D609-phosphatidylcholine-specific phospholipase C inhibitor attenuates thapsigargin-induced sodium influx in human lymphocytes

Previously, we reported that the phosphatidylcholine-specific phospholipase C (PC-PLC) inhibitor tricyclodecan-9-yl xanthogenate (D609) potentiates thapsigargin-induced Ca(2+) influx in human lymphocytes. In the present study we examined the effect of D609 on the thapsigargin-induced Na(+) entry. We found that the early phase of the thapsigargin-induced increase in the intracellular Na(+) concentration (approx. 1-2 min after stimulation) was attenuated after preincubation of lymphocytes with D609. By contrast, thapsigargin-induced Na(+) influx was not affected in the presence butan-1-ol, which inhibits phosphatidylcholine-specific phospholipase D (PC-PLD). The thapsigargin-induced Na(+) influx could be mimicked by PC-PLC exogenously added to the lymphocyte suspension, whereas addition of PC-PLD had no effect. In addition, thapsigargin stimulated formation of the physiological PC-PLC products, diacylglycerol. Cell-permeable diacylglycerol analogue, dioctanoyl-glycerol (DOG), produced time- and concentration-dependent increase in the intracellular Na(+) concentration. Both thapsigargin- and DOG-induced Na(+) increases were not affected in the presence of Na(+)/H(+) antiport inhibitor, HOE609, or Na(+)/Ca(2+) antiport inhibitor, dimethylthiourea, as well as in the presence of Co(2+) and Ni(2+), which block store-operated Ca(2+) entry. By contrast, markedly reduced thapsigargin- and DOG-induced Na(+) influx were noted in the presence of flufenamic acid, which blocks the non-selective cation current (I(CRANC)). In conclusion, our results suggest that diacylglycerol released due to the PC-PLC activation contributes to the thapsigargin-induced Na(+) entry.

Phospholipase A(2) is involved in thapsigargin-induced sodium influx in human lymphocytes

Previously, we reported that emptying of intracellular Ca(2+) pools with endoplasmatic Ca(2+)-ATP-ase inhibitor thapsigargin leads to the Na(+) influx in human lymphocytes (M. Tepel et al., 1994, J. Biol. Chem. 269, 26239-26242). In the present study we examined the mechanism underlying the thapsigargin-induced Na(+) entry. We found that the thapsigargin-induced increase in Na(+) concentration was effectively inhibited by three structurally unrelated phospholipase A(2) (PLA(2)) inhibitors, p-bromophenacyl bromide, 3-(4-octadecyl)-benzoylacrylic acid (OBAA), and bromoenol lactone (BEL). The thapsigargin-induced Na(+) influx could be mimicked by PLA(2) exogenously added to the lymphocyte suspension. In addition, thapsigargin stimulated formation of arachidonic acid (AA), the physiological PLA(2) product. AA induced Na(+) entry in a time- and concentration-dependent fashion. Both, thapsigargin-induced Na(+) influx and AA liberation were completely inhibited in the presence of tyrosine kinase inhibitor genistein but not in the absence of extracellular Ca(2+). Collectively, these data show that thapsigargin-induced Na(+) entry is associated with tyrosine kinase-dependent stimulation of PLA(2).

Reactive oxygen species in essential hypertension and non-insulin-dependent diabetes mellitus

