Determination of the driving force of the Na(+) pump in toad bladder by means of vasopressin

Multiple functions of the crustacean gill: osmotic/ionic regulation, acid-base balance, ammonia excretion, and bioaccumulation of toxic metals

The crustacean gill is a multi-functional organ, and it is the site of a number of physiological processes, including ion transport, which is the basis for hemolymph osmoregulation; acid-base balance; and ammonia excretion. The gill is also the site by which many toxic metals are taken up by aquatic crustaceans, and thus it plays an important role in the toxicology of these species. This review provides a comprehensive overview of the ecology, physiology, biochemistry, and molecular biology of the mechanisms of osmotic and ionic regulation performed by the gill. The current concepts of the mechanisms of ion transport, the structural, biochemical, and molecular bases of systemic physiology, and the history of their development are discussed. The relationship between branchial ion transport and hemolymph acid-base regulation is also treated. In addition, the mechanisms of ammonia transport and excretion across the gill are discussed. And finally, the toxicology of heavy metal accumulation via the gill is reviewed in detail.

SLC26A9 is a constitutively active, CFTR-regulated anion conductance in human bronchial epithelia

Human bronchial epithelial (HBE) cells exhibit constitutive anion secretion that is absent in cells from cystic fibrosis (CF) patients. The identity of this conductance is unknown, but SLC26A9, a member of the SLC26 family of CF transmembrane conductance regulator (CFTR)-interacting transporters, is found in the human airway and exhibits chloride channel behavior. We sought differences in the properties of SLC26A9 and CFTR expressed in HEK 293 (HEK) cells as a fingerprint to identify HBE apical anion conductances. HEK cells expressing SLC26A9 displayed a constitutive chloride current that was inhibited by the CFTR blocker GlyH-101 (71 +/- 4%, 50 microM) and exhibited a near-linear current-voltage (I-V) relation during block, while GlyH-101-inhibited wild-type (wt)CFTR exhibited a strong inward-rectified (IR) I-V relation. We tested polarized HBE cells endogenously expressing either wt or DeltaF508-CFTR for similar activity. After electrical isolation of the apical membrane using basolateral alpha-toxin permeabilization, wtCFTR monolayers displayed constitutive chloride currents that were inhibited by GlyH-101 (68 +/- 6%) while maintaining a near-linear I-V relation. In the absence of blocker, the addition of forskolin stimulated a current increase having a linear I-V; GlyH-101 blocked 69 +/- 7% of the current and shifted the I-V relation IR, consistent with CFTR activation. HEK cells coexpressing SLC26A9 and wtCFTR displayed similar properties, as well as forskolin-stimulated currents that exceeded the sum of those in cells separately expressing SLC26A9 or wtCFTR, and an I-V relation during GlyH-101 inhibition that was moderately IR, indicating that SLC26A9 contributed to the stimulated current. HBE cells from CF patients expressed SLC26A9 mRNA, but no constitutive chloride currents. HEK cells coexpressing SLC26A9 with DeltaF508-CFTR also failed to exhibit SLC26A9 current. We conclude that SLC26A9 functions as an anion conductance in the apical membranes of HBE cells, it contributes to transepithelial chloride currents under basal and cAMP/protein kinase A-stimulated conditions, and its activity in HBE cells requires functional CFTR.

Kinetics of urothelial ATP release

Recent reports have proposed that the urothelium can sense mechanical stretch and communicate this information to sensory afferent neurons by the release of ATP into the vicinity of P2X-containing neurons. This report investigates the bidirectional release of ATP by in vitro rabbit urothelium. ATP was measured using the luciferin-luciferase assay. Immediately after washing of both sides of the epithelium, there was a linear increase in ATP content in the mucosal compartment with a rate of 23 +/- 6.5 fmol x min(-1) x cm(-2) (n = 18). Serosal ATP content increased as a saturating exponential function, suggesting a constant rate of release and degradation of ATP by ectonucleotidases/exonucleotidases. The presence of a serosal ectonucleotidase/exonucleotidases was demonstrated by the time-dependent decrease in exogenously added ATP. The maximum rate of hydrolysis was 11 pmol x min(-1) x cm(-2) with a K(m) of 0.49 microM. The time course of serosal ATP release was modeled as a constant rate of release (d: mol x min(-1) x cm(-2)) and rate constant of hydrolysis (k(h): min(-)). In control conditions d was 18 fmol x min(-1) x cm(-2) and k(h) of 0.056 +/- 0.01 min(-) (n = 18). Steady-state serosal chamber content is 370 +/- 90 fmol/cm(2), and concentration is 50 +/- 1.2 x 10(-12) M. Stretching the tissue resulted in a transient fivefold increase in the rate of mucosal ATP release and a transient sixfold increase in serosal ATP release. Half-osmotic strength solutions increased mucosal release by 10-fold and serosal release by 5-fold. Tissue damage resulted in a step-increase in mucosal chamber ATP content by 6.6 +/- 1 pmol/cm(2) and serosal chamber ATP by 0.1 +/- 0.06 pmol/cm(2) (n = 5).

The isolated anterior stomach of larval mosquitoes (Aedes aegypti): Voltage-clamp measurements with a tubular epithelium

The anterior stomach of larval Aedes aegypti was isolated and perfused via two pipettes. For transepithelial voltage (V(te)) measurement, the inflow pipette and the bath were connected via agar bridges to calomel electrodes. For voltage-clamping, the lumen of the tissue contained an Ag/AgCl wire held by the outflow pipette, and the preparation was placed in a bath within a spiral of Ag/AgCl wire. After equilibrating the tissue in mosquito saline on both sides, a V(te) of -8+/-1 mV was measured (+/-S.E.M., N=32). Current-voltage curves (+/-100 mV) demonstrated ohmic behaviour of the epithelium. Short-circuiting resulted in a current (I(sc)) of 103+/-16 microA cm(-2) and a mean transepithelial conductance (G(te)) of 11.8+/-1.3 mS cm(-2) (+/-S.E.M., N=32). A Yonath-Civan plot of G(te) of individual preparations over the corresponding I(sc) resulted in a straight line (r(2)=0.8422), indicating that the difference in I(sc) of individual preparations is mainly based on different transcellular conductances (G(c)). This analysis allowed to estimate the mean leak conductance (G(l) approximately 3.9 mS cm(-2)) and the mean transcellular electromotive force (E(c) approximately 13 mV). After administering 0.2 micromol L(-1) serotonin, I(sc) and G(te) significantly increased, to 457+/-49 microA cm(-2) and to 21.3+/-2.3 mS cm(-2) (+/-S.E.M., N=31, P<0.05), respectively. The Yonath-Civan plot after serotonin resulted again in a straight line (r(2)=0.8219), indicating a mean G(l) of about 1 mS cm(-2) and a mean E(c) of about 22 mV. Dinitrophenol (2.5 mmol L(-1)) almost abolished I(sc) and significantly reduced G(te) (N=6). Concanamycin A (100 micromol L(-1)) reduced I(sc) by more than 90% without significantly affecting G(te).

