Ca++ regulation of paracellular permeability in the middle intestine of the eel, Anguilla anguilla

The role of Ca++ on the regulation of the paracellular pathway permeability of the middle intestine of Anguilla anguilla was studied by measuring the transepithelial resistance and the dilution potential, generated when one half of NaCl in the mucosal solution was substituted iso-osmotically with mannitol, in various experimental conditions altering extracellular and/or intracellular calcium levels. We found that removal of Ca++ in the presence of ethylene glycol-bis(beta-aminoethyl ether) (EGTA) from both the mucosal and the serosal side, but not from one side only, reduced both the transepithelial resistance and the magnitude of the dilution potential. The irreversibility of this effect suggests a destruction of the organization of the junction in the nominal absence of Ca++. However a modulatory role of extracellular Ca++ cannot be excluded. The decrease of the intracellular Ca++ activity, produced by using verapamil to block the Ca++ entry into the cell, or by adding 3,4,5-trimethoxybenzoic acid 8-(diethylamino) octyl ester (hydrochloride) (TMB-8), an inhibitor of Ca++ release from the intracellular stores, reduced both the transepithelial resistance and the magnitude of the dilution potential, indicating a role of cytosolic Ca++ in the modulation of the paracellular permeability. However the rise of calcium activity produced by the Ca++ ionophore calcimycin (A23187) evoked an identical effect, suggesting that any change in physiological intracellular Ca++ activity alters the paracellular permeability.     

Increase of (Ca2++Mg2+)-ATPase activity of renal basolateral membrane by parathyroid hormone via cyclic AMP-dependent membrane phosphorylation

Studies were made on the mechanism of the effect of parathyroid hormone (PTH) on the activity of (Ca2++Mg2+)-ATPase, a membrane bound Ca2+-extrusion pump enzyme from the basolateral membranes (BLM) of canine kidney (Km for free Ca2+ = 1.3 X 10(-7) M, Vmax = 200 nmol Pi/mg/min). At 1 X 10(-7) M free Ca2+, both PTH (10(-7)-10(-6) M) and cAMP (10(-6)-10(-4) M) stimulated (Ca2++Mg2+)-ATPase activity dose-dependent and their stimulatory effects were inhibited completely by 5 microM H-8, an inhibitor of cAMP-dependent protein kinase. PTH (10(-7) M) also caused 40% increase in 32P incorporation into the BLM and 5 microM H-8 inhibited this increase too. PTH (10(-7) M) was found to stimulate phosphorylation of a protein of Mr 9000 by cAMP dependent protein kinase and 5 microM H-8 was found to block this stimulation also. From these results, it is proposed that PTH stimulates (Ca2++Mg2+)-ATPase activity by enhancing its affinity for free Ca2+ via cAMP-dependent phosphorylation of a BLM protein of Mr 9000.     

Effects of delta 9-tetrahydrocannabinol on testosterone production in vitro: influence of Ca++, Mg++ or glucose

The major psychoactive component of marihuana, delta 9-tetrahydrocannabinol (THC), influences testicular function. In the present experiments, the addition of THC to incubations of whole decapsulated mouse testes altered testosterone (T) production differentially, depending on the specific gonadotropin used, the dose of THC and/or the amount of divalent cation present in the media. In the presence of luteinizing hormone (LH; 10 ng/ml), and a dose of 25 micrograms THC/ml, T production was significantly decreased, compared to that by testes incubated with LH and vehicle at all Ca++ levels, except at 0.127 or 1.0 mM Ca++. The production of T by these paired testes exposed to either THC or vehicle (ethanol; ETOH), increased as Ca++ concentration approached physiological levels. In contrast, in the presence of follicle-stimulating hormone (FSH; 1 microgram/ml), THC-induced suppression of T production was significant in the absence of Ca++ from the media, and at 12.7 mM Ca++. However, it appeared that the levels of Ca++ did not differentially affect T production in the presence of FSH, whether or not THC was also added. In the presence of human chorionic gonadotropin (hCG; 12.5 mIU/ml), a lower dose of THC (25 ng/ml), stimulated T production at 0.25 to 1 mM Ca++, but had no effect as Ca++ reached 2.5 mM. Without additional Ca++ in the media, this dose of THC significantly reduced T secretion. In contrast, in the presence of hCG, a higher THC dose (25 micrograms/ml), suppressed T accumulation at 0.127, and from 1.0 to 12.7, but had no effect at 0.25 mM, or in the absence of Ca++. In the presence of hCG, the high 25 micrograms/ml dose of THC stimulated T production, in the absence of additional Mg++, and at 0.01 mM Mg++, but THC had no effect at 0.1 mM Mg++, but inhibited T production at 1.1 mM Mg++. In the presence of hCG, 25 micrograms THC/ml produced a consistent suppression of T production across glucose concentrations examined. These findings suggest that the mechanisms by which THC effects testicular steroidogenesis may involve Ca++- and/or Mg++-dependent processes. Differential requirements for these divalent cations by the gonadotropins may explain the interactive effects of THC with LH, hCG or FSH.     

