Pathways of ion movement in the canine tracheal epithelium

The pathways of ion movement across canine tracheal epithelium, a Cl-secreting tissue, were examined by three techniques. First, the measurement of simultaneous, unidirectional fluxes of Na or Cl and mannitol, a large hydrophilic molecule that serves as a marker of the paracellular pathway, indicated that a significant fraction of both the Na flux from submucosa to mucosa (J Na sm) and the flux of Cl from mucosa to submucosa (J Cl ms) traverse the cellular pathway. The ratio of the Na-to-Cl diffusion coefficients through the paracellular pathway was 0.23, in contrast to the free solution ratio of 0.63. Second, in voltage-clamp experiments we examined the effect of transepithelial voltage differences on the unidirectional fluxes of Na and Cl. The results agree with the previous findings, suggesting that there are voltage-independent, or transcellular, backfluxes of Na and Cl, and that the relative permeability of Na to Cl through the voltage-dependent (presumably paracellular) pathway was 0.28. Third, measurement of transepithelial diffusion potentials gave a Na-to-Cl permeability ratio of 0.31 +/- 0.02 (mean +/- SE). These results suggest that there are significant transcellular backfluxes of Na and Cl and that the paracellular pathway in the canine trachea is anion selective. An anion-selective pathway would tend to shunt the secreted Cl back through the paracellular pathway, thus minimizing the net ion and fluid movement across the tissue in the open-circuit condition.

Anion selectivity of the chloride-transport process in dog tracheal epithelium

We have measured the fluxes of 82Br or 131I across short-circuited dog tracheal epithelium following complete replacement of chloride in the bathing medium by either bromide or iodide. In addition, we have partly replaced the bathing chloride with bromide, iodide, or thiocyanate and studied the effects of these anions on 36Cl fluxes. Our results show that the affinity sequence for the chloride transport process is I > Br congruent to Cl, that Br is transported, but that I is not. Thiocyanate is a potent inhibitor of chloride transport, and its effects are greater than those predicted by a simple competitive interaction with chloride for transport sites.

A possible role of linked Na and Cl movement in active Cl uptake in smooth muscle

Smooth muscles actively accumulate Cl. Abolition of the transmembrane Na gradient has been shown to lead to a passive distribution of Cl. This suggested to us that Na and Cl may move together across the smooth muscle cell membrane, Cl accumulation resulting due to [Na]i being less than [Na]o. In favour of this view are the findings that Cl uptake is reduced in Na-free medium, Na uptake is reduced in Cl-free medium, reducing [Cl]i reduces Na efflux, and increasing [Na]i increases Cl efflux.

The uptake and efflux of alpha-aminoisobutyric acid by the smooth muscle of the guinea pig's taenia coli

The uptake of α-aminoisobutyric acid (AIB) by guinea pig taenia coli was linear with time over a 5-h period. There was a saturable component with Km of 0.7 mM and a maximal value of 4.75 mmol/kg tissue per hour. A second nonsaturable component of uptake became important at high concentrations of AIB (> 5 mM). Preincubation in K+-free or Na+-free (high-K+) media greatly reduced uptake. The dependence of AIB uptake on outside [Na+] ([Na+]0) was approximately linear over the range 0–140 mM. With [Na+]0 equal to 25 mM the uptakes with various substitutes were in the order sucrose > Mg2+ > choline+ > Li+ > Cs+ > Rb+ = K+. Addition of ouabain or removal of K+ during efflux of AIB led to marked increases in the efflux rate, ouabain being the more effective treatment. Changes in tissue Na+ and K+ levels were found to be slightly greater with ouabain than with K+-free medium. Lanthanum (5 mM) prevented the uptake of Na+ seen in K+-free medium and also abolished the increase in the rate of loss of AIB. It also reduced the rise in tissue Na+ produced by ouabain; there was a corresponding reduction in the ouabain-induced increase in the rate of loss of AIB. It is concluded that both the influx and efflux of AIB in this tissue are Na+ dependent, and that the accumulation of AIB relies on the transmembrane Na+ gradient.