Estimation of paracellular conductance of primary rat alveolar epithelial cell monolayers

Freshly isolated rat type II pneumocytes, when grown on permeable tissue culture-treated polycarbonate filters, form confluent alveolar epithelial cell monolayers (RAECM). Cells in RAECM undergo transdifferentiation, exhibiting over time morphological and phenotypic characteristics of type I pneumocytes in vivo. We recently reported that transforming growth factor-β1 (TGF-β1) decreases overall monolayer resistance ( Rte) and stimulates short-circuit current in a dose-dependent manner. In this study, we investigated the effects of TGF-β1 (50 pM) or 10% newborn bovine serum (NBS) on modulation of paracellular passive ion conductance and its contribution to total passive ion conductance across RAECM. On days 5–7 in culture, tight-junctional resistance ( Rtj, kΩcm2) of RAECM, cultured in minimally defined serum-free medium (MDSF) with or without TGF-β1 or NBS, was estimated from the relationship between observed transmonolayer voltage and resistance after addition of gramicidin D to apical potassium isethionate Ringer solution under open-circuit conditions. NaCl Ringer solution bathed the basolateral side throughout the experimental period. Results showed that transmonolayer conductance (1/ Rte) and tight-junctional conductance (1/ Rtj) are 0.59 and 0.14 mS/cm2 for control monolayers in MDSF, 1.59 and 0.38 mS/cm2 for monolayers exposed to TGF-β1, and 0.38 and 0.18 mS/cm2 for monolayers grown in the presence of NBS. The contributions to total transepithelial ion conductance by the paracellular pathway are estimated to be 23, 23, and 47% for control, TGF-β1-exposed, and newborn bovine serum (NBS)-treated RAECM, respectively.

Colistin interactions with the mammalian urothelium

Here we describe the effect of colistin on the barrier function of the mammalian urinary bladder epithelium. Addition of colistin to the mucosal solution of the rabbit urinary bladder epithelium (urothelium) resulted in an increase in the transepithelial conductance. The magnitude of the increase in transepithelial conductance was dependent on the membrane voltage, concentration of colistin, and presence of divalent cations in the bath solution. The initial site of action of colistin was at the apical membrane. Colistin increased the membrane conductance only when the apical membrane potential was cell interior negative. The more negative the membrane potential, the larger the conductance increase. The concentration dependence of the conductance increase saturated, suggesting a membrane binding site. Divalent cations decreased the magnitude of the conductance increase. This divalent cation action occurred at two sites: one in competition with colistin for a membrane binding site, and the other by rapidly blocking the induced conductance. At short exposure times, the increase in conductance was reversed by either removing colistin from the bath or changing the voltage so that the apical membrane was cell interior positive. At long exposure times, the increase was only partially reversible by voltage or removal from the bath. This finding suggests that at long exposure times, there is a toxic effect of colistin on the urothelium.

The N-Terminal Domain of Vimentin Alters Bladder Permeability

PURPOSE

We assessed the effect of the vimentin amino terminal polypeptide (NT1) on barrier function of rabbit bladder epithelium.

MATERIALS AND METHODS

The effect of NT1 on the properties of rabbit bladder epithelium were studied using Ussing chambers and electrophysiological methods.

RESULTS

NT1 increased transepithelial conductance (Gt) in a voltage dependent manner. At a transepithelial voltage (Vt) of -70 mV (serosal solution ground) the addition of NT1 to mucosal solution did not result in a change in Gt. When Vt was clamped to 0 mV, there was a time dependent increase in Gt. The increase in Gt was reversed by clamping Vt back to -70 mV or by removing NT1 from the mucosal bath at 0 mV. The polypeptide acts primarily at the apical membrane with a conductance increase that is concentration dependent. Induced conductance is nonselective for small monovalent cations and anions. The ability of NT1 to increase membrane conductance was decreased in the presence of bath calcium.

CONCLUSIONS

The data suggest that the amino terminus of vimentin can interact with the plasma membrane of bladder epithelium and increase ion permeability in a voltage dependent manner.

Prostanoids stimulate K secretion and Cl secretion in guinea pig distal colon via distinct pathways

Short-circuit current (I(sc)) and transepithelial conductance (Gt) were measured in guinea pig distal colonic mucosa isolated from submucosa and underlying muscle layers. Indomethacin (2 microM) and NS-398 (2 microM) were added to suppress endogenous production of prostanoids. Serosal addition of PGE2 (10 nM) stimulated negative I(sc) consistent with K secretion, and concentrations >30 nM stimulated positive I(sc) consistent with Cl secretion. PGE2 also stimulated Gt at low and high concentrations. Dose responses to prostanoids specific for EP prostanoid receptors were consistent with stimulating K secretion through EP2 receptors, based on a rank order potency (from EC50 values) of PGE2 (1.9 nM) > 11-deoxy-PGE1 (8.3 nM) > 19(R)-hydroxy-PGE2 (13.9 nM) > butaprost (67 nM) > 17-phenyl-trinor-PGE2 (307 nM) >> sulprostone (>10 microM). An isoprostane, 8-iso-PGE2, stimulated K secretion with an EC50 of 33 nM. Cl secretory response was stimulated by PGD2 and BW-245C, a DP prostanoid receptor-specific agonist: BW-245C (15 nM) > PGD2 (30 nM) > PGE2 (203 nM). Agonists specific for FP, IP, and TP prostanoid receptors were ineffective in stimulating I(sc) and Gt at concentrations <1 microM. These results indicate that PGE2 stimulated electrogenic K secretion through activation of EP2 receptors and electrogenic KCl secretion through activation of DP receptors. Thus stimulation of Cl secretion in vivo would occur either via physiological concentrations of PGD2 (<100 nM) or pathophysiological concentrations of PGE2 (>100 nM) that could occur during inflammatory conditions.

PGE(2) activation of apical membrane Cl(-) channels in A6 epithelia: impedance analysis

Measurements of transepithelial electrical impedance of continuously short-circuited A6 epithelia were made at audio frequencies (0.244 Hz to 10.45 kHz) to investigate the time course and extent to which prostaglandin E(2) (PGE(2)) modulates Cl(-) transport and apical membrane capacitance in this cell-cultured model epithelium. Apical and basolateral membrane resistances were determined by nonlinear curve-fitting of the impedance vectors at relatively low frequencies (<50 Hz) to equations (Păunescu, T. G., and S. I. Helman. 2001. Biophys. J. 81:838--851) where depressed Nyquist impedance semicircles were characteristic of the membrane impedances under control Na(+)-transporting and amiloride-inhibited conditions. In all tissues (control, amiloride-blocked, and amiloride-blocked and furosemide-pretreated), PGE(2) caused relatively small (< approximately 3 microA/cm(2)) and rapid (<60 s) maximal increase of chloride current due to activation of a rather large increase of apical membrane conductance that preceded significant activation of Na(+) transport through amiloride-sensitive epithelial Na(+) channels (ENaCs). Apical membrane capacitance was frequency-dependent with a Cole-Cole dielectric dispersion whose relaxation frequency was near 150 Hz. Analysis of the time-dependent changes of the complex frequency-dependent equivalent capacitance of the cells at frequencies >1.5 kHz revealed that the mean 9.8% increase of capacitance caused by PGE(2) was not correlated in time with activation of chloride conductance, but rather correlated with activation of apical membrane Na(+) transport.

