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A Bäcker

Publications and source records attributed to A Bäcker.

At least 37 records · Page 2Linked to original sources
PubMed1995

Vanadium-diascorbates are strong candidates for endogenous ouabain-like factors in human urine: effects on Na-K-ATPase enzyme kinetics.

Recently, we isolated from the urine of salt-loaded healthy subjects a more polar ouabain-like factor OLF-1 and a more apolar OLF-2, the latter cross-reacted with a digoxin anti-body. They were purified to single compounds with dose-dependent Na-K-ATPase inhibition. Mass-spectroscopy (MS) showed a Mr of around 400 and 1H-NMR- and IR-spectroscopy suggested diascorbic acid salts, i.e., vanadium (V) diascorbates (Mr 403) with similar elution times from RP-HPLC as OLFs. IC50 was 9 x 10(-5)M for VIV-diascorbate as compared to 2 x 10(-6)M for Vv-diascorbate. Enzyme inhibition was non-competitive with respect to sodium and Mg-ATP; p-NPPase assay showed strong inhibition in its E2-configuration. We suggest that V-diascorbates represent endogenous OLFs excreted in human urine.

4-Nitrophenylphosphatase↗
PubMed1995

Endothelin synthesis by porcine inner medullary collecting duct cells. Effects of hormonal and osmotic stimuli.

Previously, we have shown in porcine inner medullary collecting duct (IMCD) cells that endothelin (ET), probably in an autocrine fashion, suppresses arginine vasopressin (AVP)-induced synthesis of cAMP and thereby, may modify the action of AVP on IMCD fluid transport. In the present study we investigated the effects of various stimuli including extracellular tonicity on ET synthesis in porcine IMCD cells in culture. IMCD cells produced ET in a saturationlike time-dependent manner over a period of 24 h. Neither AVP (10(-7) mol/L), bradykinin (10(-7) mol/L), nor atrial natriuretic peptide (10(-7) mol/L) affected basal ET synthesis of IMCD cells at extracellular isotonicity (323 mOsm/kg H2O). The calcium ionophore A23187 (10(-7) mol/L) increased ET production by 38% within 2 h (P < .05). Preincubation for 48 h with increased osmolality in the incubation media from 323 to 600 mOsm/kg H2O by raising the concentrations of 1) NaCl (n = 6), 2) urea (n = 6), or 3) NaCl+urea (n = 6) increased ET synthesis from a control value of 225 +/- 25 pg/mg cell protein/2 h in isotonic medium to 1) 555 +/- 13 pg/mg cell protein/2 h (P < .01), 2) 354 +/- 18 pg/mg cell protein/2 h (P < .05), and 3) 448 +/- 22 pg/mg cell protein/2 h (P < .05), respectively, in hypertonic media. These data suggest that increases in papillary osmolality are associated with enhanced ET synthesis possibly involving a calcium-dependent process and attenuating AVP-dependent fluid absorption in a short-loop feedback fashion.

Animals↗
PubMed1995

Endogenous sodium pump inhibitors in human urine. Further identification of inhibitors of Na-K-ATPase.

We investigated the presence of endogenous Na-K-ATPase inhibitor(s), ie, ouabain-like factors (OLFs), in the urine of salt-loaded healthy subjects. For this purpose 24-h urine was collected on days 3, 4, and 5 of high sodium intake (> 30 g NaCl/day). The samples then were lyophilized. Redissolved urine concentrates were acidified (pH 3.5) and subjected to gelchromatography on a Sephadex G-25 column where the OLFs eluted in the post-salt fraction IV. When lyophilized fraction IV was rechromatographed on Sephadex G-10, OLFs with molecular mass (M(r) of approximately 400 eluted in a late fraction IV/8 separate from added ouabain, ouabagenin (or digoxin), which eluted shortly after void volume. With the subsequent reverse-phase HPLC of fraction IV/8 a polar OLF-1 eluted in fraction IV/8a after the void volume in the water phase and a more apolar OLF-2 eluted at 20% acetonitrile in fraction IV/8d. Only the more apolar OLF-2 cross-reacted with a digoxin antibody. By preparative thin-layer chromatography OLF-1 and OLF-2 were purified as single compounds with potent dose-dependent Na-K-ATPase inhibition and Ki-values approximating 1.5 x 10(-5) mol/L and 1.5 x 10(-4) mol/L, respectively. Mass-spectroscopy (MS) showed M(r) of 391 and 1H-NMR characterized the endogenous urinary apolar OLF-2 as a compound that is structurally totally unrelated to ouabain; infrared (IR) spectroscopy of OLF-1 and OLF-2 also revealed no similarity with ouabain.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1995