Depolarizing response of rat parathyroid cells to divalent cations

Membrane potentials were recorded from rat parathyroid glands continuously perfused in vitro. At 1.5 mM external Ca++, the resting potential averages -73 +/- 5 mV (mean +/- SD, n = 66). On exposure to 2.5 mM Ca++, the cells depolarize reversibly to a potential of -34 +/- 8 mV (mean +/- SD). Depolarization to this value is complete in approximately 2-4 min, and repolarization on return to 1.5 mM Ca++ takes about the same time. The depolarizing action of high Ca++ is mimicked by all divalent cations tested, with the following order of effectiveness: Ca++ greater than Sr++ greater than Mg++ greater than Ba++ for alkali-earth metals, and Ca++ greater than Cd++ greater than Mn++ greater than Co++ greater than Zn++ for transition metals. Input resistance in 1.5 mM Ca++ was 24.35 +/- 14 M omega (mean +/- SD) and increased by an average factor of 2.43 +/- 0.8 after switching to 2.5 mM Ca++. The low value of input resistance suggests that cells are coupled by low-resistance junctions. The resting potential in low Ca++ is quite insensitive to removal of external Na+ or Cl-, but very sensitive to changes in external K+. Cells depolarize by 61 mV for a 10- fold increase in external K+. In high Ca++, membrane potential is less sensitive to an increase in external K+ and is unchanged by increasing K+ from 5 to 25 mM. Depolarization evoked by high Ca++ may be slowed, but is unchanged in amplitude by removal of external Na+ or Cl-. Organic (D600) and inorganic (Co++, Cd++, and Mn++) blockers of the Ca++ channels do not interfere with the electrical response to Ca++ changes. Our results show remarkable parallels to previous observations on the control of parathormone (PTH) release by Ca++. They suggest an association between membrane voltage and secretion that is very unusual: parathyroid cells secrete when fully polarized, and secrete less when depolarized. The extraordinary sensitivity of parathyroid cells to divalent cations leads us to hypothesize the existence in their membranes of a divalent cation receptor that controls membrane permeability (possibly to K+) and PTH secretion.     

The paracellular pathway in the lepidopteran larval midgut: modulation by intracellular mediators

The features of the paracellular pathway, an important route for the transfer of ions and molecules in epithelia, are in insects still poorly investigated and it has not yet been elucidated how the septate junction (SJ) acts as a transepithelial barrier. In this study, some properties of the paracellular pathway of Bombyx mori larval midgut, isolated in Ussing chambers, were determined and the modulation of SJ permeability by intracellular events disclosed. Diffusion potentials evoked by transepithelial gradients of different salts indicated that the junction bore weak negative charges and that the paracellular pathway was selective with respect to ion charge and size. In standard conditions, the transepithelial resistance was 28.2+/-2.1 Omega cm(2), a value indicating that the midgut is a low resistance epithelium. The modulation of midgut SJ by typical enhancers of mammalian tight junction permeability known to act on the cytoskeleton was studied by measuring the shunt resistance and the lumen-to-haemolymph flux of sucrose. An increase of the intracellular level of cAMP and Ca(2+) caused a significant decrease of the shunt resistance and an increase of SJ permeability. The attenuation of Ca(2+) effect in the presence of the calcium channel blocker nifedipine indicated that the influx of external Ca(2+) into the cytoplasm was important for the opening of the SJ, as well as the release of Ca(2+) from the intracellular stores.     