Voltage clamping single cells in intact malpighian tubules of mosquitoes

Principal cells of the Malpighian tubule of the yellow fever mosquito were studied with the methods of two-electrode voltage clamp (TEVC). Intracellular voltage (V(pc)) was -86.7 mV, and input resistance (R(pc)) was 388.5 kOmega (n = 49 cells). In six cells, Ba(2+) (15 mM) had negligible effects on V(pc), but it increased R(pc) from 325.3 to 684.5 kOmega (P < 0.001). In the presence of Ba(2+), leucokinin-VIII (1 microM) increased V(pc) to -101.8 mV (P < 0.001) and reduced R(pc) to 340.2 kOmega (P < 0.002). Circuit analysis yields the following: basolateral membrane resistance, 652. 0 kOmega; apical membrane resistance, 340.2 kOmega; shunt resistance (R(sh)), 344.3 kOmega; transcellular resistance, 992.2 kOmega. The fractional resistance of the apical membrane (0.35) and the ratio of transcellular resistance and R(sh) (3.53) agree closely with values obtained by cable analysis in isolated perfused tubules and confirm the usefulness of TEVC methods in single principal cells of the intact Malpighian tubule. Dinitrophenol (0.1 mM) reversibly depolarized V(pc) from -94.3 to -10.7 mV (P < 0.001) and reversibly increased R(pc) from 412 to 2,879 kOmega (P < 0.001), effects that were duplicated by cyanide (0.3 mM). Significant effects of metabolic inhibition on voltage and resistance suggest a role of ATP in electrogenesis and the maintenance of conductive transport pathways.

Central role of the apical membrane H+-ATPase in electrogenesis and epithelial transport in Malpighian tubules

The effects of bafilomycin A(1), a blocker of V-type H(+)-ATPases, were investigated in Malpighian tubules of Aedes aegypti. Bafilomycin A(1) reduced rates of transepithelial fluid secretion and the virtual short-circuit current (vI(sc)) with an IC(50) of approximately 5 micromol l(−)(1). As vI(sc) decreased, the electrical resistance increased across the whole epithelium and across the apical membrane, indicating effects on electroconductive pathways. Bafilomycin A(1) had no effect when applied from the tubule lumen, pointing to the relative impermeability of the apical membrane to bafilomycin A(1). Thus, bafilomycin A(1) must take a cytoplasmic route to its blocking site in the proton channel of the H(+)-ATPase located in the apical membrane of principal cells. The inhibitory effects of bafilomycin A(1) were qualitatively similar to those of dinitrophenol in that voltages across the epithelium (V(t)), the basolateral membrane (V(bl)) and the apical membrane (V(a)) depolarized towards zero in parallel. Moreover, V(bl)always tracked V(a), indicating electrical coupling between the two membranes through the shunt. Electrical coupling allows the H(+)-ATPase to energize not only the apical membrane, but also the basolateral membrane. Furthermore, electrical coupling offers a balance between electroconductive entry of cations across the basolateral membrane and extrusion across the apical membrane to support steady-state conditions during transepithelial transport.

Effects of substance P on human colonic mucosa in vitro

Previous studies indicated that the peptide substance P (SP) causes Cl--dependent secretion in animal colonic mucosa. We investigated the effects of SP in human colonic mucosa mounted in Ussing chamber. Drugs for pharmacological characterization of SP-induced responses were applied 30 min before SP. Serosal, but not luminal, administration of SP (10(-8) to 10(-6) M) induced a rapid, monophasic concentration and Cl--dependent, bumetanide-sensitive short-circuit current (Isc) increase, which was inhibited by the SP neurokinin 1 (NK1)-receptor antagonist CP-96345, the neuronal blocker TTX, the mast cell stabilizer lodoxamide, the histamine 1-receptor antagonist pyrilamine, and the PG synthesis inhibitor indomethacin. SP caused TTX- and lodoxamide-sensitive histamine release from colonic mucosa. Two-photon microscopy revealed NK1 (SP)-receptor immunoreactivity on nerve cells. The tyrosine kinase inhibitor genistein concentration dependently blocked SP-induced Isc increase without impairing forskolin- and carbachol-mediated Isc increase. We conclude that SP stimulates Cl--dependent secretion in human colon by a pathway(s) involving mucosal nerves, mast cells, and the mast cell product histamine. Our results also indicate that tyrosine kinases may be involved in this SP-induced response.

Ca-sensitive Na transport in sheep omasum

Na transport across a preparation of sheep omasum was studied. All tissues exhibited a serosa-positive short-circuit current (Isc), with a range of 1-4 microeq. h-1. cm-2. A Michaelis-Menten-type kinetic was found between the Na concentration and the Isc (Michaelis-Menten constant for transport of Na = 6.7 mM; maximal transport capacity of Na = 4.16 microeq. h-1. cm-2). Mucosal amiloride (1 mM), phenamil (1 or 10 microM), or serosal aldosterone (1 microM for 6 h) did not change Isc. Removal of divalent cations (Ca and Mg) enhanced Isc considerably from 2.61 +/- 0.24 to a peak value of 11.18 +/- 1.1 microeq. h-1. cm-2. The peak Isc (overshoot) immediately declined to a plateau Isc of approximately 6-7 microeq. h-1. cm-2. Na flux measurements showed a close correlation between changes in Isc and Na transport. Transepithelial studies demonstrated that K, Cs, Rb, and Li are transported, indicating putative nonselective cation channels, which are inhibited by divalent cations (including Ca, Mg, Sr, Ba) and by (trivalent) La. Intracellular microelectrode recordings from the luminal side clearly showed changes of voltage divider ratio when mucosal divalent cations were removed. The obtained data support the assumption of a distinct electrogenic Na transport mechanism in sheep omasum.

Eosinophil peroxidase increases membrane permeability in mammalian urinary bladder epithelium

Eosinophil peroxidase (EPO), a cationic protein found in eosinophils, has been reported to be cytotoxic independent of its peroxidase activity. This study investigated with electrophysiological methods whether EPO is toxic to mammalian urinary bladder epithelium. Results indicate that EPO, when added to the mucosal solution, increases apical membrane conductance of urinary bladder epithelium only when the apical membrane potential is cell interior negative. The EPO-induced conductance was concentration dependent, with a maximum conductance of 411 microseconds/cm2 and a Michaelis-Menten constant of 113 nM. The EPO-induced conductance was nonselective for K+ and Cl-. The conductance was partially reversed using voltage but not by removal of EPO from the bulk solution. Mucosal Ca2+ reversed the EPO-induced conductance by a mechanism involving reversible block of the conductance. Prolonged exposure (up to 1 h) to EPO was toxic to the urinary bladder epithelium, as indicated by an irreversible increase in transepithelial conductance. These results suggest that EPO is indeed toxic to urinary bladder epithelium via a mechanism that involves an increase in membrane permeability.

Eosinophil major basic protein increases membrane permeability in mammalian urinary bladder epithelium

The eosinophil granule protein major basic protein (MBP) is toxic to a wide variety of cell types, by a poorly understood mechanism. To determine whether the action of MBP involves an alteration in membrane permeability, we tested purified MBP on rabbit urinary bladder epithelium using transepithelial voltage-clamp techniques. Addition of nanomolar concentrations of MBP to the mucosal solution caused an increase in apical membrane conductance only when the voltage across the apical membrane was cell interior negative. The magnitude of the MBP-induced conductance was a function of MBP concentration, and the rate of the initial increase in conductance was a function of the transepithelial voltage. The MBP-induced conductance was nonselective for K+ and Cl-. Mucosal Ca2+ reversed the induced conductance, whereas mucosal Mg2+ partially blocked the induced conductance and slowed the rate of the increase in conductance. The induced conductance was partially reversed by changing the voltage gradient across the apical membrane to cell interior positive. Prolonged exposure resulted in an irreversible loss of the barrier function of the urinary bladder epithelium. These results suggest that an increase in cell membrane ion permeability is an initial step in MBP-induced loss of barrier function.

