A characteristic response of the isolated frog skin potential to neurohypophysial principles and its relation to the transport of sodium and water

Pathophysiology of gastro-esophageal reflux disease: a role for mucosa integrity?

BACKGROUND

Gastro-esophageal reflux disease (GERD) is very prevalent and has a high burden on health security system costs. Nevertheless, pathophysiology is complex and not well-understood. Several mechanisms have been proposed: decreased salivation, impaired esophageal clearance, decreased lower esophageal sphincter pressure resting tone, presence of hiatal hernia, increased number of transient lower esophageal sphincter relaxations (TLESRs), increased acid, and pepsin secretion, pyloric incompetence provoking duodeno-gastro-esophageal reflux of bile acids and trypsin. Independent of the relevance of each mechanism, the ultimate phenomenon is that mucosal epithelium is exposed for a longer time to agents as acid and pepsin or is in contact to luminal agents not commonly present in gastric refluxate as trypsin or bile acids. This leads to a visible damage of the epithelium (erosive esophagitis -EE) or impairing mucosal integrity without any sign of macroscopic alteration as occurs in non-erosive reflux disease (NERD). Luminal factors are not the only responsible for such impairment; more recent data indicate that endogenous factors may also play a role.

PURPOSE

This review will update the most recent findings on the putative pathophysiological mechanisms and specially will focus on the role of esophageal mucosal integrity in GERD. Methodologies used for the evaluation of mucosal integrity, its relevance in EE and NERD, its involvement in symptoms perception and the effect of luminal and endogenous factors will be discussed.

The effect of antidiuretic hormone on human sweating

1. Changes in insensible perspiration and sweating were followed in normal subjects by continuously monitoring total body weight loss in environmental temperatures of 18, 29 and 37 degrees C.2. Pharmacological doses of ADH had no effect on cutaneous water loss at 18 degrees C.3. Pharmacological doses of ADH are capable of increasing the rate of cutaneous water loss in human subjects who are close to or above the thermal sweating threshold.4. Physiological doses of ADH had no effect on cutaneous water loss in either cool or hot environments.5. At normal rates of secretion in the body, ADH probably does not influence human sweat secretion.

The effect of intravenous injections of oxytocin during milking and the removal of residual milk on the composition of cow's milk

After an injection of oxytocin and the removal of residual milk, the milk obtained at the next milking had an increased content of sodium, chloride and whey proteins and a decreased content of lactose. The effect persisted for several milkings. It is suggested that this was due to a direct effect of oxytocin on the permeability of the mammary epithelium and that oxytocin should be used with caution in certain types of milk secretion experiment.

The effect of Ca and antidiuretic hormone on Na transport across frog skin. I. Examination of interrelationships between Ca and hormone

Ca added to the solution bathing the outside of isolated frog skin causes a decrease in net Na transport across the skin while antidiuretic hormone (ADH) causes an increase. Possible interrelations between the effects of these agents have been examined. The decrease in Na transport caused by Ca was the same before and after treatment of the skin with ADH and the increase in transport caused by ADH was unaffected by the presence of Ca. The relationship between Ca concentration and degree of inhibition of Na transport was not appreciably altered by ADH. These results indicate that Ca and ADH do not compete but act independently at two different sites and these sites appear to be located on the same barrier to Na movement in the skin. Further, Ca causes a decrease in Cl influx across the short-circuited skin but ADH has no effect on Cl movement, again suggesting that the actions of these agents are independent.

EXPOSURE OF THE ISOLATED FROG SKIN TO HIGH POTASSIUM CONCENTRATIONS AT THE INTERNAL SURFACE. II. CHANGES IN EPITHELIAL CELL VOLUME, RESISTANCE, AND RESPONSE TO ANTIDIURETIC HORMONE

Isolated frog skin epithelia undergo marked, but reversible swelling when the external skin surface is bathed with conventional NaCl Ringer's and the internal surface with KCl Ringer's solutions. In 2 hours, epithelial thickness increased by over twofold. When NaCl Ringer's was replaced on both sides of the skin, volume returned to control levels in less than 1 hour. When sulfate, rather than chloride, was the predominant anion, exposure of the internal surface to high potassium concentrations did not evoke changes in epithelial cell volume. With both KCl and K(2)SO(4) Ringer's, an immediate drop in DC resistance across the skin occurred. This was followed by partial recovery. Both the immediate drop and partial recovery were unrelated to changes in volume. A slow, sustained secondary drop in resistance was observed with KCl but not K(2)SO(4) Ringer's. This slower drop was associated temporally with swelling. When epithelial cell swelling occurred (i.e. with KCl Ringer's), the characteristic response of the skin to vasopressin was abolished. However, with sulfate as anion, vasopressin elicited an increase in short-circuit current and/or in cell volume despite high internal potassium concentrations. It is concluded that epithelial swelling increased the permeability of the sodium-selective barrier at the external surface of the cells; and the possibility exists that stretching of cell membranes altered dimensions of pathways through which Na and water move, thereby mimicking the effects of vasopressin.