Control of mitochondrial Mg++-efflux]

Inorganic phosphate stimulates the release of Mg++ from liver mitochondria, depending on concentration; a concentration as low as 0.1 mM phosphate is already effective. The process is dependent on the electron transfer of the respiratory chain, and its rate is highest under conditions of endogenous respiration and with ascorbate and TMPD as substrates, respectively. The phosphate stimulated release of Mg++ is followed, with a pronounced delay, by a Ca++ efflux and a swelling of mitochondria. Addition of EGTA strongly reduced the rate of Mg++ liberation in the presence and absence of inorganic phosphate. Exogenous Ca++ is able to abolish the EGTA effect. ADP and ATP inhibit the phosphate stimulated release of Mg++. Phosphoenol pyruvate and free fatty acids enhance the rate of Mg++ and Ca++ efflux from the mitochondria. The results permit the conclusion that inorganic phosphate, Ca++ and various metabolites of the cell metabolism influence the Mg++ distribution between the extra- and intramitochondrial space, thus controlling the permeability of the mitochondrial inner membrane for monovalent cations.     

Role of Mg++ in the mithramycin-DNA interaction: evidence for two types of mithramycin-Mg++ complex

Mithramycin(MTR, structure shown in Figure 1) [and the related compound Chromomycin A3(CHRA3)] are antitumor antibiotics which inhibit DNA dependent RNA polymerase activity via reversible interaction with DNA only in the presence of divalent metal ion such as Mg++. In order to understand the role of Mg++ in MTR-DNA interaction, absorbance and CD spectroscopic techniques are employed to study the binding of MTR to Mg++. These studies show: i) the drug alone binds to Mg++ and ii) two different types of drug-Mg++ complexes are formed at low(Complex I) and high(Complex II) ratios of the concentration of Mg++ and MTR. We propose that these two complexes would bind to the same DNA with different affinities and rates. This result suggests that the relative concentration of Mg++ is an important factor to be taken into account to understand the molecular basis of MTR-DNA interaction.     

High affinity binding of divalent cation to actin monomer is much stronger than previously reported

Monomeric actin is known to bind tightly one divalent cation per molecule. We have quantitatively reinvestigated the affinity of actin for Ca++ and Mg++ using the fluorescent Ca++ chelator Quin2 to induce and measure the dissociation of Ca++ from Ca-actin, supporting these studies with measurements using 45Ca. We found that the KD for Ca-actin is actually 1.9 +/- 0.7 nM. Kinetic analysis supported this result and demonstrated a dissociation rate constant (k-) of 0.013 s-1 and an association rate constant (k+) of 6.8 X 10(6)M-1 s-1 for Ca-actin. Competitive binding studies indicated that the binding affinity of actin for Ca++ is 5.4 times that for Mg++, yielding a calculated KD for Mg-actin of about 10 nM. Thus, the tight-binding of divalent cations to actin is 3-4 orders of magnitude stronger than previously thought.     

Difructose anhydride III and sodium caprate activate paracellular transport via different intracellular events in Caco-2 cells

A nondigestible disaccharide, difructose anhydride (DFA) III, is known to activate calcium transport via tight junctions (TJs); however, the characteristics of and mechanisms for the increase in paracellular transport induced by DFAIII have not been clarified. We compared the effect of DFAIII with that of sodium caprate (C10), a well-known enhancer of TJ permeability, on the changes in TJ proteins, transport of paracellular markers, and effects of nine cellular signaling blockers using Caco-2 monolayers. The addition of DFAIII (0-100mmol/L) and C10 (0-10mmol/L) to the apical medium of the Caco-2 monolayers dose-dependently decreased transepithelial electrical resistance (TER), which is an indicator of TJ permeability. The reduction with C10 was much faster than that with DFAIII. Transport of the paracellular markers of various molecular weights (182-43,200) was elevated by the addition of 100mmol/L DFAIII and 10mmol/L C10. The transport rates were much in the presence of C10 than of DFAIII, while the reduction in TER by two treatments was similar (from 1000 to 300Omega cm(2)). Treatment with DFAIII and C10 changed the distribution of actin filament and claudin-1, but not occludin, junctional adhesion molecule-1, or zonula occludens-1; however, alterations in the patterns of the TJ proteins differed according to treatment. An inhibitor of myosin light chain kinase and a chelator of intracellular calcium ion ([Ca(2+)](i)) attenuated the TER reduction by C10, but not by DFAIII. These data demonstrate that the increase in TJ permeability induced by DFAIII results from the alterations to actin and claudin-1 via [Ca(2+)](i)-independent mechanisms.     