To evaluate whether increased levels of reactive oxygen species (ROS) are involved in the pathogenesis of essential hypertension (EH) and non-insulin-dependent diabetes mellitus (NIDDM), both resting and stimulated levels of intracellular ROS were measured in lymphocytes from patients with EH (n = 10), NIDDM (n = 16) and age-matched healthy individuals (control subjects, n = 19). ROS was monitored with the dye, dihydrorhodamine-123 (DHR; 1 micromol/L) in the presence or absence of superoxide dismutase (superoxide scavenger), sodium azide (singlet oxygen/hydrogen peroxide scavenger), genistein (tyrosine kinase inhibitor), or bisindolylmaleimide (protein kinase C inhibitor). Simultaneous monitoring of cytosolic [Ca2+]i was done with fura-2. Resting ROS levels were significantly higher in NIDDM (4.71+/-0.25 nmol/10(6) cells; mean +/- SEM, P<.05) compared with EH (4.03+/-0.22 nmol/10(6) cells) or controls (4.05+/-0.15 nmol/10(6) cells). The formyl-Met-Leu-Phenylalanine-(fMLP)-induced ROS generation was significantly higher in NIDDM (21.92+/-2.23 nmol/10(6) cells; P<.05) compared with EH (14.58+/-1.90 nmol/10(6) cells) or control (16.06+/-1.22 nmol/10(6) cells). The fMLP-induced ROS increase was significantly reduced in the presence of sodium azide in all groups (P<.01) but was largely unaffected in the presence of SOD. Genistein and bisindolylmaleimide significantly inhibited the fMLP-induced ROS in all groups. The fMLP-induced [Ca2+]i increase was significantly higher in NIDDM (71+/-12 nmol/L, P <.01) compared with EH (42+/-4 nmol/L) and control subjects (35+/-3 nmol/L). Phytohemagglutinin was more effective in increasing [Ca2+]i than ROS. It is concluded that ROS may play a role in the metabolic syndrome of NIDDM but not in EH.

Tyrosine and calcium/calmodulin kinases are common signaling components in the generation of reactive oxygen species in human lymphocytes

This study examined the signaling mechanism involved in the generation of reactive oxygen species (ROS) in human lymphocytes activated by formyl-Met-Leu-Phenylalanine (fMLP; 200 nmol/L) or phorbol-myristate-acetate (PMA; 100 nmol/L). ROS were monitored spectrophotometrically using dichlorofluorescin diacetate. fMLP and PMA significantly increased ROS above the control levels (p<0.05 and 0.001, respectively). These increases were significantly inhibited by catalase, sodium azide, and dimethylsulfoxide but not by superoxide dismutase, suggesting that the ROS apparently included hydrogen peroxide, singlet oxygen and hydroxyl ion but not superoxide anion. PMA-induced responses were reduced by tyrphostin (p<0.01), ST-638 (p<0.05), KN-62 (p<0.001), bisindolylmaleimide (p<0.001), RO-31-8220 (p<0.001), and by LY-83583 (p<0.001), suggesting significant involvement of tyrosine kinase, calcium/calmodulin kinase II, protein kinase C and guanylyl cyclase. fMLP-induced responses were significantly reduced by only tyrphostin (p<0.001), ST-638 (p<0.05), and KN-62 (p<0.01). The results show that tyrosine kinase and calcium/calmodulin kinase II are common signalling components in the production of reactive oxygen species in activated lymphocytes.

Evidence of altered homocysteine metabolism in chronic renal failure

The fasting serum concentrations of total homocysteine and metabolites of transsulfuration (cystathionine, cysteine, methylmalonic acid, 2-methylcitric acid) and remethylation (methionine) were determined by gas chromatography-mass spectrometry in 40 nondialyzed patients with chronic renal disease and in 50 patients with end-stage renal disease requiring chronic maintenance hemodialysis. The nondialyzed patients and 28 of the dialysis patients did not receive additional vitamin supplementations. Twenty-two of the dialysis patients received daily oral vitamin preparations containing 10 mg pyridoxine (vitamin B(6)), 6 microg cyanocobalamin (vitamin B(12)), and 1 mg folic acid. In the nondialyzed patients, linear regression analysis showed positive correlations between serum concentrations of creatinine and total homocysteine (r = 0.68, p < 0.0001), cystathionine (r = 0.73, p < 0. 0001), methylmalonic acid (r = 0.77, p < 0.0001), and 2-methylcitric acid (r = 0.81, p < 0.0001). Serum homocysteine was positively correlated with serum concentrations of cystathionine (r = 0.59, p < 0.0001), cysteine (r = 0.69, p = 0.004), methylmalonic acid (r = 0. 64, p = 0.0001), and 2-methylcitric acid (r = 0.64, p < 0.0001). There was no significant correlation between serum concentrations of homocysteine and methionine (r = -0.14, p = 0.63). In the hemodialysis patients receiving oral vitamin supplementation, serum homocysteine and cystathionine concentrations were significantly lower than in hemodialysis patients not receiving vitamins (homocysteine 21.8 +/- 1.1 vs. 33.2 +/- 3.7 micromol/l, p = 0.0004; cystathionine 2,075.9 +/- 387.1 vs. 3,171.3 +/- 680.2 nmol/l, p = 0. 02; mean +/- SEM). In summary, our results show increased intermediate products of the transsulfuration pathway, but no increase in remethylation of homocysteine in chronic renal disease, including end-stage renal disease requiring chronic maintenance dialysis.

