Electrogenic bicarbonate secretion in gallbladder: induction by barium via neuronal, possibly VIP-ergic pathways.

In guinea-pig gallbladder epithelium, cAMP converts electroneutral HCO3- secretion into an electrogenic process. The effects of blood side Ba2+ (5 mmol/l) on HCO3- transport were investigated in vitro, using pH-stat and voltage clamp techniques to determine unidirectional fluxes of HCO3- and transepithelial electrical characteristics. Serosal, not mucosal addition of Ba2+ elevated short-circuit current (Isc), transepithelial potential difference, and tissue conductance; it inhibited the absorptive HCO3- flux while leaving the secretory flux unchanged. The Isc effect of Ba2+ was inhibited or prevented by tetrodotoxin; D- and L-propranolol; the Cl- channel blocker 4-N-methyl-N-phenylaminothiophene-3-carboxylic acid; the intracellular Ca2+ antagonist, 3,4,5-trimethoxybenzoic acid 8-(diethylamino)ocytl ester; noradrenaline, by a yohimbine-sensitive action; somatostatin; HCO3(-)-free solutions. Thus Ba2+ appeared to release a neurotransmitter that gives rise to cAMP synthesis sufficient to turn part of electroneutral HCO3- secretion electrogenic. In a search for the involved signalling pathways, the H1-receptor antagonist, cetirizine, largely and hexamethonium, atropine, atenolol, indomethacin, and trifluoperazine entirely failed to antagonize the Isc effect of Ba2+. Similarly, carbachol, dobutamine, salbutamol, and serotonin were unable to mimic the action of Ba2+ and Isc effects of histamine were small and short-lived. By contrast, vasoactive intestinal peptide (VIP; 3 x 10(-7) mol/l) completely transformed HCO3- secretion into an electrogenic process. The VIP receptor antagonist (4Cl-DPhe6, Leu17) VIP, delayed and reduced the Isc responses to Ba2+ and VIP. As guinea-pig gallbladder epithelial cells possess cAMP-coupled VIP receptors close to VIPergic neurons, Ba2+ is likely to act by releasing VIP from neural terminals.

Electrogenic and electroneutral HCO3-secretion by guinea pig gallbladder epithelium: discriminatory abilities of Cl- channel blockers.

The specificity of Cl- channel blockers (including 5-nitro-2-(3-phenylpropylamino-)benzoic acid (NPPB), diphenylamine-carboxylic acid and 4-N-methyl-N-phenylaminothiophene-3-carboxylic acid (MPTC)) was studied in guinea pig gallbladder. This epithelium performs electroneutral HCO3- secretion, due to cell-to-lumen exit by Cl-/HCO3- exchange. At high intracellular cyclic AMP levels [produced by prostaglandin E1, (PGE1)], secretion turns electrogenic, secondary to induction of apical membrane Cl- and HCO3- conductances. Part of HCO3- secretion remains Cl(-)-dependent. Microelectrode, voltage-clamp and pH-stat techniques were used to determine membrane potentials, short-circuit current (Isc) and secretory HCO3- fluxes. With one exception, effective agents (concentration range 10(-5)-10(-4) M, luminal bath) including NPPB and diphenylamine-carboxylic acid inhibited secretory HCO3- fluxes in the presence and absence of PGE1 as well as PGE1-induced Isc. At 10(-4) M, inhibition of Isc was not attenuated by Cl- removal; however, NPPB effects were less at concentrations < 10(-4) M. MPTC (3 x 10(-5)-3 x 10(-4) M) only partly inhibited PGE1-induced Isc, was almost ineffective in Cl(-)-free solutions, and did not affect secretory HCO3- flux under any condition. Effects of NPPB and MPTC were distinct from those of the mitochondrial uncoupler, carbonyl cyanide p-trifluoromethoxy-phenylhydrazone. This agent (10(-7)-10(-5) M), failed to discriminate between Cl(-)-dependent and -independent Isc. In control tissues, 10(-5) M carboyl cyanide p-trifluoromethoxy-phenylhydrazone practically abolished membrane potentials, whereas 10(-4) M NPPB produced no change.(ABSTRACT TRUNCATED AT 250 WORDS)

Secretin and somatostatin as modulators of electrolyte transport in guinea pig gallbladder epithelium.

