Structural responses to voltage-clamping in the toad urinary bladder. I. The principal role of granular cells in the active transport of sodium

Effects of potassium-free media and ouabain on epithelial cell composition in toad urinary bladder studied with X-ray microanalysis

The technique of X-ray microanalysis was used to study the composition of toad urinary bladder epithelial cells incubated in Na Ringer's and K-free medium, with and without ouabain. Following incubation under short-circuit conditions, portions of tissue were coated with an external albumin standard and plunge-frozen. Cryosections were freeze-dried and analyzed. In Na Ringer's, granular and basal cells, and also the basal portion of the goblet cells, had similar water and ion compositions. In contrast, mitochondria-rich cells contained less Cl and Na. On average, the granular cells and a subpopulation of the basal cells lost K and gained Na after ouabain and in K-free medium alone. However, there was considerable variation from cell to cell in the responses, indicating differences between cells in the availabilities of ion pathways, either as a consequence of differences in the numbers of such pathways or in their control. In contrast, the compositions of both the low Cl, mitochondria-rich cells and a sub-population of the basal cells were little affected by the different incubation conditions. This is consistent with a comparatively low Na permeability of these cells. The results also indicate that (i) much, if not all, of the K in the dominant cell type, the granular cells, is potentially exchangeable with serosal medium Na, and (ii) Na is accumulated from the serosal medium under K-free conditions. They also provide information about the role of the (Na-K)-ATPase in the maintenance of cellular K in K-free medium, being consistent with other evidence that removal of serosal medium K inhibits transepithelial Na transport by decreasing Na entry to the cells from the mucosal medium, rather than solely by inhibiting the basolateral membrane (Na-K)-ATPase.

Effects of basolateral ouabain, amphotericin B, cyanide and potassium on amiloride noise during voltage clamp of Rana pipiens skin support sodium-amiloride competition

In a previous study, the amiloride-induced corner frequency (fc) was found to decrease as apical sodium was increased. This effect was small or absent when the basolateral surface was exposed to high potassium. It has been suggested that the apical sodium effect may be indirect, due either to increased intracellular [Na+] which repelled amiloride or to an increased potential at the apical surface which reduced amiloride affinity. High basolateral K+ might then suppress the sodium effect either by preventing intracellular [Na+] from increasing or by allowing a better clamp of the apical membrane potential by reducing basolateral membrane resistance and potential. We checked the effects of basolateral [K+], of cyanide and of ouabain at concentrations known to increase intracellular [Na+]. We found only negligible effects on fc. In addition, amphotericin B added to the basolateral bathing solution either in 115 mM Na+ or in 120 mM K+ had no significant effect on fc. We found that relatively wide variation in clamp potential under all conditions, even with active transport severely inhibited, left fc virtually constant. Since the amiloride kinetics were independent of clamp potential, we were able to measure paracellular and transcellular conductances separately by examining the voltage dependence of clamp current (linear) and amiloride noise power (quadratic). This made possible estimation of channel density and single-channel current.

Intracellular solute gradients during osmotic water flow: an electron-microprobe analysis

In an attempt to quantify possible intracellular water activity gradients during ADH-induced osmotic water flow, we employed energy dispersive X-ray microanalysis to thin, freeze-dried cryosections obtained from fresh, shock-frozen tissue of the toad urinary bladder. The sum of all detectable small ions (Na + K + Cl) in the cellular water space was taken as an index of the intracellular osmolarity. Presuming that all ions are osmotically active, they comprise about 90% of the cellular solutes. When the cells were exposed to dilute serosal medium, the reduction in the sum of the ions agreed well with the expected reduction in osmolarity. After inducing water flow by addition of ADH and dilution of the mucosal medium, all epithelial cells showed a fall in osmolarity. The change was more pronounced in granular cells than in basal or mitochondria-rich cells, consistent with the notion that granular cells represent the main transport pathway. Most significantly, intracellular osmolarity gradients, largely caused by an uneven distribution of K and Na, were detectable in granular cells. The gradients were not observed after ADH or mucosal dilution alone, or when the direction of transepithelial water flow was reversed. We conclude from these results that there is a significant cytoplasmic resistance to water flow which may lead to intracellular gradients of water activity. Concentration gradients of diffusible cations can be explained by a flow-induced Donnan-type distribution of fixed negative charges. With regard to transepithelial Na transport, the data suggest that ADH stimulates transport by increasing the Na permeability of the apical membranes of granular cells specifically.