Action of capsaicin and related peptides on the ionic transport across the skin of Rana esculenta

Capsaicin at low concentrations increases the short circuit current (SCC) across frog skin. Simultaneous measurements of both transepithelial fluxes of 22Na or 36Cl demonstrate that the SCC increase is due to stimulation of sodium active absorption. Capsaicin acts through the liberation of several peptides; thus these peptides were tested on the SCC across frog skin. Those more active are, in order of potency: Cyclic Calcitonin Gene Related Peptide (CGRP), Kassinin and Eledoisin, Substance P (SP) and Neurokinin A. Neurokinin B and Vasoactive Intestinal Peptide (VIP) have no effect. Also the actions of SP and CGRP are due mainly to stimulation of Na+ active absorption. A strict parallelism regarding the sensitivity to inhibitors (Naproxen, SQ22536 and CP96345) between SP, CGRP and Capsaicin strengthens the hypothesis that SP and CGRP are liberated by Capsaicin in this tissue.

Vasopressin and protein kinase A activate G protein-sensitive epithelial Na+ channels

To determine the molecular steps involved in the vasopressin-induced renal Na+ reabsorption, the patch-clamp technique was utilized to study the role of this hormone in the regulation of apical Na+ channels in renal epithelial A6 cells. Addition of arginine vasopressin (AVP) induced and/or enhanced Na+ channel activity within 5 min of addition under cell-attached conditions. The AVP-induced channel activity was a reflection of both an increase in the average apparent channel number (0.2-1.7) and the percent open time (2-56%). Addition of the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine, the adenosine 3',5'-cyclic monophosphate (cAMP) analogues, 8-(4-chlorophenylthio)-cAMP and 8-bromo-cAMP, or forskolin elicited a comparable effect to that of AVP. The induced channels had similar properties to Na+ channels previously reported, including a channel conductance of 9 pS, Na(+)-to-K+ selectivity of 3-5:1, and high amiloride sensitivity. The cAMP-dependent protein kinase A (PKA) in the presence of ATP induced and/or enhanced Na+ channel activity in excised inside-out patches with a change in average apparent channel number and percent open probability similar to those observed with either AVP or cAMP analogues in intact cells. Addition of activated pertussis toxin (100 ng/ml) completely blocked the AVP- or PKA-induced Na+ channel activity in excised inside-out patches, whereas incubation of intact cells with the toxin completely prevented the effect of both activators. The data indicate that AVP mediates its effect through a cAMP-dependent pathway involving PKA activation whose target is the G protein pathway that regulates apical epithelial Na+ channel activity.

Sodium flux in the apical membrane of the toad skin: aspects of its regulation and the importance of the ionic strength of the outer solution upon the reversibility of amiloride inhibition

Injection of small pulses of concentrate solutions of salts or drugs into the outer bathing fluid led to sudden increases of its solute concentration. Vigorous stirring of the outer bathing solution was used to minimize the thickness of the unstirred layer adjacent to the outer skin surface. Pulses of 1 M NaCl injected into the outer compartment induced sharp increases of the SCC following a time course variable with the magnitude of the pulse and the particular condition of each skin. Comparison of the spontaneous decline of the SCC with the decline induced by a small dose of amiloride, where an increase in R was observed, indicates that the spontaneous decline cannot be explained simply as a reduction of the Na permeability of the apical membrane by self-inhibition of feedback inhibition of the apical membrane Na channels. Reduction of the driving force for Na movement into the epithelial cells must play an important role in the process. Reversibility of the amiloride inhibition of the SCC was highly dependent upon the ionic strength of the solution used to rinse and wash out the inhibitor from the outer skin surface. With H2O, the amiloride molecules washed out slowly as compared to NaCl or KCl solutions. Na or K have the same ability to dislodge the amiloride molecules from their binding sites. This effect is apparently of a purely electrostatic nature.