Effect of dexamethasone on the lymphocytic Na+/H+ antiporter activity

OBJECTIVE

The effects of short-term administration of dexamethasone on sodium/proton antiporter (Na+/H+ antiporter) in human lymphocytes were investigated in vitro.

METHODS

Cytosolic pHi in lymphocytes was measured spectrophotometrically using the pH-sensitive fluorescent dye 2'-7'-bis-carboxyethyl-5(6)-carboxyfluorescein at 530 nm with excitation wavelengths of 440 and 530 nm. The Na+/H+ antiporter activity was determined after intracellular acidification using 100 mmol/l propionic acid.

RESULTS

The addition of 1 micromol/l dexamethasone significantly reduced cytosolic pHi from 7.44+/-0.03 to 7.25+/-0.05 (mean +/- SEM; P<0.01). Incubation with dexamethasone for 40 min significantly reduced the Na+/ H+ antiporter activity from (9.19+/-0.61)x10(-3) pHi/s (n = 22) to (7.23+/-0.49)x10(-3) pHi/s (n = 22; P<0.01). The effect of dexamethasone was time and concentration dependent The apparent affinity of the Na+/H+ antiporter was not significantly different in the absence or presence of dexamethasone. The inhibition of the Na+/H+ antiporter by dexamethasone was abolished in the presence of the glucocorticoid receptor blocker, mifepristone.

CONCLUSION

Dexamethasone directly inhibits the Na+/H+ antiporter using a receptor-dependent pathway. This effect may be important for pharmacological side effects such as glucocorticoid-induced hypertension.

Identification of diadenosine hexaphosphate in human erythrocytes

Diadenosine polyphosphates have been identified as important regulators of vascular tone and blood pressure. In reference to the background of the well-known vasoconstriction induced by hemolysate, we questioned whether this action may be due in part to the presence of diadenosine polyphosphates in human erythrocytes. Therefore, erythrocytes were separated from other blood cells and deproteinated. To concentrate and purify nucleotides, the extract was chromatographed by anion exchange, affinity, and reversed-phase columns. In one of the purified fractions, diadenosine hexaphosphate (diadenosine 5', 5'-P(1), P(6) hexaphosphate [AP(6)A]) was identified by matrix-assisted laser desorption/ionization mass spectrometry, ultraviolet spectroscopy, and enzymatic analysis. Hemolysate of erythrocytes injected into the isolated perfused rat kidney induced a dose-dependent vasoconstriction, which was partially inhibited by P(2)-purinoceptor antagonist. The data document the existence of AP(6)A in erythrocytes. AP(6)A may be involved in the pathogenesis of vasospasm induced by free hemoglobin.

Identification and characterization of P(1), P(7)-Di(adenosine-5')-heptaphosphate from human platelets

Diadenosine pentaphosphate and diadenosine hexaphosphate have been isolated in human platelets and have been postulated to play an important role in the control of vascular tone. Here we describe the isolation and identification of diadenosine heptaphosphate from human platelets. Dinucleoside polyphosphates were concentrated by affinity chromatography from a nucleotide-containing fraction from deproteinated human platelets. Dinucleoside polyphosphates were purified by anion-exchange and reversed phase high performance liquid chromatography to homogeneity. Analysis of one of these fractions with matrix-assisted laser desorption/ionization mass spectrometry revealed a molecular mass of 1076.4 (1077.4 = [M + H](+)) Da. UV spectroscopic analysis of this fraction showed the spectrum of an adenosine derivative. Comparison of the postsource decay matrix-assisted laser desorption/ionization mass spectrum of the fraction minus that of diadenosine heptaphosphate (Ap(7)A) demonstrated that the isolated substance was identical to Ap(7)A. The identity of the retention times of the authentic and the isolated compound confirmed this result. Enzymatic analysis demonstrated an interconnection of the phosphate groups with the adenosines in the 5'-positions of the riboses. With thrombin-induced platelet aggregation, Ap(7)A is released from the platelets into the extracellular space. The vasoconstrictive action of Ap(7)A on the vasculature of the isolated perfused rat kidney Ap(7)A was slightly less than that of Ap(6)A. The threshold of the vasoconstrictive action of Ap(7)A was 10(-5) mol/liter. The vasoconstrictive effect was abolished by suramin and pyridoxal phosphate 6-azophenyl-2', 4'-disulfonic acid, suggesting an activation of P(2x) receptors. Furthermore, Ap(7)A inhibits ADP-induced platelet aggregation. Thus, the potent vasoconstrictor Ap(7)A derived from human platelets, like other diadenosine polyphosphates, may play a role in the regulation of vascular tone and hemostasis.

