Removal of ambient K+ inhibits net Na+ transport in toad bladder by reducing Na+ permeability of the luminal border

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.

Potassium dependence of sodium transport in frog skin

22Na+ and 42K+ fluxes across the basolateral membrane of the isolated epithelium of frog skin were investigated with regard to dependence on K+ in the basolateral solution. When K+ was removed from the basolateral solution (K+-free Ringer), there was a transient rise in short circuit current (Isc) that could be eliminated by pretreatment with ouabain. Concurrently, the apparent sodium efflux across the basolateral membrane (JNa*13) showed either no change or an immediate (1-2 min) small decrease (approximately equal to 10%) that was followed by a small transient increase. K+ fluxes showed either no change or a small decrease under these conditions. JNa*13 was partially ouabain sensitive during all of the above treatments. Furosemide partially inhibited both sodium and potassium flux after K+-free treatment. The pump, as defined by ouabain sensitivity of Na+ flux, continued to work even after 20 minutes of K+-free treatment. Pump activity may be maintained by potassium leaking from the cells that is recycled by the pump. However, the ouabain-sensitive transient rise in Isc after K+-free treatment cannot readily be explained by changes in either Na+ or K+ flux. A change in pump coupling ratio provides one explanation for these data.

Interaction between the basolateral K+ and apical Na+ conductances in Necturus urinary bladder

Experimental modulation of the apical membrane Na+ conductance or basolateral membrane Na+-K+ pump activity has been shown to result in parallel changes in the basolateral K+ conductance in a number of epithelia. To determine whether modulation of the basolateral K+ conductance would result in parallel changes in apical Na+ conductance and basolateral pump activity, Necturus urinary bladders stripped of serosal muscle and connective tissue were impaled through their basolateral membranes with microelectrodes in experiments that allowed rapid serosal solution changes. Exposure of the basolateral membrane to the K+ channel blockers Ba2+ (0.5 mM/liter), Cs+ (10 mM/liter), or Rb+ (10 mM/liter) increased the basolateral resistance (Rb) by greater than 75% in each case. The increases in Rb were accompanied simultaneously by significant increases in apical resistance (Ra) of greater than 20% and decreases in transepithelial Na+ transport. The increases in Ra, measured as slope resistances, cannot be attributed to nonlinearity of the I-V relationship of the apical membrane, since the measured cell membrane potentials with the K+ channel blockers present were not significantly different from those resulting from increasing serosal K+, a maneuver that did not affect Ra. Thus, blocking the K+ conductance causes a reduction in net Na+ transport by reducing K+ exit from the cell and simultaneously reducing Na+ entry into the cell. Close correlations between the calculated short-circuit current and the apical and basolateral conductances were preserved after the basolateral K+ conductance pathways had been blocked. Thus, the interaction between the basolateral and apical conductances revealed by blocking the basolateral K+ channels is part of a network of feedback relationships that normally serves to maintain cellular homeostasis during changes in the rate of transepithelial Na+ transport.

Microelectrode study of K+ accumulation by tight epithelia: II. Effect of inhibiting transepithelial Na+ transport on reaccumulation following depletion

The effects of restoring serosal potassium to potassium-depleted toad urinary bladders have been re-examined using double-barrelled microelectrodes. The data confirm the existence of a time-lag phenomenon, a dissociation between potassium reaccumulation and restoration of short-circuit current. Returning serosal potassium stimulates an increase in intracellular potassium activity 21-26 min before any increase can be detected in short-circuit current. The reaccumulation of potassium has been further studied using split frog skin, a far more suitable preparation for electrophysiologic study than toad bladder. Under baseline short-circuited conditions, potassium is accumulated against an electrochemical gradient of 22 +/- 4 mV. Reaccumulation of potassium by potassium-depleted tissues can be blocked by inhibiting the Na,K-exchange pump with high concentrations of ouabain. On the other hand, blocking apical sodium entry by the addition of 10(-4) M amiloride to the outer bathing medium does not interfere with reaccumulation of potassium. The data support the concept that the time-lag phenomenon of toad bladder reflects stimulation of potassium reaccumulation by the sodium pump in exchange for the extrusion of excess cell sodium collected during the period of potassium depletion. This reaccumulation of potassium can proceed before the entry of significant added amounts of sodium across the apical plasma membrane.