Effect of extracellular ATP level on flow-induced Ca++ response in cultured vascular endothelial cells

Cultured vascular endothelial cells loaded with the highly fluorescent Ca(++)-sensitive dye Fura-2 were exposed to the flow of a fluid containing various concentrations of ATP (0, 0.5, 1, 5 microM) in an apparatus designed on the basis of fluid dynamics, and simultaneous changes in intracellular free Ca++ concentration were monitored by photometric fluorescence microscopy. The flow rate of the perfusate was altered from 0 to 6.3 to 22.8 to 39.0 cm/sec, inducing shear stress on the cell surface of 0, 2.9, 10.4, and 17.9 dynes/cm2, respectively. Although no significant change in intracellular Ca++ level was observed at ATP levels below 100 nM, at an ATP level of 500 nM, the intracellular Ca++ level increased together with an increase in the flow rate of the perfusate. At this level of ATP, the intracellular Ca++ levels at flow rates of 0, 6.3, 22.8, and 39.0 cm/sec were 44.8 +/- 7.3, 60.3 +/- 10.7, 74.0 +/- 5.8 and 89.4 +/- 6.4 nM (mean +/- SD; n = 8), respectively. At ATP levels over 1 microM, the flow-rate dependency of Ca++ response became less clear than that observed at the ATP level of 500 nM. These Ca++ responses to changes in flow rate disappeared when extracellular Ca++ was chelated by adding 2 mM of EGTA to the perfusate. These results suggest that the vascular endothelial cell has a mechanism that elevates the intracellular Ca++ level in accord with the flow rate at appropriate ATP concentrations, and that changes in intracellular Ca++ level under this mechanism seem to be chiefly caused by the influx of extracellular Ca++ into cells.     

The effect of conjugated linoleic acid on transepithelial calcium transport and mediators of paracellular permeability in human intestinal-like Caco-2 cells

Conjugated linoleic acid (CLA) increases paracellular permeability across human intestinal-like Caco-2 cell monolayers, which transport Ca predominantly by the transcellular route. In vivo, however, paracellular Ca transport is the predominant route of Ca transport. Therefore, the objective of this study was to investigate the effect of CLA on transepithelial Ca transport in Caco-2 cells transporting Ca predominantly by the paracellular route. Cells were seeded onto permeable transport membranes and allowed to differentiate, over 14 d, into intestinal-like cell monolayers. Monolayers (n=9/treatment) were exposed to 0 (control) or 80 microM- 18:2, -cis-9, trans-11 CLA or -trans-10, cis-12 CLA for 14 d prior to Ca transport studies. Overall transepithelial Ca transport as well as transcellular and parcellular Ca transport was significantly increased (P<0.001) by exposure of Caco-2 cells to both isomers of CLA, an effect which appeared to be related to altered localization of zona occludens 1 (a tight junction protein).     

Phosphorylation of the red blood cell membrane during the active transport of C++

The phosphorylation of red blood cell membrane fragments (RBCMF) during Ca++ transport was investigated. When red cell membrane fragments are incubated with [gamma-32P]ATP under the experimental condition which minimizes the phosphorylation of Na+-K+-ATPase, RBCMF are labeled in the presence of Mg++ without Ca++. When Ca++ is added, the labeling decreases due to dephosphorylation of RBCMF. The initial reaction of phosphorylation is reversed in the presence of excess ADP. The treatment of RBCMF with n-ethylmaleimide (NEM) does not interfere with the initial phosphorylation reaction, but blocks the dephosphorylation in the presence of Ca++. These data suggest that the enzymatic sequence of the Ca++ transport mechanism may be very similar to that of the Na+ transport mechanism.     

