Interpretation of hexose-dependent electrical potential differences in small intestine

The mechanism of cation permeation in rabbit gallbladder : Conductances, the current-voltage relation, the concentration dependence of anion-cation discrimination, and the calcium competition effect

The questions underlying ion permeation mechanisms, the types of experiments available to answer these questions, and the properties of some likely permeation models are examined, as background to experiments designed to characterize the mechanism of alkali cation permeation across rabbit gallbladder epithelium. Conductance is found to increase linearly with bathing-solution salt concentrations up to at least 400MM. In symmetrical solutions of single alkali chloride salts, the conductance sequence is K(+)>Rb(+)>Na(+)>Cs(+)∼Li(+). The current-voltage relation is linear in symmetrical solutions and in the presence of a single-salt concentration gradient up to at least 800 mV. The anion/cation permeability ratio shows little change with concentration up to at least 300MM. Ca(++) reduces alkali chloride single-salt dilution potentials, the magnitude of the effect being interpreted as an inverse measure of cation equilibrium constants. The equilibrium-constant sequence deduced on this basis is K(+)>Rb(+)>Na(+)∼Cs(+)∼Li(+). These results suggest (1) that the mechanism of cation permeation in the gallbladder is not the same as that in a macroscopic ion-exchange membrane; (2) that cation mobility ratios are closer to one than are equilibrium-constant ratios; (3) that the rate-limiting step for cation permeation is in the membrane interior rather than at the membrane-solution interface; and (4) that the rate-controlling membrane is one which is sufficiently thick that it obeys microscopic electroneutrality.

A mechanistic explanation of the effect of potassium on goldfish intestinal transport

Partial replacement of sodium by potassium or rubidium in the solution used to perfuse isolated intestinal segments of goldfish causes an increase in transmural electrical resistance. Serosal replacements have a stronger effect than mucosal replacements. A 70% inhibition of the glucose-evoked transmural electrical current is brought about by serosal replacement of 40 mM sodium by potassium. Transmural mucosal to serosal flux of 3-O-methyl-D-glucose is also strongly inhibited by serosal potassium. These inhibitory effects of potassium do not occur when the intestinal mucosa is stripped free from the intestinal muscular layers. It is concluded that potassium-induced muscular contractures cause a decrease in transport area by pressing the mucosal folds closer against each other. Certain effects of high potassium concentrations that have been reported in mammalian intestinal preparations may involve a similar mechanism.

Polarized monolayers formed by epithelial cells on a permeable and translucent support

An epithelial cell line (MDCK) was used to prepare monolayers which, in vitro, develop properties of transporting epithelia. Monolayers were formed by plating cells at high densities (10(6) cells/cm2) on collagen-coated nylon cloth disks to saturate the area available for attachment, thus avoiding the need for cell division. An electrical resistance developed within 4-6 h after plating and achieved a steady-state value of 104 +/- 1.8 omega-cm2 after 24 h. Mature monolayers were morphologically and functionally polarized. They contained junctional complexes composed of desmosomes and tight junctions with properties similar to those of "leaky" epithelia. Monolayers were capable of maintaining a spontaneous electrical potential sensitive to amiloride, produced a net water flux from the apical to basal side, and discriminated between Na+ and Cl- ions. The MDCK permeability barrier behaves as a "thin" membrane with negatively charged sites. It has: (a) a linear conductance/concentration relationship; (b) an asymmetric instantaneous current/voltage relationship; (c) a reduced ability to discriminate between Na+ and Cl- caused by lowering the pH; and (d) a characteristic pattern of ionic selectivity which suggests that the negatively charged sites are highly hydrates and of medium field strength. Measurements of Na+ permeability of electrical and tracer methods ruled out exchange diffusion as a mechanism for ion permeation and the lack of current saturation in the I/deltapsi curves does not support the involvement of carriers. The discrimination between Na+ and Cl- was severely but reversibly decreased at low pH, suggesting that Na+-specific channels which exclude Cl- contain acidic groups dissociated at neutral pH. Bound Ca++ ions are involved in maintaining the integrity of the junctions in MDCK monolayers as was shown by a reversible drop of resistance and opening of the junctions in Ca++-free medium containing EGTA. Several other epithelial cell lines are capable of developing a significant resistance under the conditions used to obtain MDCK monolayers.

Characteristics of sodium flux from serosa to mucosa in rabbit ileum

Sodium flux from serosa to mucosa, J(sm) (Na) in rabbit ileum in vitro has been studied as a function of applied electrical potential at equal sodium concentrations in the bathing solutions. The results indicate that J(sm) (Na) involves two pathways, a diffusional flux through a paracellular shunt pathway and a flux that is independent of applied potential and presumably involves a transcellular pathway. The latter pathway comprises approximately 25 % of J(sm) (Na) in Ringer's solution containing 10 mM glucose and 25 mM bicarbonate. It is stimulated significantly by theophylline unaffected by removal of glucose or addition of ouabain but is reduced to negligible values by anoxia, dinitrophenol, and replacement of all chloride and bicarbonate by isethionate. Thus this component of J(sm) (Na) has a number of characteristics consistent with involvement in a specific secretory process mediating an electrically neutral secretory transport of sodium plus anion from serosa to mucosa. In addition to stimulating this process, theophylline significantly reduced the permeability of the paracellular shunt pathway to sodium.

