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Biomedical subjects

M Abramow

Publications and source records attributed to M Abramow.

At least 19 recordsLinked to original sources
PubMed1995

Aldosterone stimulation of GTP hydrolysis in membranes from renal epithelia.

Specific hydrolysis of GTP catalyzed by membranes prepared from A6 epithelial cells grown on porous supports was measured. Aldosterone treatment of the cells for 4 h increased Na+ transport and stimulated GTP hydrolysis by apical membranes in vitro more than twofold over basal levels. This stimulation was attributed to an increase in maximum velocity with little change in Michaelis-Menten constant values. Na+ transport rate and GTP hydrolysis were linearly correlated after aldosterone. This relationship was maintained when aldosterone's response was blunted by various inhibitors. Spironolactone decreased both the hormone-stimulated guanosinetriphosphatase (GTPase) and the Na+ transport rate. Pertussis toxin, which exerted minimal effects on basal rates, reduced the increase of Na+ current normally observed after aldosterone and the hormone stimulation of GTPase activity. The expression of classical Gi/Go-type G proteins was not increased after hormone treatment. When A6 cells were grown on nonporous plastic dishes, aldosterone neither stimulated GTPase activity nor increased amiloride-blockable 22Na+ fluxes. We propose that activation of one or more G proteins in the apical membrane of A6 cells is directly involved in the natriferic action of aldosterone.

Aldosterone↗
PubMed1995

Drugs activating G proteins disturb cycling of ADH-dependent water channels in toad urinary bladder.

In the toad urinary bladder, antidiuretic hormone (ADH)-mediated changes in water permeability depend on exocytic insertion and endocytic retrieval of water channels into and from the apical membrane, respectively. Because GTP-binding proteins (G proteins) are well-recognized regulators of vesicular trafficking throughout the cell, we tested the hypothesis that drugs interfering with G protein would modify the hydrosmotic response to ADH and the ADH-regulated formation of endosomes, as assessed by luminal incorporation of a fluid-phase marker [fluorescein isothiocyanate (FITC)-dextran, 70 kDa]. Mastoparan (4 microM) and compound 48/80 (poly-p-methoxyphenylethylmethylamine; 50 micrograms/ml), added to the luminal side of the toad urinary bladder, as well as AlF3 added to the serosal side (400 microM), inhibited ADH- and 8-bromoadenosine 3',5'-cyclic monophosphate-induced transepithelial water flow by > 50% and simultaneously enhanced cellular incorporation of FITC-dextran by > 200%. The pattern of FITC-dextran uptake observed using fluorescence microscopy both in scraped cells and in the intact bladder was granular, suggesting fluid-phase endocytosis. Mastoparan and AlF3, which are both probes of G proteins, increased FITC-dextran uptake only in the presence of ADH and a transepithelial osmotic gradient, i.e., under conditions where water channel-carrying endosomes presumably cycle. Therefore, we suggest that the ADH-dependent cycling of water channels could be controlled by one or more G proteins associated with the apical membrane and/or the water channel-carrying vesicles.

Aluminum Compounds↗
PubMed1995

Adaptation of inner medullary collecting duct to dehydration involves a paracellular pathway.

Prolonged fluid restriction in rats is accompanied by functional modifications of the terminal part of the inner medullary collecting duct (IMCD) revealed by a sustained increase in arginine vasopressin (AVP)-independent transepithelial osmotic water permeability (PTE) in vitro. The cellular basis of this adaptation was explored in isolated and perfused terminal IMCDs of Sprague-Dawley rats using video and fluorescence microscopy. Basolateral membrane osmotic water permeability (Posm), transcellular Posm, and PTE were measured in quick sequence in every tubule. They were expressed per unit area of basolateral membrane corrected for infoldings, based on previous stereological studies and assuming no major change in membrane surface area between hydrated and dehydrated animals. Compared with IMCDs of rats with a high water intake, IMCDs of rats deprived of fluid for 36 h displayed a significantly higher basal PTE (24.9 +/- 5.1 vs. 6.1 +/- 0.6 microns/s), a similar basolateral Posm, and a higher transcellular Posm, implying a higher permeability of the apical membrane, despite the absence of exogenous AVP. However, when IMCDs of thirsted rats were exposed to AVP in vitro, their transcellular Posm (36.0 +/- 2.4 microns/s) was significantly smaller than their PTE determined simultaneously (51.8 +/- 7.1 microns/s), suggesting that part of the water flow may follow a paracellular route. A change in paracellular pathways was supported by higher apparent permeabilities to [14C]sucrose (0.85 +/- 0.27 vs. 0.28 +/- 0.04 x 10(-5) cm/s) and to [methoxy-3H]inulin (0.25 +/- 0.04 vs. 0.14 +/- 0.03 x 10(-5) cm/s) in IMCDs of thirsted rats. The nonelectrolyte permeabilities were affected neither by AVP nor by urea-rich bathing solutions. We conclude that in vivo factors related to dehydration produce a conditioning effect on terminal IMCD, which includes stabilization of the apical membrane in a state of high Posm and opening up of paracellular pathways revealed by a higher permeability to water and nonelectrolytes. The role of these adaptive phenomena remains unclear but may pertain to the sudden transitions between antidiuresis and diuresis.

