Proscillaridin A immunoreactivity: its purification, transport in blood by a specific binding protein and its correlation with blood pressure

A material crossreacting with antibodies against the bufadienolide proscillaridin A and inhibiting the sodium pump was found in human blood plasma. The concentration of the material with a retention time similar to ouabain in a reversed phase HPLC correlated to systolic blood pressure and pulse pressure. Affinity purification of this compound from bovine adrenals resulted in the isolation of a compound with molecular mass of 600 Da that was not identical with ouabain. Consistent with the postulate that endogenous ouabain and proscillaridin A immunoreactivities may belong to a new class of cardiotonic steroid hormones, a protein of Mr = 60 kDa has been found in bovine serum by affinity-labeling with N-hydroxysuccimidyl digoxigenin-3-O-methylcarbonyl-epsilon-aminocaproate.

HDL3-mediated inhibition of thrombin-induced platelet aggregation and fibrinogen binding occurs via decreased production of phosphoinositide-derived second messengers 1,2-diacylglycerol and inositol 1,4,5-tris-phosphate

We demonstrate that physiological concentrations of HDL3 inhibit the thrombin-induced platelet fibrinogen binding and aggregation in a time- and concentration-dependent fashion. The underlying mechanism includes HDL3-mediated inhibition of phosphatidylinositol 4,5-bis-phosphate turnover, 1,2-diacylglycerol and inositol 1,4,5-tris-phosphate formation, and intracellular calcium mobilization. The inhibitory effects of HDL3 on inositol 1,4,5-tris-phosphate formation and intracellular calcium mobilization were abolished after covalent modification of HDL3 with dimethylsuberimidate. Furthermore, they could be blocked by calphostin C and bis-indolylmaleimide, 2 highly selective and structurally unrelated protein kinase C inhibitors. However, the inhibitory effects of HDL3 were not blocked by H89, a protein kinase A inhibitor. In addition, HDL3 failed to induce cAMP formation but stimulated the phosphorylation of the protein kinase C 40- to 47-kD major protein substrate. We observed a close temporal relationship between the HDL3-mediated inhibition of thrombin-induced inositol 1,4,5-tris-phosphate formation, intracellular calcium mobilization, and fibrinogen binding and the phosphorylation of the protein kinase C 40- to 47-kD major protein substrate. Taken together, these findings indicate that the HDL3-mediated inhibition of thrombin-induced fibrinogen binding and aggregation occurs via inhibition of phosphatidylinositol 4,5-bis-phosphate turnover and formation of 1,2-diacylglycerol and inositol 1,4,5-tris-phosphate. Protein kinase C may be involved in this process.

Adenosine(5') oligophospho-(5') guanosines and guanosine(5') oligophospho-(5') guanosines in human platelets

We isolated and identified nucleoside(5') oligophospho-(5') nucleosides containing adenosine and guanosine (ApnG; n = 3-6) as well as diguanosine polyphosphates (GpnG; n = 3-6) in human platelets. For identification, UV spectrometry, matrix-assisted laser desorption/ionization, postsource decay matrix-assisted laser desorption/ionization mass spectrometry, and enzymatic cleavage experiments were used. The adenosine(5') oligophospho-(5') guanosines act as vasoconstrictors and growth factors. The diguanosine polyphosphates are potent modulators of growth in vascular smooth muscle cells, but do not affect vascular tone.

Sodium-dependent Cl-/HCO3- exchange in patients with chronic renal failure: correlation with renal function

