Carbamyl phosphate and glutamine stimulation of the gallbladder salt pump

Effect of dietary carbamyl phosphate on dentine apposition in rat molars

In rats, sucrose increases dental caries and impairs odontoblastic function by reducing dentine apposition during primary dentinogenesis. A high-sucrose diet also affects negatively a pulp or dentine function that appears to regulate solute or fluid movement within rat dentinal tissue. In earlier work it was found that carbamyl phosphate could significantly reverse sucrose-induced cariogenesis and also stimulate sucrose-depressed movement of dentinal fluid through a mechanism involving parotid function(s). In the current study, the possibility that carbamyl phosphate could overcome the sucrose-induced reduction in dentine apposition was examined. Weanling rats were fed a high-sucrose diet supplemented or not with carbamyl phosphate for 5 weeks. Dentine apposition was measured planimetrically in sagittal sections of the molars. The effect of carbamyl phosphate was similarly tested in parotidectomized animals. Carbamyl phosphate significantly reduced the deleterious effect of sucrose on dentine apposition by 58% in the first molars. However, the reduction in dentine apposition that followed parotidectomy was not altered by carbamyl phosphate supplementation. The possibility that the beneficial effect of carbamyl phosphate on dentinogenesis involves a parotid function is entertained.

Bicarbonate effects, electromotive forces and potassium effluxes in rabbit and guinea-pig gall-bladder

The stimulating effect of external HCO3- on Na+ salt transport has been examined in rabbit and guinea-pig gall-bladder by electrophysiological methods, as a sequel to a previous study carried out by radiochemical techniques. At steady state, cell K+ activity was found to be significantly reduced in the presence of HCO3-, whereas cell Na+ activity significantly increased; in parallel the apical membrane p.d. was depolarized; K+ equilibrium potential was higher than membrane p.d. in every case. The apical p.d. dependence on K+ was unaffected by HCO3-, but in the guinea-pig it was affected by Cl-. Rapid increases in HCO3- concentration on the luminal side caused a depolarization of the apical p.d. of the guinea-pig within about 30 sec, an effect that did not occur if the tissue was pre-treated with 10(-4) M-acetazolamide; the epithelial resistance and apical/basolateral resistance ratio were unchanged in all cases. The primary action of HCO3- is confirmed to be on the apical membrane; an HCO3- conductance does not seem to be present at this level, either in the rabbit or guinea-pig, nor does HCO3- affect Na+ influx through the apical conductive pathway, so that all the stimulating effects of the anion are confirmed to be on the neutral transports of Na+ salts; in spite of this, the apical electromotive force is modified due to the changed cell K+ activity. The rapid depolarization caused by the anion in the guinea-pig is in agreement with an HCO3- electrogenic secretion and/or a basolateral conductance for the anion. Polyelectrolyte dissociation from protons increases in the absence of external HCO3-: the negative charges are mainly counterbalanced by bound Na+ in the rabbit and by free K+ in the guinea-pig. K+ leakage from the cell into the lumen is calculated to be minimal in the rabbit and all K+ lost could be reabsorbed through the paracellular pathways; K+ efflux to the subepithelial layer via conductive routes is insufficient to account for the over-all K+ efflux.

Effects of bicarbonate on fluid and electrolyte transport by guinea pig and rabbit gallbladder: stimulation of absorption

The effect of bicarbonate (HCO3) on fluid absorption by guinea pig gallbladder was investigated in vitro. Stimulation of fluid absorption was concentration dependent resulting in a fourfold increase in transport over the range 1 to 50 mM. Phosphate, Tris, glycodiazine and glutamine buffers failed to substitute for HCO3 in stimulating absorption. Unidirectional 22Na fluxes were measured across short-circuited sheets of guinea pig and rabbit gallbladders mounted in Ussing-type chambers. In both species the net Na flux was unaffected by serosal HCO3 alone but was stimulated by addition of HCO3 to the mucosal bathing solution. Transepithelial electrical potential difference in rabbit gallbladder was about 1.4 mV (lumen positive) when HCO3 was present in the mucosal or in both compartments. This fell to 0.2 mV under HCO3-free conditions or when HCO3 was present only in the serosal solution. The respective values for guinea pig gallbladder were -1.6 and -0.6 mV (lumen negative). HCO3 stimulation of Na absorption by guinea pig gallbladder was abolished by increasing the bathing pH from 7.4 to 7.8, an effect resulting mainly from a reduction in JNams. Tris buffer (25 mM) inhibited HCO3-dependent fluid absorption in this species completely at pH 8.5 and partially at 7.5. These results indicate that HCO3 stimulates gallbladder transport in both species by an action from the mucosal side. This effect cannot be attributed to simple buffering of H+ but may be explained by the participation of HCO3 in the maintenance of intracellular H+ for a Na/H-exchange.

Electrical properties of the cellular transepithelial pathway in Necturus gallbladder: III. Ionic permeability of the basolateral cell membrane