Acclimatization↗
PubMed1994

Subtypes of Madin-Darby canine kidney (MDCK) cells defined by immunocytochemistry: further evidence for properties of renal collecting duct cells.

The Madin-Darby canine kidney (MDCK) cell line has been proposed as a model for studying intercalated (IC) cells of the renal cortical collecting duct. The IC cells are characterized by peanut lectin (PNA) binding capacity, carbonic anhydrase (CA) activity and Cl(-)-HCO3- exchange mediated by a band 3-related protein. It has been suggested that these properties are also expressed in MDCK cells. So far however, the nature of the specific protein involved in Cl(-)-HCO3- exchange, the type of CA isozyme and the relationship between these two characteristics and PNA binding, have not been investigated in MDCK cells by immunocytochemical methods. Using two antibodies raised against human erythrocyte band 3 protein and two against human erythrocyte CA I and II isozymes, our study provides evidence that a protein related to band 3 is expressed in about 5% of cultured MDCK cells; these band 3-positive cells do not bind PNA and are not reactive for CAI or CAII. About 30% of the MDCK cells bind PNA, two-thirds of which are also CAII-positive. A majority (about 65%) of MDCK cells is not reactive for the three markers used; their density is increased after incubation with aldosterone. These data indicate (i) that the Cl(-)-HCO3- exchange of the MDCK cells could be related to human erythrocyte band 3, (ii) that the CA activity of the MDCK cell line bears antigenic identity with the erythrocyte CA II isozyme and (iii) that the latter is always co-localized with PNA binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

Aldosterone-induced and GTP-stimulated methylation of a 90-kDa polypeptide in the apical membrane of A6 epithelia.

Aldosterone treatment of A6 cultured renal epithelial cells methylates the apical membrane, and we examined the aldosterone-induced carboxymethylation of the apical membrane of these cells to determine the targeted polypeptides. Methionine-deprived A6 cells were incubated with aldosterone and [3H]methionine. Homogenates and apical membranes were solubilized and analyzed by SDS-polyacrylamide gel electrophoresis. Label incorporation in a 90-kDa polypeptide was more intense (4-fold) in membranes after aldosterone compared to control. For in vitro methylation, membranes were isolated, incubated with S-adenosyl-L-[methyl-3H]methionine, and analyzed for 3H-methyl uptake. Label incorporation was low in control membranes but markedly stimulated (4-fold) in membrane preparations from aldosterone-treated cells. Guanosine 5'-O-(3-thiotriphosphate) increased in vitro methylation of a 90-kDa polypeptide 5-fold in control membranes but after aldosterone, where methylation was already stimulated, little change was observed. We conclude that aldosterone induces methylation of an apical membrane 90-kDa polypeptide, possibly a subunit of the epithelial Na+ channel, in a GTP-dependent manner, and this may be one of the final steps in a cascade of reactions leading to the natriferic action of this hormone.

Aldosterone↗
PubMed1992

The iodide channel of the thyroid: a plasma membrane vesicle study.

The uptake of radioactive iodide or chloride by plasma membrane vesicles of bovine thyroid was studied by a rapid filtration technique. A Na(+)-I- cotransport was demonstrated. When this Na(+)-I- cotransport is inactive (i.e., at 4 degrees C and in the absence of Na+), an uptake of iodide above chemical equilibrium could be induced, driven by the membrane potential. The latter was set up by allowing potassium to diffuse into the membrane vesicles in the presence of valinomycin and of an inward K+ gradient. This potential difference (positive inside) induced the uptake of iodide (or other anion present). The data support the existence of two anionic channels. The first one, observed at low near-physiological iodide concentration (micromolar range), which exhibits a high permeability and specificity for iodide (hence called the iodide channel), has a Km of 70 microM. The other one appears similar to the epithelial anion channel as described by Landry et al. (J. Gen. Physiol. 90: 779-798, 1987); it is still about fourfold more permeable to iodide than to chloride and presents a Km of 33 mM. Under physiological conditions the latter channel would mediate chloride transport, and the iodide channel, which is proposed to be restricted to the apical plasma membrane domain of the thyrocyte, transports iodide from the cytosol to the colloid space.

