Aldosterone does not stimulate the Na:K pump in isolated turtle colon

Basolateral ion transporters involved in colonic epithelial electrolyte absorption, anion secretion and cellular homeostasis

Electrolyte transporters located in the basolateral membrane of the colonic epithelium are increasingly appreciated as elaborately regulated components of specific transport functions and cellular homeostasis: During electrolyte absorption, Na(+) /K(+) ATPase, Cl⁻ conductance, Cl⁻/HCO₃⁻ exchange, K(+) /Cl⁻ cotransport and K(+) channels are candidates for basolateral Na(+) , Cl⁻ and K(+) extrusion. The process of colonic anion secretion involves basolateral Na(+) /K(+) /2Cl⁻ , and probably also Na(+) /HCO₃⁻ cotransport, as well as Na(+) /K(+) ATPase and K(+) channels to supply substrate, stabilize the membrane potential and generate driving force respectively. Together with a multitude of additional transport systems, Na(+) /H(+) exchange and Na(+) /HCO₃⁻ cotransport have been implicated in colonocyte pH(i) and volume homeostasis. The purpose of this article is to summarize recently gathered information on the molecular identity, function and regulation of the involved basolateral transport systems in native tissue. Furthermore, we discuss how these findings can help to integrate these systems into the transport function and the cellular homoeostasis of colonic epithelial cells. Finally, disturbances of basolateral electrolyte transport during disease states such as mucosal inflammation will be reviewed.

Basolateral K conductance: role in regulation of NaCl absorption and secretion

In this review we explore the possible role of basolateral K conductance (gK) in the regulation of salt absorption and secretion. This inquiry is prompted by a growing body of evidence which, taken together, suggests that basolateral gK is very labile and that alterations in basolateral gK may be a key feature in both stimulatory and inhibitory regulatory mechanisms. We first consider the role of basolateral gK in relation to models for salt absorption and secretion, particularly in relation to the maintenance of cellular charge balance and the obligatory coupling between the apical and basolateral membranes that is produced by transcellular current flow. Next, we review some of the experimental evidence that suggests that changes in basolateral gK are associated with transport regulation. The cellular mechanisms that are known to impact on K channel regulation are considered in a general way, and finally, we consider the use of integrated models for understanding possible coordinate regulation of apical and basolateral cell membranes.

Effects of corticosteroid hormones on the electrophysiology of rat distal colon: implications for Na+ and K+ transport

1. Conventional microelectrodes, the Na+ channel blocker amiloride (0.1 mM), and the K+ channel blocker tetraethylammonium chloride (TEA, 30 mM) were used to examine the effects of corticosteroid hormones administered in vivo on the Na+ and K+ transport properties of isolated rat distal colon. The cell membrane changes induced by aldosterone (a specific mineralocorticoid), RU 28362 (a synthetic glucocorticoid with negligible affinity for mineralocorticoid receptors), and dexamethasone (an activator of both mineralocorticoid and glucocorticoid receptors) were compared. 2. In control animals, there was no amiloride-sensitive apical Na+ conductance, and only a relatively small TEA-sensitive apical K+ conductance. 3. Hyperaldosteronism secondary to dietary Na+ depletion for 10-14 days, dexamethasone (600 micrograms 100 g-1 day-1 for 3 days), and RU 28362 (600 micrograms 100 g-1 day-1 for 3 days) induced amiloride-sensitive electrogenic Na+ transport, with the potency of aldosterone greater than dexamethasone greater than RU 28362. 4. With each corticosteroid, increased electrogenic Na+ transport reflected enhanced apical Na+ conductance, and in the case of aldosterone and dexamethasone, 3.3-fold and 2-fold increases respectively in the maximum activity of the basolateral Na+-K+ pump. In contrast, RU 28362 suppressed the maximum activity of the basolateral Na+-K+ pump by 45%. 5. All three corticosteroids enhanced the K+ conductance of the apical membrane, with the potency of aldosterone greater than dexamethasone greater than RU 28362. 6. Co-administration of spironolactone (5 mg 100 g-1 day-1) inhibited the effects of aldosterone on Na+ and K+ transport, but in dexamethasone-treated animals spironolactone resulted in a pattern of response similar to that found in RU 28362-treated animals. 7. The results support the view that mineralocorticoid receptors mediate changes in colonic Na+ and K+ transport which differ quantitatively and qualitatively from those mediated by glucocorticoid receptors. Dexamethasone and similar 'glucocorticoids' activate both types of receptor, with an overall epithelial response which mimics that induced by aldosterone.