The significance of changes in thermodynamic affinity induced by aldosterone in sodium-transporting epithelia

The energetics of sodium transport were examined in toad (and occasionally frog) skin, with particular emphasis on the effect of aldosterone. Thermodynamic affinity was computed according to Essig and Caplan. Following treatment with antidiuretic hormone or drugs believed to affect only the apical membrane barrier, no change in thermodynamic affinity was observed either acutely (after one to two hours) or chronically (after 18-odd hours hours). By contrast, following treatment with aldosterone overnight, thermodynamic affinity was considerably increased, whether or not incubation was conducted in the presence of sodium in the outer solution; addition of glucose at the end of incubation, whereby sodium transport was stimulated further, failed to influence affinity as measured. The stoichiometry between sodium transport and oxygen consumption was, however, unchanged by aldosterone treatment in short-circuit conditions, neither was that fraction of aerobic metabolism unrelated to sodium transport influenced. It is concluded that the change observed with aldosterone can be directly ascribed to the hormone, as it is independent of glucose availability and of sodium transport. Aldosterone action, at least following prolonged incubation, therefore does not involve only an increase in apical conductance for sodium.

The role of sodium-channel density in the natriferic response of the toad urinary bladder to an antidiuretic hormone

Urinary bladders of Bufo marinus were depolarized, by raising the serosal K concentration, to facilitate voltage-clamping of the apical membrane. Passive Na transport across the apical membrane was then studied with near-instantaneous current-voltage curves obtained before and after eliciting a natriferic response with oxytocin. Fitting with the constant-field equation showed that the natriferic effect is accounted for by an increase in the apical Na permeability. It is accompanied by a small increase in cellular Na activity. Furthermore, fluctuation analysis of the amiloride-induced shot-noise component of the short-circuit current indicated that the permeability increase is not due to increased Na translocation through those Na channels which were already conducting prior to hormonal stimulation. Rather, the natriferic effects is found to be based on an increase in the population of transporting channels. It appears that, in response to the hormone, Na channels are rapidly "recruited" from a pool of electrically silent channels.

Effect of vasopressin on electrical potential difference and chloride transport in mouse medullary thick ascending limb of Henle's loop

Medullary thick ascending limbs of Henle's loop of the Swiss-Webster mouse were perfused in vitro with an isotonic perfusate and a Ringer's bathing medium. In five studies, addition of a supramaximal concentration of synthetic arginine vasopressin (AVP) to the bathing medium resulted in an increase in electrical potential difference (PD) from 5.0 +/- 1.5 mV, lumen positive, to 10.7 +/- 1.4 mV (P < 0.001). When AVP was removed, the PD returned to 2.6 +/- 0.9 mV (P < 0.001), then increased again to 6.9 +/- 1.7 mV (P < 0.01) when AVP was added a second time. A significant, but submaximal, increase in PD of 2.3 +/- 0.6 MV (P < 0.05) was observed in five medullary thick ascending limbs when AVP was added to the bathing medium at a concentration of 10 microunits/ml. This increase was approximately one-third of the response observed at a concentration of 100 microunits/ml in the same tubule. No further increment in PD was observed in five medullary thick ascending limbs when the AVP concentration was increased from 100 to 1,000 microunits/ml. In seven thick ascendcing limbs, the effect of AVP on PD was reproduced by the addition of 8-[p-chlorophenylthio]-cyclic 3',5'-adenosine monophosphate to the bathing medium at a final concentration of 0.1 mM. AVP increased unidirectional chloride flux from lumen to bath from 29.3 +/- 3.2 to 69.8 +/- 6.2 peq/cm per s (P < 0.001) in spite of an increase in the lumen positive PD from 1.6 +/- 0.5 mV to 7.0 +/- 0.6 mV (P < 0.001). Unidirectional chloride flux from bath to lumen was not affected by AVP. In another series of experiments, net chloride flux increased from 15.6 +/- 3.0 to 41.7 +/- 5.3 peq/cm per s (P < 0.05) after addition of AVP. The effect of AVP on hydraulic water permeability (Lp) was examined by adding raffinose to the bathing medium in both the presence and the absence of AVP. The calculated Lp of 16 +/- 2 nm/s per atm in the absence of AVP, although very low, was significantly different from zero (P < 0.01). However, the Lp did not increase significantly when AVP was added to the bathing medium. These results suggest that AVP has a second site of action in the kidney to increase chloride transport by the medullary thick ascending limb in addition to its well-known effect on the water permeability of the collecting tubule. The former effect would contribute to urinary concentrating ability by increasing the axial osmotic gradient in the renal medulla.