Histone-induced damage of a mammalian epithelium: the role of protein and membrane structure

In a previous report [T. J. Kleine, A. Gladfelter, P. N. Lewis, and S. A. Lewis, Am. J. Physiol. 268 (Cell Physiol. 37): C1114-C1125, 1995], we found that the cationic DNA-binding proteins histones H4, H1, and H5 caused a voltage-dependent increase in the transepithelial conductance in rabbit urinary bladder epithelium. In this study, results from lipid bilayer experiments suggest that histones H5-H1 and H4 form variably sized conductive units. Purified fragments of histones H4 and H5 were used to determine the role of histone tertiary structure in inducing conductance. Isolated COOH- and NH2-terminal tails of histone H4, which are random coils, were inactive, whereas the central alpha-helical domain induced a conductance increase. Although the activities of the central fragment and intact histone H4 were in many ways similar, the dose-response relationships suggest that the isolated central domain was much less potent than intact histone H4. This suggests than the NH2- and COOH-terminal tails are also important for histone H4 activity. For histone H5, the isolated globular central domain was inactive. Thus the random-coil NH2- and COOH-terminal tails are important for H5 activity as well. These results indicate that histone molecules interact directly with membrane phospholipids to form a channel and that protein tertiary structure and the degree of positive charge play an important role in this activity.

Substrate-dependent expression of Na+ transport and shunt conductance in A6 epithelia

A6 epithelia grown in tissue culture vary enormously in their baseline rates of Na+ transport due to differences in growth media, serum, and other unknown factors. To evaluate the effect(s) of substrates on expression of Na+ transport, we determined short-circuit currents, open-circuit voltages, and electrical resistances of mature confluent A6 epithelia grown on a variety of commercially available permeable supports. Because the cells, growth conditions, and all other factors were the same, differences in transport could be attributed alone to the substrate on which the cells were grown. Tissues were grown on both large- and small-diameter inserts of the same type with differing ratios of edge length to area so that the contribution of the edge and tight junction conductances to the combined shunt conductance of the inserts could be evaluated. Shunt and cellular conductances and the cellular Thévenin electromotive force were determined after aldosterone stimulation and amiloride inhibition of Na+ transport. Marked and extreme differences were observed not only for expression of Na+ transport (controls, 0.09-3.94 microA/cm2; aldosterone, 1.53-28.2 microA/cm2) due to changes of apical membrane conductance but also for the development of junctional conductances (3,250 to < infinity omega.cm2) and edge conductances (13,175 to < infinity omega.cm) among substrates.

Evidence for a rapid, direct effect on epithelial monolayer integrity and transepithelial transport in response to Salmonella invasion

In cultured monolayers of high-resistance Madin-Darby Canine Kidney (MDCK) cells, infection with Salmonella typhimurium SL1344 resulted in a dose- and time-dependent increase in transepithelial conductance (Gt) and short-circuit current (Isc). There was a direct linear relationship between the S. typhimurium-induced increments in Isc and Gt suggesting that this early change in epithelial parameters is, in part, the result of a cellular conductance change most probably at the apical membrane. An additional wild-type S. typhimurium strain, SR11, and an invasion-deficient isogenic mutant SB111 carrying a non-polar mutation in invA were used to confirm that the S. typhimurium-induced change in epithelial electrical parameters is directly linked to the invasion process. The S. typhimurium-induced change in epithelial electrical parameters was markedly attenuated in Na+-free choline medium. Addition of piretanide (10(-4) M, basal side) failed to affect the increased epithelial conductance and Isc after a 40-min incubation with S. typhimurium. NPPB (5x10(-4) M) added to the apical medium reduced the S. typhimurium-stimulated Isc by 28%, but Gt was not significantly reduced. It is unlikely that the S. typhimurium-induced Isc is due to Cl- secretion. Staining of S. typhimurium-infected MDCK I monolayers with TRITC-phalloidin revealed marked alterations of F-actin; diffuse intracellular accumulations of F-actin corresponding to the presence of invading bacteria were observed by 15 min. After 60 min, prominent extrusions of the apical membrane corresponding to previously described "membrane ruffles" were noted. Marked accumulations of perijunctional F-actin in infected cells corresponded to contraction of the perijunctional actin ring at the apical pole. In adjacent cells marked distortion and stretch of the apical surface was evident. The invasion-deficient invA mutant SB111 failed to induce these morphological changes. These data demonstrate that S. typhimurium invasion induces increased transcellular conductance which does not result from stimulation of Cl- secretion but instead appears to be predominantly due to increased Na+ permeability. The increased membrane conductance is coincident with increased transepithelial inulin permeability indicating that the increment in Gt has an additional "paracellular" component. The S. typhimurium-induced alterations in epithelial parameters may be related to "membrane ruffling" and/or to the accompanying changes in cell shape.

Effect of oxytocin on transepithelial transport of water and Na+ in distinct ventral regions of frog skin (Rana catesbeiana)

Thoracic, abdominal, and pelvic fragments of ventral skin of Rana catesbeiana were analysed regarding the effect of oxytocin on: (1) transepithelial water transport; (2) short-circuit current; (3) skin conductance and electrical potential difference; (4) Na+ conductance, the electromotive force of the Na+ transport mechanism, and shunt conductance; (5) short-circuit current responses to fast Na+ by K+ replacement in the outer compartment, and (6) epithelial microstructure. Unstimulated water and Na+ permeabilities were low along the ventral skin. Hydrosmotic and natriferic responses to oxytocin increased from thorax to pelvis. Unstimulated Na+ conductance was greater in pelvis than in abdomen, the other electrical parameters being essentially similar in both skin fragments. Contribution of shunt conductance to total skin conductance was higher in abdominal than in pelvic skin. Oxytocininduced increases of total skin conductance, Na+ conductance, and shunt conductance in pelvis were significantly larger than in abdomen. An oscillatory behaviour of the short-circuit current was observed only in oxytocin-treated pelvic skins. Decrease of epithelial thickness and increase of mitochondria-rich cell number were observed from thorax to pelvis. Oxytocin-induced increases of interspaces were more conspicuous in pelvis and abdomen than in thorax.

Histone-induced damage of a mammalian epithelium: the conductive effect

Human semen has been reported to be cytotoxic to rat descending colon by a mechanism involving polyamines (cationic molecules) and collagenase. In this study, we report that histones, cationic proteins found in human semen, can contribute to semen's cytotoxicity. Histones H1, H4, and H5, when added to the mucosal side of rabbit urinary bladder epithelium, were found to alter the transepithelial conductance (Gt) in a voltage-sensitive manner. When the cell interior was negative, the conductance rapidly increased and plateaued. When the cell interior was positive, the induced conductance decreased to control values. Histone increased the Gt by increasing the apical membrane conductance rather than the tight junction conductance. The magnitude of the Gt increase was dose dependent, and the histone-induced conductance was nonselective for Na+, K+, and Cl-. The induced conductance could be reversed by either increasing mucosal Ca2+ concentration or by removal of histone from the mucosal solution. Prolonged exposure of the epithelium to histone was toxic as determined by the irreversible loss of transepithelial resistance. These results indicate that histone increases membrane ionic permeability, is cytotoxic, and thus may contribute to human semen's toxic effect on colonic epithelium.

