Alkaline secretion by Necturus proximal duodenal mucosa.

Proximal duodenum from the amphibian Necturus was stripped of muscle layers and the mucosa was mounted as a tube for studies of alkali transport or as a flat sheet for intracellular impalement by voltage-sensitive glass micro-electrodes. The mucosa alkalinized the unbuffered luminal perfusate at a high rate (3.4 muequiv. cm-1 h-1) and developed a transepithelial electric potential difference of 5.7 mV (lumen negative). Transport was inhibited by 2,4-dinitrophenol (10(-4) M) and by furosemide (10(-3) M) and SITS (10(-3) M) on the seros but not on the mucosal side, indicating dependence on tissue metabolism and on serosal membrane Cl-/HCO3- exchange. Prostaglandin E2 (10(-7)-10(-5) M) and dibutyryl cyclic AMP (10(-6)-10(-4) M) had no effects on the secretion or transepithelial electrical potential difference. removal of serosal HCO3- decreased luminal alkalinization by 75%, indicating a contribution by passive migration of HCO3- and/or a dependence of transcellular transport on the nutrient supply of this ion. Administration of HCO3- (17.8 mM) to the luminal perfusate affected neither the transepithelial nor transmembrane electrical potential differences nor the resistance ratio. It is thus unlikely that the luminal membrane possesses any major HCO3- conductance.

Electrophysiological characteristics of the Necturus proximal duodenal mucosa: effects of ion substitutions.

Voltage-sensitive glass micro-electrodes were used to determine the electrical characteristics of Necturus proximal duodenal epithelium. Some comparative experiments with amiloride were performed with gastric antrum. The apical and the basolateral cell membrane potential differences in duodenum averaged -32 mV and -34 mV (cell negative) respectively. The transepithelial potential difference was -2 mV (lumen negative). The EMF across the apical cell membrane was -29 mV and that across the basolateral cell membrane -39 mV. The transepithelial resistance (Rt) of 63 omega cm2 and the paracellular pathway resistance (Rs) of 80 omega cm2 are of magnitudes similar to that previously reported for more distal amphibian small intestine. The apical and basolateral cell membrane resistances, however, were lower than those reported for distal small intestine. Ion permeabilities for Na+, K+ and Cl- across the apical cell membrane were calculated from ion substitution experiments. The permeability sequence across the apical cell membrane was PK:PCl:PNa 3.02:1.31:1.00. Luminal amiloride (10(-4)M) was without significant effect, further indicating a low duodenal membrane conductance for Na+. The low conductances for K+, Na+ and Cl- suggest that the major ion transport modes across the apical duodenal cell membrane are electroneutral in nature. In contrast, amiloride caused a marked increase in the transmembrane potentials in the antrum.

Gastroduodenal bicarbonate secretion in mucosal protection. Possible role of vasoactive intestinal peptide and opiates.

HCO3- secretion by surface epithelium in duodenum devoid of Brunner's glands was titrated in situ in anesthetized rats. Intravenous injection of small amounts (20 ng/kg) of the endogenous opioid peptide beta-endorphin significantly increased secretion. Naloxone prevented this effect, suggesting that stimulation is mediated by mu-opiate receptors. Morphine 50 microgram/kg had a similar stimulatory action. Vasoactive intestinal peptide (VIP) 0.5-100 microgram/kg dose-dependently increased secretion and this response was independent of simultaneous cholinergic stimulation. The HCO3- secretion maintained pH in the mucus gel adherent to the luminal surface at neutrality for long periods of time (greater than or equal to 60 min); even when the pH in the terminal bulk solution was as low as 2.0. Mucosal HCO3- secretion is thus very probably important in mucosal protection and VIP and endogenous opioid peptides may have a role in its control.

Cysteamine and propionitrile inhibit the rise of duodenal mucosal alkaline secretion in response to luminal acid in rats.

Effects of subulcerogenic doses of cysteamine (100 mg/kg s.c.) and propionitrile (5 mg/kg) on alkaline secretion by duodenal surface epithelium and pH at the surface of this mucosa were assessed in duodenum of anesthetized rats. Alkaline secretion was titrated in situ, using segments of duodenum just distal to the Brunner's glands area and devoid of pancreatic HCO3-. Surface pH was measured by advancing pH-sensitive microelectrodes from the luminal solution to the epithelial cell surface. Proximal duodenum from bullfrogs was used to study effects of cysteamine on alkaline secretion in vitro. Cysteamine caused an increase in alkaline secretion in the rat during the first hour after administration, but rates after 5 and 20 h were the same as in controls and cysteamine (1 mg/ml) had no effect on secretion in vitro. Neither in vitro nor in vivo did cysteamine affect the rise in alkaline secretion in response to exogenous prostaglandin E2 (and dibutyryl-cyclic adenosine monophosphate). Luminal acid is a potent stimulant of duodenal mucosal alkaline secretion. By delayed (5 h) actions, both cysteamine and propionitrile inhibited the rise in alkaline secretion in response to a 5-min exposure to luminal acid with pH 2.00 in the rat. Cysteamine also depressed the ability of this mucosa to maintain a high rate of alkaline secretion during sustained exposure at pH 2.00 but had no such effect at pH 5.00. The former resulted in acidification of the pH gradient at the mucosal surface. Cysteamine is thus probably without effect on the HCO3- secretory process itself but impairs the ability of the duodenal mucosa to respond to acid. Inhibition of mechanisms mediating this response may contribute to the duodenal ulcerogenic actions of cysteamine and propionitrile.

Stimulation of mucosal bicarbonate secretion in rat duodenum in vivo by BW755C.

Bicarbonate secretion from 12 mm segments of duodenum just distal to the Brunner's gland area was titrated (pH 7.60) in situ in anesthetized rats. Intravenous BW755C (10-20 mg/kg) increased both bicarbonate secretion and the transmucosal electrical potential difference and pretreatment with indomethacin (3 mg/kg intravenously) prevented these effects. Indomethacin also inhibited stimulation of HCO3- secretion by luminal acid (10 mM HCl) but had no effect on the rise in secretion in response to exogenous (luminal) prostaglandin E2. The results support previous suggestions of a role for endogenous prostaglandins in mediation of the HCO3- response to acid and are consistent with the recent demonstration that BW755C increased prostaglandin formation in homogenates of rat intestinal mucosa. Stimulation of HCO3- secretion by BW755C was not enhanced but attenuated by preexposure to luminal acid, suggesting that the latter increases secretion by effects other than mucosal mobilization of arachidonate.

Stimulation by BW755C and inhibition by cysteamine of duodenal epithelial alkaline secretion suggest a role of endogenous prostaglandin in mucosal protection.

Alkaline secretion by 12 mm segments of duodenum just distal to the Brunner's glands area was titrated in situ in anesthetized rats. Intravenous injection of the lipoxygenase inhibitor BW755C (10-20 mg/kg) increased the surface epithelial HCO3- secretion and pretreatment with indomethacin prevented this effect. This supports the view that endogenous production of prostaglandins is important in control of duodenal epithelial HCO3- secretion and ulceroprotection . Pretreatment with cysteamine (100 mg/kg) inhibited the ability of the duodenal surface epithelium to respond to luminal acid with a compensatory rise in alkaline secretion. Measurement of pH at the mucosal surface with microelectrodes revealed that acidification of this surface occurred simultaneously with the decline in alkaline secretion.