Impaired renal function in obstructive jaundice: enhanced glomerular thromboxane synthesis and effects of thromboxane receptor blockade in bile duct-ligated rats.

1. Patients with obstructive jaundice are especially susceptible to acute renal failure. We have previously observed that in rats with bile duct ligation impaired renal function is associated with increased urinary thromboxane excretion. 2. In the present study we therefore investigated, in rats with bile duct ligation, renal function, urinary thromboxane excretion and thromboxane B2 synthesis by isolated glomeruli as well as the effects of the thromboxane A2/prostaglandin H2 receptor antagonist Daltroban on renal function in rats with bile duct ligation as compared with sham-operated rats. 3. On the fourth day after bile duct ligation (n = 7 rats) endogenous creatinine clearance as an estimate of glomerular filtration rate was significantly reduced to 0.74 +/- 0.05 (SEM) as compared with 1.06 +/- 0.09 ml min-1 g-1 kidney weight in sham-operated rats (n = 7, P < 0.01). In rats with bile duct ligation, urine volume was slightly increased, whereas urinary sodium (Na+) (P < 0.001) and potassium (K+) (P < 0.01) excretion as well as urine osmolarity (P < 0.05) were significantly reduced and lower than in sham-operated rats. 4. Urinary thromboxane excretion was significantly higher in rats with bile duct ligation than in sham-operated rats: 116.6 +/- 22.3 versus 56.8 +/- 10.2 pmol 24 h-1 100 g-1 body weight (P < 0.05). Thromboxane B2 synthesis in glomeruli isolated from rats with bile duct ligation was also significantly higher than in sham-operated rats: 12.6 +/- 2.0 versus 6.4 +/- 0.9 pmol h-1 mg-1 protein (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Acute Kidney Injury↗
PubMed1994

Cyclosporine A enhances total cell calcium independent of Na-K-ATPase in vascular smooth muscle cells.

The effect of cyclosporine A in enhancing vasconstrictor-induced calcium (Ca2+) mobilization in vascular smooth muscle cells may contribute to important side effects in cyclosporine therapy such as hypertension and nephrotoxicity. As we have previously shown, cyclosporine A stimulates transmembrane Ca2+ influx. Since Ca2+ efflux was not affected by cyclosporine A, we concluded that cyclosporine augments angiotensin II induced Ca2+ mobilization in vascular smooth muscle cells by an increased amount of Ca2+ in angiotensin II sensitive intracellular Ca2+ stores. The present study was therefore designed to examine the effect of cyclosporine A on cellular calcium content and on membrane calcium transport mechanisms. An important mechanism of Ca2+ extrusion from the cell is the Na-Ca exchanger. Its activity is closely related with that of the Na-K-ATPase. By increasing cellular sodium concentration the blockade of Na-K-ATPase would in turn activate cellular calcium uptake bx the Na-Ca exchanger. Therefore, we hypothesized that cyclosporine A might exert its effects in the same manner as a circulating Na-K-ATPase inhibitor. Total cell calcium was measured by atomic absorption and activity of Na-K-ATPase was estimated by an assay measuring phosphate production. Preincubation of the cells with cyclosporine (10 micrograms/ml) for 15 min increased total cell calcium from 31.4 +/- 5.0 to 46.5 +/- 5.3 nmol/mg protein (P < 0.05). Activity of Na-K-ATPase was not affected by cyclosporine A (3.9 +/- 0.2 vs. 4.3 +/- 0.2 mumol Pi h-1 mg-1 protein). Therefore, cyclosporine A induced Ca2+ influx is not mediated by an inhibition of the Na-K-ATPase.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

Atrial natriuretic peptide inhibits cyclosporin A-induced endothelin production and calcium accumulation in rat vascular smooth muscle cells.