Na(+)/Ca(2+) exchange inhibitors modulate thapsigargin-induced Ca(2+) and Na(+) influx in human lymphocytes

Thapsigargin has been shown the elevate intracellular Na(+) concentration in human lymphocytes, but mechanisms underlying thapsigargin-induced Na(+) entry are little understood. In the present study we investigated thapsigargin-induced changes in cytosolic free Na(+) and Ca(2+) concentration in human lymphocytes after inhibition of the Na(+)/Ca(2+) exchange with two structurally unrelated compounds, dimethylthiourea ad bepridil. The intracellular Na(+) increase induced by 5 microM thapsigargin was significantly enhanced in the presence of 5 mM dimethylthiourea or 40 microM bepridil. In contrast, both compounds significantly decreased the thapsigargin-induced intracellular Ca(2+) elevation. No effect of dimethylthiourea or bepridil on thapsigargin-induced Ca(2+) influx was observed in the absence of extracellular Na(+). These observations are consistent with the hypothesis that thapsigargin stimulates Na(+)/Ca(2+ )exchange in human lymphocytes. However, Na(+)/Ca(2+) exchange does not mediate Na(+) influx in human lymphocytes.

Regulation of the Na+/H+ antiporter in patients with mild chronic renal failure: effect of glucose

BACKGROUND

The aim of this study was to determine the glucose-dependent regulation of the sodium-proton-antiporter (Na+/H+ antiporter) in patients with mild chronic renal failure (CRF).

METHODS

We measured plasma glucose concentrations, plasma insulin concentrations, plasma C peptide concentrations, arterial blood pressure, cytosolic pH (pHi), cellular Na+/H+ antiporter activity, and cytosolic sodium concentration ([Na+]i) in 19 patients with CRF and 41 age-matched healthy control subjects (control) during a standardized oral glucose tolerance test. Intracellular pHi, [Na+]i, and Na+/H+ antiporter activity was measured in lymphocytes using fluorescent dye techniques.

RESULTS

Under resting conditions, the pHi was significantly lower, whereas the Na+/H+ antiporter activity was significantly higher in CRF patients compared with controls (each P < 0.0001). The oral administration of 100 g glucose significantly increased the Na+/H+ antiporter activity in CRF patients from 13.35 +/- 1.26 x 10-3 pHi/second to 16.44 +/- 1.37 x 10-3 pHi/second after one hour and to 14.06 +/- 1.36 x 10-3 pHi/second after two hours (mean +/- SEM, P = 0.008 by Friedmans's two-way analysis of variance). In controls, the administration of 100 g glucose significantly increased the Na+/H+ antiporter activity from 4.23 +/- 0.20 x 10-3 pHi/second to 6.00 +/- 0.56 x 10-3 pHi/second after one hour and to 6.65 +/- 0.64 x 10-3 pHi/second after two hours (P = 0.0003). The glucose-induced enhancement of the Na+/H+ antiporter activity was more pronounced in CRF patients compared with controls (P = 0.011). Resting [Na+]i was not significantly different between the two groups.

CONCLUSIONS

CRF patients show an intracellular acidosis leading to an increased Na+/H+ antiporter activity. In addition, high glucose levels exaggerate the differences in Na+/H+ antiporter activity already present between cells from patients with mild CRF and those from control subjects.

