Evidence implicating a prostaglandin as the mediator of intestinal fluid secretion induced by pithing: inhibition by indomethacin, morphine and pentobarbitone

A reconsideration of the evidence for Escherichia coli STa (heat stable) enterotoxin-driven fluid secretion: a new view of STa action and a new paradigm for fluid absorption

A review of the evidence for Escherichia coli STa causing fluid secretion in vito leads to the conclusion that the concept of STa acting through enhanced chloride secretion in order to derange intestinal function is unproven. However, a consistent effect of STa in the small intestine is on Na+/H+ exchange, leading to interruption of luminal acidification. A model for the action of STa, involving inhibition of Na+/H+ exchange, is proposed which explains the ability of STa to reduce absorption in vito but its inability to cause secretion in vito in contrast to its apparent secretory effect in vitro. The apparent ability to demonstrate secretion in vitro is shown to derive from methodologies which do not involve measurement of mass transport of water but instead, infer it from in vitro and in vivo proxy measurements. The in vitro demonstration of notional secretion after STa exposure can be reconciled with the proposed new model for fluid absorption in that cell swelling is argued to arise as a transient consequence of STa challenge followed by regulatory volume decrease. Evidence for this derangement model is presented in the form of observations derived from acute in vivo physiological studies and clinical studies on patients without the exchanger. This process of appraisal of the evidence for the mechanism of action of STa has led to a new model for fluid absorption. This is based on the formation of hypotonicity at the brush border luminal surface rather than hypertonicity within the lateral spaces as required by the present standing gradient model of fluid absorption. Evidence from the literature is presented for this new paradigm of water absorption, which may only be relevant for small intestine and other tissues that have Na+/H+ exchangers in contact with HCO-3-containing solutions but which may also be generalizable to all mammalian absorbing epithelial membranes.

Studies investigating the possible involvement of adenosine in the antisecretory action of morphine

1. Fluid secretion was induced in the jejunum of anesthetised rats using vasoactive intestinal peptide. 2. The adenosine antagonist, DPCPX (0.1 mg/kg), suppressed the antisecretory action of morphine (10 mg/kg), but naloxone (80 micrograms/kg) did not inhibit the antisecretory response of the adenosine agonist, NECA (40 micrograms/kg), at a dose previously shown to antagonize the antisecretory response of morphine. 3. NECA (40 (micrograms/kg) reversed secretion in pithed and reserpine-pretreated (5 mg/kg subcutaneously) rats. 4. It is proposed that adenosine acts as a mediator of the morphine antisecretory effect at a site distal to the noradrenergic neurons involved in the action of morphine.

Insights into opioid action in the intestinal tract

Opioids have been used for centuries as antidiarrhoeal drugs. In recent years, their mechanism and sites of action in exerting their antidiarrhoeal effect have been studied intensely. Attempts have been made to propose their general mode of action. Whilst there are numerous similarities in their general effects on motility, fluid secretion, and neuroeffector transmission, the differences between species, in some cases, can be remarkable. We highlight and contrast the similarities and differences in the commonly examined species and compare them to humans. Insights into mechanisms of opioid antidiarrhoeal action now also provide some new perspectives of opioid action in the intestine.

Mechanisms of corneal drug penetration. I: In vivo and in vitro kinetics

Corneal penetration studies were conducted in unanesthetized albino rabbits using various organic compounds representing both polar and nonpolar species. Very low molecular weight compounds demonstrate rapid uptake into the aqueous humor despite the lipid-like barrier imposed by the corneal epithelium. Evidence that these compounds may have access to a diffusional channel for corneal transport is presented. In vitro permeability studies were also conducted in an effort to quantitate the corneal diffusion of compounds covering a range of molecular weights and partition coefficients; the results corresponded well with the results of in vivo experiments. Calculations of energies of activation, taken from Arrhenius plots, indicate that the diffusion of drug across the cornea may be by two different mechanisms that depend on the physical-chemical characteristics of the perfusant. One mechanism appears similar to drug movement in an aqueous environment and is characterized by an activation energy similar to that for diffusion in water. The other relates to the expected partitioning of a compound across cellular membranes represented by a relatively high activation energy for diffusion. For hyrdophilic compounds, the epithelium appears to be rate limiting to drug movement, whereas for hydrophobic compounds, the stroma is rate limiting. In the presence of calcium-chelating agents, glycerol demonstrated an increase in corneal penetration in vivo. This effect appears to be reversible at specific concentrations of chelator. In contrast, divalent cations reduced corneal penetration of glycerol. The known calcium chelator EDTA was shown to penetrate the cornea, conjunctiva, and iris/ciliary body from a topically applied dose. The implications of this observation pertain to toxicity effects when EDTA is incorporated into ocular drug products for stability purposes, or novel stratagems for improving ocular bioavailability of topically applied drugs are employed. The addition of calcium-chelating agents to in vivo mounted corneas demonstrated increases in permeability of the cornea to glycerol which were directly related to the concentration of chelating agent used. These results paralleled the findings of similar in vivo studies. The results of these studies are consistent with a currently proposed 'pore' model for the penetration of drugs through the cornea which demonstrates both a partition coefficient and molecular weight dependency on the permeability of the cornea to transported compounds.