Functional analysis of tight junction organization

The functional basis of tight junction design has been examined from the point of view that this rate-limiting barrier to paracellular transport is a multicompartment system. Review of the osmotic sensitivity of these structures points to the need for this sort of analysis for meaningful correlation of structure and function under a range of conditions. A similar conclusion is drawn with respect to results from voltage-clamping protocols where reversal of spontaneous transmural potential difference elicits parallel changes in both structure and function in much the same way as does reversal of naturally occurring osmotic gradients. In each case, it becomes necessary to regard the junction as a functionally polarized structure to account for observations of its rectifying properties. Lastly, the details of experimentally-induced junction deformation are examined in light of current theories of its organization; arguments are presented in favor of the view that the primary components of intramembranous organization (as viewed with freeze-fracture techniques) are lipidic rather than proteinaceous.

Electrophysiology of Necturus urinary bladder: II. Time-dependent current-voltage relations of the basolateral membranes

As reported previously (S.R. Thomas et al., J. Membrane Biol. 73:157-175, 1983) the current-voltage (I-V) relations of the Na-entry step across the apical membrane of short-circuited Necturus urinary bladder in the presence of varying mucosal Na concentrations are (i) time-independent between 20-90 msec and (ii) conform to the Goldman-Hodgkin-Katz constant field flux equation for a single cation over a wide range of voltages. In contrast, the I-V relations of the basolateral membrane under these conditions are (i) essentially linear between the steady-state, short-circuited condition and the reversal potential (Es); and (ii) are decidedly time-dependent with Es increasing and the slope conductance, gs, decreasing between 20 and 90 msec after displacing the transepithelial electrical potential difference. Evidence is presented that this time-dependence cannot be attributed entirely to the electrical capacitance of the tissue. The values of gs determined at 20 msec are linear functions of the short-circuit current, Isc, confirming the relations reported previously, which were obtained using a more indirect approach. The values of Es determined at 20 msec are significantly lower than any reasonable estimate of the electromotive force for K across the basolateral membrane, indicating that this barrier possesses a significant conductance to other ions which may exceed that to K. In addition, these values increase linearly with decreasing Isc and approach the value of the electrical potential difference across the basolateral membrane observed when Na entry across the apical membrane is blocked with amiloride or when Na is removed from the mucosal solution. A possible explanation for the time-dependence of Es and gs is offered and the implications of these findings regarding the interpretation of previous microelectrophysiologic studies of epithelia are discussed.

Energetics of sodium transport in the urinary bladder of the toad. Effect of aldosterone and sodium cyanide

Experiments were designed to determine whether the stimulatory effect of aldosterone on sodium transport involves an increase in tissue ATP. Urinary bladders that were removed from toads presoaked in 0.6% saline for 48-72 h, mounted as sacs, and maintained in open circuit except for brief observation of short circuit current every 30 min responded to 100 nM aldosterone added to the serosal bath with an increase in short circuit current to 170% of control hemibladders, which plateaus at 2-3 h. Tissue (ATP)/(ADP) X (Pi) measured in perchloric acid extracts increased to a maximum of 208% of controls (P less than 0.001) and ATP increased to 116% of controls (P less than 0.01) at 180 min. The short circuit current response to aldosterone paralleled the increase in ATP and (ATP)/(ADP) X (Pi) measured at 75, 120, 180, and 240 min. In bladders clamped at -150 mV, the short circuit current response to aldosterone was greater: 280% of controls (P less than 0.001) and tissue (ATP)/(ADP) X (Pi) increased to 191% of controls (P less than 0.001). In continuously short circuited bladders and bladders clamped at +75 mV, the short circuit current response to aldosterone and the change in ATP, ADP, or Pi were markedly diminished. 100 microM amiloride added to mucosal bath decreased the short circuit current to zero and inhibited the short circuit current response to aldosterone, whereas tissue ATP increased to 141% (P less than 0.05). 100, 250, and 500 microM NaCN dropped the short circuit current to 59, 35, and 24% of control values, respectively. Concurrently, tissue ATP measured at 60 min after the addition of NaCN dropped to 79, 66, and 56% of control values, respectively, and tissue ATP/ADP dropped to 68, 50, and 40%, respectively. The data revealed significant correlation between the change in the rate of sodium transport produced by aldosterone or NaCN as measured by the short circuit current and the concentration of ATP (r = 0.96, P less than 0.001), as well as ATP/ADP (r = 0.95, P less than 0.001). In conclusion, these results support the view that the stimulatory effects of aldosterone on sodium transport involve an increase in ATP or (ATP)/(ADP) X (Pi).

