Intracellular ion concentrations in the frog cornea epithelium during stimulation and inhibition of Cl secretion

Membrane potential difference and intracellular cation concentrations in human placental trophoblast cells in culture

1. The electrochemical gradients for Na+ and K+ were assessed in a cell culture model of trophoblast differentiation. 2. Membrane potential difference (Em), intracellular water and Na+ and K+ contents were measured in choriocarcinoma cells (JAr cell line; 96% of which are undifferentiated trophoblast cells) and in mononucleate and multinucleate (differentiated) cytotrophoblast cells isolated from the human placenta at term. 3. There was a significant fall in Em from -57 mV in JAr cells, to -48 and -40 mV in mono-and multinucleate cytotrophoblast cells, respectively. Treatment with ouabain (1 mM for 15 min) depolarized the JAr cell membrane by 15 mV but did not affect cytotrophoblast cell membrane potential. 4. Intracellular K+ concentration was similar in JAr, mono- and multinucleate cytotrophoblast cells but Na+ concentration was higher in mononucleate cytotrophoblast cells compared with JAr cells. 5. Ouabain treatment (3 mM for 15 min) caused a small increase (4.5%) in cell water in mononucleate cytotrophoblast cells but lowered K+ (approximately 30%) and increased Na+ concentration (approximately 125%) in all the trophoblast cells studied. 6. The K+ equilibrium potential (EK) was more negative than Em in all cells and the difference between EK and Em was smaller in JAr cells (-25 mV) than in mono- and multinucleate cytotrophoblast cells (-33 and -43 mV, respectively). 7. The Na+ equilibrium potential (ENa) was positive in the trophoblast cells and the difference between ENa and Em was 122, 100 and 100 mV in JAr, mono- and multinucleate cytotrophoblast cells, respectively. 8. These results suggest that the electrochemical gradient for K+ is affected by the stage of trophoblast cell differentiation. In contrast, the electrochemical gradient for Na+ is similar in mono- and multinucleate cytotrophoblast cells.

Changes in cellular membrane and paracellular conductances by amphotericin B in the epithelium of the bullfrog cornea

In the isolated bullfrog cornea, measurements of DC electrical parameters in conjunction with AC impedance and ultrastructural analyses were used to determine the effects of 10(-5) M amphotericin B on epithelial cellular membrane and paracellular conductances. In NaCl Ringers, amphotericin B elicited a 3.5-fold increase in the specific apical membrane conductance (Ga/Ca); where Ga and Ca are the apical membrane conductance and capacitance, respectively. The basolateral membrane conductance (Gb) and the basolateral membrane capacitance (Cb) fell by 57% and 50%, respectively. In the paracellular pathway, the tight junctional complex (Gj) was unchanged whereas the lateral intercellular space resistance (Rp) decreased by 55%. The declines in Gb and Cb were suggestive of cell volume shrinkage because these changes were consistent with a previously described decline in intracellular K+ content and reduction in exposed basolateral membrane area to current flow. Ultrastructural analysis validated that amphotericin B caused cell volume shrinkage because there was: (1) increased folding of the basolateral membrane and waviness of the basal aspects- of the plasma membrane; (2) dilatation of the lateral intercellular spaces. This agreement suggests that intracellular activity decreased following exposure to amphotericin B which resulted in cell volume shrinkage and an impairment of Cl- uptake across the basolateral membrane.

Intracellular ion concentrations and cell volume during cholinergic stimulation of eccrine secretory coil cells

Methacholine (MCh)-induced changes in intracellular concentrations of Na, K, and Cl [( Na]i, [K]i, and [Cl]i, respectively) and in cellular dry mass (a measure of cell shrinkage) were examined in isolated monkey eccrine sweat secretory coils by electron probe X-ray microanalysis using the peripheral standard method. To further confirm the occurrence of cell shrinkage during MCh stimulation, the change in cell volume of dissociated clear and dark cells were directly determined under a light microscope equipped with differential interference contrast (DIC) optics. X-ray microanalysis revealed a biphasic increase in cellular dry mass in clear cells during continuous MCh stimulation; an initial increase of dry mass to 158% (of control) followed by a plateau at 140%, which correspond to the decrease in cell volume of 37 and 29%, respectively. The latter agrees with the MCh-induced cell shrinkage of 29% in dissociated clear cells. The MCh-induced increase in dry mass in myoepithelial cells was less than half that of clear cells. During the steady state of MCh stimulation, both [K+]i and [Cl]i of clear cells decreased by about 45%, whereas [Na]i increased in such a way to maintain the sum of [Na] i + [K]i constant. There was a small (12-15 mM) increase in [Na]i and a decrease in [K]i in myoepithelial cells during stimulation with MCh. Dissociated dark cells failed to significantly shrink during MCh stimulation. The decrease in [Cl]i in the face of constant [Na]i + [K]i suggests the accumulation of unknown anion(s) inside the clear cell during MCh stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Secretagogues increase apical membrane conductance of isolated bullfrog corneal epithelium pretreated with loop diuretics