Animals↗
PubMed1992

Single-channel behavior of a purified epithelial Na+ channel subunit that binds amiloride.

The apical membrane of high electrical resistance epithelia, which is selectively permeable to Na+, plays an essential role in the maintenance of salt balance. Na+ entry from the apical fluid into the cells is mediated by amiloride-blockable Na(+)-specific channels. The channel protein, purified from both amphibian and mammalian sources, is composed of several subunits, only one of which the 150-kDa polypeptide, specifically binds the Na+ transport inhibitor amiloride. The goal of the present study was to investigate whether the isolated amiloride-binding subunit of the channel could conduct Na+. The patch-clamp technique was used to study the 150-kDa polypeptide incorporated into a lipid bilayer formed on the tip of a glass pipette. Unitary conductance jumps averaged 4.8 pS at 100 mM Na2HPO4. Open times ranged from 24 ms to several seconds. The channel spent most of the time in the closed state. Channel conductance and gating were independent of voltage between -60 and +100 mV. Amiloride (0.1 microM) decreased the mean open time of the channel by 98%. We conclude that the 150-kDa subunit of the amiloride-blockable Na+ channel conducts current and may be sufficient for the Na+ transport function of the whole channel.

Amiloride↗
PubMed1990

Impaired hydroosmotic response to vasopressin of cortical collecting tubules from lithium-treated rabbits.

The hydroosmotic action of [arginine]vasopressin (vasopressin, 25 microU/ml) and of 8-Br-cAMP (10(-4)M) was studied in vitro in perfused cortical collecting tubules (CCT) isolated from rabbits fed with lithium chloride for 3 weeks. Vasopressin-dependent water reabsorption was significantly inhibited by 65% although no lithium was used in the in vitro experiments. The hydroosmotic action of 8-Br-cAMP was also inhibited by previous Li treatment, but the effect was smaller in magnitude. Water intake, diuresis, and urinary osmolality were no different in the lithium-treated animals as compared with respective pretreatment values or with control animals given an equivalent amount of sodium chloride. Neither the creatinine clearance nor the maximal urinary concentrating ability were modified by lithium treatment. A mathematical model simulating water reabsorption along the CCT predicts that a 65% reduction of vasopressin-stimulated hydraulic conductivity, as observed in the Li group, may not be sufficient to prevent a complete osmotic equilibration at the end of the CCT in vivo. We conclude that: (a) in the rabbit, lithium administration induces an impairment of the hydroosmotic action of vasopressin in the CCT, which is due to an inhibition of pre- and post-cAMP events. (b) The inhibition of vasopressin action can be demonstrated in vitro at a time when no detectable impairment of the water conservation process occurs in vivo.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1990

Atrial natriuretic peptides in canine hypoxic pulmonary vasoconstriction.

STUDY OBJECTIVE: The aim of the study was to investigate whether atrial natriuretic peptides have a physiological role in regulation of the pulmonary circulation. DESIGN: Plasma concentrations of immunoreactive atrial natriuretic peptide and guanosine-3',5'-cyclic monophosphate (cGMP) were measured during evaluation of pulmonary vascular tone by multipoint pulmonary arterial pressure-cardiac index (Ppa/Q) relationships. SUBJECTS: Experimental animals were 17 mongrel dogs of either sex, 21-35 kg weight, anaesthetised with pentobarbitone. MEASUREMENTS AND MAIN RESULTS: Measurements of Ppa/Q relationships and atrial natriuretic peptide/cGMP were made during hyperoxia (Fio2 0.4) and hypoxia (Fio2 0.1). Hypoxic pulmonary vasoconstriction, defined as hypoxia induced increase in pulmonary artery pressure over the entire range of Q studied from 2-5 litre.min-1.m-2, was elicited in nine dogs ("responders"). In the other eight dogs, hypoxia did not change pulmonary artery pressure over the entire range of Q studied ("non-responders"). At neither the highest nor the lowest Q in hyperoxia did atrial natriuretic peptide and cGMP concentrations differ between these two groups, nor did acute reduction in Q affect the concentrations in either group. At the highest Q, plasma atrial natriuretic peptide increased in hypoxia from 11(SEM 2) to 15(3) pmol.litre-1 in the responders (p less than 0.05), and from 15(2) to 20(2) pmol.litre-1 in the non-responders (p less than 0.05). However at the lowest Q, atrial natriuretic peptide was increased in non-responders only, from 17(3) to 23(4) pmol.litre-1 (p less than 0.05). CGMP did not vary significantly in any experimental condition. CONCLUSIONS: Hypoxia slightly increased plasma atrial natriuretic peptides without any relationship with associated pulmonary haemodynamic changes. These data do not support the hypothesis that atrial natriuretic peptides play a physiological role in the regulation of the pulmonary circulation in dogs.