Effects of hormonal and electrical stimulation of sodium transport on metabolism of toad urinary bladder

The carbon dioxide produced by toad urinary bladders bathed on their mucosal surfaces by sodium Ringer solution and on their serosal surfaces by modified Leibovitz tissue culture medium was analysed by multiple regression on both sodium transport and time. The fractions contributed by metabolism related to transport and by basal metabolism were assessed, and the extent to which these might vary with time was determined. This analytical method, which improves the accuracy with which suprabasal metabolism is estimated, was used to examine the effects on metabolism of vasopressin, aldosterone, and mucosa-positive voltage-clamping. Vasopressin (0.05 u./ml), which on average increased sodium transport 2.9 times and concurrently increased the rate of carbon dioxide production in these transporting tissues, also altered the carbon dioxide production of non-transporting, amiloride-treated control hemibladders. For each hemibladder the ratio of sodium transported to suprabasal carbon dioxide produced after vasopressin was compared with that observed before vasopressin. Differences between the ratios were much reduced when the carbon dioxide productions of the paired transporting hemibladders were corrected for the effects of vasopressin on basal carbon dioxide production. With such analysis, it was confirmed that vasopressin did not alter the stoichiometry of sodium transport. A 30 mV, mucosa-positive voltage clamp, applied near the peak of the response to vasopressin, further increased both sodium transport and carbon dioxide production. No alterations of the ratio of sodium to suprabasal carbon dioxide were seen under these conditions where the maximal rate of active sodium transport in this tissue must have been approached. Active sodium transport was more than doubled some 4 h after adding aldosterone (10(-7) M). However, the related increase in suprabasal carbon dioxide production was greater than threefold. Therefore, whereas the stimulation resulting from vasopressin and voltage clamping had no effect on the ratio of sodium transported to suprabasal carbon dioxide produced, this ratio was reduced significantly by aldosterone. When the sodium transport of aldosterone-treated bladders was increased further by voltage clamping, the ratio of sodium transported to suprabasal carbon dioxide production remained at the reduced value. Sodium transport was increased by approximately 35% more when aldosterone-treated hemibladders were voltage clamped after vasopressin, the control paired hemibladders being exposed to vasopressin and voltage clamping alone.(ABSTRACT TRUNCATED AT 400 WORDS)

Localization of ionic pathways in the teleost opercular membrane by extracellular recording with a vibrating probe

We have adapted the vibrating probe extracellular recording technique to use on an epithelium under voltage clamp in an Ussing chamber. The vibrating probe allows very low drift measurements of current density immediately over the epithelial surface. These measurements allowed sites of electrogenic transport in the epithelium to be localized with a spatial resolution of 5 micrometers. The technique was applied to the opercular membrane of the teleost fish, the tilapia, Sarotherodon mossambicus. The mitochondrion-rich "chloride cells" were shown to be the only sites of electrogenic ion transport in this heterogeneous epithelium. Cell sampling experiments demonstrated variable negative short-circuit currents associated with nearly all of approximately 300 chloride cells examined, which appeared to account for all of the tissue short-circuit current. Current-voltage relations for individual cells were also measured. Conductance associated with chloride cells (i.e. cellular and junctional pathways) accounted for all but 0.5 mS/cm2 of the tissue conductance, with the balance apparently accounted for by leak pathways near the edge of the tissue. Current and conductance associated with other cell types was at least 50-fold smaller than for the chloride cell. Chloride-free solutions reduced chloride cell current and conductance by 98 and 95%, respectively.

Evaluation by capacitance measurements of antidiuretic hormone induced membrane area changes in toad bladder

A technique for estimating effective transepithelial capacitance in vitro was used to investigate changes in epithelial cell membrane area in response to antidiuretic hormone (ADH) exposure in toad bladder. The results indicate that transepithelial capacitance increases by about 30% within 30 min after serosal ADH addition and decreases with ADH removal. This capacitance change is not blocked by amiloride and occurs whether or not there is a transepithelial osmotic gradient. It is blocked by methohexital, a drug which specifically inhibits the hydro-osmotic response of toad bladder to ADH. We conclude that the hydro-osmotic response of toad bladder to ADH is accompanied by addition of membrane to the plasmalemma of epithelial cells. This new membrane may contain channels that are permeable to water. Stimulation of Na+ transport by ADH is not related to membrane area changes, but appears to reflect activation of Na+ channels already present in the cell membrane before ADH challenge.