1. As we have previously shown, cyclosporin A enhances the vasoconstrictor-induced rise in intracellular free calcium in vascular smooth muscle cells. This effect may contribute to important side-effects in cyclosporin therapy, such as hypertension and nephrotoxicity. Atrial natriuretic peptide has been shown to inhibit this effect as well as the cyclosporin-stimulated transmembrane calcium influx in smooth muscle cells. 2. The present study, therefore, was designed to examine the effect of cyclosporin and atrial natriuretic peptide on total cellular calcium content in the rat. Furthermore, since cyclosporin was recently shown to induce endothelin production in smooth muscle cells, we investigated the effect of atrial natriuretic peptide on this potentially adverse cellular effect of cyclosporin therapy. 3. Total cell calcium was measured by atomic absorption, and cellular endothelin production was estimated by radioimmunoassay. 4. Preincubation of the cells with cyclosporin (10 micrograms/ml) for 30 min increased total cell calcium from 2.6 +/- 0.5 to 6.9 +/- 0.3 nmol/mg of protein (P < 0.01). Within 24 h endothelin production was significantly enhanced in the presence of cyclosporin (52.2 +/- 2.5 versus 65.9 +/- 2.7 fmol/mg of protein, P < 0.05). Therefore, the cyclosporin-induced rise in total cell calcium in smooth muscle cells is associated with enhanced production of endothelin. Thus, it is tempting to speculate that the cyclosporin-induced changes in calcium kinetics may be mediated by endothelin. 5. In the presence of atrial natriuretic peptide (10(-8) mol/l), the cyclosporin-induced rise in total cell calcium was significantly reduced (6.9 +/- 0.3 versus 5.1 +/- 0.2 nmol/mg of protein, P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

Rat renal, aortic and pulmonary endothelin-I receptors: effects of changes in sodium and water intake.

1. In the present study we investigated, first, the effects of high Na+ intake and, second, the effects of water deprivation on plasma endothelin-1 concentration and urinary endothelin-1 excretion and on endothelin receptors in membranes of renal glomeruli and papillae and of aortic smooth muscle and lung tissue from 32 female Sprague-Dawley rats. 2. After 5 weeks of high Na+ intake (n = 8) urinary Na+ excretion was 10.5 +/- 1.3 compared with 1.6 +/- 0.2 mmol/24 h in controls. Body weight, plasma osmolarity, plasma endothelin-1 concentration (23 +/- 6 versus 28 +/- 3 fmol/ml) and urinary endothelin-1 excretion (6.1 +/- 1.3 versus 4.7 +/- 0.3 pmol/24 h) remained unchanged. 3. The characteristics of endothelin-1 receptors in glomeruli, papillae, aortic smooth muscle and lung tissue from salt-loaded rats were not different from those of controls. 4. After 48 h water deprivation (n = 8) body weight had decreased, whereas packed cell volume and plasma and urine osmolarities had increased compared with controls (n = 8) (P < 0.05). Plasma endothelin-1 concentration (40 +/- 6 versus 21 +/- 2 fmol/ml) was higher (P < 0.01) and urinary endothelin-1 excretion (1.0 +/- 0.2 versus 2.8 +/- 0.3 pmol/24 h) was lower than in controls (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

Characteristics of endothelin receptors and intracellular signalling in porcine inner medullary collecting duct cells.

