The effect of theophylline on intestinal bicarbonate transport measured by pH stat in Amphiuma

Pharmacological profile of duodenal alkaline secretion

Stimulation of mucosal alkaline secretion represents an opportunity for discovering novel drugs of potential benefit in maintenance therapy of duodenal ulcer disease. We screened over 200 agents representing the full spectrum of pharmacological categories in order to characterize stimulatory pathways and identify mechanistic leads. A variety of eicosanoids, phospho-diesterase inhibitors and adrenoreceptor agonists together with forskolin, 6-hydroxy-dopamine, 2-chloroadenosine, diazepam, testosterone, dipyridamole and dihydropyridazinone caused a reproducible increase in the metabolism-dependent component of alkaline secretion in bullfrog proximal duodenum. PGE2 (ED50 0.02 microgram/ml) was the most potent agent in vitro and was also the most effective stimulant of duodenal alkalinization in vivo in an anaesthetized cat preparation. Agents without effect on spontaneous alkaline secretion by amphibian duodenum included agonists and antagonists of histamine, 5-hydroxy-tryptamine, gamma-aminobutyric acid, dopamine, muscarinic and nicotinic receptors, inhibitors of amine uptake, monoamine oxidase and cholinesterase, plus various corticoids, diuretics, oestrogens, chemotherapeutic (anticancer) and antimicrobial agents. The major mechanism of stimulating alkaline secretion in the isolated duodenum is by increasing intracellular cyclic AMP levels. This may occur by either inhibiting metabolism of the nucleotide or by stimulating its formation. Additionally, many stimulants appear to act indirectly via liberation of endogenous prostaglandins as judged from the marked attenuation of responses in the presence of indomethacin to all agonists apart from exogenous PGE2, forskolin, ICI 63197 (PDE inhibitor), 2-chloroadenosine and diazepam. Whether purinergic agonists and benzodiazepines act directly on the enterocyte or by releasing other paracrine mediators is unknown.

Effect of heat-stable enterotoxin of Escherichia coli and theophylline on ion transport in porcine small intestine

The effect of a heat-stable enterotoxin of Escherichia coli was compared with that of theophylline on ion transport in the pig jejunum, using both in vivo and in vitro techniques. The maximal electrical response to heat-stable enterotoxin was only one-half that of theophylline even though the magnitude of the net secretory response was similar. A net, active secretion of HCO3 was partially responsible for the secretory response induced by heat-stable enterotoxin, whereas theophylline induced an active secretion of chlorine which could account for the entire secretory response. Heat-stable enterotoxin elevated tissue cyclic guanosine monophosphate levels, whereas theophylline elevated both cyclic adenosine monophosphate and cyclic guanosine monophosphate. Cyclic guanosine monophosphate levels induced by heat-stable enterotoxin were markedly potentiated by theophylline. Results suggest that HCO3 secretion in the pig jejunum may be controlled by the cyclic guanosine monophosphate system and this system also activates a neutral secretory process which at high heat-stable enterotoxin doses accounts for the bulk of the net secretion observed. Conversely, the chlorine secretion elicited by theophylline is entirely electrogenic and is consistent with results obtained in other species.

Bicarbonate transport across the frog choroid plexus and its control by cyclic nucleotides

We have studied the effects of ions, hormones, diuretics and cyclic nucleotides on the short-circuit current (Isc) across the frog choroid plexus. In normal HCO3 Ringer solution, the trans-epithelial potential difference (p.d.), Isc and resistance (Rt) were -0.8 mV (ventricular side negative with respect to to the blood (serosal) side), 6 microA/cm2 and 170 omega cm2, respectively. Removal of Na, Cl or HCO3 from the solution decreased the Isc to 1.9, 2.3 or -4.6 microA/cm2, respectively. Theophylline, dibutyryl cyclic AMP, isoproterenol, prostaglandin E1, ACTH, cholera toxin and forskolin all significantly increased the Isc. The theophylline-induced change in Isc (delta Itheosc) was reduced by 50% upon Cl substitution with gluconate, and was abolished to less than 12% by Na-free and HCO3-free solutions. pH monitoring of the bathing solutions showed that acidification of the serosal bathing fluid was enhanced by theophylline while that of the ventricular solution was retarded. Ouabain, acetazolamide, SITS, DIDS and furosemide inhibited both Isc and delta Itheosc. We conclude that HCO3 secretion by the choroid plexus into the c.s.f. is controlled by hormones which stimulate the adenylate cyclase system, and propose a model of ion transport across the choroidal epithelium.

Intestinal electrogenic HCO3- absorption localized to villus epithelium

Isolated segments of jejunum from Amphiuma absorb HCO3+ electrogenically by a process sensitive to acetazolamide. Using a tissue chamber of special design which permits isolation of villus or intervillus epithelium the transepithelial electrical potential (psi ms) of each region was measured. The serosa-negative psi ms generated by the villus epithelium was more negative than that of the intervillus and exhibited greater sensitivity to acetazolamide. Both regions were responsive to other agents which alter epithelial ion transport. The results indicate that intestinal HCO3- absorption is localized predominantly within cells lining the villus epithelium.

Chloride transport and intracellular chloride activity in the presence of theophylline in Amphiuma small intestine

1. Simultaneous bidirectional tracer fluxes of 22Na and 36Cl were measured under short-circuited conditions in isolated stripped segments of Amphiuma duodenum incubated in media containing Cl- and HCO3-. 2. Theophylline (10 mM) increased the short-circuit current (Isc), eliminated net Cl- absorption and elevated the residual flux by more than 50%. 3. Subsequent addition to acetazolamide lowered the Isc and reduced the residual flux. 4. Using double-barreled Cl- specific micro-electrodes the intracellular activity of Cl- averaged 27.1 mM in the presence of theophylline, consistent with active accumulation of Cl-. 5. Theophylline depolarized the mucosal membrane of villus epithelial cells by an average of 6.9 mV. 6. The findings indicate that the inhibition of Cl- absorption by theophylline does not result from complete inhibition of mucosal Cl- uptake and provide evidence that HCO3- secretion is enhanced by theophylline.