Opioid action of the intestine: the importance of the intestinal mucosa

Drug effects on the intestine are traditionally explained in terms of action on the muscle layers and the nerves that control them. This is particularly true in the case of the opioids but research starting two decades ago has identified the intestinal mucosa as the site of action of the antidiarrhoeal opioids. Continued research using the intestinal mucosa offers a fresh approach to solving some old problems. For example it could lead to more confident predictions to be made about the wanted and unwanted effects of opioid drugs on the intestine and may help to find better drug treatments for alleviating withdrawal diarrhoea in addicts. Eventually it may help to explain how the general process of opioid dependence occurs at a cellular level.

Evidence for tryptaminergic and noradrenergic involvement in the antisecretory action of morphine in the rat jejunum

Experiments have been performed to determine whether the antisecretory (antidiarrhoeal) effect of morphine in the intestine is mediated by a direct action of morphine on enteric nerves. Rats were pretreated with 6-hydroxydopamine (6-OHDA) or p-chlorophenylalanine (PCPA) to deplete intestinal stores of noradrenaline and 5-hydroxytryptamine (5-HT). Intraperitoneal injection of 6-OHDA (3 doses at 50 mg kg-1) caused a selective reduction in the level of noradrenaline in the jejunum to 7.3% of control. Intraperitoneal injection of PCPA (200 mg kg-1) selectively reduced the jejunal level of 5-HT to 30.5% of control. Groups of rats that had been treated as described above were anaesthetized and then injected intravenously with saline or with blocking doses of either atropine (0.25 mg kg-1), hexamethonium (20 mg kg-1), ketanserin (30 micrograms kg-1), methysergide (30 micrograms kg-1), phentolamine (2 mg kg-1) or propranolol (1 mg kg-1). Following perfusion of the lumen of the jejunum, the rate of glucose absorption was measured to assess the integrity of the mucosa. Glucose absorption was unaltered in animals pretreated with hexamethonium and propranolol but there was a small enhancement in animals pretreated with atropine, PCPA, methysergide, 6-OHDA and phentolamine. The rate of net water absorption from the lumen of the jejunum and the rate of fluid secretion into the lumen following intra-arterial infusion of vasoactive intestinal peptide (VIP, 0.8 microgram min-1) were unaltered by any of the drug treatments. Intravenous injection of morphine (10 mg kg-1) did not alter the levels of noradrenaline or 5-HT in the whole jejunum. However, this dose of morphine did cause a 63.5% decrease in the VIP-induced change in water transport. This antisecretory effect of morphine was unaltered in animals pretreated with atropine, hexamethonium and propranolol. In contrast, methysergide, ketanserin and 6-OHDA abolished the antisecretory effect of morphine. PCPA and phentolamine produced a partial inhibition of morphine's antisecretory effect. It is concluded that morphine produces its antisecretory effect in the jejunum by activation of noradrenergic and tryptaminergic systems.

Tetrodotoxin inhibits directly acting stimulants of intestinal fluid secretion

Intravenous injection of tetrodotoxin (20 micrograms kg-1) to anaesthetized rats blocks PGE1- and inhibits VIP-induced fluid secretion from the mucosa of the jejunum without affecting the physiological rates of net water and glucose absorption. It is suggested that the toxin might release an endogenous inhibitor of secretion or that it might possess direct antisecretory activity on the epithelial cells of the jejunum. It is further suggested that results where tetrodotoxin has been used to investigate secretagogue action should be treated with caution.

Choice of anesthetic for intestinal absorption and secretion experiments using rats

The net rates of glucose and water absorption from, and secretion of water into, the lumen of the rat small intestine were compared using alpha-chloralose, halothane, pentobarbitone, and urethane as anesthetics. Water and glucose transport were determined by recirculation of perfusion fluid through the lumen of the jejunum. Net fluid secretion was stimulated by intraarterial (i.a.) infusion of prostaglandin E1 (PGE1) and vasoactive intestinal peptide (VIP). The results show that the anesthetics used exert only minimal influence on the physiological rates of glucose and water absorption and there is no correlation between the small differences in glucose and water absorption among the anesthetics. However, when the fluid secretory mechanism of the mucosa is stimulated with either PGE1 or VIP, pentobarbitone allows the greatest rates of secretion, but halothane prevents secretion. It is suggested that of the anesthetics used, pentobarbitone is the most appropriate for intestinal secretion experiments.