In porcine kidneys we investigated the characteristics of endothelin (ET) receptors that are present in papillary tissue but not in glomeruli. Therefore, porcine inner medullary collecting duct (IMCD) cells were separated by Percoll density gradient centrifugation after enzymatic and hypotonic treatment of minced papillary tissue. Studies were performed in fresh cell suspensions and in cells in primary culture. Changes in cytosolic free Ca2+ concentration [Ca2+]i were measured by the use of fura-2. Optimum binding of ET-1 was obtained by incubation for 120 min at 37 degrees C, pH 7.0 when maximal protein content was 40 micrograms. Analysis with the LIGAND program showed an average number of binding sites (Bmax) of 26.0 +/- 30.5 fmol/mg protein and dissociation constant (Kd) of 90.5 +/- 28.6 pmol/L for ET-1 and Bmax of 246.9 fmol/mg protein and Kd of 162.5 pmol/L for ET-3. ET-1, 10(-9) to 10(-6) mol/L, dose dependently raised [Ca2+]i four to tenfold, respectively, from a mean basal level of 41 nmol/L. This rise was significantly attenuated by TMB-8 and by verapamil. Preincubation with Ni2+ almost completely prevented the increment in [Ca2+]i. ET-1 slightly suppressed basal and significantly attenuated arginine vasopressin (AVP)-induced cyclic adenosine monophosphate (cAMP) synthesis. Thus, porcine IMCD cells possess a single class of super high affinity ETB receptors (ETB1). ET-1 raises [Ca2+]i through release from intracellular stores, activation of L-type calcium channels and, probably to a larger extent, through stimulation of other channels, eg, T-type calcium channels or unselective cation channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

Cellular mechanisms of action of a ouabain-like factor in vascular smooth-muscle cells.

Ouabain-like factor (OLF) has been implicated to play an important role in certain forms of hypertension. We isolated OLF from the urine of salt-loaded healthy subjects by stepwise chromatographic procedures. The post-salt fraction (F IV) eluted from Sephadex G-25 was rechromatographed on Sephadex G-10. A late small-molecular-weight fraction F8 inhibited Na-K-ATPase in vitro (OLF activity). The effects of OLF on intracellular Ca2+ concentrations ([Ca2+]i) and pH (pHi) were examined in cultures of vascular smooth-muscle cells using the fluorescent probes fura-2 and 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF), respectively. Preincubation with OLF increased basal [Ca2+]i from 87 +/- 6 to 160 +/- 8 nM (p < 0.001) and enhanced arginine vasopressin-stimulated maximal [Ca2+]i (418 +/- 11 vs. 523 +/- 14 nM, p < 0.01). This effect was similar to that of ouabain. OLF also induced a rapid transient increase of [Ca2+]i (82 +/- 9 vs. 253 +/- 23 nM, p < 0.01); [Ca2+]i returned to levels slightly above baseline within approximately 4 min. OLF-stimulated [Ca2+]i was attenuated by verapamil (126 +/- 5 nM, p < 0.01) and was also reduced in Ca(2+)-free medium (104 +/- 9 nM, p < 0.01). As opposed to OLF, ouabain did not exhibit this fast transient effect on [Ca2+]i. Amiloride (10(-3) M) blocked the sustained effect of OLF on [Ca2+]i (77 +/- 11 vs. 86 +/- 12 nM, NS). OLF induced an increase of pHi from 7.09 +/- 0.03 to 7.28 +/- 0.04 (p < 0.002).(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1993

Effects of thromboxane A2 receptor blockade on oliguric ischemic acute renal failure in conscious rats.

To investigate the potential pathogenetic and therapeutic roles of thromboxane A2 (TXA2) and its receptor blockade, respectively, in the early phase of ischemic acute renal failure (ARF), renal function, TXB2 excretion, and the effects of the specific TXA2 receptor antagonist sulotroban (SU) in a model of unilateral renal artery occlusion in conscious female Sprague-Dawley rats were studied. Occlusion of the left renal artery for 1 h in untreated (i.e., vehicle-treated) rats (N = 8) resulted in oliguric ARF. In SU-treated rats (N = 8), the drug was given as an i.v. bolus of 5 mg/kg body wt, followed by a continuous infusion of 0.5 mg/min.kg body wt from 1 h before and during ischemia and for 6 h after reflow. After 1 h of ischemia, urine volume of left ischemic kidneys from untreated rats had decreased from 13.2 +/- 2.8 to 1.0 +/- 0.3 and 0.5 +/- 0.2 microL/min.100 g at 2 and 6 h of reflow, respectively, and GFR had decreased from 0.32 +/- 0.04 mL/min.100 g body wt to undetectable values. At 6 h of reflow, medullary Na-K-ATPase was slightly (P < 0.05) reduced in left ischemic kidneys, whereas medullary and papillary enzyme activities were compensatorily increased (P < 0.01) in right intact kidneys. The ADP/O ratio of cortical mitochondria was 41% (P < 0.05) and ATP synthesis was 77% (P < 0.01) lower than in right intact kidneys.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute Kidney Injury↗
PubMed1992