Localization of Ca++-containing antimonate precipitates during mitosis

Intracellular bound Ca++ has been localized throughout mitosis and cytokinesis in two plant species by means of in situ precipitation with potassium antimonate and electron microscope visualization. Identification of Ca++ as the major cation precipitated was made by comparing solubility properties in water, EDTA, and EGTA of the intracellular deposits with respect to those of K+-, Mg++-, and Ca++- antimonate standards. In spermatogenous cells of the water fern, Marsilea vestita, and stomatal complex cells of barley, Hordeum vulgare, antimonate deposits have been found associated with the endoplasmic reticulum (ER), vacuoles, euchromatin/nucleoplasm, and mitochondria. The last contain a much higher density of precipitates in Marsilea than in Hordeum. Dictyosomes and the nuclear envelope of Marsilea also contain antimonate deposits, as do the plasmalemma, cell wall, and phragmoplast vesicles of Hordeum. Microtubule-organizing centers such as kinetochores and the blepharoplast of Marsilea do not stain. In spite of differences in associated antimonate between certain organelles of the two species, the presence of antimonate aong the ER throughout the cell cycle is common to both. Of particular interest are those precipitates seen along the tubules and cisternae of the extensive smooth ER that surrounds and invades the mitotic spindle in both species. The ability to bind divalent cations makes the mitotic apparatus (MA)-associated ER a likely candidate for regulation of free Ca++ levels in the immediate vicinity of structural components and processes that are Ca++-sensitive and proposed to be Ca++-regulated.     

Apical membrane ionic channels in the rabbit cortical thick ascending limb in primary culture.

Cortical thick ascending limbs of Henle's loop (cTAL) were microdissected from rabbit kidneys and cultured in a hormonally-defined medium. The cultured cells grew as a monolayer and retained the morphological and biochemical characteristics of the original tubule. Cyclic AMP production of the cultured cells was increased by human calcitonin (x13) and parathyroid hormone (x2). The cultured epithelial developed a transepithelial potential of 4.1 +/- 1.3 mV that was orientated positively towards the apical compartment. The basolateral membrane of the cells exhibited a chloride conductance sensitive to diphenylamine 2-carboxylate (DPC) and the apical membrane a barium-sensitive K+ permeability. Patch clamp analysis conducted on the apical membrane of the cells revealed the presence of three types of ionic channel. The first is a large conductance Ca(2+)-activated K+ channel (95 pS). The second K+ channel has a much smaller conductance (18.3 pS) and is insensitive to Ca2+. It may represent the conductive pathway for K+ recycling into the lumen in the original tubule. The last channel is cation selective, does not discriminate between Na+ and K+ and was found to have a conductance of 20.5 pS. Channel activity required a high cytoplasmic calcium concentration (1 mM), and was blocked by ATP (10 microM) applied on its cytoplasmic face.     

Effects of divalent cations on dynein cross bridging and ciliary microtubule sliding

We recently demonstrated that addition of the divalent cation Mg++ to demembranated cilia causes the dynein arms to attach uniformly to the B subfibers. We have now studied the dose-dependent relationship between Mg++ or Ca++ and dynein bridging frequencies and microtubule sliding in cilia isolated from Tetrahymena. Both cations promote efficient dynein bridging. Mg++-induced bridges become saturated at 3 mM while Ca++-induced bridges become saturated at 2 mM. Double reciprocal plots of percent bridging vs. the cation concentration (0.05-10 mM) suggest that bridging occurs in simple equilibrium with the cation concentration. When microtubule sliding (spontaneous disintegration in 40 mM N-2-hydroxyethylpiperazine-N'-2-ethane sulfonic acid (HEPES), 0.1 mM ATP at pH 7.4) is assayed (A350 nm) relative to the Mg++ or Ca++ concentration, important differential effects are observed. 100% Disintegration occurs in 0.5-2 mM Mg++ and the addition of 10 mM Mg++ does not inhibit the response. The addition of 0.05-10 mM Ca++ to cilia reactivated with 0.1 mM ATP causes a substantial reduction in disintegration at low Ca++ concentrations and complete inhibition at concentrations greater than 3 mM. When Ca++ is added to cilia reactivated with 2 mM Mg++ and 0.1 mM ATP, the percent disintegration decreases progressively with the increasing Ca++ concentration. The addition of variable concentrations of Co++ to Mg++-activated cilia causes a similar but more effective inhibition of the disintegration response. These observations, when coupled with the relatively high concentrations of Ca++ or Co++ needed to inhibit disintegration, suggest that inhibition results from simple competition for the relevant cation-binding sites and thus may not be physiologically significant. The data do not yet reveal an interpretable relationship between percent disintegration, percent dynein bridging, and percent ATPase activity of both isolated dynein and whole cilia. However, they do illustrate that considerable (sliding) disintegration (60%) can occur under conditions that reveal only 10-15% attached dynein cross bridges.     

