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At least 19 recordsLinked to original sources
PubMed1987

Spontaneous opening at zero membrane potential of sodium channels from eel electroplax reconstituted into lipid vesicles.

The voltage-dependent sodium channel from the eel electroplax was purified and reconstituted into vesicles of varying lipid composition. Isotopic sodium uptake experiments were conducted with vesicles at zero membrane potential, using veratridine to activate channels and tetrodotoxin to block them. Under these conditions, channel-dependent uptake of isotopic sodium by the vesicles was observed, demonstrating that a certain fraction of the reconstituted protein was capable of mediating ion fluxes. In addition, vesicles untreated with veratridine showed significant background uptake of sodium; a considerable proportion of this flux was blocked by tetrodotoxin. Thus these measurements showed that a significant subpopulation of channels was present that could mediate ionic fluxes in the absence of activating toxins. The proportion of channels exhibiting this behavior was dependent on the lipid composition of the vesicles and the temperature at which the uptake was measured; furthermore, the effect of temperature was reversible. However, the phenomenon was not affected by the degree of purification of the protein used for reconstitution, and channels in resealed electroplax membrane fragments or reconstituted solely into native eel lipids did not show this behavior. The kinetics of vesicular uptake through these spontaneously-opening channels was slow, and we attribute this behavior to a modification of sodium channel inactivation.

Animals↗
PubMed1986

Electrogenic sodium/bicarbonate cotransport in rabbit renal cortical basolateral membrane vesicles.

The present studies examined the mechanism of bicarbonate transport across basolateral membrane vesicles prepared from rabbit renal cortex. Isotopic sodium uptake was stimulated by bicarbonate when compared with gluconate (2.5 nmol/mg protein per 5 s versus 1.4 nmol/mg protein per 5 s), and this process was inhibited by disulfonic stilbenes. Imposition of an interior-positive potassium diffusion potential further stimulated isotopic sodium uptake to 3.4 nmol/mg protein per 5 s, an effect that occurred only in the presence of bicarbonate and was blocked by disulfonic stilbenes. Kinetic analysis of the rate of bicarbonate-dependent sodium uptake as a function of sodium concentration revealed saturable stimulation with a Vmax of 2.7 nmol/mg protein per 2 s and a Km of 10.4 mM. The effect of bicarbonate concentration on bicarbonate-dependent sodium uptake was more complex. The present results demonstrate an electrogenic (negatively charged) sodium/bicarbonate cotransporter in basolateral membrane vesicles from the rabbit renal cortex. The electrogenicity implies a stoichiometry of at least two bicarbonate ions for each sodium ion.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1975

Inhibition of active sodium transport by radiographic contrast media.

We demonstrate that salts of diatrizoate and iothalamate, radiographic contrast agents, depress the active transport of sodium in the urinary bladder of the Columbian toad, Bufo marinus. Isolated toad bladders were incubated in isotonic Ringer's solutions with isosmolar displacement of sodium chloride by contrast media in experimental solutions. Sodium transport as measured both by short-circuit current (SCC) and by isotopic sodium flux was significantly depressed in the presence of sodium diatrizoate. Sodium transport measured by SCC was significantly depressed with sodium iothalamate and meglumine iothalamate. Equimolar methylsulfate Ringer's solution did not depress SCC. Although contrast media in isotonic Ringer's solutions depressed basal SCC, the vasopressin-stimulated increment in SCC was not depressed by contrast media. Separate experiments with hyperosmolar solutions (786 mM, as utilized in angiography) demonstrated equivalent suppression of SCC by contrast media and by other solutions made hyperosmolar with glucose or sodium methylsulfate, implying a general or nonspecific effect of hyperosomolarity. Inhibition of SCC by contrast media was reversible when the agents were removed by serial changes with standard Ringer's solution. Inhibition of sodium transport by contrast media might provide a basis for studies on some of the clinical toxicities of these agents.

Animals↗
PubMed1977

Inhibition of sodium transport by prostaglandin E2 across the isolated, perfused rabbit collecting tubule.

This study was designed to examine whether prostaglandin E2 can directly affect sodium transport across isolated perfused rabbit renal collecting tubules. Changes in transepithelial potential and isotopic sodium fluxes in response to peritubular prostaglandin E2 were measured. In addition, changes in transepithelial potential of the outer medullary collecting tubule in response to prostaglandin E2 were also measured. With few exceptions, all rabbits received 5 mg/day desoxycorticosterone acetate for 4-11 days before experimentation. The results of the experiments show that: (a) prostaglandin E2 inhibits the negative transepithelial potential in the cortical collecting tubule as well as the outer medullary collecting tubule; (b) prostaglandin E2 inhibits net sodium transport out of the lumen by inhibiting efflux while backflux is unaffected; (c) prostaglandin E2 produces this inhibition within 15 min, and the effects are dose dependent and reversible. These results suggest that prostaglandin E2 may modulate sodium transport in vivo and may contribute to the final regulation of sodium excretion.