Calcium reduces the sodium permeability of luminal membrane vesicles from toad bladder. Studies using a fast-reaction apparatus

Regulation of the sodium permeability of the luminal membrane is the major mechanism by which the net rate of sodium transport across tight epithelia is varied. Previous evidence has suggested that the permeability of the luminal membrane might be regulated by changes in intracellular sodium or calcium activities. To test this directly, we isolated a fraction of the plasma membrane from the toad urinary bladder, which contains a fast, amiloride-sensitive sodium flux with characteristics similar to those of the native luminal membrane. Using a flow-quench apparatus to measure the initial rate of sodium efflux from these vesicles in the millisecond time range, we have demonstrated that the isotope exchange permeability of these vesicles is very sensitive to calcium. Calcium reduces the sodium permeability, and the half-maximal inhibitory concentration is 0.5 microM, well within the range of calcium activity found in cells. Also, the permeability of the luminal membrane vesicles is little affected by the ambient sodium concentration. These results, when taken together with studies on whole tissue, suggest that cell calcium may be an important regulator of transepithelial sodium transport by its effect on luminal sodium permeability. The effect of cell sodium on permeability may be mediated by calcium rather than by sodium itself.

Control of sodium permeability of the outer barrier in toad skin

The 24Na efflux (JNaeff) (i.e., the rate of appearance of 24Na in the outer compartment) in the isolated short-circuited toad skin bathed by NaCl-Ringer's solution on both sides is composed of para- and transcellular components of almost equal magnitudes. This relies on the assumption that amiloride acts on the transcellular component only and could block it completely. Ouabain induces a large transient increase of the transcellular component. This increase, which starts within a few minutes after the addition of ouabain, is due to electrical depolarization of the outer barrier, rather than a consequence of blocking Na recirculation across the inner barrier. The subsequent decline of JNaeff, which takes place after the ouabain-induced JNaeff peak, is due to a progressive block of outer barrier Na channels with time, which can eventually be complete, depending on the duration of action of ouabain. As the external Na concentration was always kept high and constant in these experiments, the results indicate that a rise in cell Na concentration, and not in the outer bathing solution, is the signal that triggers the reduction of outer barrier Na permeability (PNao). Ouabain has no effect upon JNaeff with Na-free solution bathing the outer and NaCl-Ringer's solution the inner skin surface, showing the importance of Na penetration across the outer barrier, and not across the inner barrier due to its low Na permeability, in the process of closing the Na channels of this structure. Step changes from Na 115 mM to Na-free external solution, or vice-versa, may affect both the outer barrier electrical potential difference (PDo) and cell Na concentration (Na)c. Therefore, the behavior of JNaeff depends on which variable (if PDo or (Na)c regulated outer barrier Na permeability) is most affected by step changes in outer bathing solution Na concentration. Amiloride in the control condition blocks the transcellular component of JNaeff. However, in the condition of approximate short-circuiting of the outer barrier and high cellular Na concentration induced by long term effects of ouabain, when the Na channels of the outer barrier are already blocked by elevated cell Na concentration, amiloride may induce the opposite effect, increasing Na permeability of the outer barrier. With outer barrier Na channels completely blocked by high cell Na concentration, PCMB in the outer bathing medium induces a large increase of JNaeff, rendering these channels again amiloride sensitive. The results are consistent with the notion that Na efflux from cell compartment to the outer bathing solution goes through the amiloride-sensitive Na channels of the apical border of the superficial cell layer of toad skin, with an apparent Na permeability modulated by cell ionic environment, most probably the cell Na concentration.

Regulation of the sodium permeability of the luminal border of toad bladder by intracellular sodium and calcium: role of sodium-calcium exchange in the basolateral membrane

Sodium movement across the luminal membrane of the toad bladder is the rate-limiting step for active transepithelial transport. Recent studies suggest that changes in intracellular sodium regulate the Na permeability of the luminal border, either directly or indirectly via increases in cell calcium induced by the high intracellular sodium. To test these proposals, we measured Na movement across the luminal membrane (th Na influx) and found that it is reduced when intracellular Na is increased by ouabain or by removal of external potassium. Removal of serosal sodium also reduced the influx, suggesting that the Na gradient across the serosal border rather than the cell Na concentration is the critical factor. Because in tissues such as muscle and nerve a steep transmembrane sodium gradient is necessary to maintain low cytosolic calcium, it is possible that a reduction in the sodium gradient in the toad bladder reduces luminal permeability by increasing the cell calcium activity. We found that the inhibition of the influx by ouabain or low serosal Na was prevented, in part, by removal of serosal calcium. To test for the existence of a sodium-calcium exchanger, we studied calcium transport in isolated basolateral membrane vesicles and found that calcium uptake was proportional to the outward directed sodium gradient. Uptake was not the result of a sodium diffusion potential. Calcium efflux from preloaded vesicles was accelerated by an inward directed sodium gradient. Preliminary kinetic analysis showed that the sodium gradient changes the Vmax but not the Km of calcium transport. These results suggest that the effect of intracellular sodium on the luminal sodium permeability is due to changes in intracellular calcium.