To investigate the effects of uremia on cellular function the activity of the sodium-dependent chloride-bicarbonate exchanger (sodium-dependent Cl-/HCO3- exchanger) and the sodium-independent chloride-bicarbonate exchanger (sodium-independent Cl-/HCO3- exchanger) were examined in lymphocytes from 25 patients with mild chronic renal failure, 9 patients with end-stage chronic renal failure on regular hemodialysis, and from 25 age-matched healthy control subjects. Cytosolic pH (pHi) and the activity of the sodium-dependent Cl-/HCO3- exchanger and the sodium-independent Cl-/HCO3- exchanger were measured spectrophotometrically using the pH-sensitive fluorescent dye 2'7'-bis-carboxyethyl-5 [6]-carboxyfluorescein acetoxy-methylester (BCECF-AM). The activation of the sodium-dependent Cl-/HCO3- exchanger by removal of extracellular chloride was prevented in the presence of 500 micromol/liter 4,4' diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) or in the absence of extracellular sodium, but was not affected by the specific inhibitor of the sodium/proton exchanger, ethyl isopropyl amiloride (EIPA). The sodium-dependent Cl-/HCO3- exchangers were significantly different in lymphocytes from healthy control subjects, patients with mild chronic renal failure, and patients with end-stage chronic renal failure (X2 = 6.43, P = 0.040 by Kruskal-Wallis-test). The sodium-dependent Cl-/HCO3- exchanger was significantly lower in patients with end-stage chronic renal failure compared to patients with mild chronic renal failure or compared to healthy control subjects (each P < 0.05). In patients with chronic renal failure a significantly negative correlation between sodium-dependent Cl-/HCO3- exchanger and the serum creatinine concentration (r = -0.507; P = 0.0022) could be observed. On the other hand, resting pHi in lymphocytes and sodium-independent Cl-/HCO3- exchanger were not significantly different in lymphocytes from healthy control subjects, patients with mild chronic renal failure or patients with end-stage chronic renal failure. The present study suggests that the activity of the sodium-dependent Cl-/HCO3- exchanger is progressively impaired in chronic renal failure.

Hypertensive crisis: pathophysiology, treatment and handling of complications

Hypertensive crisis is a potentially life-threatening complication of nearly all forms of hypertension. Vascular damage causing end-organ failure and direct complications such as intracerebral hemorrhage determine the outcome of hypertensive crisis. In the case of apparent end-organ damage immediate and effective lowering of blood pressure is necessary. In addition to drugs used traditionally in hypertensive crisis, a number of alternative antihypertensives such as urapidil or enalaprilat have been recently introduced for treatment of hypertensive crisis.

Transgenic hypertensive rats show a reduced angiotensin II induced [Ca2+]i response in glomerular mesangial cells

The effects of angiotensin II (Ang II) induced changes of cytosolic free calcium concentration ([Ca2+]i) and growth response were investigated in transgenic TGR(mREN2)27 rats, a strain showing fulminant hypertension after the mouse Ren-2d renin gene has been integrated into its genome, in age-matched normotensive Sprague-Dawley rats (SD), in spontaneously hypertensive rats of the Münster strain (SHR), and in normotensive Wistar-Kyoto rats (WKY). In each strain the Ang II induced changes of [Ca2+]i were measured in cultured glomerular mesangial cells (MC) using the calcium-sensitive fluorescent dye, fura2. Resting [Ca2+]i was not significantly different between the strains tested. The Ang II induced [Ca2+]i rise was significantly less in MC from TGR(mREN2)27 compared to SD (peak level at 200 seconds: 161 +/- 15 nmol/L vs 217 +/- 43 nmol/L; mean +/- SEM; p<0.05). In the absence of external calcium, the Ang II induced [Ca2+]i increase was similar in MC from TGR(mREN2)27 and SD, indicating that the Ang II induced trans-plasma membrane calcium influx but not the calcium release is impaired in TGR(mREN2)27. The arginine vasopressin or endothelin induced [Ca2+]i increase were not significantly different in MC from TGR(mREN2)27 and SD. The Ang II or PDGF induced 3H-thymidine incorporation was not significantly different in MC from TGR(mREN2)27 and SD, indicating that the early growth response to Ang II is not impaired in TGR(mREN2)27. The Ang II induced peak [Ca2+]i increase was significantly enhanced in MC from SHR compared to WKY (215 +/- 30 nmol/L, n=17; vs 161 +/- 35 nmol/L, n=17; p<0.05). It is concluded that TGR(mREN2)27 show a selective defect in the cellular calcium response to Ang II.