Relationship of transepithelial electrical potential to membrane potentials and conductance ratios in frog skin

Previous studies in anuran epithelia have shown that, after clamping the transepithelial voltage in symmetrical sequences for 4-6 min there is near-constancy of the rate of active Na transport and the associated oxidative metabolism, with a near-linear potential dependence of both. Here we have investigated in frog skin the cellular electrophysiolgical events associated with voltage clamping (Vt = inside-outside potential). Increase and decrease of Vt produced converse effects, related directly to the magnitude of Vt. Hyperpolarization resulted in prompt decrease in inward transepithelial current It and increase in fractional outer membrane resistance fRo (as evaluated from small transient voltage perturbations) and in outer membrane potential Vo. Overshoot of Vo was followed by relaxation to a quasi-steady state in minutes. Changes in fRo were progressive, with half times of some 1-5 sec. Changes in transepithelial slope conductance gt were more variable, usually preventing precise evaluation of the outer and inner cell membrane conductances go and gi. Nevertheless, it was shown that go is related inversely to Vt and Vo. Presuming insensitivity of gi to Vt, the dependence of Go on Vo in the steady state much exceeds that predicted by the constant field equation. Apparent inconsistencies with earlier results of others may be attributable to differences in protocol and the complex dependence of go on Vo and/or cellular-current. In contrast to previous findings in tight epithelia at open circuit, differences in Vt were associated with substantial differences in fRo and inner membrane potential Vi. Hyperpolarization of Vt over ranges commonly employed in studies of active transport ad metabolism appears to increase significantly the electrochemical work per Na ion transported.

Transepithelial Na+ transport and the intracellular fluids: a computer study

Computer simulations of tight epithelia under three experimental conditions have been carried out, using the rheogenic nonlinear model of Lew, Ferreira and Moura (Proc. Roy. Soc. London. B 206:53-83, 1979) based largely on the formulation of Koefoed-Johnsen and Ussing (Acta Physiol. Scand. 42: 298-308. 1958). First, analysis of the transition between the short-circuited and open-circuited states has indicated that (i) apical Cl- permeability is a critical parameter requiring experimental definition in order to analyze cell volume regulation, and (ii) contrary to certain experimental reports, intracellular Na+ concentration (ccNa) is expected to be a strong function of transepithelial clamping voltage. Second, analysis of the effects of lowering serosal K+ concentration (csK) indicates that the basic model cannot simulate several well-documented observations; these defects can be overcome, at least qualitatively, by modifying the model to take account of the negative feedback interaction likely to exist between the apical Na+ permeability and ccNa. Third, analysis of the strongly supports the concept that osmotically induced permeability changes in the apical intercellular junctions play a physiological role in conserving the body's stores of NaCl. The analyses also demonstrate that the importance of Na+ entry across the basolateral membrane is strongly dependent upon transepithelial potential, cmNa and csK; under certain conditions, net Na+ entry could be appreciably greater across the basolateral than across the apical membrane.

Intra- and transepithelial analytical techniques

Epithelia transport a variety of solutes and water. Study of such transport requires a determination of the driving forces responsible for transport, of the pathways through which transport occurs, and of the factors controlling such transport. Transepithelial driving forces are readily determined where the composition of the bathing media can be altered and electrical forces negated. Where substances move only through a paracellular pathway such manipulations may be adequate to define the permeability and selectivity of the pathways. For substances utilizing a cellular pathway, driving forces and permeabilities across the two dissimilar apical and basolateral cellular membranes must be determined. Where a substance can be shown to move across a membrane against its electrochemical potential gradient, the source of the energy for such movement must be assessed. This review focuses on the applicability and validity of a variety of techniques utilized for the study of epithelial transport to answer these questions. These include microelectrode techniques, chemical analyses, microprobe analysis, microscopy, and techniques for assessing the coupling of metabolism to transport.

Structural responses to voltage-clamping in the toad urinary bladder. II. Granular cells and the natriferic action of vasopressin

The natriferic action of vasopressin has been investigated with morphological studies of voltage-clamped toad urinary bladders. Granular cell swelling can be induced in the presence of isoosmotic solutions when the orientation of the transmural potential is reversed by voltage clamping (V.A. Bobrycki, J. W. Mills, A.D.C. Macknight & D. R. DiBona, J. Membrane Biol., 60:21, 1981) and results from an increased rate of sodium entry across the mucosal membrane; under these conditions the active transport mechanism at the basal-lateral membrane becomes rate-limiting. Vasopressin exacerbated the voltage-reversal-induced swelling of granular cells while other cell types were unaffected. Granular cell swelling appeared to be dependent upon sodium entry from the mucosal medium since it was completely prevented by amiloride. There was no evidence for an effect of vasopressin on tight junction permeability; voltage-reversal induced the same amount of junction blistering whether or not vasopressin was present. It is concluded that the predominant effect of vasopressin on transepithelial sodium transport is to increase the sodium conductance of the mucosal plasma membrane. As is the case with the hydroosmotic effect of the hormone, the natriferic action of vasopressin seems to be exerted primarily, if not entirely, on the granular cells.