Polycation-induced enhancement of epithelial paracellular permeability is independent of tight junctional characteristics

The nature of polycation-induced change in transepithelial permeability was investigated in strains I (tight) and II (leaky) MDCK epithelial monolayers. Apical exposure to poly(L-lysine) (PLL, mol. wt. (MW) approximately 20,000) induced a dose-dependent increase in transepithelial conductance (GT) in both strains which correlated with increasing transepithelial flux of extracellular markers (thiourea/inulin) indicating that PLL enhanced paracellular permeability in these epithelia. Coincident with the increase in GT, PLL also induced an inward short circuit current (Isc) which was associated with the early phase of the increase in GT and may be responsible for part of it. Morphological studies showed that immunofluorescent staining of the tight junction protein, ZO-1, was abolished following PLL exposure. In addition, F-actin staining in monolayers challenged with PLL demonstrated breaks in the zonulae occludentes at the apical surface. PLL had similar effects on monolayers of T84 and HCT-8 human intestinal cells indicating that polycation action may be general for a range of epithelial types. We conclude that epithelial exposure to polycations results in opening of the paracellular route by mechanisms which are independent of tight junction characteristics.

Effects of aldosterone on the impedance properties of cultured renal amphibian epithelia

The cultured renal amphibian cell line A6 has proven advantageous for studies of Na+ transport regulation. In the present study, the effects of aldosterone action on the transepithelial electrical properties of this epithelium were assessed. Specifically, the time course of aldosterone action was determined and the effects of chronic (10-18 day) aldosterone elevation were assessed using transepithelial equivalent circuit methods and impedance analysis techniques. Short-term (< 4 hr) exposure to aldosterone (0.1 microM) stimulated the amiloride-sensitive short-circuit current (Isc) by over twofold and increased the transepithelial conductance (GT) by approximately 12%. The increases in Isc and GT were maintained in epithelia subjected to chronic aldosterone exposure. In contrast to previous reports, paracellular resistance (Rj) was not altered by aldosterone. This difference may be related to the longer time of exposure or different basal Na+ transport rates in the present study. The apical membrane conductance was significantly increased for aldosterone-treated epithelia compared to aldosterone-depleted (i.e., serum-deprived) controls. Apical membrane area (capacitance) was not significantly affected. This finding is consistent with a higher density (number of channels per membrane area) of conducting Na+ channels in this membrane following aldosterone stimulation. Basolateral membrane properties were not significantly altered for aldosterone-treated tissues compared to serum-treated control tissues. In contrast, basolateral membrane-specific conductance (i.e., basolateral membrane conductance normalized to basolateral membrane capacitance) was significantly lower for serum-deprived epithelia than for serum-treated controls or aldosterone-treated tissues. The effects of chronic aldosterone exposure were also evaluated for the A6 subclonal cell line, 2F3. Similar to A6 epithelia, Isc was essentially doubled following aldosterone stimulation while Rj and cellular driving force (Ec) were not affected. Apical membrane conductances under control conditions for 2F3 epithelia were higher than those for A6, but were not significantly different from A6 following aldosterone exposure or serum deprivation. These findings suggest possible differences in the regulation of apical membrane Na+ channels for 2F3 and A6 epithelia.

Transport-related modulation of the membrane properties of toad urinary bladder epithelium

Impedance analysis and transepithelial electrical measurements were used to assess the effects of the apical membrane Na+ channel blocker amiloride and anion replacement on the apical and basolateral membrane conductances and areas of the toad urinary bladder (Bufo marinus). Mucosal amiloride addition decreased both apical and basolateral membrane conductances (Ga and Gbl, respectively) with no change in membrane capacitances (Ca and Cbl). Consequently, the specific conductances of these membranes decreased without significant changes in membrane area. Following amiloride removal, an increase was obtained in the steady-state rate of sodium transport compared to values before amiloride addition. This increase was independent of the initial transport rate, suggesting activation of a quiescent pool of apical sodium channels. Chloride replacement by acetate or gluconate had no significant effects on apical or basolateral membrane capacitances. The effects of these replacements on membrane conductances depended on the anion species. Gluconate (which induces cell shrinkage) decreased both membrane conductances. In contrast, acetate (which induces cell swelling) increased Ga and had no effect on Gbl. The increase in the apical membrane conductance was due to an increase in the amiloride-sensitive Na+ conductance of this membrane. In summary, mucosal amiloride addition or chloride replacements led to changes in membrane conductances without significant effects on net membrane areas.

Amiloride-sensitive Na+ transport across cultured renal (A6) epithelium: evidence for large currents and high Na:K selectivity

Electrical techniques were used to determine the Na:K selectivity of the amiloride-sensitive pathway and to characterize cellular and paracellular properties of A6 epithelium. Under control conditions, the mean transepithelial voltage (VT) was -57 +/- 5 mV, the short-circuit current (Isc) averaged 23 +/- 2 microA/cm2 and the transepithelial resistance (RT) was 2.8 +/- 0.3 k omega cm2 (n = 13). VT and Isc were larger than reported in previous studies and were increased by aldosterone. The conductance of the amiloride-sensitive pathway (Gamil) was assessed before and after replacement of Na+ in the mucosal bath by K+, using two independent measurements: (1) the slope conductance (GT), determined from current-voltage (I-V) relationships for control and amiloride-treated tissues and (2) the maximum amiloride-sensitive conductance (Gmax) calculated from the amiloride dose-response relationship. The ratio of Gamil in mucosal Na+ solutions to Gamil for mucosal K+ solutions was 22 +/- 6 for GT measurements and 15 +/- 2 for Gmax data. Serosal ion replacements in tissues treated with mucosal nystatin indicated a potassium conductance in the basolateral membrane. Equivalent circuit analyses of nystatin and amiloride data were used to resolve the cellular (Ec) and paracellular (Rj) resistances (approximately 5 k omega cm2 and 8-9 k omega cm2, respectively). Analysis of I-V relationships for tissues depolarized with serosal K+ solutions revealed that the amiloride-sensitive pathway could be described as a Na+ conductance with a permeability coefficient (PNa) = 1.5 +/- 0.2 x 10(-6) cm/s and the intracellular Na+ concentration (Nai) = 5 +/- 1 mM (n = 5), similar to values from other tight epithelia. We conclude that A6 epithelia are capable of expressing large amiloride-sensitive currents which are highly Na+ selective.

Electrophysiology of newt skin: effects of prolactin

Integumental transepithelial potential (TEP) was measured by both in vivo and in vitro (Ussing chamber) techniques in the same adult terrestrial-phase California newts (Taricha torosa). In both types of preparation, TEP showed a logarithmic relation to external sodium (as Na2SO4) concentrations between 0.1 and 10 meq/liter, with in vivo values exceeding in vitro values at all points. KM was ca. 1 and 5 meq/liter for the in vivo and in vitro preparations, respectively. When terrestrial-phase newts were treated with prolactin (PRL; 50 mIU/g/day), in vivo TEP declined significantly within 3 days. In vivo TEP versus [Na+]ext curves of recently collected aquatic-phase newts were found to be not significantly different from those of PRL-treated terrestrial-phase animals. Current/voltage (I/V) plots likewise showed that skin from aquatic-phase and PRL-treated terrestrial-phase newts was electrophysiologically alike. Estimates and calculations based on the I/V relationship suggest that PRL acts primarily to increase the epithelial electrical resistance in the transcellular pathway, with no effect on the electromotive force of the sodium pump.

