Components of sodium and chloride flux across toad bladder

Catecholamine stimulation of ion transport in the toad urinary bladder

We have observed that serosal catecholamines increase the amplitude of the short-circuit current (Isc) in the toad urinary bladder by as much as 450%. Chemical sympathectomy with 10(-6) M 6-hydroxydopamine and the sympathomimetic effects of 10(-5) M tyramine indicate a reservoir of amines in the serosal stroma of the tissue. The urinary epithelium from the toad responds to six adrenoceptor agonists: (-)-epinephrine, (-)-norepinephrine, (-)-phenylephrine, clonidine, methoxamine and oxymetazoline. The alpha 2-adrenoceptor agonist clonidine is most potent for stimulating Isc. Some agonists were found to diminish Isc. Apparently this is related to a simultaneous increase in the transepithelial flux of both chloride and sodium. The Isc response to the catecholamines is also inhibited by several adrenoceptor antagonists. The alpha 2-adrenoceptor antagonist yohimbine is more effective than the alpha 1-antagonist prazosin for blocking the stimulation of epithelial transport. As a result of these studies, we have tentatively classified the serosal adrenoceptor of the toad urinary bladder as alpha 2.

Characteristics of proton excretion in normal and acidotic toad urinary bladder

This study, performed on the urinary bladder of Bufo marinus, was to investigate the characteristics of H+ excretion in the normal and metabolic acidotic toad. Experiments were run in modified Ussing chambers in the presence and absence of exogenous CO2. Amiloride in the mucosal medium (1.5 X 10(-4) M) inhibited H+ excretion in the acidotic bladder but this inhibition was reversed in the presence of 5% CO2. Na+-free mucosal medium revealed a component of H+ excretion in the normal toad that is Na+-dependent and not reversed by 5% CO2. Acetazolamide (10(-3) M) had no effect on H+ excretion in normal toad but inhibits excretion in the acidotic toad both in the presence and absence of exogenous CO2. In both the presence and absence of exogenous CO2 and in the presence of varying pH gradients, the bladders from acidotic toads excreted H+ at a greater rate and were able to generate greater pH gradients than in normal bladders. Voltage clamp experiments revealed that H+ excretion in the acidotic toad was inhibited by a mucosal potential difference of -20 to -100 mV but this inhibition was completely reversed by 5% CO2. In the normal toad bladder H+ excretion was stimulated by a mucosal potential difference of -20 to -100 mV in both the presence and absence of exogenous CO2. Our evidence suggests the possibility of two H+ excretory mechanisms in toad urinary bladder each with its' own set of characteristics dependent on the acid-base state of the animal. Proposed models are presented for these mechanisms.

A simple model for early morphogenesis

Despite the large amount of knowledge which continues to accumulate about early developmental events, very little is known about the processes which control them. Part of the problem may lie in that workers applying different approaches and techniques have different points of view and appear to be reluctant to read each others' literature. My aim in this paper is not to give a generative, formal model for early development, but rather to suggest several connecting strands between the physiological, biochemical, cell biological and experimental embryological approaches which may stimulate new research in fields between those already exploited.

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).

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.

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.

On the mechanism of lithium-induced renal tubular acidosis: studies in the turtle bladder

The mechanism whereby lithium impairs urinary acidification was investigated in vitro with the bladder of the fresh water turtle. The effect of lithium was compared to that of choline or cesium. In the presence of 1% carbon dioxide, lithium (40 mM in the mucosa) failed to alter hydrogen ion secretion. Lithium also caused a significant decrease in sodium transport and in potential difference. Lithium failed to increase the passive loss of hydrogen ions from the mucosal solution, indicating that it does not affect the permeability of the mucosa to hydrogen ions. Bicarbonate secretion also was unchanged in lithium-treated hemibladders. Under open-circuit conditions, lithium caused a significant decrease in potential difference and, hence, caused a significant decrease in hydrogen ion secretion. Restoration of the potential difference, in the presence of lithium, to control levels caused a return of hydrogen ion secretion to the original value. These data demonstrate that in the turtle baldder lithium inhibits hydrogen ion secretion solely by reducing the potential difference and suggest that in vivo lithium causes an acidification defect not by directly inhibiting the hydrogen ion pump or causing a hydrogen ion backleak but by inhibiting sodium transport, thereby decreasing the favorable electric gradient for hydrogen ion secretion.