The ionic permeability of the basolateral membrane of Necturus gallbladder epithelium was studied with intracellular microelectrode techniques. After removal of most of the subepithelial tissue (to reduce unstirred layer thickness), impalements were performed from the serosal side, and ionic substitutions were made in the serosal solution while a microelectrode was kept in a cell. Thus, it was possible to obtain continuous (and reversible) records of transepithelial and cell membrane potentials and to measure intermittently the transepithelial resistance and the ratio of cell membrane resistances. From these data and the mean value of the equivalent resistance of the cell membranes in parallel (obtained from cable analysis in a different group of tissues), absolute cell membrane and shunt resistances and equivalent electromotive forces (emfs) were calculated. From the changes of basolateral membrane emf (Eb) produced by the substitutions, the conductance (G) and permeability (P) of the membrane for K, Cl and Na were estimated. Potassium-for-sodium substitutions produced large reductions of both cell membrane potentials, of Eb, and of the resistance of the basolateral membrane (Rb), indicating high GK and PK. Chloride substitution with isethionate or sulfate resulted in smaller changes of cell membrane potentials and Eb and in no significant change of Rb, indicating small but measurable values of GCl and PCl. Sodium substitutions with N-methyl-D-glucamine (NMDG) resulted in cell potential changes entirely attributable to the biionic potential produced in the shunt pathway (PNa greater than PNMDG), and in no significant changes of Rb or Eb, indicating that GNa and PNa are undetectable. The question of the mechanism of Cl transport across the basolateral membrane was addressed by comparing the mean rate of transepithelial Cl transport : formula, see text: and the predicted passive Cl flux across the basolateral membrane (from the membrane Cl conductance, potential, and Cl equilibrium potential). The conclusion is that only a very small fraction of the Cl flux across the basolateral membrane can be electrodiffusional. Since the paracellular Cl conductance is also too low to account for : formula, see text:, these results suggest the presence of a neutral mechanism of Cl extrusion from the cells. This could be a NaCl pump, a downhill KCl transport mechanism, or a Cl-HCO3 exchange mechanism.

Stimulation by HCO3- of Na+ transport in rabbit gallbladder

Bicarbonate presence in the bathing media doubles Na+ and fluid transepithelial transport and in parallel significantly increases Na+ and Cl- intracellular concentrations and contents, decreases K+ cell concentration without changing its amount, and causes a large cell swelling. Na+ and Cl- lumen-to-cell influxes are significantly enhanced, Na+ more so than Cl-. The stimulation does not raise any immediate change in luminal membrane potential and cannot be due to a HCO3(-)-ATPase in the brush border. The stimulation goes together with a large increase in a Na+-dependent H+ secretion into the lumen. All of these data suggests that HCO3- both activates Na+--Cl- cotransport and H+--Na+ countertransport at the luminal barrier. Thiocyanate inhibits Na+ and fluid transepithelial transport without affecting H+ secretion and HCO3(-)-dependent Na+ influx. It reduces Na+ and Cl- conentrations and contents, increases the same parameters for K+, causes a cell shrinking, and abolishes the lumen-to-cell Cl- influx. It enters the cell and is accumulated in the cytoplasm with a process which is Na+-dependent and HCO3(-)-activated. Thus SCN- is likely to compete for the Cl- site on the cotransport carrier and to be slowly transferred by the cotransport system itself.

Intracellular chloride activities in rabbit gallbladder: direct evidence for the role of the sodium-gradient in energizing "uphill" chloride transport

Intracellular chloride activities, (Cl)c, in rabbit gallbladder were determined by using conventional (Kcl-filled) microelectrodes and Cl-selective, liquid ion-exchanger, microelectrodes. The results indicated that in the presence of a normal Ringer's solution, (Cl)c averages 35mM; this value is 2.3 times that predicted for an equilibrium distribution across the mucosal and baso-lateral membranes. On the other hand, when the tissue is bathed by Na-free solutions, (Cl)c declines to a value that does not differ significantly from that predicted for an equilibrium distribution. These results, together with those of Frizzell et al. (J. Gen. Physiol. 65:769, 1975) provide, for the first time, compelling evidence that (i) the movement of Cl from the mucosal solution into the cell is directed against an electrochemical potential difference (23mV); and (ii) this movement is energized by coupling to the entry of Na down a steep electrochemical potential difference. Finally, our data suggest that (i) Cl exit from the cell across the basolateral membrane may be coupled to the co-transport of a cation or the countertransport of an anion; and (ii) the mechanism responsible for active Na extrusion from the cell across the baso-lateral membrane is rheogenic (electrogenic), and is not the result of a neutral Na-K exchange.

Na+ transport by rabbit urinary bladder, a tight epithelium

By in vitro experiments on rabbit bladder, we reassessed the traditional view that mammalian urinary bladder lacks ion transport mechanisms. Since the ratio of actual-to-nominal membrane area in folded epithelia is variable and hard to estimate, we normalized membrane properties to apical membrane capacitance rather than to nominal area (probably 1 muF approximately 1 cm2 actual area). A new mounting technique that virtually eliminates edge damage yielded resistances up to 78,000 omega muF for rabbit bladder, and resistances for amphibian skin and bladder much higher than those usually reported. This technique made it possible to observe a transport-related conductance pathway, and a close correlation between transepithelial conductance (G) and short-circuit current (Isc) in these tight epithelia. G and Isc were increased by mucosal (Na+) [Isc approximately 0 when (Na+) approximately 0], aldosterone, serosal (HCO-3) and high mucosal (H+); were decreased by amiloride, mucosal (Ca++), ouabain, metabolic inhibitors and serosal (H+); and were unaffected by (Cl-) and little affected by antidiuretic hormone (ADH). Physiological variation in the rabbits' dietary Na+ intake caused variations in bladder G and Isc similar to those caused by the expected in vivo changes in aldosterone levels. The relation between G and Isc was the same whether defined by diet changes, natural variation among individual rabbits, or most of the above agents. A method was developed for separately resolving conductances of junctions, basolateral cell membrane, and apical cell membrane from this G--Isc relation. Net Na+ flux equalled Isc. Net Cl- flux was zero on short circuit and equalled only 25% of net Na+ flux in open circuit. Bladder membrane fragments contained a Na+-K+-activated, ouabain-inhibited ATPase. The physiological significance of Na+ absorption against steep gradients in rabbit bladder may be to maintain kidney-generated ion gradients during bladder storage of urine, especially when the animal is Na+-depleted.