Evidence for two different P2X-receptors mediating vasoconstriction of Ap5A and Ap6A in the isolated perfused rat kidney

The activation of various P2-receptor subtypes in rat renal vasculature by P1, P5-diadenosine pentaphosphate (ApsA) and P1, P6-diadenosine hexaphosphate (Ap6A) were studied by measuring their effects on perfusion pressure during continuous perfusion in a rat isolated perfused kidney. Permanent perfusion with Ap5A and Ap6A elicited both a transient and sustained vasoconstriction with both vasoconstrictions to be different: the transient vasoconstriction can be elicited with concentrations > or = 10 nM, whereas the sustained vasoconstriction is observed with concentrations > or = 1 nM. ApsA and Ap6A act via the same receptors as alpha,beta-methylene ATP (alpha,beta-meATP). The rank order of potency for transient vasconstriction was alpha,beta-meATP = ApsA>Ap6A>B,gamma-meATP, and for sustained vasoconstriction alpha,beta-meATP = Ap5A > beta,gamma-meATP > or = Ap6A. Suramin, a non-selective P2-receptor antagonist, and pyridoxal-phosphate-6-azophenyl-2;4-disulphonic acid (PPADS) a highly selective P2X-receptor antagonist antagonized both the transient and the sustained vasoconstriction. Taken together the results of the agonist profile of Ap5A and Ap6A and comparing its findings to literature it can be demonstrated that the transient but not the sustained vasoconstriction is mediated via the P2X1-receptor which is present in rat renal vasculature. It is demonstrated that the agonist profile of the sustained vasoconstriction induced by ApsA and Ap6A does not fit to any currently known P2X- or P2Y-receptor subtype. We conclude a yet unidentified P2X-receptor or chimeric P2X-receptor may contribute to the effects on rat renal vasculature produced by Ap5A and Ap6A and which may play an important role in glomerular perfusion pressure and blood pressure control.

Thapsigargin-insensitive calcium pools in vascular smooth muscle cells

Since sarcoplasmic Ca2+-ATPase may play an important role for the regulation of cytosolic free calcium concentration ([Ca2+]i) and may be altered in primary hypertension, the effects of thapsigargin and bradykinin on intracellular calcium pools in cultured vascular smooth muscle cells (VSMC) from spontaneously hypertensive rats of the Münster strain (SHR) and normotensive Wistar-Kyoto (WKY) rats were investigated. VSMC were cultured on glass cover slips and [Ca2+]i was measured using the fluorescent dye fura2. To exclude transplasmamembrane calcium influx all experiments were performed in a calcium free medium. Thapsigargin, a selective inhibitor of the sarcoplasmic Ca2+-ATPase, and bradykinin, that is known to induce inositol trisphosphate release, dose dependently caused an increase of [Ca2+]i by emptying intracellular Ca2+ stores. The peak increase of [Ca2+]i after addition of saturation doses of thapsigargin (1 micromol/L) was not significantly different in the two strains (SHR: 69 +/- 11 nmol/L, n=24; WKY: 58 +/- 12 nmol/L, n=20; mean +/- SEM). When 10 micromol/L bradykinin was added after depletion of the thapsigargin-sensitive pools, still a release of [Ca2+]i could be observed. The bradykinin-induced [Ca2+]i increase was similar in the absence and presence of thapsigargin in VSMC from SHR (62 +/- 12 nmol/L, n=20; vs 52 +/- 18 nmol/L, n=22). In contrast, in the VSMC from WKY a significant reduction of the bradykinin induced [Ca2+]i-increase could be observed after the depletion of the thapsigargin sensitive calcium pools (70 +/- 8 nmol/L, n=21, vs. 33 +/- 7, n=20; p<0.002). It is concluded that bradykinin releases calcium from a pool that is not refilled by the common, thapsigargin-sensitive Ca2+-ATPase. In contrast to VSMC from normotensive WKY, in VSMC from spontaneously hypertensive rats thapsigargin and bradykinin sensitive pools may be regulated separately.