Localization of Na pumps in the tracheal epithelium of the dog

Binding of [3H]ouabain by the dog's tracheal epithelium shows a nonspecific component depending linearly on ouabain concentration, and a specific saturable component with a Km of 10(-7) M. Control experiments showed that the tracer taken up was not trapped within the extracellular space nor bound to tissue collagen. Inhibition of the saturable uptake by high K, metabolic inhibition, low Na, and low temperature indicated that binding was to Na/K ATPase. One-sided exposures of tissue sheets to tracer showed that the submucosal side took up 10 X as much tracer as the luminal. Autoradiography localized tracer uptake under all conditions to the cells' basolateral membranes.

The effects of sodium substitution and ouabain on ion transport by dog tracheal epithelium

Using ouabain and sodium removal, we studied the mechanism of chloride transport by dog tracheal epithelium. When applied to the submucosal side of the tissue, 2 x 10(-4) M ouabain decreased the short-circuit current, the net chloride flux toward the lumen, and the net sodium flux toward the submucosa to zero after 30 min. Applied to the luminal side, ouabain had little effect. Potassium-free medium, like ouabain, decreased the short-circuit current when present in the submucosal bath; on the luminal side ti produced a slight increase (7.0 +/- 1.5 per cent; n = 6) in short-circuit current. Replacement of sodium in the submucosal bathing medium by choline led to a decrease in net chloride flux of 84 per cent. Sodium replacement in the luminal bath produced no change in net chloride flux. It is proposed that chloride secretion by this tissue depends on active basolateral sodium pumps, and that a component of chloride entry into the transporting cells from the submucosal medium may be sodium-linked.

Fine structure and innervation of smooth muscle in the monkey oviduct

The smooth muscle of the monkey oviduct is similar to that described for other species and organs. Cell-to-cell contacts, in the form of simple appositions, were numerous. The adrenergic nerve supply was sparse, and close contacts between nerve and muscle were extremely rare. There was no difference in the density of innervation between the isthmus and ampulla. The rabbit appeared to have a greater denisty of innervation than the monkey.

Ouabain-sensitive ion fluxes in the smooth muscle of the guinea-pig's taenia coli

1. Tissues with raised intracellular Na levels, produced by incubation in K-free media, were used throughout. The uptake of 42K by these Na-loaded tissues was followed for 10 min in the presence and absence of 1-37 X 10(-4) M ouabain, this being sufficient to inhibit Na pumping maximally. Subtraction of the uptake seen in the presence from that seen in the absence of ouabain gave estimates of the pumped ouabain-sensitive K uptake. 2. In Na-free (MgCl2) medium this depended on the [K]0 in a sigmoidal fashion with a half maximal [K]0 for activation of some 4mM. The maximal uptake of K was 3 m-mole/kg.min corresponding to a transmembrane flux of some 12-5 p-mole. cm-2.sec-1. 3. In the presence of Na the K activation curve became more obviously sigmoid and higher concentrations of K were needed to achieve a given active K influx. The results were well fitted by assuming that Na and K competed for two identical, non-interacting sites on the external pump face. 4. Addition of K during the efflux of 24Na into a Na-free (MgCl2) medium led to an increased rate of tracer loss. The magnitude of this increase depended on the [K] used in a hyperbolic fashion and it was abolished by addition of ouabain. The [K] causing half-maximal activation of ouabain-sensitive Na efflux was in the order of 1-2 mM. 5. When the [K] in the uptake media was 1-5 mM; Na, Li, Rb and Cs all inhibited ouabain-sensitive K uptake, the order of effectiveness being Rb greater than Cs greater than Na greater than Li. With a E1TKA10 OF 0-15 MM low concentrations of Cs and Rb were shown to stimulate K uptake. Such an effect is predicted by assuming two ion binding sites on the pump's outer face, and that the pump can translocate mixtures of K and either Rb or Cs...