Influence of end-stage renal failure and hemodialysis on event-related potentials

Chronic renal failure frequently causes uremic encephalopathy with impairment of different cognitive functions, but the pathophysiology of uremic encephalopathy is still unknown. We measured visually evoked event-related potentials (ERPs) in 33 neurologically asymptomatic patients before and after they underwent hemodialysis and compared their data with those of a strictly age-matched healthy control group. Before hemodialysis, the patients' P3 latency was significantly increased and P3 amplitude was significantly decreased as compared with that of the healthy control group. After hemodialysis, P3 latency of the patients showed a significant decrease (457+/-56 before and 438+/-54 ms after hemodialysis) and the P3 latency habituation during the ERP measurement was also significantly decreased. Patients with higher levels of blood urea nitrogen (BUN), creatinine, and uric acid performed better in ERP measurement than did patients with lower levels. Hemoglobin did not influence ERP latencies and amplitudes. Our data suggest that impaired cognitive processing can be disclosed by ERP even in neurologically asymptomatic chronic renal disease. Removal of uremic toxins by hemodialysis leads to an improvement in cognitive processing.

Phosphatidylcholine-specific phospholipase C regulates thapsigargin-induced calcium influx in human lymphocytes

The involvement of phosphatidylcholine-specific phospholipase C (PC-PLC) and D (PC-PLD) in the regulation of the thapsigargin-induced Ca2+ increase was investigated. Pretreatment of human lymphocytes with the PC-PLC inhibitors D609 or U73122 enhanced the thapsigargin-induced Ca2+ influx. By contrast, no effect was observed in the presence of phospholipase D inhibitor butanol. Addition of exogenous PC-PLC but not PC-PLD to lymphocytes prestimulated with thapsigargin led to a decrease of intracellular Ca2+. In addition, thapsigargin was shown to release diacylglycerol (DAG) from cellular phosphatidylcholine pools. The thapsigargin-induced DAG formation was inhibited by U73122 and D609 but not by butanol. Moreover, no formation of the PC-PLD activity marker phosphatidylbutanol was detected. Thapsigargin-induced DAG formation was dependent on the Ca2+ entry, as it was abolished in the absence of extracellular Ca2+ or in the presence of Ni2+. Further investigations demonstrated that the inhibition of the cellular DAG target, protein kinase C (PKC), enhanced thapsigargin-induced Ca2+ increase, whereas direct PKC activation had an inhibitory effect. Taken together, our results reveal the involvement of PC-PLC in the regulation of the thapsigargin-induced Ca2+ increase and point to the existence of a physiologic feedback mechanism activated by Ca2+ influx and acting via consecutive activation of PC-PLC and PKC to limit the rise of intracellular Ca2+.

Diadenosine polyphosphates' action on calcium and vessel contraction

The effects of the endogenous, platelet-derived vasoactive compounds, diadenosine tetraphosphate (AP4A), diadenosine pentaphosphate (AP5A), and diadenosine hexaphosphate (AP6A) on the vasoconstriction of isolated rat renal resistance vessels and rat aortic strips were measured using a vessel myograph. In addition, the effects of AP4A, AP5A, and AP6A on the cytosolic free calcium concentration ([Ca2+]i) were evaluated in cultured rat vascular smooth muscle cells (VSMC) using the fluorescent dye technique. Diadenosine polyphosphates dose-dependently increased the force of renal resistance vessels and isolated aortic strips. The administration of 10 mumol/L AP4A, AP5A, or AP6A significantly increased the force of isolated renal resistance vessels by 3.48+/-0.43 mN (n = 8), 2.14+/-0.40 mN (n = 12), or 2.70+/-0.31 mN (n = 11, each P < .01 compared with resting tension), respectively. The administration of 10 micromol/L AP4A, AP5A, or AP6A significantly increased the force of isolated aortic strips by 2.45+/-0.97 mNewton (n = 10), 2.70+/- 0.30 mN (n = 6), or 1.48+/-0.20 mN (each P < .01 compared with resting tension), respectively. The administration of 10 micromol/L AP4A, AP5A, or AP6A significantly increased [Ca2+]i in VSMC to a peak concentration of 314+/-60 nmol/L (n = 6), 247+/-25 nmol/L (n = 15), or 332+/-100 nmol/L (n = 5), respectively (each P < .01 compared with resting value). Both the diadenosine polyphosphate-induced vasoconstriction and [Ca2+]i increase was significantly reduced in the absence of extracellular calcium or after administration of a specific inhibitor of P2 purinoceptors. It is concluded that diadenosine polyphosphates increase [Ca2+]i and hence cause vessel constriction.

