Sodium transport by the colon of Bufo marinus: Na uptake across the mucosal border

Changes in the electrophysiological parameters of the posterior intestine of Anguilla anguilla (Pisces) induced by oxytocin, urotensin II and aldosterone

In view of the importance of the intestine in the osmoregulation of freshwater fishes, we determined the effects of oxytocin, urotensin II (UII), and aldosterone added to the serosal side of the isolated posterior intestine of the freshwater-adapted teleost Anguilla anguilla on electrophysiological parameters. Oxytocin decreased the short-circuit current (SCC) and transepithelial potential difference (TPD) at concentrations of 1 and 10 mU/ml (to 50% and 42% of control values, respectively), but did not alter these parameters at a concentration of 0.1 mU/ml. UII reduced SCC and TPD at concentrations of 10 nM, 50 nM and 100 nM (to 85% of control values), but increased these parameters at the concentration of 500 nM (to 115% of control values). Aldosterone did not alter SCC or TPD at the concentrations tested (10 nM and 100 nM). Oxytocin may open Na+ channels in the apical membrane, allowing the flow of Na+ to the serosa, reducing SCC and TPD. Should this hypothesis be correct, oxytocin would be important for freshwater adaptation, since it would increase Na+ absorption. The reduction of SCC and TPD in the posterior intestine of A. anguilla induced by UII is evidenced that this neurohormone is also important for freshwater adaptation in teleosts. Aldosterone did not show this effect probably due to the lack of receptors in this organ.

Electrophysiological analysis of sodium-transport in the colon of the frog (Rana esculenta). Modulation of apical membrane properties by antidiuretic hormone

Sodium transport and apical bioelectrical membrane properties were investigated in frog colonic epithelium in the absence and presence of the antidiuretic hormone arginine-vasotocin (AVT). Apical Na-permeability and intracellular Na-activity were evaluated by analysis of current-voltage relationships in the serosally K-depolarized tissue. Tissue- and apical membrane capacitance were measured by voltages step analysis. The frog colon was found to be a tight epithelium with a transepithelial resistance of 2.63 +/- 0.25 k omega.muF (n = 17). 85-90% of short circuit current (11.2 +/- 1.1 microA.microF.l-1; n = 17) was related to electrogenic Na-transport from mucosa to serosa. Graded doses of amiloride (less than 50 mumol.l-1) induced Michaelis-Menten-type inhibition kinetics. Serosal addition of 10(-6) mol.l-1 AVT induced a significant increase in sodium current (25%), apical sodium permeability (19%) and tissue capacitance (4.3%) whereas intracellular Na-activity remained unchanged. There was a good correlation between increased Na-current and apical Na-permeability. No correlation was found between Na-current and membrane capacitance. Our results demonstrate that in contrast to other species the amphibian colon shows a natriferic reaction to AVT. We suggest that the regulation of Na-transport in frog colon is similar to that in the toad urinary bladder. It is caused by an activation of preexisting apical Na-channels and not by fusion of subapical cytoplasmic vesicles with the apical membrane.

Relationship of transmural electrical parameters to the luminal Na concentration in the colon of the fowl (Gallus domesticus)

Electrical parameters: PD, resistance, Isc and amiloride-sensitive-Isc across the fowl colon (in vitro) change in response to the Na content of the diet. On a low-Na diet these changes appear to reflect increases in ion transport, especially amiloride-inhibitable Na transport. In vitro the magnitudes of the changes are related to the Na concentration in the luminal (mucosal) fluid and in birds on a low-Na diet peak at a concentration of about 12.5 mM. Such Na concentrations are similar to those in the colonic fluid of Na-deprived birds. Typical Michaelis-Menten kinetics do not appear to apply, possibly reflecting a local adaptation of the ion transport process in response to its external Na concentration.

Effect of chronic hyperaldosteronism on the electrophysiology of rat distal colon

Microelectrodes have been used to study the effects of aldosterone on the barriers and forces controlling sodium and potassium transport in rat distal colon. Compared to control tissues, hyperaldosteronism induced by dietary sodium depletion resulted in a 7-fold increase in transepithelial voltage (VT) and a 52% decrease in total resistance (RT). Increased VT reflected both a rise in the basolateral membrane voltage (Vbl) and a fall in the apical membrane voltage (VA). RT was resolved into its separate membrane components using nystatin (585 U X ml-1), and the decrease in RT produced by aldosterone was found to be due entirely to a 66% decrease in the apical membrane resistance (RA). Amiloride had no effect on the control tissues, but restored VT, Vbl and VA in tissues from sodium deprived animals to control values. Amiloride also increased RT in the experimental tissue, but the post-amiloride values remained significantly lower than those in controls. These results indicate, therefore, that hyperaldosteronism results in an increase in VT by hyperpolarizing the basolateral membrane, as well as depolarizing apical membrane in rat distal colon. The fall in RT, however, is due only to a fall in RA since Rbl and junctional resistance (Rj) were unaffected. The data are consistent with the concept that aldosterone acts to stimulate sodium absorption by increasing the rate of cell entry of sodium, through the induction of amiloride-sensitive sodium channels in the apical membrane, and enhances the rate of potassium secretion by increasing the electrical driving force towards the mucosal solution.