Effects of secretin and somatostatin on electrolyte transport by guinea pig gallbladder epithelium were investigated in vitro. Tissues were mounted in Ussing-type chambers and continuously short-circuited to measure tissue conductance, short-circuit current (Isc) and transepithelial voltage. Unidirectional fluxes (from mucosa to serosa: Jms; from serosa to mucosa: Jsm) of Na+ and Cl- were determined simultaneously by using tracer techniques and those of HCO3- by pH-stat titration. Serosal addition of secretin caused concentration-dependent increases in tissue conductance, Isc and (lumen-negative) transepithelial voltage. At 1.3 x 10(-8) M, secretin raised net HCO3- secretion by 1.5 mumol/cm2/hr, due to inhibition of Jms (from approximately 1.2 to approximately 0.7 mumol/cm2/hr) and stimulation of Jsm (from approximately 3.7 to approximately 4.7 mumol/cm2/hr). The associated increase in Isc by approximately 3.8 mumol/cm2/hr (up from approximately 0.5 mumol/cm2/hr) was not different from net HCO3-secretion (approximately 4 mumol/cm2/hr). Neither Na+ nor Cl- net fluxes accounted for secretin-induced Isc, both being absorbed at equal rates (Na+: approximately 2.7, down from approximately 4.6 mumol/cm2/hr; Cl-: approximately 2.5, down from approximately 6.4 mumol/cm2/hr). Secretin caused a 10-fold rise of luminal efflux of cyclic AMP that was mitigated by somatostatin, added at 6 x 10(-7) M to the serosal bath. Somatostatin inhibited secretin- and prostaglandin E1-induced Isc, but was ineffective in tissues stimulated with 8-Br-cyclic AMP. It partly reverted the effects of secretin on JmsHCO3 and JsmHCO3, but had no effects in untreated tissues. Our data indicate that secretin converts HCO3-secretion from an electroneutral into an electrogenic process and blocks part of NaCl absorption.(ABSTRACT TRUNCATED AT 250 WORDS)

HCO3- reduces paracellular permeability of guinea pig duodenal mucosa by a Ca2+ (prostaglandin)-dependent action.

The mechanism of HCO3(-)-induced decrease in electrical conductance (Gt) of guinea pig duodenal mucosa was investigated in vitro. Gt and unidirectional fluxes of mannitol (M), Na+, and Cl- were straightly correlated. In the presence of HCO3- (20 mM), elevating the bath concentration of Ca2+ from 0 to 1.2 mM caused decreases in Gt and Na+ fluxes by approximately 30%. Smaller reductions were found in the absence of HCO-3. With the addition of Ca2+, Gt and unidirectional fluxes were lower with than without HCO3-. In the latter condition, PM/PNa, PM/PCl, and PNa/PCl (P is permeability) were close or equal to ratios predictable from free solution diffusion coefficients; with HCO3-, ratios became different or more different and PM/PNa was reduced. Straightly correlated were conductivities of six differently composed HCO3(-)-free salines and Gt of tissues bathed therein. This correlation did not embrace HCO3- Ringer. The Gt effect of Ca2+ (HCO3- present) was mimicked by prostaglandin E2 and reduced (by approximately 40%) by Ca2+ channel blockers (diltiazem, gallopamil), cyclooxygenase inhibitors (meclofenamate, diclofenac), cytochalasin D, and trifluoperazine. We conclude that without HCO3-, paracellular solute flow occurs mainly via structures resembling a free-solution shunt; HCO3-, through an action made more efficient by Ca(2+)-mediated prostaglandin synthesis and possibly involving microfilaments, strengthens a discriminatory barrier.