Transport of NaCl and water was examined in the rabbit medullary thick ascending limb of Henle (ALH) by perfusing isolated segments of these nephrons in vitro. Osmotic water permeability was evaluated by perfusing tubules against imposed osmotic gradients. In these experiments the net transport of fluid remained at zero when segments of thick ALH were perfused with isotonic ultrafiltrate in a bath of rabbit serum in which the serum osmolality was increased by the addition of either 239+/-8 mosmol/liter of raffinose or 232+/-17 mosmol of NaCl indicating that the thick ascending limb of Henle is impermeant to osmotic flow of water. When these tubules were perfused at slow rates with isosmolal ultrafiltrate of same rabbit serum as used for the bath, the effluent osmolality was consistently lowered to concentrations less than the perfusate and the bath. That this decrease in collected fluid osmolality represented salt transport was demonstrated in a separate set of experiments in which it was shown that the sodium and chloride concentrations decreased to 0.79+/-0.02 and 0.77+/-0.02 respectively when compared with the perfusion fluid concentrations. In each instance the simultaneously determined transtubular potential difference (PD) revealed the lumen to be positive with the magnitude dependent on the perfusion rate. At flow rates above 2 nl.min(-1), the mean transtubular PD was stable and equal to 6.70+/-0.34 mv. At stop-flow conditions this PD became more positive. Ouabain and cooling reversibly decreased the magnitude of this PD. The transtubular PD remained positive, 3.3+/-0.2 mV, when complete substitution of Na by choline was carried out in both the perfusion fluid and the bathing media. These results are interpreted to indicate that the active transport process is primarily an electrogenic chloride mechanism. The isotopic permeability coefficient for Na was 6.27+/-0.38 x 10(-5) cm.s(-1) indicating that the thick ALH is approximately as permeable to Na as the proximal convoluted tubule. The chloride permeability coefficient for the thick ALH was 1.06+/-0.12 x 10(-5) cm.s(-1) which is significantly less than the chloride permeability of the proximal tubule. These data demonstrate that the medullary thick ascending limb of Henle is water impermeable while having the capacity for active outward solute transport as a consequence of an electrogenic chloride pump. The combination of these characteristics allows this segment to generate a dilute tubular fluid and participate as the principal energy source for the overall operation of the countercurrent multiplication system.

/pdf/sodium-chloride-and-water-transport-in-the-medullary-thick-1z7vlvitqm.pdf

Sodium chloride and water transport in the medullary thick ascending limb of Henle. Evidence for active chloride transport.

https://www.amjmed.com/article/0002-9343(67)90093-9/pdf

The role of adenosine 3′,5′-phosphate in the action of antidiuretic hormone

STONER, L. C., M. B. BURG, AND J. ORLOFF. Ion transport in cortical collecting tubule; effect of amiloride. Am. J. Physiol. 227(2) : 453-459. 1974.-T b 1 u u es were dissected from rabbit kidneys and perfused in vitro. In cortical collecting tubules the transepithelial electrical potential difference (PD) was 35 mV (lumen negative). There was net active sodium reabsorption from the lumen and net active potassium secretion into the lumen. There was net chloride reabsorption that was probably passive, but the analysis of the mechanism of chloride transport was complicated by the presence of chloride exchange diffusion. Amiloride (10-j M) in the lumen inhibited sodium and potassium transport and caused a reversal of the polarity of the PD; 10B4 M amiloride in the bath had no effect. The positive PD was most likely caused by urinary acidification and decreased toward zero when acetazolamide was added to the bath, or when CO2 was eliminated from the bathing solutions. In the thick ascending limb of Henle’s loop the PD was +5.8 mV, which is attributed to active chloride reabsorption. Amiloride had no effect on either the PD or the net chloride transport. We conclude that amiloride inhibits active sodium and potassium transport in the cortical collecting tubule, but does not inhibit active chloride transport in the thick ascending limb, and that these effects account for the action of the drug previously observed in the intact kidney.

Ion transport in cortical collecting tubule; effect of amiloride.

1. Membrane potentials were recorded from single rods in the isolated retina of Bufo marinus while the ionic composition of the extracellular medium was rapidly changed. Substitution of 2 mM aspartate(-) for Cl(-) produced a prompt depolarization of horizontal cells, but no modification of either resting potential or response to light in receptor cells. This implies that feed-back from horizontal cells to receptor cells was not active.2. During substitution of choline(+) or Li(+) for Na(+), and during isosmotic substitution of sucrose for NaCl, the resting potential of receptor cells became more negative and responses to light were abolished. During exposure to K(+)-free medium, the resting potential became slightly more negative and the responses to light became larger and developed small after-depolarizations. Exposure to [K(+)](out) of four times normal resulted in permanent diminution of response magnitude and permanent change of response waveshape. Removal of Mg(2+), four times normal [Mg(2+)](out) or substitution of methylsulphate(-) for Cl(-) had no effect on resting potential or responses to light. With the exception of the small effects seen with altered [K(+)](out) these results are consistent with the receptor potential being generated by a light-induced decrease of membrane conductance to Na(+).3. Exposure to decreased [Ca(2+)](out) caused both a depolarization of the receptor membrane in the dark and an increase in the magnitude of the maximal response that could be evoked by a test stimulus. The magnitude of the increase in response equalled the magnitude of the depolarization. Exposure to increased [Ca(2+)](out) or steady background light caused both a steady hyperpolarization and a decrease in the magnitude of the maximal response that could be evoked by a test stimulus. For steady hyperpolarizations greater than 3.5 mV, whether caused by elevated [Ca(2+)](out) or steady background light, the decrease in response magnitude exceeded the magnitude of the hyperpolarization. These results imply that externally applied Ca(2+) ions mimic the effects of steady background lights, but the applied Ca(2+) ions must do more than merely decrease membrane conductance to Na(+).

Ionic mechanism for the photoreceptor potential of the retina of Bufo marinus

Mucosal hypertonicity, produced by the addition of NaCl, KCl, mannitol, urea, sucrose or raffinose, reduced the electrical resistance of toad urinary bladder and induced bullous deformations (blisters) of the most apical junctions of the mucosal epithelium: the smaller solutes were most effective in eliciting both phenomena. Study of the effect of addition and subsequent removal of mannitol from the mucosal medium indicated that both the electrical and morphologic changes are reversible and follow the same time course. Mucosal hypertonicity induced comparable changes in the tissue in the presence or absence of inhibition of active sodium transport by replacement of sodium by choline, or by addition of ouabain or amiloride. Dilution of the tonicity of the serosal medium similarly reduced the tissue resistance and induced blisters within the epithelium, demonstrating that the osmotic gradient, rather than the mucosal hypertonicity itself is the cause of the osmotically-induced resistance change. The data indicate, therefore, that the osmotic gradient reduces the electrical resistance of the tissue primarily by deforming the apical junctions. The simplest interpretation of the data is that the apical tight junctions are considerably more permeable to water and small solutes than had previously been thought. Addition of solute to the mucosal medium leads to the diffusion of solute into the junctions: the subsequent transfer of water from the lateral intercellular spaces and/or the adjacent cellular cytoplasm, deforms these structures and reduces the resistance to the passage of ions across the tissue. The results suggest that the apical junctions constitute the rate-limiting permeability barrier of the putative parallel shunt pathway across toad bladder.

AL Finn

Papers

Relation between toad bladder potassium content and permeability response to vasopressin

Active chloride transport in the isolated toad bladder