Chemoattractant- and mitogen-induced generation of reactive oxygen species in human lymphocytes: the role of calcium

This study examined the role of calcium in the generation of reactive oxygen species (ROS) in human lymphocytes activated by the chemoattractant formyl-Met-Leu-Phe (fMLP) and the T-cell mitogen phytohaemagglutinin (PHA). The concentrations of cytosolic calcium ([Ca2+]i) and ROS were monitored simultaneously with a fluorescence spectrophotometer after the cells had been incubated in fura-2 (calcium-sensitive dye) and 2',7'-dichlorofluorescein diacetate (DCF-DA, ROS-sensitive dye). The lymphocytes were stimulated with fMLP (200 nmol l-1) or PHA (10 micromol l-1) in the absence and presence of extracellular calcium. A dose-response test was also conducted for extracellular calcium. fMLP and PHA significantly increased both [Ca2+]i (P < 0.001) and ROS concentrations (P < 0. 001) above the control levels in the presence of extracellular calcium. However, such increases were abolished in the absence of extracellular calcium, suggesting total dependence of the responses to both fMLP and PHA on transplasma-membrane calcium influx. There were also graded increases in ROS with increasing concentrations of extracellular calcium. The results show that transplasma-membrane calcium influx is essential for fMLP- and PHA-induced generation of reactive oxygen species in human lymphocytes.

High-performance liquid chromatographic assay of the diadenosine polyphosphates in human platelets

Diadenosine pentaphosphate (Ap5A) and diadenosine hexaphosphate (Ap6A) were recently identified in human platelets and were shown to be important modulators of cardiovascular function. Here we describe an HPLC assay for quantitating Ap3A, Ap4A, Ap5A, and Ap6A contents in human platelets simultaneously. Di(1,N6-ethenoadenosine) hexaphosphate was used as internal standard. The extraction procedure consists of (a) deproteinization, (b) selective concentration of the diadenosine polyphosphates with a boronate affinity chromatography, and (c) desalting prior to the HPLC analysis. The assay was validated by PSD-MALDI-mass spectrometry and by addition of authentic diadenosine polyphosphate to platelet samples. The assay was carried out by an ion-pair reversed-phase perfusion chromatography. In platelets from human blood the following amounts of diadenosine polyphosphates were determined: Ap3A, 192.5 +/- 151.0 nM; Ap4A, 223.8 +/- 172.3 nM; Ap5A, 100.2 +/- 81.1 nM; Ap6A, 32.0 +/- 19.6 nM (mean +/- SD, n = 105). The described assay can be used with less than 20 ml blood and allows quantitation of the diadenosine polyphosphates in the picomole range.

Identification and characterization of diadenosine 5',5"'-P1,P2 -diphosphate and diadenosine 5',5"'-P1,P3-triphosphate in human myocardial tissue

We examined whether human cardiac tissue contains diadenosine polyphosphates and investigated their physiological role. Extracts from human cardiac tissue from transplant recipients were fractionated by size exclusion-, affinity-, anion exchange- and reversed-phase chromatography. MALDI-MS analysis of two absorbing fractions revealed molecular masses of 676.2 Da and 756.0 Da. The UV spectra of both fractions were identical to that of adenosine. Postsource decay MALDI mass spectrometry indicated that the molecules with a mass of 676.2 Da and 757.0 Da contained AMP and ATP, respectively. As shown by enzymatic cleavage, both molecules consist of two adenosines interconnected by either two or three phosphates in 5'-positions of the riboses. Two substances can be identified as 5',5"'-P1,P2-diphosphate (Ap2A) and 5',5"'-P1, P3-triphosphate (Ap3A). Ap2A and Ap3A, together with ATP and ADP, are stored in myocardial-specific granules in biologically active concentrations. In the isolated perfused rat heart, Ap2A and Ap3A caused dose-dependent coronary vasodilations. In myocardial preparations, Ap2A and Ap3A attenuated the effect of isoproterenol, exerting a negative inotropic effect. The calcium current of guinea pig ventricular myocytes, stimulated by isoproterenol, was also attenuated by Ap2A and Ap3A. The presence of Ap2A and Ap3A in cardiac-specific granules and the actions of these substances on the myocardium and coronary vessels indicate a role for these substances as endogenous modulators of myocardial functions and coronary perfusion.