Toad urinary bladder epithelial cells in culture: maintenance of epithelial structure, sodium transport, and response to hormones

Epithelial cells from the toad urinary bladder have been grown in continuous culture. Many of the cells resemble the granular cell type of the urinary bladder. They form an epithelium with typical tight junctions and gap junctions. The transport properties of two cell lines have been examined. When cells of the line designated TB-M or of line TB-6c are grown on collagen-coated nucleopore filters, epithelia are formed that have transepithelial potential differences of 40 and 20 mV, resistances of 5000 and 10,000 omega-cm2, and short-circuit currents (ISC) of 8.5 and 2.5 muA/cm2, respectively. Net mucosa to serosa sodium transport accounts for all of ISC in line TB-M and for 70% of ISC in line TB-6c. Vasopressin, which stimulates adenylate cylase and ISC in the intact bladder, has no effect on the cells in culture. Cyclic AMP stimulates ISC and lowers resistance in both lines. Aldosterone stimulates ISC in both lines. This is accompanied by a fall in resistance in line TB-M and no change in resistance in line TB-6c. Amiloride inhibits ISC in TB-M cells under basal conditions and after stimulation by aldosterone. In line TB-6c amiloride has no effect under basal conditions but lowers ISC of aldosterone-treated cells to the basal level. Thus, the cells have retained the ability to form oriented, high-resistance epithelial membranes that manifest hormone-sensitive transepithelial sodium transport.

Aldosterone receptor occupancy and sodium transport in the urinary bladder of Bufo marinus

The concentration dependence of binding of [3H]aldosterone to cytoplasmic and nuclear receptors was evaluated in urinary bladder epithelial cells of Colombian toads. One class of specific sites (sensitive to displacement by excess aldosterone) was detected in the cytosol. However, two classes of specific nuclear [3H]aldosterone binding sites were evident. In the nucleus, high-affinity (Kd = 2.7 X 10(-9) M), low-capacity (N = 15 X 10(-14) mol/mg DNA) sites (type I) were completely saturated at approximately 4 X 10(-8) M aldosterone, a concentration which gave a maximal increase in short-circuit current (SCC). Occupancy of low-affinity (Kd = 4.6 X 10(-7) M), high-capacity sites (N = 150 X 10(-14) mol/mg DNA) (type II) occurred at higher steroid concentrations and did not correlate with further increase in SCC. Binding parameters of Colombian and Dominican variants of Bufo marinus were compared. In both variants, the SCC increase elicited by aldosterone correlated with accumulation of type I complexes in the nucleus, but the relationship was markedly nonlinear. Various alternatives were considered as the basis for a curvilinear dependence of the increment in Na+ transport on abundance of nuclear type I complexes.

Pathways for movement of ions and water across toad urinary bladder. III. Physiologic significance of the paracellular pathway

Hypertonicity of the mucosal bathing medium increases the electrical conductance of toad urinary bladder by osmotic distension of the epithelial "tight" or limiting junctions. However, toad urine is not normally hypertonic to plasma. In this study, the transmural osmotic gradient was varied strictly within the physiologic range; initially hypotonic mucosal bathing media were made isotonic by addition of a variety of solutes. Mucosal NaCl increased tissue conductance substantially. This phenomenon could not have reflected soley an altered conductance of the transcellular active transport pathway since mucosal KCl also increased tissue conductance, whether or not Na+ was present in the bathing media. The effect of mucosal NaCl could not have been mediated solely by a parallel transepithelial pathway formed by damaged tissue since mucosal addition of certain nonelectrolytes also increased tissue conductance. Finally, the osmotically-induced increase in conductance could not have occurred soley in transcellular transepithelial channels in parallel with the active pathway for Na+, since the permeability to 22Na from serosa to mucosa (s to m) was also increased by mucosal addition of NaCl; a number of lines of evidence suggest that s-to-m movement of Na+ proceeds largely through paracellular transepithelial pathways. The results thus establish that the permeability of the limiting junctions is physiologically dependent on the magnitude of the transmural osmotic gradient. A major role is proposed for this mechanism, serving to conserve the body stores of NaCl from excessive urinary excretion.