Na+/H+ exchange during an oral glucose challenge in patients with essential hypertension

To determine the effects of an oral glucose challenge on cellular Na+/H+ exchange in vivo we measured plasma glucose concentrations, plasma insulin concentrations, plasma C-peptide concentrations, arterial blood pressure, cytosolic pH (pHi) and cellular Na+/H+ exchange in 24 patients with essential hypertension (HT) and 41 age-matched healthy normotensive control subjects (NT) during a standardized oral glucose tolerance test. Under resting conditions, the plasma glucose concentrations, plasma insulin concentrations, plasma C-peptide concentrations and Na+/H+ exchange activity were significantly higher in HT compared with NT (P < 0.05 in each case). A significant increase in lymphocytic Na+/H+ exchange activity was only seen in NT (resting 0 h: (4.23 +/- 0.2) x 10(-3) pHi/s; mean +/- S.E.M.; 1 h after glucose administration: (6.00 +/- 0.56) x 10(-3) pHi/s; 2 h after glucose administration: (6.65 +/- 0.64) x 10(-3) pHi/s; P = 0.0003 by Friedman's two-way ANOVA), but not in HT (resting 0 h: (6.07 +/- 0.36) x 10(-3) pHi/s; 1 h after glucose administration: (6.72 +/- 1.02) x 10(-3) pHi/s; 2 h after glucose administration: (6.71 +/- 0.62) x 10(-3) pHi/s; P = 0.7470). During an oral glucose challenge the systolic (P < 0.0001) and diastolic (P < 0.0001) blood pressure significantly decreased in HT but not in NT. Essential hypertension shows abnormal in vivo regulation of Na+/H+ exchange and blood pressure following oral glucose intake.

Effect of diadenosine polyphosphates on Ca2+ ATPase activity

Diadenosine tri-, tetra-, penta-, and hexaphosphate (Ap3A, Ap4A, Ap5A and Ap6A) have been described as having various effects on vascular tone depending on the number of phosphate groups. This study examined the effect of diadenosine polyphosphates on Ca2+ ATPase activity. The activity of the enzyme was measured spectrophotometrically as the difference in hydrolysis of ATP in the presence and absence of Ca2+ with various concentrations of ATP and diadenosine polyphosphates. The diadenosine polyphosphates increased the activity of the Ca2+ ATPase. The effect tended to be stronger with Ap5A and Ap6A than with Ap3A and Ap4A in the order of potency: Ap3A approximately AP4A < Ap5A approximately AP6A. The stimulatory effect of diadenosine polyphosphates was not competitive with that of ATP, suggesting an allosteric activation of Ca2+ ATPase by diadenosine polyphosphates. This effect may be physiologically relevant for limiting the increase in cytosolic free Ca2+ concentration elicited by diadenosine polyphosphates by receptor activation and modulating Ca2+ ATPase function under resting conditions.