Selective downregulation of rat renal clearance receptors for atrial natriuretic peptide by chronic high-salt intake: study on isolated membranes using 125I-labelled c-atrial natriuretic peptide-(4-23).

1. 125I-labelled c-atrial natriuretic peptide-(4-23) was used as radioligand for the direct quantification of atrial natriuretic peptide clearance receptors, and 125I-labelled rat atrial natriuretic peptide-(1-28) was used for the determination of total and biologically active atrial natriuretic peptide receptors, in renal glomerular and papillary membrane preparations of chronically salt-loaded and control rats. 2. The membrane preparation technique included acid-washing (pH 5), and receptor binding was assessed by saturation studies at 4 degrees C for 3 h. Chronic salt loading revealed a 35% decrease in clearance receptor number in the glomerular membrane and a 42% decrease in the papillary membranes. The absolute decrease in clearance receptor number was almost identical with the observed reduction in absolute number of total atrial natriuretic peptide receptors. Biologically active receptors were not affected by chronic salt loading. 3. Selective downregulation of clearance receptors for atrial natriuretic peptide may reflect an important role for the rat renal atrial natriuretic peptide system in long-term fluid and electrolyte regulation.

Animals↗
PubMed1990

[Endogenous natriuretic and ouabain-like factors. Their potential role in volume and blood pressure regulation].

The existence of an endogenous natriuretic hormone and ouabain-like factors (OLF) has been postulated for many years. This postulate was based on our original observation that a small M.W. fraction in the serum after acute expansion of the extracellular fluid volume (ECFV) not only exhibited natriuretic activity but also inhibited the Na-K-ATPase enzyme in vitro similar to ouabain. Since then, numerous studies confirmed the presence of OLFs in serum, urine, cerebrospinal fluid, and various organs including the heart and hypothalamus. Some of these OLFs are well-known endogenous compounds, such as free unsaturated fatty acids, which inhibit in vitro transmembranous sodium transport, Na-K-ATPase and 3H-ouabain binding to its membrane receptor or cross-react with digoxin antibodies. Chemically yet undefined OLFs of potentially hypothalamic origin were detected in various models of experimental and clinical hypertension and are suggested to play a pathophysiological role especially in salt- and volume-dependent forms of hypertension. Our results show that OLFs isolated from the urine of salt-loaded healthy subjects strongly enhance basal and vasopressin-stimulated release of calcium in vascular smooth muscle cells and platelets similar to the effects we had observed with endothelin. This urine fraction also exhibits natriuretic activity which increases in parallel with sodium intake. Further chromatographic separation and amino acid analysis confirmed the peptidic nature (M.W. less than 1000) of the natriuretic factor(s). However, the two biological activities, namely natriuretic and ouabain-like activities, reside in distinct and chemically different compounds.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Plasma concentrations of endothelin in patients with abnormal vascular reactivity. Effects of ergometric exercise and acute saline loading.

