence against a direct action of triamterene on transport of K+ and H+/HCO3-. Triamterene rapidly and reversibly reduced the transepithelial electrical potential difference. This was due to almost complete abolition of Na+ conductance of the luminal membrane at 10(-4) M triamterene, whereas K+ conductance was not altered. Triamterene, administered in vitro from the interstitial side of the isolated duct epithelium was ineffective even at the highest concentrations. The activities of the Na-K-ATPase, the Mg-ATPase and the microsomal HCO3-ATPase were influenced by 10(-4) M triameterene in a similiar fashion. These effects were clearly demonstrated only in the homogenate of the duct tissue and not in intact cells in the isolated duct preparation. Therefore they were considered unspecific. The transport studied demonstrate a primary effect of triamterene on Na+ entry from lumen to cell. Influences on net K+ and H+/HCO3 transport are secondary consequences of functional coupling between movement of Na+ and movement of K+ and H+ across the luminal cell membrane.

Evidence for Na+ independent active secretion of K+ and HCO - 3 by rat salivary duct epithelium.

In order to elucidate whether or not active secretion of potassium and bicarbonate by the rat submaxillary duct epithelium operates independently of sodium reabsorption, Na+ transport was blocked by amiloride, which is known to inhibit Na+ entry from lumen into cell. With 10(-4) M amiloride in HCO - 3 -Ringer at the luminal side, the transepithelial electrical potential difference approached zero, the Na+ conductance of the luminal cell membrane was drastically reduced, and the K+ conductance was significantly reduced. Net K+ secretion was reduced by 80%, whereas net HCO - 3 secretion was significantly increased. The remaining 20% of net K+ secretion proceeded at zero net Na+ transport and in the absence of significant chemical and electrical potential differences between lumen and interstitium of the duct. This active component of net K+ secretion was accompanied by an equal rate of active HCO - 3 secretion. These findings confirm the independence of this active secretion of K+ and HCO - 3 from Na+ transport. They indicate an electrically neutral secretion of K+ and HCO - 3, probably by the postulated luminal K+ -H+ -exchange mechanism. The 80% of net K+ secretion, which were abolished by amiloride together with Na+ reabsorption, seem to be functionally coupled with Na+ transport. The linkage of K+ -to- Na+ is probably mediated by a luminal carrier exchanging Na+ for K+ and H+.