Interpretation of fluctuation of transmural potential difference across the proximal small intestine

The duodenal transmural potential difference (pd) has been studied in isolated vascular-perfused preparations of canine stomach and duodenum. There was no quantitative correlation between the magnitude of the intraluminal pressure change and pd but fluctuations of pd in these preparations were related to duodenal slow wave activity.

Ionic conductances of extracellular shunt pathway in rabbit ileum. Influence of shunt on transmural sodium transport and electrical potential differences

The unidirectional influxes of Na, K, and Cl into isolated strips of rabbit ileum are comprised of movements across the mucosal membrane of the epithelial cells and ionic diffusion into an extracellular shunt pathway. A large fraction of the Na influx across the mucosal membrane alone is inhibited by Li, suggesting the participation of a carrier mechanism in the influx process. The partial ionic shunt conductances of Na, K, and Cl account for at least 82% of the total tissue conductance. The calculated shunt permeabilities (P) are (in centimeters per hour) P(K) = 0.040, P(Na) = 0.035, and P(Cl) = 0.019, so that P(K):P(Na):P(Cl) = 1.14:1.00:0.55. Diffusion potentials across the tissue resulting from isotonic replacement of NaCl in the mucosal solution with mannitol or KCl are described by the Goldman constant-field equation together with the above permeabilities of the shunt pathway. These observations are not consistent with permeation through a fixed-charge pore but can be explained by a model featuring constant ionic partition into a neutral-polar pore that traverses the tight junction. Such a pore may be lined with either fixed dipoles or fixed dipolar ions oriented such that electronegative groups influence the permselective properties of the diffusion pathway. The essential feature of both models is that electroneutrality is maintained by means of fixed membrane components and does not depend upon the presence of mobile counterions.

The mechanism of cation permeation in rabbit gallbladder : Dilution potentials and biionic potentials

The experimental measurements of passive ion permeation in rabbit gallbladder presented in this paper include: single-salt dilution potentials as a function of concentration gradient; comparison of dilution potentials for different alkali chlorides; comparison of biionic potentials for different alkali chlorides; and biionic mixture potentials as a function of cation concentration gradient. Both dilution potentials and biionic potentials yield the permeability sequence K(+)>Rb(+)>Na(+)>Li(+)>Cs(+), a sequence consistent with simple considerations of ion-site interactions and ion hydration energies. Construction of empirical selectivity isotherms for alkali cation permeation in epithelia shows that permeability ratios are nearer one in the gallbladder and other epithelia than in most other biological membranes, indicating a relatively hydrated permeation route. Evaluation of the results of this and the preceding paper suggests the following: that cations permeate gallbladder epithelium via channels with fixed neutral sites; that the rate-controlling membrane is thick enough that microscopic electroneutrality must be obeyed; that virtually all anion conductance is in a shunt which develops with time after dissection; that apparent permeability changes with solution composition are due to the non-ideal activity factorn being less than 1.0; that effects of pH, Ca(++), and ionic strength may involve changes in the anion/cation mobility ratio owing to changes in wall charges or dipoles; and that the permeation route may reside in the tight junctions. A similar mechanism may be applicable to cation permeation in other epithelia.

Studies on the electrical potential profile across rabbit ileum. Effects of sugars and amino acids on transmural and transmucosal electrical potential differences

When isolated strips of mucosal rabbit ileum are bathed by physiological electrolyte solution the electrical potential difference (PD) across the brush border (psi(mc)) averages 36 mv, cell interior negative. Rapid replacement of Na in the mucosal solution with less permeant cations, Tris or choline, results in an immediate hyperpolarization of psi(mc). Conversely, replacement of choline in the mucosal solution with Na results in an abrupt depolarization of psi(mc). These findings indicate that Na contributes to the conductance across the brush border. The presence of actively transported sugars or amino acids in the mucosal solution brings about a marked depolarization of psi(mc) and a smaller increase in the transmural PD (Deltapsi(ms)). It appears that the Na influx that is coupled to the influxes of amino acids and sugars is electrogenic and responsible for the depolarization of psi(mc). Under control conditions Deltapsi(ms) can be attributed to the depolarization of psi(mc) together with the presence of a low resistance transepithelial shunt, possibly the lateral intercellular spaces. However, quantitatively similar effects of amino acids on psi(mc) are also seen in tissues poisoned with metabolic inhibitors or ouabain. Under these conditions Deltapsi(mc) is much smaller than under control conditions. Thus, the depolarization of psi(mc) might not account for the entire Deltapsi(ms), observed in nonpoisoned tissue. An additional electromotive force which is directly coupled to metabolic processes might contribute to the normal Deltapsi(ms).