The use of lanthanum to estimate the numbers of extracellular cation-exchanging sites in the guinea-pig's taenia coli, and its effects on transmembrane monovalent ion movements

1. Tissues were allowed to equilibrate in a Tris-buffered Krebs solution and were then exposed to similar solutions containing up to 5 mM-La. La caused shrinkage and significant losses of tissue K, Na, Mg and Ca. The shrinkage was exactly accountable for by a reduction in the extracellular space (e.c.s.) as measured by [14C]sucrose. No significant change was seen in tissue Cl content. Exposure for 1 hr to 5 mM-La led to a total cation loss of 24-3 +/- 1-6 m-equiv/kg or, correcting for the small change in Cl content, a loss of positive charge of 23-8 +/- 2-2 m-equiv/kg fresh wt. 2. Using the radioisotope 140La it was shown that this loss of cation was balanced by an uptake of La3+. 3. Subtraction of the ions in the measured [14C]sucrose space from the total tissue ion contents led to estimates of the "cellular" ion contents. The effects of 1 hr exposure to 5 mM-La on these were a loss of 12-9 +/- 2-4 m-equiv/kg of cation and a gain of 10-0 +/- 2-6 m-equiv/kg of Cl. 4. Similar changes in ion content were produced by La on "Na-loaded" and "K-loaded" tissues, these being tissues which by exposure to K-free or Na-free (high K) solutions had replaced all their K with Na or vice versa. 5. The uptakes of 24Na and 36Cl by Na-loaded tissues were both describable as the sum of two exponentail processes: a fast component (t 1/2 congruent to 1/2 min), which was presumed to be extracellular and a slower, presumed transmembrane, component. La reduced the rapid component of uptake of 24Na by an amount greater than that predicted by the reduction in the e.c.s., the extra amount lost being some 10--15 m-equiv/kg. La also reduced the amount of rapidly exchanging 36Cl, bwt this reduction was entirely accounted for by the change in the e.c.s. La reduced the rate constant of the slow component of 24Na uptake. 6. La reduced the rapidly exchanging component of 42K uptake by normal tissues by an amount equivalent to about 0-5 m-mole/kg fresh wt. of K in excess of the change in the extracellular space. 7. La had little effect on the effluxes of 36Cl and 42K from normal tissues. However, it reduced the size of the fastest component of exchange of 42K efflux from K-loaded tissues by an amount equal to some 10-15 m-equiv/kg in excess of the reduction in the e.c.s. A similar reduction in the rapidly exchanging component of 24Na efflux from normal tissues was also seen. La slowed the efflux of 24Na from Na-loaded tissues at times when the tracer lost could safely be regarded as intracellular. 8. The taenia coli when exposed to K-free solutions gains Na and loses K. In the presence of La the gain in Na was completely blocked. K was still lost, however, being accompanied by Cl and increased shrinkage. La also prevented the uptake of Na from high Na media by ion-depleted tissues (produced by exposure to sucrose media), while having little effect on the uptake of K from high K media by such tissues. 9 Tedia by such tissues. 9. The cation displaced by La in excess of that lost due to the reduction of the e.c.s...

An estimate of sodium-potassium pump activity and the number of pump sites in the smooth muscle of the guinea-pig taenia coli, using (3H)ouabain

1. Binding of tritiated ouabain to the smooth muscle of the guinea-pig taenia coli showed two components, one saturable at lower glycoside concentrations and the other linear with increasing concentrations.2. The saturable component alone was affected by extracellular potassium concentrations. This component seems to be bound to sodium pumping sites, and when completely saturated binds 1.1 x 10(11) molecules per mg fresh wt. of tissue, or 250-300 molecules per square micron of membrane, assuming a volume:surface area ratio of 1.5 mum.3. Only a fraction of (42)K uptake by the cells can be blocked by ouabain at maximal concentrations. In normal Krebs solution two thirds can be blocked. The remaining one third is presumably passive uptake. The fraction blocked is reduced as the extracellular potassium concentration, and thus passive uptake, is increased.4. The amount of potassium pumped into the cells at various concentrations of extracellular potassium has been calculated. In normal Krebs solution the amount pumped in 45 min was 20.0 m-mole/kg fresh wt., and this was increased at higher potassium concentrations.5. On the assumption of a stoichiometry of 3Na: 2K, the pump sites in normal Krebs solution have a turnover rate of 1320 min(-1).6. Indirect calculations of sodium movements suggest that the sodium permeability may be about 0.9 x 10(-8) cm sec(-1) and the pump may generate a current of 0.9 x 10(-7) A cm(-2). This crossing an Ohmic membrane resistance of 30-60 kOmega cm(2) would be equivalent to a potential difference of 3-5 mV.