Na transport stimulation by novobiocin: transepithelial parameters and evaluation of ENa

The action of the antibiotic novobiocin on transepithelial Na transport was studied in isolated skins obtained from two different frog species. In Rana esculenta addition of novobiocin to the outer bath (1 mM) resulted in a sustained and reversible stimulation of the short-circuit current, transepithelial potential, and transepithelial conductance. Similar, though more variable and much less pronounced changes were observed in Rana temporaria. In the presence of amiloride (0.1 mM) novobiocin had no effect on any of the investigated transport parameters and all novobiocin induced changes were fully reversed when amiloride was given subsequently. At reduced external Na concentration or low pH the action of novobiocin was found to be greatly attenuated. In the presence of novobiocin an increased affinity to amiloride and a linearization of the transepithelial current-voltage relationship was observed. The results are consistent with the view that novobiocin increases the Na permeability of the outer membrane, possibly by an attenuation of an Na self-inhibition mechanism. In addition, the driving force of transepithelial Na transport was estimated by means of novobiocin. Several different methods were employed, providing varying results. As shown in an Appendix, for the most part the discrepancies can be explained by changes in the intracellular Na and K concentration. In some cases, novobiocin induced large secondary increases in the skin conductance which can be referred to an increased Cl permeability.

Determination of the driving force for the sodium pump (ENa) and of active and passive conductances (GNa and Gsh) in isolated toad skin: influence of antidiuretic hormone

1. Values of the sodium potential (ENa), active conductance (GNa) and passive conductance (Gsh) were measured in the isolated skin of the toad Pleurodema thaul placed in an Ussing chamber, and Isaacson's test was performed with 2,4,6-trieminopyrimidine (TAP) and with amiloride. 2. The numerical estimates obtained in the presence of TAP were ENa 122.85 +/- 15.17 mV, GNa 0.493 +/- 0.09 mS/cm2 and Gsh 1.145 +/- 0.23 mS/cm2. 3. After exposure to ADH these values were as follows: ENa 85.76 +/- 12.17 mV, GNa 1.191 +/- 0.20 mS/cm2 and Gsh 0.935 +/- 0.14 mS/cm2. 4. Addition of 0.5 x 10(2-) TAP produced a 53.90 +/- 5.10% decrease in transepithelial potential and a 37.90 +/- 4.90% fall in short-circuit current. 5. Exposure to ADH increased the transepithelial potential difference 34.20 +/- 13.20% and the short-circuit current to 78.00 +/- 20.50% above the control values. 6. Comparison of the efficiency and mechanism of action of TAP and amiloride in the determination of electrical parameters shows that both agents induce a similar decrease in Gsh, a finding which could indicate that TAP blocks toad skin apical membrane Na+ channels without affecting tight junction conductance.

Effects of anions on cellular volume and transepithelial Na+ transport across toad urinary bladder

The effects of complete substitution of gluconate for mucosal and/or serosal medium Cl- on transepithelial Na+ transport have been studied using toad urinary bladder. With mucosal gluconate, transepithelial potential difference (VT) decreased rapidly, transepithelial resistance (RT) increased, and calculated short-circuit current (Isc) decreased. Calculated ENa was unaffected, indicating that the inhibition of Na+ transport was a consequence of a decreased apical membrane Na+ conductance. This conclusion was supported by the finding that a higher amiloride concentration was required to inhibit the residual transport. With serosal gluconate VT decreased, RT increased and Isc fell to a new steady-state value following an initial and variable transient increase in transport. Epithelial cells were shrunken markedly as judged histologically. Calculated ENa fell substantially (from 130 to 68 mV on average). Ba2+ (3 mM) reduced calculated ENa in Cl- Ringer's but not in gluconate Ringer's. With replacement of serosal Cl- by acetate, transepithelial transport was stimulated, the decrease in cellular volume was prevented and ENa did not fall. Replacement of serosal isosmotic Cl- medium by a hypo-osmotic gluconate medium (one-half normal) also prevented cell shrinkage and did not result in inhibition of Na+ transport. Thus the inhibition of Na+ transport can be correlated with changes in cell volume rather than with the change in Cl-per se. Nystatin virtually abolished the resistance of the apical plasma membrane as judged by measurement of tissue capacitance. With K+ gluconate mucosa, Na+ gluconate serosa, calculated basolateral membrane resistance was much greater, estimated basolateral emf was much lower, and the Na+/K+ basolateral permeability ratio was much higher than with acetate media. It is concluded the decrease in cellular volume associated with substitution of serosal gluconate for Cl results in a loss of highly specific Ba2+-sensitive K+ conductance channels from the basolateral plasma membrane. It is possible that the number of Na+ pump sites in this membrane is also decreased.

The role of anions in cellular volume regulation

Cellular volume can be varied substantially by replacing medium Cl- isosmotically by other univalent anions. Since K+ content changes in parallel, cellular K+ concentration is well maintained. Gluconate behaves as an impermeant anion so cells shrink. Acetate enters cells apparently by non-ionic diffusion causing marked cellular swelling. These changes in volume are fully reversed when Cl- is again restored to the medium. However, ouabain (10(-3) M) largely prevents this reversal when Cl- replaces acetate, arguing against a ouabain-insensitive volume regulating mechanism. In toad urinary bladder, serosal gluconate inhibits transepithelial Na+ transport and cells shrink. Analysis suggests that cell shrinkage results in a loss of Ba2+-sensitive highly selective basolateral membrane K+ conductance channels.

Chloride secretion by canine tracheal epithelium: IV. Basolateral membrane K permeability parallels secretion rate

We evaluated the K conductance properties of the basolateral membranes of the surface cells of canine tracheal epithelium using microelectrode techniques. Studies were conducted under basal conditions (indomethacin, 10(-6) M, mucosal solution) and after stimulation of electrogenic Cl secretion with epinephrine (10(-6) M, serosal solution). Elevated serosal solution [K] depolarized the electrical potential differences across the apical (psi a) and basolateral (psi b) membranes in both the presence and absence of epinephrine. Serosal barium (0.5 mM) also depolarized psi a and psi b and selectively increased basolateral membrane resistance threefold. We also used K-selective microelectrodes to determine cell K activity (acK) and the driving force for K transport across the limiting membranes under basal and stimulated conditions. Stimulation of Cl secretion was not associated with significant changes in psi b or acK so that the driving force for K exit from cell to serosal solution (ca. 20 mV) was not altered. There was close agreement between the basolateral membrane electromotive force (Eb) determined from prior studies (M.J. Welsh, P.L. Smith and R.A. Frizzell, J. Membrane Biol. 71:209-218, 1983) and the chemical potential difference for K across this barrier (EbK) in the presence and absence of epinephrine. These findings support the notion that the basolateral membrane is characterized by a high conductance to K under both secreting and nonsecreting conditions and indicate that the decrease in basolateral membrane resistance that accompanies stimulation of Cl secretion results from an increase in its K conductance. This obviates changes in acK that would otherwise accompany increased Na/K pump activity and, by hyperpolarizing psi a, establishes the electrical driving force for Cl secretion across the apical membrane.