Mediation of the vasoactive properties of diadenosine tetraphosphate via various purinoceptors

OBJECTIVE AND METHODS

The vasoactive properties of P1,P4-diadenosine tetraphosphate (Ap4A) were studied by measuring the effects of perfusion pressure of a rat isolated perfused kidney.

RESULTS

The vasoconstrictive response to Ap4A was mediated to a large extent to a P2X receptor which could be shown by inhibition with pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid tetrasodium. The remaining vasoconstriction of Ap4A could be blocked by a 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective A1 receptor antagonist In raised tone preparation Ap4A evoked vasodilation when P2 receptors were blocked by suramin. The dilation was not mediated by a P2Y receptor as the effect could not be blocked by suramin.

CONCLUSION

Ap4A induces vasoconstriction via A1 and P2X receptors and vasodilatation via an unidentified receptor which is not a P2Y receptor. Ap4A may play an important role in kidney perfusion and, thus, in blood-pressure control.

Decreased membrane Mg2+ concentrations in a subgroup of hypertensives: membrane model for the pathogenesis of primary hypertension

Intracellular Mg2+ measurements were performed in erythrocyte membranes of 18 untreated normotensive and 19 untreated essential hypertensive patients. Mg2+ concentrations were determined by atomic absorption spectroscopy using a Video 12 apparatus. The results show that in patients with essential hypertension total Mg2+ content in erythrocyte membranes was significantly decreased as compared with the control group (0.28 +/-0.05 v 0.52+/-0.15 mmol/g membrane protein; mean+/-SD, P < .001). Additionally, plasma and free intracellular Mg2+ content of lymphocytes and platelets showed no significant difference in normotensives and hypertensives. Lowered total membrane Mg2+ concentrations in a subgroup of primary hypertensives may contribute to the development of this disorder, perhaps due to different buffering or membrane transport systems.

Identification of oxidized low-density lipoprotein in human serum by NMR spectroscopy

1. In this study we compared the 500 MHz 1H-NMRs from native and oxidized low-density lipoproteins. 2. The measurements revealed a characteristic pattern of three resonances in spectra from oxidized, but not from native low-density lipoprotein at 1.17 p.p.m., 1.18 p.p.m. and 1.20 p.p.m. (relative to 3-trimethylsilyl-[2,2,3, 3-2H4]-propionate).3.A quantitative comparison between these resonances in sera from patients with coronary heart disease and healthy control subjects revealed that the intensity was significantly higher in patients with coronary heart disease (1.17 p.p.m.: 0.026+/-0.014 versus 0.015+/-0.019; 1.18 p.p.m.: 0.032+/-0.011 versus 0.017+/-0.021; 1.20 p.p.m.: 0.030+/-0.066 versus 0.010+/-0.005; P<0.05 compared with healthy control subjects for each resonance).4.Fractionation of sera from patients with coronary heart disease revealed that the resonances equal to those obtained from experimentally oxidized low-density lipoprotein are indeed caused by the low-density lipoprotein fraction of the sera.5. When the NMRs from sera were calibrated with oxidized low-density lipoprotein prepared by Cu2+ oxidation, a concentration of 66.5+/-28.6 microgram/ml and 36.3+/-23.7 microgram/ml (P<0.05) was estimated in patients with coronary heart disease and healthy subjects respectively. Elevated levels of oxidized low-density lipoprotein also occurred in those patients with normal serum concentrations of total low-density lipoprotein.6. The study shows a simple method to measure oxidized low-density lipoprotein in human serum and may gain interest to assess the cardiovascular risk factor profiles more completely.

Activation of Na+, H+ exchanger produces vasoconstriction of renal resistance vessels