Animals↗
PubMed1989

Pharmacological doses of atrial natriuretic factor do not inhibit canine hypoxic pulmonary vasoconstriction.

It has been recently suggested that atrial natriuretic factor (ANF) might be involved in the physiological regulation of pulmonary circulation. Therefore, we investigated the pulmonary hemodynamic response to 20-min infusions of 0.05, 0.1, and 0.2 micrograms kg-1 min-1 of alpha human ANF in five dogs alternatively ventilated with hyperoxic (FIO2 0.4) and hypoxic (FIO2 0.1) gas mixtures. Cardiac output was held constant by the inflation of a balloon in the inferior vena cava or by opening of an arteriovenous femoral fistula, in order to discriminate between active and passive changes in pulmonary arterial pressure (Ppa). Hypoxia increased Ppa from 14 +/- 3 to 24 +/- 3 mm Hg (mean +/- SE, p less than 001). Circulating ANF and guanosine 3',5'-cyclic monophosphate (cGMP) were increased to 1,326 +/- 299 pmol L-1 (normal is less than 10 pmol L-1) and 75.5 +/- 5.8 pmol ml-1 (normal is less than 15 pmol ml-1) respectively, at the highest infused dose. After ANF infusion, heart rate (HR), Ppa, pulmonary capillary wedge pressure (Ppw), and right atrial pressure (Pra) did not change either in hyperoxia or hypoxia. Systemic arterial pressure (Psa) decreased after ANF, but only in hypoxia. Thus, ANF at pharmacological doses associated with a 100-150-fold increase in plasma levels proved to be a poor vasodilator and, in particular, did not inhibit hypoxic pulmonary vasoconstriction (HPV). These results do not support the speculation that ANF might be an endogenous vasodilating modulator of pulmonary vascular tone in the dog.

Animals↗
PubMed1988

Natriuresis and atrial natriuretic factor secretion during inappropriate antidiuresis.

The mechanisms responsible for the natriuresis encountered in the syndrome of inappropriate secretion of antidiuretic hormone (SIADH) are not fully understood. The present study explores the role of atrial natriuretic factor (ANF). Eight subjects unable to excrete ingested free water normally (three patients with SIADH and five healthy humans after intranasal administration of desmopressin) underwent a standard oral water loading test. Plasma ANF level and urinary sodium excretion increased during water retention, whereas plasma aldosterone value decreased later. The increment of urinary sodium excretion rate was significantly correlated with that of plasma ANF. In two patients with hyponatremia due to SIADH, plasma ANF levels were increased during the hyponatremic phase of their condition and decreased under water restriction. In one of them, marked natriuresis was observed when the plasma ANF level was high. It is concluded that secretion of ANF is acutely and chronically stimulated during water retention in SIADH and that ANF may be in part responsible for the natriuresis encountered in inappropriate antidiuresis.

Adult↗
PubMed1987

Mechanisms of lithium-vasopressin interaction in rabbit cortical collecting tubule.

In single cortical collecting tubules (CCT) of the rabbit, guanosine 5'-triphosphate (GTP) increased the arginine vasopressin (AVP)-stimulated adenylate cyclase (AC) by 60% (P less than 0.05). In contrast, guanosine 5' O-(2-thio)-diphosphate (GDP-beta S), a competitive inhibitor of GTP action on the stimulatory guanine regulatory protein (Ns), reduced the AVP-stimulated AC activity by 72% (P less than 0.001), indicating the presence of endogenous GTP in the cells under study. That inhibitory effect was reversed by the addition of GTP to the incubation medium. In isolated perfused CCT, cholera toxin (CT) induced a significant increase in water permeability in the absence of AVP. In contrast, Bordetella pertussis toxin (BPT) did not modify the low AVP-independent water permeability. Lithium, an inhibitor of the hydrosmotic action of AVP, also inhibits the hydrosmotic action of CT by 70% (P less than 0.05) but not that of forskolin. The conclusions of the present study are Ns is required for AVP stimulation of AC in the CCT; Ns is functionally active in this system as evidenced by the hydrosmotic effect of CT; the lack of effect of BPT suggests that the low AVP-independent water permeability in the CCT is not the result of a tonic inhibition of the AC operating through the inhibitory guanine nucleotide regulatory protein; and the inhibition by lithium of the hydrosmotic action of AVP in the CCT appears to involve an interaction with the regulatory proteins (probably Ns) or with their binding to the catalytic unit of AC.

Adenylyl Cyclases↗