Sodium uptake across the apical border of the isolated turtle colon: confirmation of the two-barrier model

The initial rate of Na uptake by the turtle colon from the mucosal bathing solution consists of two operationally distinct components. One component is a linear function of mucosal Na concentration, is unaffected by amiloride, and appears to represent Na uptake into the paracellular shunt path. The major component of Na uptake is abolished by amiloride and is virtually equal to the short-circuit current over a wide range of mucosal Na concentrations, suggesting that this portion of Na uptake represents Na movement into Na-transporting cells of the colon. The amiloride-sensitive component of Na uptake, at low mucosal Na concentrations, was unaffected if net Na transport was abolished by ouabain. Similarly, at low mucosal Na concentrations the amiloride-sensitive conductance of the colon was identical in the presence and in the absence of net Na transport. These results show that the isolated turtle colon behaves as two distinct barriers to transmural Na transport, an apical barrier blocked by amiloride and a more basal-lying barrier where active, transmural Na transport is blocked by ouabain. In addition, these experiments appear to provide the first unambiguous demonstration that the initial-rate isotope uptake technique can provide a direct measure of the properties of the amiloride-sensitive barrier to transmural Na movement, presumably the apical membranes of the Na-transporting cells. The results are consistent with the notion that the rate of transmural active Na transport and the conductance of the active Na-transport path are determined by the properties of the apical membrane.

Cation selectivity of the apical membrane of the turtle colon: sodium entry in the presence of lithium

Exposure of the apical surface of the isolated turtle colon to Li produced a marked transient in short-circuit current (ISC) and total tissue conductance (GT) which was abolished by amiloride but was unaffected by ouabain or by removing Na or Cl from the mucosal bathing solution. Despite marked changes in Isc, Na uptake across the apical membrane was a linear function of time during exposure to Li-containing solutions, and except at very high Li concentrations, the initial rate of Na uptake, JiNa, was identical to its pre-Li value. In the presence of Li, however, JiNa was significantly less than the total Isc. The apparent "transference number" for Na in the apical membranes was a function of the Li:Na concentration ratio in the mucosal bathing solution. These results suggest that Li can carry substantial amounts of current through amiloride-sensitive channels in the apical membrane of the colon without having any effect on the rate coefficient for Na entry. This behavior is not consistent with "competition" of Na and Li for a membrane "carrier" but rather suggests that the Na entry mechanism may be a population of pores or channels through which Na and Li may pass with negligible interaction.

Ion transport by rabbit colon: II. Unidirectional sodium influx and the effects of amphotericin B and amiloride

The unidirectional influx of Na from the mucosal solution into the epithelium of in vitro descending rabbit colon (JName) determined under short-circuit conditions, is comprised of two components: one represents entry of Na into transporting epithelial cells and is abolished by amiloride which also abolishes Na absorption (JNanet). The other represents diffusional Na entry into paracellular pathways traversing the epithelium. In all instances, exposure of the mucosal surface to amphotericin B increased tissue conductance and JName and elicited K secretion. Tissues showing a spontaneous Isc of approximately 4 microneq/cm2hr did not respond to amphotericin B with increased Isc and JNanet. However, in tissues characterized by a lower Isc under control conditions, amphotericin B increased Isc and JNanet to approximately 4 microneq/cm2 hr. These findings suggest that amphotericin increases JNanet and elicits K secretion by disrupting the normal perm-selectivity of the mucosal membrane. Under these conditions the extrusion of Na from cell-to-serosal solution becomes the rate limiting step in transepithelial Na transport. Finally, a close correlation between JName and JNanet was was observed when the rate of Na absorption varied either spontaneously or experimentally with amiloride, suggesting that the backflux of Na from cell-to-mucosal solution is undetectably small.

Effect of aldosterone on ion transport by rabbit colon in vitro

Segments of descending colon obtained from rabbits, that had been maintained on drinking water containing 25 mM NaCl and an artificial diet which contains 1% Na and is nominally K-free, respond to aldosterone in vitro (after a 30 to 60-min lag period) with a marked increase in the short-circuit current (Isc), an equivalent increase in the rate of active Na absorption (JNa net) and a decline in tissue resistance (Rt). Aldosterone also brings about a marked increase in the unidirection influx of Na into the cells across the mucosal membrane ("zero-time" rate of uptake) which does not differ significantly from the inrease m Isc. Treatment of control tissues with amphotericin B brings about sustained increases in Isc and JNa net to levels observed in aldosterone-treated tissues. However, addition of amphotericin B to the mucosal solution of aldosterone-treated tissues does not result in a sustained increase in Isc or JNa net and these values do not differ markedly from those observed in control tissues treated with amphotericin B. These findings, together with other evidence that Na entry in the presence of amphotericin B is sufficiently rapid to saturate the active Na extrusion mechanism at the baso-lateral membrane, are consistent with the notion that the aldosterone-induced protein increases the permeability of the mucosal membrane to Na but does not increase the "saturation level" of the active Na "pump" within the time-frame of these studies (3 hr). Finally, aldosterone has no effect on the bidirectional or net transepithelial movements of K under short-circuit conditions, suggesting that the enhanced secretion of K observed in vivo is the result of increased diffusion of K from plasma to lumen via paracellular pathways in response to an increased transepithelial electrical potential difference (lumen negative).