Animals↗
PubMed1976

On the influence of the natriuretic factor from patients with chronic uremia on the bioelectric properties and sodium transport of the isolated mammalian collecting tubule.

A gel filtration fraction of urine from patients with chronic renal disease (natriuretic factor) has been shown previously to cause natriuresis in rats and to inhibit sodium transport in the isolated toad bladder. The effect of this fraction on transtubular potential difference and sodium transport was examined on the isolated perfused cortical collecting tubule of the rabbit. A rapid inhibition of potential difference from -22.5 mV to -12 mV (P less than 0.001) was observed when the fraction was applied to the peritubular surface. This effect was accompanied by a decrease in net sodium flux from 6.29 to 3.21 pmol/cm per s (P less than 0.001). Unidirectional fluxes using isotopic sodium revealed that the inhibition of net sodium transport was due to a decrease in flux from the lumen to the peritubular surface, i.e., an inhibition of active sodium transport. There was no change in sodium flux in the reverse direction. These changes were all rapidly reversed by removal of the fraction from the peritubular surface. The addition of the fraction to the lumen had no effect on potential difference or net sodium flux. Control studies using the same fraction from the urine of normal subjects had no effect on any of the parameters studies. Where both a uremic and a normal fraction were sequentially applied to the peritubular surface of the same tubule, inhibition of potential difference was obtained only with the former. In the light of evidence implicating the collecting duct fraction from normal animals, the data are consistent with the view that the natriuretic factor may be biologically important in the regulation of sodium balance via it's regulatory role in active sodium transport in the collecting tubule.

Animals↗
PubMed1987

Neurotoxin-modulated uptake of sodium by highly purified preparations of the electroplax tetrodotoxin-binding glycopeptide reconstituted into lipid vesicles.

Using the dialysable detergent CHAPS (3-[(3-cholamidopropyl)-dimethylammonio]-1-propane sulfonate), the tetrodotoxin-binding protein from the electroplax of the electric eel has been purified to a high degree of both chemical homogeneity and toxin-binding activity. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the best preparations showed only a single microheterogeneous band at Mr approximately 260,000, despite attempts to visualize smaller bands by sample overloading. Upon dialysis, this material became incorporated into the membranes of small unilamellar vesicles, and in this form the purified protein exhibited tetrodotoxin-binding properties similar to the component in the original electroplax membrane. Furthermore, in the presence of activator neurotoxins the vesicles were able to accumulate isotopic sodium in a manner similar to that previously described for less active or less pure preparations of vesicles containing either mammalian or eel electroplax toxin-binding proteins. Quantitative consideration of the isotopic transport activity of this pure material, along with the high degree of purity of the protein, strongly suggests that the 260-kDa glycopeptide from electroplax is necessary and sufficient to account for the sodium channel function seen in these studies, and eliminates the possible involvement of smaller peptides in the channel phenomena observed.

Animals↗
PubMed1981

Effects of catecholamines on electrolyte transport in cortical collecting tubule.

We examined the direct effects of isoproterenol (ISO) and L-norepinephrine (NE) on electrolyte transport in isolated rabbit cortical collecting tubules (CCT) perfused in vitro. The addition of either ISO (10(-6) M) or NE (10(-6) M) to the bath decreased transepithelial potential difference (PD), on average by 51 and 25%, respectively. These effects of ISO and NE were abolished by prior addition of the beta-adrenergic blocker, L-propranolol. ISO (10(-5) M) had no effect from lumen. Also, osmotic water permeability was not influenced by ISO. Ouabain and ISO had additive effects on PD. Elimination of chloride from both perfusate and bath, or addition of acetazolamide, abolished the effect of ISO on PD. Although isotopic sodium flux for lumen to bath was not influenced by ISO, chemical net chloride absorption increased from 1.1 +/- 0.4 to 2.7 +/- 0.6 peq . cm-1 . sec-1 (n = 8, p less than 0.005). In conclusion, both ISO and NE and capable of decreasing PD in rabbit CCT perfused in vitro. This effect is mediated by beta-adrenergic receptors and is accompanied by the increase in net chloride absorption. Although the mechanism responsible for this decrease in PD with ISO in unclear, active chloride absorption, active hydrogen secretion, or membrane chloride permeability changes may account for the effects of ISO.

Acetazolamide↗