Physiology of transport regulation

The regulation of biological transport is discussed on the basis of studies on sodium transport through amphibian skin. The following types of regulation are briefly considered: 1) Hormonal regulation 2) Regulation of Na entry by apparent or real saturation of entry path by outside Na 3) Regulation of Na transport by changes in resistance to the counter ion (mostly chloride) 4) Role of cellular Na concentration which may act both by controlling the passive entry of Na and by influencing the pumping rate. 5) Dependence of Na entry upon cell volume. It is shown that a moderate osmotic swelling of ouabain-poisoned skins leads to excessive swelling of the whole epithelium when NaCl is present on the outside. This indicates that cell swelling leads to opening of the Na channels, but it also indicates coupling between the different layers of the epithelium.

A comparative study of the effects of norepinephrine and vasopressin on Na transport and O2 consumption in frog skin

Experiments were designed to compare the effects of two hormones-vasopressin and norepinephrine-on the energetics of Na transport in frog skin. Simultaneous measurements of O2 consumption and net Na flux were performed in the same skins by means of O2 cathodes and the short circuit current technique. The results showed that both hormones induced similar increments in Na transport. In contrast, there was a conspicuous difference in O2 consumption values, norepinephrine having a very small stimulatory effect compared to the one induced by vasopressin. Thus, despite the fact that both hormones increase Na permeability of frog skin by similar mechanisms and to a similar extent, they appear to exert very different effects on cell metabolism.

Active sodium uptake by the skin of foetal sheep and pigs

1. Simultaneous measurements of unidirectional sodium fluxes across foetal skin incubated in vitro with identical solutions ([Na] = 150 mM) bathing either side showed a flux ratio (influx/efflux) of 1-40+/-0-08 in twenty-seven sheep skins, which was significantly different from unity (P less than 0-001). The gestational ages ranged from 47 to 98 days (term = 147 days). Similar experiments on eight foetal pig skins at 58 days gestation (term = 114-118 days) gave a mean flux ratio of 1-10 +/- 0-03 (P less than 0-02). 2. Unidirectional sodium fluxes measured with dilute Ringer solution on the outside (mucosal) surface ([Na]0 = 100mM) gave influx to efflux ratios of 0-86 +/- 0-09 in seventeen sheep (P less than 0-05) and 1-07 +/- 0-26 in five foetal pigs; the value predicted for passive movement was 0-67. 3. Incubation with inhibitors, ouabain (10-4 M) or dinitrophenol (DNP) (10-4 M) gave a flux ratio for sodium which was not significantly different from unity in the absence of a gradient, or from 0-67 when the concentration gradient was applied. 4. Sequential measurement of unidirectional diffusional fluxes of tritiated water across foetal skin gave flux ratios of 0-98 +/- 0-02 in six sheep skins and 1-06 +/- 0-11 for four pig skins in control conditions. When the outside solution was diluted to give an osmotic gradient of 100 m-osmole. kg-1 across the skin a flux ratio of 0-95 +/- 0-07 was obtained for seven sheep and was not measured in pig skin. Hormones and inhibitors had no effect on the diffusional flux ratio for water in the presence or absence of an osmotic gradient. 5. Lysine vasopressin (ADH) (200 mu./ml.) increased influx and efflux of water in the presence and, to a lesser extent in the absence of an osmotic gradient in sheep skin. In pig skin prolactin (1 u./ml.) increased both influx and efflux, but ADH had no effect on diffusional water fluxes. 6. ADH increased sodium influx in sheep skin slightly but vasotocin (5-5 mu./ml.) was more potent, particularly in the presence of an opposing diffusion gradient. Vasotocin (55 mu./ml.) reduced sodium influx in pig skin ADH had no effect on influx or efflux and prolactin reduced sodium influx and efflux. Ouabain and DNP generally reduced permeability to both sodium and water in sheep skin but had no effect in pig skin.

A new model for regulation of sodium transport in high resistance epithelia

A new three barrier, four compartment model for sodium transport in high resistance urinary epithelia is presented. This model provides a unified and simplified mechanistic explanation for sodium transport and its quantitative regulation. Sodium enters the epithelial cell by passive diffusion. Active extrusion occurs across the lateral cell membrane into the lateral intercellular space (LICS). Sodium movement from the LICS into the serosal compartment is not free and unobstructed as in the models for low resistance epithelia, but rather occurs through a regulatory channel of the LICS passing through desmosomes and the basilar slit. The exact configuration of this regulatory channel controls the rate of sodium movement from the LICS into the serosal compartment. Thus, the configuration of the regulatory channel controls the afterload on the sodium pump and thus ultimately controls the rate of transepithelial sodium transport. Antidiuretic hormone could act by increasing the effective width of this regulatory channel by contraction of intracellular microtubules or microfilaments. Present theories for regulation of transepithelial sodium transport in high resistance epithelia invoke a regulatory barrier at the apical cell membrane or at the active sodium pump located in the basolateral cell membrane. The hypothetical model presented here invokes a new alternative: regulation of the active pump rate by the sodium concentration in the LICS serving as an afterload on the pump; sodium escape from the LICS into the serosal compartment thus becomes the regulatory step for transepithelial transport.