Decreased cellular Mg2+ concentrations in a subgroup of hypertensives--cell models for the pathogenesis of primary hypertension

A new method to determine total Mg2+ content in lymphocytes was developed, offering advantages for routine measurements as compared to fluorescence methods. Intracellular Mg2+ measurements were performed in lymphocytes of 18 untreated normotensive and 19 untreated essential hypertensive patients. Mg2+ content was referred to lymphocytic protein, which was determined according to Bradford's method. Mg2+ measurements were performed by atomic absorption spectroscopy using a Video 12 apparatus from Thermo Electron Instrumentation Laboratory, Andover, MA, USA. The results show that in patients with essential hypertension, total intralymphocytic Mg2+ content is significantly lower (0.07 +/- 0.05 mmol/g lymphocytic protein, mean +/- s.d.) as compared to controls (0.11 +/- 0.04 mmol/g lymphocytic protein, mean +/- s.d., P < 0.05). Free intracellular Mg2+ content was measured in lymphocytes by the fluorescent indicator mag-fura-II, showing no significant difference in normotensives and hypertensives (0.30 +/- 0.16 vs 0.38 +/- 0.17 mmol/l). In platelets free intracellular Mg2+ concentrations were not found of significant difference in normotensive and hypertensive patients (0.52 +/- 0.23 vs 0.47 +/- 0.27 mmol/l) using mag-fura-II. In plasma Mg2+ concentrations there was no significant difference in the normotensive and hypertensive group (0.92 +/- 0.07 vs 0.88 to 0.07 mmol/l). There was no correlation between plasma, free or total cellular magnesium concentrations in each group. Furthermore this method also seems suitable for routine measurements of total intracellular Mg2+ concentrations in even larger groups of patients in comparison with fluorescent indicator measurements like mag-fura-II. Lowered total intracellular Mg2+ concentrations in a subgroup of primary hypertension may contribute to the development of this disorder, perhaps due to different buffering systems.

Low-density lipoproteins inhibit the Na+/H+ antiport in human platelets. A novel mechanism enhancing platelet activity in hypercholesterolemia

BACKGROUND

LDL have been reported to augment platelet activation, and increased platelet reactivity has been observed in familial hypercholesterolemia. However, the underlying mechanisms of this putatively atherogenic effect is unknown. Because intracellular pH (pHi) may play an important role in platelet function, we examined the influence of LDL on pHi and Na+/H+ antiport activity in human platelets and compared it with the effect of [3-methylsulfonyl-4-piperidinobenzoyl] guanidine hydrochloride (HOE 694), a selective Na+/H+ antiport inhibitor.

METHODS AND RESULTS

Using a fluorescent dye technique, we demonstrated that incubation of platelets with physiological concentrations of LDL or with HOE 694 decreased pHi. In addition, both LDL and HOE 694 inhibited the Na+/H+ antiport in platelets treated with sodium propionate or thrombin. The inhibitory effect of LDL was observed both in normal and in glycoprotein (GP)IIb/IIIa-as well as in GPIIIb (CD36)-deficient platelets and was not influenced by the covalent modification of apolipoprotein B lysine residues, suggesting that specific LDL binding sites were not involved. Thrombin-induced phosphoinositide breakdown, diacylglycerol formation, and Ca2+ mobilization, as well as platelet aggregation and granule secretion, were potentiated by both LDL and HOE 694. pHi and Na+/H+ antiport activity were significantly reduced in platelets from patients with familial hypercholesterolemia. Both parameters were normalized after normalization of LDL levels by apheresis treatment.

CONCLUSIONS

LDL inhibits the Na+/H+ antiport most likely via receptor-independent mechanisms, thereby augmenting platelet reactivity. This novel mechanisms explains increased platelet reactivity in patients with familial hypercholesterolemia and may contribute to the atherogenic potential of LDL.