Studies of sodium channels in rabbit urinary bladder by noise analysis

Sodium channels in rabbit urinary bladder were studied by noise analysis. There are two components of short-circuit current (Isc) and correspondingly two components of apical Na+ entry, one amiloride-sensitive (termed IA and the A channel, respectively) and one amiloride-insensitive (IL and the leak pathway, respectively). The leak pathway gives rise to l/f noise, while the A channel in the presence of amiloride gives rise to Lorentzian noise. A two-state model of the A channel accounts well for how the corner frequency and plateau value of Lorentzian noise vary with amiloride concentration. The single-channel current is 0.64 pA, and the conducting channel density is on the order of 40 copies per cell. Triamterene blocks the A channel alone, and increasing external Na+ decreases the number but not the single-channel permeability of the A channel. Hydrostatic pressure pulses ("punching") increase the number of both pathways. Repeated washing of the mucosal surface removes most of the leak pathway without affecting the A channel. Properties of the A channel revealed by noise analysis of various tight epithelia are compared, and the mechanism of l/f noise is discussed. It is suggested that the A channel is synthesized intracellularly, stored in intracellular vesicles, transferred with or from vesicular membrane into apical membrane under the action of microfilaments, and degraded into the leak pathway, which is washed out into urine or destroyed. The A channel starts with PNa/PK approximately 30 and loses selectivity in stages until PNa/PK reaches the free-solution mobility ratio (approximately 0.7) for the leak pathway. This turnover cycle functions as a mechanism of repair and regulation for Na+ channels, analogous to the repair and regulation of most intracellular proteins by turnover. Vesicular delivery of membrane channels may be operating in several other epithelia.

Cellular and paracellular pathway resistances in the "tight" Cl- -secreting epithelium of rabbit cornea

The high transverse resistance of the isolated rabbit cornea (6-12 l omega . cm2) is associated with the corneal epithelium, a Cl- -secreting tissue which is modulated by beta-adrenergic and serotonergic receptors. Three methods were employed to determine the resistances for the apical membrane, basolateral membrane, and paracellular conductive pathways in the epithelium. In the first method, the specific resistance of the apical membrane was selectively and reversibly changed. Epinephrine was used to increase apical cation permeability. The second method utilized a direct measure of the spontaneous cellular ionic current. The third method obtained estimates of shunt resistance using transepithelial electrophysiological responses to changes in apical membrane resistance. The results of the first method were largely independent of the agent used. In addition, the three methods were in general agreement, and the ranges of mean values for apical membrane, basolateral membrane, and shunt resistances were 23-33, 3-4, and 12-16 k omega . cm2, respectively, for the normal cornea. The apical membrane was the major, physiologically-modulated barrier to ion permeation. The shunt resistance of the corneal epithelium was comparable to that found previously for other "tight" epithelia. Experiments using Ag+ in tissues that were bathed in Cl- and HCO3-free solutions indicated that under resting conditions the apical membrane is anion-selective.

Effects of sodium pentobarbitone on the bioelectric properties of isolated toad skin

The effects of sodium pentobarbitone on the transepithelial potential difference and on the short-circuit current in the isolated toad skin were studied under different experimental conditions. Pentobarbitone was consistently effective when placed in the solution bathing the inside surface of the skin. The response to small concentrations (1.2, 3.6 and 6.0 x 10(-5) M) was a transient dose-related increase in the transepithelial potential difference and short-circuit current, and a decrease in both parameters at larger concentrations (3.6 x 10(-4) M). Total skin resistance decreased at small and increased at large concentrations. Analysis of the response in terms of the equivalent electrical circuit indicated that in small concentrations the effect was due to an increase in the driving force of the sodium (Na+) pump although the rate of entry of Na+ through the outer barrier was reduced. The activity of (Na+-K+)ATPase was increased within the first 7 min of the effect of pentobarbitone, thus confirming a direct action of the drug on Na+ transport. In large concentrations the electromotive force of the Na+ pump was decreased and inactivation of (Na+-K+)ATPase became evident. The effect of pentobarbitone was unchanged in six reserpinized toads, which would seem to rule out a catecholamine-mediated effect of the drug. The reduction of Na+-influx could indirectly increase transport efficiency since the rate of transport work should be reduced.

Bretylium opens mucosal amiloride-sensitive sodium channels

Addition of the quanternary ammonium compound, bretylium, to the outer surface of a frog skin leads to an increase in the potential difference and in the short circuit current across the skin. Bretylium does not have any effect when applied to the inside face of the frog skin. The effect of bretylium is dependent upon the presence of sodium ions in the outer medium; it is depressed when sodium is replaced by choline or potassium but not when lithium substitutes for sodium. The bretylium effect is blocked by the specific sodium channel blocker, amiloride. It is proposed that bretylium opens mucosal, amiloride-sensitive sodium channels.

Electrical properties of the rabbit urinary bladder assessed using gramicidin D

Recently, antibiotics have enjoyed widespread usage as tools in studies of epithelial transport. In the present study we assess the usefulness of the pore-forming antibiotic gramicidin D as a means for probing the electrical properties of the tight epithelium rabbit urinary bladder. Addition of 50 micron gramicidin to the mucosal bath (either a NaCl or KCl Ringer's solution) led to a large irreversible increase in the transepithelial conductance (GT) with 800 sec. GT increased by approximately 1200% and 500% in KCl and NaCl Ringer's solutions, respectively. Microelectrode measurements of the resistance ratio (the ratio of apical membrane resistance to basolateral membrane resistance) showed that apical membrane resistance is decreased by the drug. Measurements of the basolateral membrane resistance (Rbl) and tight junctional resistance (Rj) using a new and independent method (based on the perturbation of basolateral membrane electrogenic Na+ pump) demonstrated that Rbl and Rj were unaffected, suggesting that the effects of gramicidin are restricted to the apical membrane for periods of at least 2 hours after drug addition. The selectivity of the gramicidin-induced permeability in the apical membrane was calculated from measurements of the apical membrane potential after ion substitutions using a modified version of the constant field equation. The selectivity sequence for cations was Cs+ greater than K+ greater than Na+ greater than Li+ greater than choline. Unlike the commonly used polyene antibiotics nystatin and amphotericin B, gramicidin did not induce a significant Cl- permeability. In addition, the dose-response curve had a slope of 1. A method is described for calculating membrane resistances directly from transepithelial measurements under some conditions of gramicidin use, without requiring the use of microelectrode measurements.