Bullfrog (Rana catesbeiana) corneas were mounted in an Ussing type chamber and impaled with an intracellular microelectrode and the short circuit current was inhibited by pretreatment with the loop diuretics furosemide (0.3 to 1 mM) or bumetanide (10 to 100 microM). Subsequent addition of the secretagogues prostaglandin E2, forskolin, or 3-isobutyl-1-methylxanthine (IBMX) caused the fractional voltage drop of the apical barrier to decrease from 0.72 +/- 0.05 to 0.48 +/- 0.04 and the chloride-dependent conductance to increase by 0.15 +/- 0.03 mS/cm2, but caused only a small, transient increase in short circuit current. The loop diuretics by themselves always greatly reduced the short circuit current but did not consistently reduce conductance or fractional voltage drop of the apical membrane. Because the secretagogues were able to increase the apical membrane conductance of diuretic-inhibited corneas without large effects on the short circuit current, the loop diuretics must have a major effect at a site other than the apical membrane Cl- conductance, presumably at the basolateral membrane. An additional effect of the loop diuretics at the apical membrane is also possible.

Mechanism of chloride uptake in rabbit corneal epithelium

The mechanism of chloride uptake at the basal membrane (stromal side) of rabbit corneal epithelium was examined by observing the effects of ion transport inhibitors and ion concentrations on the stimulated epithelial short-circuit current (Isc). Loop diuretics inhibited the theophylline-stimulated peak and sustained Isc. Treatment with 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS, 0.2 mM) and/or 5-(N,N-dimethyl)amiloride (0.1 mM) as well as the potent anion exchange inhibitor, 5c(+)[(2,3,9,9a-tetrahydro-1H-fluoren-7-yl)oxy]acetic acid (0.01 mM), had no significant effect on Isc. These results are consistent with Cl- uptake by a Na+-Cl- or Na+-K+-2Cl- cotransport mechanism rather than Cl(-)-HCO3(-)-OH- exchange coupled to Na+-H+ exchange. Incubation in low [Na+] or [Cl-] before stimulation with forskolin (0.1 mM) reduced both peak and sustained Isc, and saturation kinetics were exhibited. Hill coefficients for [Na+] and [Cl-] were 0.99 and 1.04, respectively, for peak Isc and 0.66 and 1.18, respectively, for sustained Isc. Apparent ion affinities for Na+ and Cl- were 13.5 and 18 mM, respectively, for peak Isc and 15 and 22 mM, respectively, for sustained Isc. These results favor Cl- uptake by a 1 Na+:1 Cl- cotransport mechanism for the rabbit corneal epithelium, but involvement of K+ in this process has not been eliminated.

Effect of loop diuretics on bullfrog cornea epithelium

The effect of loop diuretics on Cl transport was studied on an in vitro preparation of the bullfrog cornea. Bumetanide (10(-4) M) or furosemide (10(-3) M) added to the stromal solution decreased Cl transport measured as the short-circuit current (Isc) to values near zero. Concomitantly, transepithelial conductance (gt) decreased, whereas the intracellular potential (Vo) hyperpolarized and the fractional resistance of the apical membrane (fRo) increased. Substitution of SO4 for Cl in the tear-side solution led to prompt changes in Isc, gt, Vo, and fRo, characteristic of appreciable passive Cl movement across the apical membrane before and after inhibition. Epinephrine (10(-4) M) was similarly effective on apical membrane conductance in inhibited tissues as under control conditions, but the effective electromotive force for transepithelial Cl transport was reduced to approximately 25%. Intracellular Cl activity, measured with ion-selective microelectrodes, decreased so much that the difference in electrochemical Cl potential divided by the Faraday constant (delta mu Cl/F) was close to zero after inhibition of Isc by bumetanide. Apical Cl permeability remained essentially unchanged. Accordingly, loop diuretics inhibit Cl transport in the Cl-secreting cornea epithelium by blocking the Na-Cl symport without secondary apical effects, as believed for other Cl-reabsorbing epithelia.

Extracellular volume in the bovine corneal epithelium

Three different substances, [14C]inulin, [14C]polyethylenglycol (PEG, molecular weight 4000) and [3H]sucrose have been examined for suitability as markers for determination of the extracellular volume (ECV) in the bovine corneal epithelium and stroma. In both tissues, sucrose was the best marker reaching equilibrium of distribution within 90 min. Inulin and 4000 PEG have been found to be unacceptable because of a slow rate of penetration of the ECV. In addition, both inulin and 4000 PEG gave considerably lower values for the ECV than sucrose. Using the value for ECV determined with sucrose as a marker in the bovine corneal epithelium, i.e. 21.6 +/- 1.4% of tissue water, and the results obtained previously by the chemical analysis of this tissue (Midelfart 1987) the intracellular concentrations of 34.4 +/- 8.6 mmol Na/kg H2O and 154.3 +/- 11.1 mmol K/kg H2O were determined. These results were compared with the results obtained by other methods in corneae from other species.