Ion transport during cholera-induced ileal secretion in the dog

To assess the ion transport mechanism by which cholera causes the small bowel to secrete, ion transport rates and electrical potential difference (PD) were determined simultaneously in the normal and choleragen-treated dog ileum in vivo. The results indicate that, during cholera, HCO(3) is actively secreted (i.e., against both an electrical and a concentration gradient); Cl is also actively secreted, against a modest electrochemical gradient. Electrogenic pumping of one or both of these anions is probably responsible for an observed PD change of approximately 13 mv (lumen negative). Na secretion can be accounted for entirely by passive ion movement. K secretion can be partly explained by passive diffusion secondary to the negative intraluminal PD; however, its concentration in the secreted fluid is two to three times higher than expected on the basis of passive forces, suggesting a component of active K secretion. The PD response of the choleragen-treated ileum is normal in response to glucose, but there was no PD response to saline-free mannitol perfusion. This suggests that the normal differential permeability of the ileum to anions and cations may be altered by choleragen, although other explanations of this finding are also possible.

The effect of adenosine triphosphate on the transmural potential in rat small intestine

1. ATP either in the mucosal or serosal fluid caused a transient increase in the potential difference and short-circuit current across the wall of rat jejunum in vitro, the serosal solution becoming more positive.2. Similar responses were observed in the ileum and colon, and in in vivo preparations of small intestine.3. The response is relatively specific for ATP.4. The transient nature of the response is not due to rapid hydrolysis of extracellular ATP.5. High concentrations of K(+) in the mucosal medium, serosal ouabain or mucosal 2,4-dinitrophenol all inhibit the response without altering the time course.6. Significant inhibition was not observed in the presence of ADP or in Mg(2+)-free salines.7. The results are consistent with an intracellular action of ATP in the epithelium to stimulate net ion transport. The results do not demonstrate whether or not extracellular ATP can act as an energy donor for an electrogenic ion pump.8. Theophylline prolongs the time course of the response, and the involvement of the adenyl cyclase system cannot be excluded as an explanation for the findings.

Lysine transport across isolated rabbit ileum

Lysine transport by in vitro distal rabbit ileum has been investigated by determining (a) transmural fluxes across short-circuited segments of the tissue; (b) accumulation by mucosal strips; and (c) influx from the mucosal solution across the brush border into the epithelium. Net transmural flux of lysine is considerably smaller than that of alanine. However, lysine influx across the brush border and lysine accumulation by mucosal strips are quantitatively comparable to alanine influx and accumulation. Evidence is presented that the "low transport capacity" of rabbit ileum for lysine is due to: (a) a carrier-mediated process responsible for efflux of lysine out of the cell across the serosal and/or lateral membranes that is characterized by a low maximal velocity; and (b) a high "backflux" of lysine out of the cell across the mucosal membrane. A possible explanation for the latter observation is discussed with reference to the relatively low Na dependence of lysine transport across the intestinal brush border.

The ontogeny of electrical activity associated with absorption of solutes across the developing small intestine of the chick (Gallus domesticus)

1. Electrical potentials across the developing intestine of the chick were measured in vitro 3-4 days before and up to 32 days after hatching.2. Embryonic intestine had a very high endogenous p.d. (serosa positive to mucosa) which decreased up to hatching. This decrease continued during intestinal maturation, reaching a basal level several days after hatching.3. Transfer potential differences were caused only by the actively transferred hexoses both in embryonic and post-embryonic intestine. These potentials decreased concomitantly with the decrease in the endogenous p.d. Kinetic analysis showed that the ;apparent K(m)' of glucose for the glucose transfer p.d. generating mechanism did not change over the period of hatching.4. Both the embryonic and post-embryonic transmural p.d. either in the presence or absence of metabolizable hexose was dependent upon aerobic metabolism.5. The transmural p.d. of hatched chick small intestine (with and without hexoses) was linearly related to log(10)[Na(+)] of the mucosal fluid. The magnitude of the glucose transfer p.d. both in embryonic and post-embryonic small intestine, however, was not lowered by decreasing the mucosal [Na(+)].6. Amino acid transfer potentials were of small magnitude at all stages of development.

Relation between sodium concentration, electrical potential and transfer capacity of rat small intestine

1. A study has been made of the effect of Na replacement in the incubation media on the hexose-dependent potential and the capacity of sacs of rat intestine to transfer fluid and galactose.2. Replacement of NaCl with mannitol or Tris Cl had little effect on the hexose-dependent potential, while replacement with LiCl and KCl had an inhibitory effect on the potential.3. Replacement with KCl, LiCl, mannitol and Tris Cl all reduced the capacity to transfer fluid and galactose, but the effects of Li and K replacement were greater than mannitol or Tris replacement.4. There is not a fixed relation between the magnitude of the potential and the amount of galactose transferred.