The effect of antidiuretic hormone on Na movement across frog skin

1. The effect of antidiuretic hormone (ADH) on the movement and distribution of Na was studied. This was done using three different approaches: (a) the measurement of Na and (22)Na in slices of epithelium of skins which were exposed to Ringer of varied composition containing (22)Na, (b) the measurement of the influx of Na from the outer to the inner bathing solution with (22)Na added to the outside, and (c) the use of a recently introduced technique which permits the direct evaluation of the flux from the outer solution --> epithelium, (J(OT)), i.e. the flux across the barrier which is generally regarded as the site of ADH activity.2. ADH increased the influx from the outer to the inner bathing solution of Na (50%) not only when the concentration of Na on the outside was 115 mM (i.e. higher than in the epithelium) but even when the concentration was 1 mM (67%).3. When the skin was bathed with 1mM-Na Ringer on the outside, ADH increased the unidirectional Na flux J(OT) by 56% (Rana pipiens) and 71% (Leptodactylus ocellatus). When the concentration was 115 mM a small increase (17%) was observed in paired skins of R. pipiens. Under this condition no change was observed in L. ocellatus.4. The amount of epithelial sodium which is labelled by (22)Na added to the outside was taken to reflect the amount of Na involved in Na transport across the epithelium. Depending on whether the concentration of Na on the outside was high (115 mM) or low (1 mM), ADH produced an increase, or a decrease, of both the total Na content and the amount of (22)Na exchanged.5. When the concentration of Na on the outside was low, ADH increased the total influx and J(OT) in spite of the fact that it lowers the total Na content and does not affect the exchangeable pool of Na. This observation is inconsistent with the view that the effect of ADH is due to the fact that the increased permeability of the outer barrier allows more Na into the cell, and that the resulting increase of Na concentration in the cytoplasm accelerates the Na pumps at the inner side of the cells.6. It is concluded that ADH speeds up Na movements at the outward facing barrier, and that this exchange which facilitates the penetration of Na into a transporting compartment produces also a gain or a loss of Na in compartments not directly involved in Na transport across the epithelium. One compartment which is not involved in Na transport might be the cytoplasm of the epithelial cells.

A study of the mechanism of water transfer across frog skin by a comparison of the permeability of the skin to deuterated and tritiated water

1. Frog abdominal skins were placed between the two half-cells of a modified version of Ussing's frog skin apparatus.2. The rate of equilibration of deuterated and tritiated water across the skin was followed at pH 5.0, 6.9, and 8.0 with Ringer solution bathing both surfaces of the skin; and at pH 7.0 with sulphate Ringer solution bathing both surfaces of the skin.3. The ratio of the permeability coefficients for the influx of deuterated and tritiated water had a mean value over all the experiments of 0.9954 +/- 0.0207, n = 27, indicating that no isotope effect was observable within the limits of accuracy of the experimental method.4. The thickness of the unstirred layers associated with the frog skin was measured, and, from the data obtained, values of the true permeability coefficients for each isotope were calculated.

Independent action of antidiuretic hormone, theophylline and cyclic 3',5'-adenosine monophosphate on cell membrane permeability in frog skin

1. The effects of antidiuretic hormone (ADH), theophylline and cyclic 3',5'-adenosine monophosphate (AMP) on membrane potentials in frog skin have been investigated.2. Membrane potentials across the outer and inner facing membranes were recorded in both normal and current clamped skins. In the latter condition active transport of sodium had been abolished by ouabain or metabolic inhibitors, but ionic gradients were maintained by passing current through the skin from the inside.3. ADH increases the potential across the outer facing membranes and reduces the skin resistance. The results are consistent with ADH causing an increase in permeability of the outer facing membranes to sodium ions.4. Theophylline reduces the skin potential by reducing specifically the potential across the outer facing membranes. At the same time the skin resistance is reduced. Theophylline acts by increasing the permeability of the outer facing membranes to chloride ions.5. Cyclic 3',5'-AMP causes a biphasic potential change accompanied by an increase in skin resistance.6. Metabolic inhibitors block the response of the skin to ADH but not to theophylline.7. Separate explanations for the increase in sodium transport by ADH, theophylline and cyclic 3',5'-AMP are discussed. It is not necessary to involve cyclic AMP in order to explain the effects of either ADH or theophylline.