Cellular protective action of angiotensin converting enzyme inhibitors

In several studies organ protective effects of ACE inhibitors independent from their antihypertensive action have been demonstrated. The mechanisms of the protective effects of angiotensin converting enzyme (ACE) inhibitors on vasculature and kidney are largely unknown. In the present study the modulatory action of captopril on the angiotensin II (AngII), arginine vasopressin (AVP), and platelet derived growth factor (PDGF)-induced increase of cytosolic free calcium concentration ([Ca2+]i) was investigated in cultured vascular smooth muscle cells (VSMC) and cultured glomerular mesangial cells (MC) from rats using the fluorescent dye technique. Resting [Ca2+]i in VSMC or MC was not significantly affected by captopril. The preincubation of VSMC with 1 mumol/l captopril significantly reduced the AngII-induced [Ca2+]i increase in VSMC from 90 +/- 10 nmol/l (n = 78; mean +/- SEM) to 51 +/- 16 nmol/l (n = 53; p < 0.05) and in MC from 102 +/- 42 nmol/l (n = 14) to 43 +/- 12 nmol/l (n = 7; p < 0.05). In the absence of extracellular calcium captopril produced no effect on AngII induced changes of [Ca2+]i. Captopril significantly attenuated the AVP-induced [Ca2+]i increase in VSMC and MC. The preincubation of MC with 1 mumol/l captopril for 40 min significantly reduced the PDGF-induced [Ca2+]i increase in MC from 127 +/- 31 nmol/l (n = 11) to 61 +/- 32 nmol/l (n = 5, p < 0.05). The present results may indicate that part of the protective effects of ACE inhibitors on vasculature and kidney may be promoted by inhibition of growth-factor induced changes of calcium homeostasis.

Vascular actions of diadenosine phosphates

1. Diadenosine phosphates were isolated from platelets, adrenal gland and autonomic nerves. The presence of diadenosine phosphates in storage pools releasable into the circulation suggests an important role in the control of blood pressure, and potentially to a modulation of the actions of catecholamines. 2. Besides a role of the diadenosine phosphates in platelet aggregation, these agents have potent vasoactive properties. Vasoactive actions of the diadenosine phosphates were demonstrated in numerous vascular models including most of the physiologically important elements of blood pressure regulation. Mostly, the vasoactive action depends on the number of phosphates in the diadenosine phosphates. Vasodilation can be observed in intact vessels after administration of Ap2A, Ap3A and Ap4A whereas contraction is affected by Ap5A and Ap5A and Ap6A. Vasocontraction induced by the diadenosine phosphates in vascular smooth muscle cells is mediated by an increase in intracellular free Ca2+. 3. In vivo, intravenous injection of Ap4A lowers blood pressure whereas injections of Ap5A and Ap6A caused a prolonged increase in blood pressure. In blood, in contrast to ATP, diadenosine phosphates are relatively long-lived molecules, suggesting that the action of the latter is of intermediate time span. In a similar manner to the vasoconstrictor angiotensin II, diadenosine phosphates also act as mitogens. It can be assumed that diadenosine phosphates may be involved in pathophysiological events of circulation including hypertension and atherosclerosis.

Diadenosine polyphosphates induce transplasma membrane calcium influx in cultured glomerular mesangial cells

The effects of diadenosine tetraphosphate (AP4A) diadenosine pentaphosphate (AP5A) and diadenosine hexaphosphate (AP6A) on the cytosolic-free calcium concentration ([Ca2+]i) were evaluated in cultured rat glomerular mesangial cells (MCs) using the fluorescent dye technique. The addition of 10 mumol L-1 AP4A, AP5A or AP6A significantly increased [Ca2+]i in MCs by 57 +/- 9 nmol L-1 n = 17; P < 0.01), 76 +/- 27 nmol L-1 (n = 9; P < 0.01) or 65 +/- 12 nmol L-1 (n = 18; P < 0.01) respectively. In the absence of extracellular calcium, there was no significant change in [Ca2+]i in MCs after administration of diadenosine polyphosphates, indicating that these agents induce transplasma membrane Ca2+ influx. AP6A significantly enhanced the angiotensin II-induced changes in [Ca2+]i in MCs. The AP5A-induced transplasma membrane Ca2+ influx was inhibited by the P2 purinoceptor blockers suramin and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), but was not affected by the adenosine A1 receptor blocker 8-cyclopentyl-1.3-dipro-pylzanthine (CPDPX). Adenosine triphosphate (ATP) and adenosine 5'-O-(3-thio)triphosphate (ATP-gamma S) increased [Ca2+]i in MCs, whereas alpha, beta-methylene ATP had no effect on [Ca2+]i in MCs. Measurements of diacylglycerol and phosphatidic acid showed that AP5A and AP6A also stimulated phospholipase C, but had no effect on phospholipase D. The inhibition of phosphatidylcholine-specific phospholipase C significantly reduced the AP5A-induced [Ca2+]i increase. In summary, diadenosine polyphosphates induce Ca2+ influx through P2 purinoceptors and may be involved in the local regulation of vascular resistance evoked by the Ca(2+)-dependent contractile response of mesangial cells.