We measured circulating concentrations of endothelin, a recently discovered vasoconstrictor peptide produced by vascular endothelial cells, in healthy subjects and in patients with abnormal vascular reactivity. Endothelin concentrations were determined by radio-immunoassay after extraction of plasma using Sep-Pak C-18 cartridges in healthy subjects (n = 20), in patients with diabetes mellitus type I (n = 10), in patients with mild to moderate essential hypertension (n = 12) and in non-dialyzed patients with stable chronic renal failure (n = 12). Plasma concentrations were similar in healthy controls, in diabetics and in hypertensive patients averaging 5.0 +/- 0.6 pg/ml, 4.7 +/- 0.2 pg/ml and 6.5 +/- 1.0 pg/ml, respectively. In contrast, plasma concentrations of endothelin were markedly elevated in patients with chronic renal failure averaging 16.6 +/- 2.9 pg/ml (p less than 0.005). No correlations were observed between serum creatinine concentrations ranging from 124 to 850 mumol/l or blood pressure and plasma concentrations of endothelin. Bicycle ergometric exercise in six healthy subjects and an acute modest i.v. saline load of 1,000 ml of 0.45% NaCl administered within 60 min in six patients with mild essential hypertension did not affect plasma concentrations of endothelin. Thus, it is unlikely that vascular synthesis of endothelin is related to acute physiological changes in systemic hemodynamics or to the circulatory and renal responses to acute extracellular fluid volume (ECFV) expansion. A potential role of endothelin, however, in the control of regional blood flow cannot be excluded. Elevated plasma concentrations of endothelin observed in patients with chronic renal failure require further investigations.

Adult↗
PubMed1990

Regulation of atrial natriuretic peptide receptors in glomeruli during chronic salt loading.

The effects of chronic salt loading on atrial natriuretic peptide (ANP) receptor density and affinity were studied in isolated renal glomeruli of male Sprague-Dawley rats, which received 0.9% saline as drinking fluid (NaCl-rats) and a normal rat chow diet for 35 days (N = 12). Animals on a low sodium intake received the same diet, but deionized water and served as controls (C) (N = 12). After 35 days blood pressure was only slightly increased to 136 +/- 9 in NaCl-rats versus 120 +/- 2 mm Hg in C (NS). Glomerular filtration rate, plasma cGMP and plasma ANP remained unaltered. Determination of total ANP receptor characteristics in these rats indicated a significant down-regulation of ANP receptors in salt loaded rats. Since ANP-stimulated cGMP formation was not affected by salt loading, the roles of clearance (C) and of biologically active (B) receptors were further evaluated at 21 degrees C on freshly isolated and acid washed (pH 5) glomeruli in seven animals after 35 days of salt loading and in seven animals on a low sodium intake. B-receptors were assessed by blocking C-receptors with 4-23 cANP. C-receptor numbers were lower in NaCl-rats (97 +/- 8 vs. 184 +/- 14 fmol/mg protein in C; N = 7; P less than 0.02), while C-receptor affinity was increased (Kd: 12 +/- 3 pM in NaCl-rats vs. 22 +/- 5 pM in C; P less than 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1989

Effects of endothelin on sodium transport mechanisms: potential role in cellular Ca2+ mobilization.

The effects of endothelin on cellular Ca2+ mobilization were examined in cultured rat vascular smooth muscle cells (VSMC). Endothelin (10(-8)M) induced a rapid transient increase of [Ca2+]i from 77 +/- 3 to 104 +/- 5 nM (p less than .05) in VSMC. Preincubation (60 min) with endothelin (2 x 10(-6)M) increased basal [Ca2+]i from 77 +/- 3 to 105 +/- 8 nM (p less than .05). Preincubation with endothelin also enhanced vasopressin (10(-7)M)-stimulated peak levels of [Ca2+]i (528 +/- 20 nM vs 969 +/- 21 nM, p less than .01). Endothelin (10(-7)M) induced an intracellular alkalinization (7.18 +/- 0.03 vs 7.37 +/- 0.04, p less than .01) which was blocked by pretreatment with amiloride. The biphasic effects of endothelin on [Ca2+]i were similar to those of an endogenous inhibitor of Na-K-ATPase that we examined in a previous study. Therefore, we examined the effects of endothelin on Na-K-ATPase in an enzyme preparation from hog cerebral cortex. At high concentrations, endothelin (10(-5)M) inhibited Na-K-ATPase in vitro. Thus, endothelin may exert its vasoconstrictor effects at least in part via alterations of cellular Ca2+ mobilization in VSMC. While the rapid transient increase of [Ca2+]i appears to reflect intracellular Ca2+ mobilization, the sustained effect on [Ca2+]i may be related to an increase of intracellular sodium mediated by inhibition of Na-K-ATPase and/or more likely by stimulation of the Na+/H+-antiport.

Animals↗