Sodium pump quantity and turnover in rabbit descending colon at different rates of sodium absorption

3H-Ouabain binding to isolated epithelia and basolateral membrane vesicles of Na+-transporting epithelial cells of rabbit descending colon was determined to quantify the number of operative Na+-pump sites at different rates of transcellular Na+ transport which was varied over a wide range by chronic dietary Na+ restriction or Na+ loading. Both in intact epithelia and in basolateral membrane vesicles the maximal number of specific ouabain binding sites was higher in preparations from animals transporting Na+ at high rates than in preparations from animals transporting Na+ at low rates. The affinity of ouabain to its binding site and the association and dissociation rate constants were not dependent on the rate of Na+ transport. In intact epithelia the Na+ turnover rate per pump unit was twice as high in tissues with high Na+ transport than in tissues with low Na+ transport. In basolateral membrane vesicles the Na+ turnover rate was considerably higher than in intact epithelia and there was no difference in turnover rate between vesicle preparations obtained from tissues transporting Na+ at high or low rates. Hence, factors within the intact cell appear to control the turnover rate of the Na+-pump.

Differential effects of aldosterone and ADH on intracellular electrolytes in the toad urinary bladder epithelium

Quantitative electron microprobe analysis was employed to compare the effects of aldosterone and ADH on the intracellular electrolyte concentrations in the toad urinary bladder epithelium. The measurements were performed on thin freeze-dried cryosections utilizing energy dispersive x-ray microanalysis. After aldosterone, a statistically significant increase in the intracellular Na concentration was detectable in 8 out of 9 experiments. The mean Na concentration of granular cells increased from 8.9 +/- 1.3 to 13.2 +/- 2.2 mmol/kg wet wt. A significantly larger Na increase was observed after an equivalent stimulation of transepithelial Na transport by ADH. On average, the Na concentration in granular cells increased from 12.0 +/- 2.3 to 31.4 +/- 9.3 mmol/kg wet wt (5 experiments). We conclude from these results that aldosterone, in addition to its stimulatory effect on the apical Na influx, also exerts a stimulatory effect on the Na pump. Based on a significant reduction in the Cl concentration of granular cells, we discuss the possibility that the stimulation of the pump is mediated by an aldosterone-induced alkalinization. Similar though less pronounced concentration changes were observed in basal cells, suggesting that this cell type also participates in transepithelial Na transport. Measurements in mitochondria-rich cells provided no consistent results.

The role of aldosterone in water and electrolyte transport across the colonic epithelium of the lizard, Gallotia galloti

1. The effects of D-aldosterone on the electrical properties and ionic transport have been analysed (in vitro) in the colonic epithelium of the lizard, Gallotia galloti. 2. The injection of 30 g of D-aldosterone induced a significant rise of plasma aldosterone concentration, sustained for 4 hr after administration. 3. Intraperitoneal injections of D-aldosterone caused a slight reduction of transmural potential difference (PD) and short-circuit current (Isc). 4. Net absorption of Na+ across the colon from aldosterone-treated lizards did not increase when compared to colon from normal lizards. Cl- absorption was increased in treated lizards. 5. Addition of mucosal barium to treated tissues brought about a sustained increase of PD and Isc. 6. The absorption of fluid in normal lizards was reversed to fluid secretion in treated lizards. 7. The effects of D-aldosterone on lizard colon are compared with those reported for the colon of mammals and birds.

Cholinergic regulation of Na absorption by turtle colon: role of basolateral K conductance

The mechanism underlying the muscarinic inhibition of colonic Na absorption is unknown. In this study the effects of carbachol on active Na transport and basolateral K conductance were compared in the isolated turtle colon. Carbachol produced a biphasic response in both Na transport and basolateral K conductance. The response consisted of a transient activation followed by a sustained inhibition and was blocked by atropine. Submucosal cholinergic neurons were implicated in the regulation of colonic transport by employing depolarizing agents to release endogenous acetylcholine. Depolarizing agents produced a carbachol-like response that was atropine-sensitive. Finally, experiments with the Ca ionophores, A23187 and ionomycin, suggested that the muscarinic response may be mediated, at least in part, by changes in cellular Ca. These experiments provide evidence that cholinergic neurons are present in the turtle colon submucosa, muscarinic agonists cause a change in basolateral K conductance that may be an important event in the regulation of colonic Na absorption, and a Ca second messenger system may be involved in mediating the response.