Increased lymphocytic Na+/H+ exchange activity after hemodialysis: evidence for an endogenous inhibitor of Na+/H+ exchange in patients with end-stage renal failure

The Na+/H+ exchange antiport activity was measured in lymphocytes from 16 patients with end-stage renal failure pre- and postdialysis. In addition the effect of the patients' plasma on lymphocytes from healthy subjects was tested. Resting pH (pHi) was not significantly different in lymphocytes pre- and postdialysis. On the other hand, the Na+/H+ exchange activity was significantly lower in lymphocytes before hemodialysis (6.22 +/- 0.73 x 10(-3) pHi/s) than after hemodialysis (9.32 +/- 1.58 x 10(-3) pHi/s; n = 16; p < 0.05). The buffer capacity was not significantly different before and after hemodialysis. The incubation of lymphocytes from healthy control subjects with plasma from patients with end-stage chronic renal failure significantly reduced the lymphocytic Na+/H+ exchange activity. The addition of ultrafiltrate also significantly reduced the Na+/H+ exchange activity in lymphocytes from healthy control subjects. The study indicates the existence of an endogenous inhibitor of the Na+/H+ exchange that is accumulated in plasma from patients with end-stage chronic renal failure.

Diadenosine polyphosphates regulate cytosolic calcium in human fibroblast cells by interaction with P2x purinoceptors coupled to phospholipase C

The effects of diadenosine pentaphosphate (AP5A), and diadenosine hexaphosphate (AP6A) on the cytosolic-free Ca2+ concentration ([Ca2+]i) were evaluated in cultured human fibroblast cells (HF cells) using the fluorescent dye technique. AP5A, and AP6A concentration-dependently increased [Ca2+]i in HF cells. The addition of 10 mumol/1 AP5A and AP6A significantly increased [Ca2+]i in HF cells from 71 +/- 3 nmol/1 (n = 184) to 241 +/- 39 nmol/1 (n = 11; P < 0.001 compared to resting value) and to 227 +/- 26 nmol/1 (n = 23; P < 0.001), respectively. The purinoceptor P2 blockers, suramin and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), inhibited the diadenosine polyphophate-induced [Ca2+]i increase, whereas the P2y purinoceptor blocker, reactive blue, had no effect. Adenosinetriphosphate (ATP) and the P2x agonist, alpha 1 beta-methylene-ATP also significantly increased [Ca2+]i in HF cells, whereas the P2y agonist methylthio-ATP showed only a small [Ca2+]i response. Diadenosine polyphosphates mainly induced transplasmamembrane Ca2+ influx as was confirmed by experiments in the absence of extracellular Ca2+ or by manganese quenching studies. Organic (verapamil) and inorganic Ca2+ channel blockers (NiCI2) significantly reduced the AP6A induced transplasmamembrane Ca2+ influx. The inhibitor of phosphatidylcholine-specific phospholipase C, D609, significantly reduced the effect of diadenosine polyphosphates on [Ca2+]i in HF cells. It is concluded that diadenosine polyphosphates regulate transplasmamembrane Ca2+ influx after occupation of P2x receptors via activation of phosphatidylcholine-specific phospholipase C and hence of voltage-operated Ca2+ channels.