Blocking by 2,4,6-triaminopyrimidine of increased tight junction permeability induced by acetylcholine in the pancreas

1. The permeability of the paracellular pathway in the isolated rabbit pancreas has been studied with the aid of 2,4,6-triaminopyrimidine. 2. Addition of 2,4,6-triaminopyrimidine (1--10 mM) to the bathing medium has no effect on the rate of fluid secretion or on protein, Na+, K+, Ca2+ and sucrose concentrations in the secreted fluid. 3. When 1 x 10(-5) M carbachol is also added to the 2,4,6-triaminopyrimidine-containing bathing medium, there is a marked reduction in the increase of the paracellular permeability for sucrose and Ca2+ found upon addition of carbachol alone. The enzyme secretion, induced by carbachol, is not affected. 4. The minimal concentration of 2,4,6-triaminopyrimidine in the bathing medium required to reach its maximal effect on the paracellular permeability is approx. 0.55 mM at pH 7.4. 5. The effect of 2,4,6-triaminopyrimidine on the paracellular permeability after carbachol stimulation is also present when 2,4,6-triaminopyrimidine is added 5 min after the addition of 1 x 10(-5) M carbachol. 6. 2,4,6-Triaminopyrimidine has no effect on the increases in enzyme secretion and sucrose permeability caused by 1 x 10(-8) pancreozymin C octapeptide. 7. 2,4,6-Triaminopyrimidine appears in the secreted fluid at a concentration of 50% of that in the bathing medium. Upon addition of 1 x 10(5) M carbachol this concentration increases up to 80%. 8. These results indicate that: (a) the increased paracellular permeability upon stimulation with carbachol is not caused by the enzyme secretion as such and (b) addition of 2,4,6-triaminopyrimidine prevents the carbachol-induced increase in permeability of a channel in the tight junction complex.     

Cellular Li+ opens paracellular path in toad skin: Amiloride blockable effect

The presence of Li in the solution bathing the outer surface of toad skin under short-circuit condition promotes an unspecific permeability increase characterized by a delayed and progressive increase in the effluxes of 24Na, 42K and 14C sucrose. The effect of Li upon sucrose permeability might indicate an increased permeability of the paracellular pathway. The Li effect is mediated by an intracellular action since blockade of Li entrance into the cell compartment by amiloride prevents the increase in Na, K and sucrose permeability. A possible mechanism of this effect is discussed in terms of a disturbance in the cellular Ca++ balance leading to an increase in cytosolic Ca++ concentration which perturbs the organization of the cytoskeleton and the interplay between cytoskeleton and tight junctions.     

The influence of Mg++ and Zn++ on polypurine-polypyrimidine multistranded helices

We have studied by X-ray diffraction fibres of complexes of polypurine-polypyrimidine with divalent cations. In the presence of Mg++, poly(dC) and poly(dG) form a very stable triple helix at neutral pH, based on G-G-C triplexes, whereas Zn++ prevents its formation, both at neutral and acidic pH. The poly(dC) . poly(dG) complex with Zn++ is of the B form, but its X-ray diffraction pattern shows an unusual intensity distribution. This is probably due to the fact that counterions occupy defined positions on the helix. The A form has not been observed. With poly[d(A-G)].poly [d(C-T)] a different triple helical structure is formed, both with Zn++ and Mg++. Direct, X-ray diffraction evidence for these triple helices is provided here for the first time.     