Serosal Na/Ca exchange and H+ and Na+ transport by the turtle and toad bladders

A Na/Ca exchange system has been described in the plasma membrane of several tissues and seems to regulate the concentration of calcium in cytosol. Replacement of extracellular Na by sucrose increases calcium uptake into and decreases calcium efflux from the cell, leading to an increase in cytosolic calcium. The effect of an increase in cytosolic calcium mediated by the Na/Ca exchange system on H+ and Na transport in the turtle and toad bladder was investigated by replacing serosal Na isosmotically by sucrose or choline. Replacement of serosal by sucrose was associated with a significant inhibition of H+ secretion or Na transport which was reversible by addition of NaCl. Replacement of mucosal Na by sucrose failed to alter H+ secretion. Removal of serosal Na was associated with a significant increase in 45Ca uptake which could be blocked by pretreatment with lanthanum chloride. Pretreatment with lanthanum chloride blunted the inhibitory effect of replacement of serosal Na by sucrose on H+ and Na transport, thus suggesting that the increase in calcium uptake and the inhibition of transport are causally related. Under anaerobic conditions the rate of H+ or Na transport are linked to the rate of lactate production. The inhibition of Na or H+ transport by removal of serosal Na was accompanied by a proportional decrease in lactate production, thus suggesting that an increase in cytosolic calcium does not inhibit transport by uncoupling glycolysis from transport. Replacement of serosal Na by sucrose did not alter the force of the H+ or Na pump but led to an increase in resistance of the active pathway of H+ and Na transport. The inhibition of Na transport by replacement of serosal Na with sucrose could be reversed by addition of amphotericin B, an agent which increases luminal permeability to Na, thus suggesting that decreased Na entry across the apical membrane is the mechanism responsible for the inhibition of Na transport. The results of the present studies strongly suggest that an increase in cytosolic calcium through the serosal Na/Ca exchange system inhibits H+ and Na transport in the turtle and toad bladder probably by increasing the resistance of the luminal membrane.

Effects of antidiuretic hormone on kinetic and energetic determinants of active sodium transport in frog skin

The effects of antidiuretic hormone (ADH) on the rate of transepithelial active Na transport JaNa and the rate of suprabasal O2 consumption of Jsbr were studied in paired hemiskins of frog. Within some 30 min following administration of ADH both JaNa and Jsbr increased to near-maximal levels and then remained stable for at least an hour. On symmetric perturbation of the transepithelial electrical potential delta psi at 6-min intervals, the dependence of JaNa and Jsbr on delta psi was near-linear, both in control and experimental hemi-skins. The stability and near-linearity of the system permitted systematic analysis of the parameters of linear non-equilibrium thermodynamic (NET) and electrical equivalent circuit (EC) formulations. ADH (100 mU/ml) stimulated two of the three NET phenomenological L coefficients, as well as A, the affinity (negative Gibbs free energy) of a metabolic reaction driving transport. Observations at partially depressed levels of transport indicated that the effects of kinetic and energetic factors are to some extent discrete. EC analysis showed stimulation of the amiloride-sensitive conductance Ka, but not of the apparent electromitive force of Na transport 'ENa'. Similar effects were produced by 10 mU/ml of ADH or by 10 mM dibutyryl cyclic AMP, although less marked effects on the L coefficients were noted with the lower concentration of hormone. It is suggested that, in contrast to EC analysis, the NET formulation distinguishes between kinetic and energetic determinants of transport, supporting a dual mechanism of action of ADH.

Action of caerulein, gastrin 17, pentagastrin, and secretin on the active transport of sodium by the frog skin

Frog skin was mounted in an Ussing chamber and the actions of caerulein, gastrin, pentagastrin, and secretin on the active transport of sodium were studied using the short-circuit current method. All polypeptides exerted their effect when placed in the solution bathing the outside surface of the skin. The response was a transient dose-related increase in the transepithelial electrical potential difference and in the short-circuit current. Analysis of the response indicated that at submaximal doses the effect was due to an increase in the rate of entry of sodium through the outer barrier to active sodium transport. At supramaximal doses the passive permeability of the skin was also increased. Th ED50 concentrations of the hormones were: caerulein, 50 pM; gastrin, 53 pM; pentagastrin, 440 pM; and secretin, 30 pM. It is argued that the large quantity of caerulein or caerulein-like peptides stored in the skin may be required either to control the entry of sodium when the amphibian is undergoing maximum stress in a freshwater environment, or that it may have a protective function for the amphibian as it could elicit a noxious hypersecretion in the gastrointestinal tract of the predator together with a marked hypotension.

Electrophysiologic changes associated with potassium depletion of frog skin

Skins from the frog Rana pipiens pipiens were studied under short-circuited conditions during the course of removing and replacing potassium in the inner bathing media in 14 experiments. The intracellular potential (Vsc), fractional resistance (FR), short-circuit current (Isc) and total tissue conductance (gr) were constantly monitored during impalements of the epithelial cells. The mean value (+/- SE) for Vsc was --79 (+/- 3) mV under baseline conditions. Removal of potassium from the inner bathing solution transiently stimulated the short-circuit current and hyperpolarized the basolateral membrane; with sufficiently long incubations, the basolateral membrane was eventually depolarized. Restoration of potassium to the inner solution within 43 min after initiating the perfusion with K+-free solution depolarized the basolateral membrane. However, restoration of potassium after at least 1 1/2 hr of incubation hyperpolarized the membrane. Ouabain consistently depolarized the basolateral membrane, even after extended periods of potassium depletion as long as 320 min. In the presence of ouabain, restoration of potassium depolarized the basolateral membrane. The data provide further evidence for the concept that the Na--K exchange pump of frog skin is rheogenic. Furthermore, the results suggest that the pump continues to be active even during prolonged periods of potassium depletion, reaccumulating potassium which has leaked out of the epithelial cells.

Protocol-dependence of equivalent circuit parameters of toad urinary bladder

Determinations o- current-voltage relationships are widely employed in the characterization of epithelial sodium transport. In order to determine the protocol dependence of transport parameters in the toad urinary bladder, studies were carried out in the presence and absence of amiloride, an inhibitor of active sodium transport. With symmetric positive and negative perturbations of the transepithelial electrical potential difference delta psi (0 leads to +/- 100 mV) for 30 sec, the amiloride-sensitive current-voltage (ia-delta psi) relationship was near linear over the range -75 leads to +100 mV, indicating constancy of the conductance ka and the apparent electromotive force "ENa", lumped parameters of the standard electrical equivalent circuit model of the active transport system. With a reverse protocol (+/- 100 leads to 0 mV) or 15 min perturbations the ia-delta psi relationships were highly nonlinear. Nonlinearity reflected voltage dependence of parameters: perturbations that increased active transport decreased "ENa" and increased ka, as evaluated from 10 sec perturbations of delta psi; slowing of active transport produced the converse changes. These effects are usefully analyzed in both quasi-steady states and true steady states by means of a detailed equivalent circuit incorporating the significant ionic currents across each plasma membrane. Precise understanding of the significance of ka and "ENa" will require characterization of the partial ionic conductances on perturbation of delta psi.

Voltage-current relation and K+ transport in tobacco hornworm (Manduca sexta) midgut

Voltage-current curves for the isolated midgut of the tobacco hornworm were determined by transient and steady voltage clamping over the range of 200 to -200 mV. Over this range the transient method yields a linear relation while the steady method usually yields a curve consisting of two lines of differing slope which intersect at zero voltage. The difference between the results of the methods is due to a slow decline in total conductance which accompanies steady voltage clamping. Holding the midgut at short circuit increases the total conductance of the tissue in a manner consistent with increasing shunt conductance; this effect was seen in both diet-reared and leaf-reared animals. When potassium transport is inhibited by substitution of choline or sodium for potassium in bathing solution the total conductance decreases and the voltage-current curve intersects the normal curve in the hyperpolarizing region. Applying a simple equivalent circuit analysis to the results from partial or total potassium replacement suggests that the electromotive force of the potassium transport system is of the order of 140-190 mV. The conductance decrease during inhibition of potassium transport by transient anoxia is of similar magnitude, suggesting that a major effect of metabolic inhibition is to decrease the active conductance of the potassium transport pathway.