Kinetics of bidirectional active sodium fluxes in the toad bladder

The kinetics of isotopic Na+ flows was studied in urinary bladders of toads from the Dominican Republic. Initial studies of the potential dependence of passive serosal to mucosal 22Na+ efflux demonstrated the absence of isotope interaction and/or other coupling with passive Na+ flow. The electrical current I and mucosal to serosal 22Na+ influx were then measured with transmembrane potential clamped at deltapsi=0, 25, 50, 75 or 100 mV. Subsequent elimination of active Na+ transport with mucosal amiloride permitted calculation of the rates of active Na+ transport JaNa and active and passive influx leads to JaNa and leads to JpNa. The results indicate that for Dominican toad bladders mounted in chambers only Na+ contributes significantly to transepithelial active ion transport; hence JaNa=Ja. Ja was abolished at deltapsi=E=96.3+/-1.9 (S.E.) mV. As deltapsi approached E, active efflux comes from Ja became demonstrable. At deltapsi=100 mV, comes from Ja exceeded leads to Ja, so that Ja was negative. Experimental values of leads to Ja agreed well with theoretical values predicted by a thermodynamic formulation: leads to Jaexp=0.985 leads to Jatheor (r=0.993). The dependence of leads to Ja on deltapsi is curvilinear.

Passive sodium fluxes across toad bladder in the presence of simultaneous transepithellal gradients of concentration and potential

Bidirectional sodium fluxes were measured across toad bladder sacs after eliminating active transport with ouabain. Transepithelial potential was clamped to 100 mV or the Nernst potential, psieq, at varying sodium concentrations, Cm, in the mucosal medium. Serosal sodium concentration, Cs, was held constant. Equations were derived for permeability, partial ionic conductance, and unidirectional fluxes as functions of Cm and Cs, based in part on the assumption that the ratio, Q, of bulk sodium permeability to tracer sodium permeability is a constant, independent of concentration and potential. The results conformed closely to these equations.

Effects of aldosterone on Na+ transport in the toad bladder. II. The anaerobic response

The action of aldosterone on active Na+ transport was assessed under aerobic and anaerobic conditions in the isolated urinary bladder of the toad, Bufo marinus. Aldosterone augmented the short-circuit current (Isc) under rigorous anaerobiosis. Four lines of evidence indicate that the increase in anaerobic Isc does not represent an equivalent increase in active Na+ transport: 1. Net Na transport, determined by isotopic fluxes, was the same in the aldosterone-treated and control quarter-bladders, and significantly greater than the simultaneously measured Isc-2. Amiloride, an inhibitor of the apical entry of Na+, did not reduce the steroid-dependent increase in the anaerobic Isc-3. Substitution of choline for Na+ in the mucosal medium reduced the magnitude of the anaerobic Isc values but did not eliminate the effect of aldosterone. 4. Addition of ouabain, a potent inhibitor of the Na+ pump, partially inhibited the effect of aldosterone on the anerobic Isc but a significant hormonal increment remained. The source of the anaerobic Isc was not identified; an effort was made, however, to determine the dependence of this current on glycolysis. During anaerobiosis, aldosterone increased the integral Isc by 42% but did not alter lactate production. These results suggest that the steroid-dependent increase in the anaerobic Isc may involve effects on permeability properties of the epithelium rather than on active transport systems.