Ionic conductance pathways in the mouse medullary thick ascending limb of Henle. The paracellular pathway and electrogenic Cl- absorption

Net Cl- absorption in the mouse medullary thick ascending limb of Henle (mTALH) involves a furosemide-sensitive Na+:K+:2 Cl- apical membrane symport mechanism for salt entry into cells, which occurs in parallel with a Ba++-sensitive apical K+ conductance. The present studies, using the in vitro microperfused mouse mTALH, assessed the concentration dependence of blockade of this apical membrane K+-conductive pathway by Ba++ to provide estimates of the magnitudes of the transcellular (Gc) and paracellular (Gs) electrical conductances (millisiemens per square centimeter). These studies also evaluated the effects of luminal hypertonicity produced by urea on the paracellular electrical conductance, the electrical Na+/Cl- permselectivity ratio, and the morphology of in vitro mTALH segments exposed to peritubular antidiuretic hormone (ADH). Increasing luminal Ba++ concentrations, in the absence of luminal K+, produced a progressive reduction in the transcellular conductance that was maximal at 20 mM Ba++. The Ba++-sensitive transcellular conductance in the presence of ADH was 61.8 +/- 1.7 mS/cm2, or approximately 65% of the total transepithelial conductance. In phenomenological terms, the luminal Ba++-dependent blockade of the transcellular conductance exhibited negative cooperativity. The transepithelial osmotic gradient produced by luminal urea produced blebs on apical surfaces, a striking increase in shunt conductance, and a decrease in the shunt Na+/Cl- permselectivity (PNa/PCl), which approached that of free solution. The transepithelial conductance obtained with luminal 800 mM urea, 20 mM Ba++, and 0 K+ was 950 +/- 150 mS/cm2 and provided an estimate of the maximal diffusion resistance of intercellular spaces, exclusive of junctional complexes. The calculated range for junctional dilution voltages owing to interspace salt accumulation during ADH-dependent net NaCl absorption was 0.7-1.1 mV. Since the Ve accompanying ADH-dependent net NaCl absorption is 10 mV, lumen positive, virtually all of the spontaneous transepithelial voltage in the mouse mTALH is due to transcellular transport processes. Finally, we developed a series of expressions in which the ratio of net Cl- absorption to paracellular Na+ absorption could be expressed in terms of a series of electrical variables. Specifically, an analysis of paired measurement of PNa/PCl and Gs was in agreement with an electroneutral Na+:K+:2 Cl- apical entry step. Thus, for net NaCl absorption, approximately 50% of Na+ was absorbed via a paracellular route.     

Interactions of sodium transport, cell volume, and calcium in frog urinary bladder

The volume of individual cells in intact frog urinary bladders was determined by quantitative microscopy and changes in volume were used to monitor the movement of solute across the basolateral membrane. When exposed to a serosal hyposmotic solution, the cells swell as expected for an osmometer, but then regulate their volume back to near control in a process that involves the loss of KCl. We show here that volume regulation is abolished by Ba++, which suggests that KCl movements are mediated by conductive channels for both ions. Volume regulation is also inhibited by removing Ca++ from the serosal perfusate, which suggests that the channels are activated by this cation. Previously, amiloride was observed to inhibit volume regulation: in this study, amiloride-inhibited, hyposmotically swollen cells lost volume when the Ca++ ionophore A23187 was added to Ca++-replete media. We attempted to effect volume changes under isosmotic conditions by suddenly inhibiting Na+ entry across the apical membrane with amiloride, or Na+ exit across the basolateral membrane with ouabain. Neither of these Na+ transport inhibitors produced the expected results. Amiloride, instead of causing a decrease in cell volume, had no effect, and ouabain, instead of causing cell swelling, caused cell shrinkage. However, increasing cell Ca++ with A23187, in both the absence and presence of amiloride, caused cells to lose volume, and Ca++-free Ringer's solution (serosal perfusate only) caused ouabain-blocked cells to swell. Finally, again under isosmotic conditions, removal of Na+ from the serosal perfusate caused a loss of volume from cells exposed to amiloride. These results strongly suggest that intracellular Ca++ mediates cell volume regulation by exerting a negative control on apical membrane Na+ permeability and a positive control on basolateral membrane K+ permeability. They also are compatible with the existence of a basolateral Na+/Ca++ exchanger.