To evaluate the influence of the sodium/proton exchanger (Na+,H+ exchanger) on the constriction of rat resistance vessels and on the iliac artery, the isometric vasoconstrictions of renal resistance vessels and strips from iliac artery derived from Wistar-Kyoto rats were measured using a vessel myograph. The Na+,H+ exchanger was activated by intracellular acidification using propionic acid. Cytosolic pH (pHi) and cytosolic free sodium concentration ([Na+]i) in vascular smooth muscle cells were measured using the fluorescent dye technique. The activation of the Na+,H+ exchanger increased the [Na+]i by 12.4 +/- 1.3 mmol/L (n = 8). The activation of the Na+,H+ exchanger caused a contractile response of the renal resistance vessels (increase of tension, 1.5 +/- 0.1 x 10(-3) N; n = 13) and of the rat iliac artery (increase of tension, 7.5 +/- 0.8 x 10(-3) N; n = 5). The contractile response after activation of the Na+,H+ exchanger was significantly inhibited in the absence of external sodium or in the presence of amiloride, confirming the involvement of the Na+,H+ exchanger. The contractile response after activation of the Na+,H+ exchanger was significantly reduced in the absence of external calcium, after inhibition of calcium channels by nifedipine, and in the presence of an intracellular calcium antagonist 8-(diethylamino-)-octyl-3,4,5-trimethoxybenzoate (TMB-8), indicating that the activation of the Na+,H+ exchanger consecutively caused transplasma membrane calcium influx. On the other hand, the inhibition of the Na+,Ca2+ exchanger by NiCl2 significantly increased the vasoconstriction of renal resistance vessels after activation of the Na+,H+ exchanger. The activation of the Na+,H+ exchanger produces vasoconstriction by an increased cytosolic sodium concentration, inhibition of the Na+,Ca2+ exchanger, and activation of transplasma membrane calcium influx through potential dependent calcium channels.

Vitamin supplementation during weight reduction--favourable effect on homocysteine metabolism

Moderately elevated homocysteine concentrations, reflecting deficiency of some nutritional factors required for homocysteine metabolism (folate, vitamin B-6, vitamin B-12) and/or less severe genetic defects, are common in the general population. Several studies have indicated the role of homocysteine as an independent risk factor for vascular disease. A pilot study published recently suggested that plasma homocysteine levels increase during weight reduction in slightly overweight, otherwise healthy subjects (group A). We examined a comparable group of 13 overweight subjects (group B) using a standardised caloric intake and defined vitamin supplementation (Medyn: folate 0.2 mg/ vitamin B-68.0 mg/ vitamin B-120.010 mg three times the day orally) to determine the effect of weight reduction on serum homocysteine levels and to compare the results with those of the pilot study. Mean body weight declined from 87.0 +/- 20.2 to 84.2 +/- 20.1 kg (P < 0.05) in group A and 85.7 +/- 11.3 to 82.5 +/- 9.9 kg (P = 0.049) in group B. Serum homocysteine levels rose from 7.9 +/- 2.0 to 8.7 +/- 2.3 mumol/l (P < 0.0001) in group A and decreased from 8.19 +/- 1.73 to 7.35 +/- 0.88 mumol/l (P = 0.0022) in group B. No correlation was found between the changes in body weight and in homocysteine levels (r = 0.02 in group A, r = 0.18 in group B). Additionally, no correlation was found between serum folate levels and changes in homocysteine levels (r = 0.03 in group A, r = 0.09 in group B). The results suggest that an adequate oral vitamin-supplementation protects against increased homocysteine production during weight reduction.

Characterization of dimethylguanosine, phenylethylamine, and phenylacetic acid as inhibitors of Ca2+ ATPase in end-stage renal failure

The activity of the plasma membrane Ca2+ ATPase of chronic renal failure patients is decreased by circulating inhibitors yet to be characterized. In this study, inhibitors of Ca2+ ATPase were isolated from ultrafiltrate of patients with end-stage renal failure. They were identified as dimethylguanosine, phenylethylamine, and phenylacetic acid by chromatography and mass spectrometry. Ca2+ ATPase activity was measured spectrophotometrically as the difference in hydrolysis of ATP in the presence and absence of Ca2+ with different concentrations of ATP and the isolated substances. All of the identified compounds are sufficiently lipophilic to penetrate the blood-brain barrier and to accumulate in cerebral tissue. The inhibitory effects of these agents were additive. The apparent K(m) values for ATP and Ca2+ were not altered by these substances, suggesting a noncompetitive mechanism of inhibition. In plasma of healthy subjects, the substances were not detectable. The Ca2+ ATPase inhibitors identified may play a role in the pathophysiology of end-stage renal failure and, potentially, in monitoring toxic effects on cellular Ca2+ metabolism in renal failure.