Sodium fluxes through the active transport pathway in toad bladder

To assess the active components of sodium flux across toad bladder as a function of transepithelial potential, unidirectional sodium fluxes between identical media were measured before and after adding sufficient ouabain (1.89 X 10(-3)M) to eliminate active transport, while clamping transepithelial potential to 0, 100 or 150 mV. Evidence was adduced that ouabain does not alter passive fluxes, and that fluxes remain constant if ouabain is not added. Hence, the ouabain-inhibitable fluxes represent fluxes through the active path. Results were analyzed by a set of equations, previously shown to describe adequately passive fluxes under electrical gradients in this tissue, here modified by the insertion of E, the potential at which bidirectional sodium fluxes (beta E, and theta E) through the active pathway are equal. According to these equations, beta E and theta E are the logarithmic mean of bidirectional fluxes through the active path at any potential, and the flux ratio in this path is modified by a constant factor Qia, which represents the ratio of the bulk diffusion coefficient to the tracer diffusion coefficient in this pathway. The data are shown to conform closely to these equations. Qia averages 2.54. Hence, serosal-to-mucosal flux vanishes rapidly as potential falls below E. Mean E in these experiments was 158 +/- 1 mV. Thus, linear dependence of net flux in both active and passive pathways on potential is present, even though the sodium fluxes in both paths fail to conform to the Ussing flux ratio equation. Qip less than 1 in the passive path (qualitatively similar to exchange diffusion) and Qia greater than 1 in the active path (as in single file pore diffusion). Both of these features tend to reduce the change in serosal-to-mucosal sodium flux induced by depolarization from spontaneous potential to zero potential ("short-circuiting").

Influence of transepithelial potential difference on the sodium uptake at the outer surface of the isolated frog skin

The unidirectional uptake of sodium across the outer surface of the isolated frog skin (J(12) (Na)) was measured in the presence of transepithelial potential difference (Delta(psi)) ranging from +100 to -100 mV. With a sodium concentration of 115 mM in the bathing solutions J(12) (Na) increases significantly when the spontaneous Delta(psi) is reduced to zero by short-circuiting the skin. With an Na concentration of 6 mM a progressive increase J(12) (Na) can be observed when Delta(psi) is decreased in several steps from +100 to -100 mV (serosal side positive and negative, respectively). The observed change J(12) (Na) amounts to a fraction only of that predicted from the shift in Delta(psi). The results suggest that under open circuit conditions the potential step across the outside surface is at most one half of Delta(psi) and that the resistance across the outside and inside barrier of the skin is ohmic. This is in agreement with measurements of intracellular potentials in the frog skin and with resistance measurements carried out in the toad skin. The data strongly support the view that the saturating component of J(psi) proceeds via a charged carrier system. Exposure to negative values of Delta(psi) of 50 mV or more for times of 24 min or more result in a marked reduction of J(12) (Na) which shows only partial or no reversibility.

Aldosterone action and sodium- and potassium-activated adenosine triphosphatase in toad bladder

Urinary hemibladders obtained from toads soaked in water or saline were treated with aldosterone, 10(-6) M, either 1(1/2) or 16 h after mounting. After 2(1/2) h exposure to the hormone, short-circuit current was increased by 110-192% and open-circuit potential by 20-44% as compared with untreated paired hemibladders. Mucosal cells were then assayed for sodium-potassium-stimulated adenosine triphosphatase (ATPase). No increase occurred in activity per milligram protein or in the portion of total activity dependent on sodium. Activity at low sodium concentrations was also measured and analyzed by means of the Hill equation in terms of K, the apparent dissociation constant of the enzyme-sodium complex, and n, a number that expresses the degree of interaction between binding sites. Neither K nor n was significantly altered by aldosterone. A few experiments were also carried out at low ATP concentrations (0.3 mM); again no change in sodium-dependent activity was noted. The results indicate that aldosterone does not stimulate sodium transport by increasing the quantity of sodium-potassium adenosine triphosphatase in mucosal cells or the dependence of this activity on sodium or ATP concentrations.