Prostacyclin binding to guinea pig pulmonary receptors

Thromboxane A2 and prostacyclin release in bleeding time blood during primary haemostasis in healthy individuals

It is generally believed that prostacyclin (PGI2) generation is greatly stimulated when blood vessels are injured, even by minor trauma, such as venepuncture. The Simplate technique for measuring skin bleeding time was adapted to quantify by radioimmunoassay PGI2 and thromboxane A2 (TXA2) in the emerging blood, as the stable degradation products 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) and thromboxane B2 (TXB2), both of which were measured in venous plasma as well as in serum (clotted at 37 degrees C for 1 h). During bleeding, when platelets aggregate to occlude the injured vessels, the median TXB2 level in the emerging bleeding time blood was 1.7 ng/ml. The median TXB2 level in plasma was less than 1 ng/ml and in serum 275 ng/ml. The levels of immunoreactive 6-keto-PGF1 alpha were always below determination limit in bleeding time blood (0.2 ng/ml) and in plasma (0.1 ng/ml), whereas in serum the levels ranged between 0.26 and 0.47 ng/ml. The fact that enhanced PGI2 production in primary haemostasis in skin incisions could not be demonstrated calls for further investigations of possible PGI2 production with more sensitive assays or in injured large vessels.

Synergistic inhibitory effects of adrenoceptor antagonists and prostacyclin, and umbilical artery-derived prostacyclin-like activity on platelet aggregation

Adrenoceptor antagonists are being used increasingly for the treatment of pregnancy-induced hypertension (PIH). Previous studies have shown that these drugs can inhibit platelet aggregation and thromboxane production. The aims of this study were to determine (i) whether adrenoceptor antagonists had any effect on vascular prostacyclin (PGI2) production and (ii) whether these drugs acted synergistically with PGI2 and the PGI2-like activity derived from umbilical artery to inhibit platelet aggregation in vitro. The adrenoceptor antagonists labetalol, pindolol and propranolol were found to have no effect on PGI2 production from the umbilical artery at low drug concentrations. These agents were also found to act synergistically with both pure PGI2 and PGI2-like activity derived from umbilical artery to inhibit platelet aggregation. This synergistic effect may be beneficial in the treatment of disorders which are associated with reduced PGI2 production, such as PIH where such synergy may help compensate for PGI2 deficiency.

Induction of prostacyclin receptor expression in human erythroleukemia cells

We have identified both high-affinity (KD = 36 +/- 3 nM) and low-affinity (KD = 2.1 +/- 0.8 microM) prostacyclin (PGI2)-receptor sites on human erythroleukemia (HEL) cells using the radiolabelled prostacyclin analogue. [3H]iloprost. The addition of the phorbol ester, TPA, to the culture medium caused a 5-10-fold increase in the number of both the low- and the high-affinity sites, without any change in their affinity constants. Iloprost stimulated HEL cell membrane adenylate cyclase activity 5-fold. This stimulation was potentiated in the presence of GTP, indicating a conventional PGI2 receptor-G2-adenylate cyclase system. HEL cells represent a source of prostacyclin receptor mRNA which may be of value in expression cloning of this receptor.

The single prostacyclin receptor of gel-filtered platelets provides a correlation with antiaggregatory potency of PGI2 mimics

Gel-filtered human platelets (GFP) display only a single binding site for [3H]-PGI2: KD = 61nM, 234 fmol/10(8) platelets (1410 sites/platelet). Platelet-rich plasma (PRP) displays the same receptor density but the KD value increases to 123 nM due to protein binding of PGI2 which lowers its effective concentration. The [3H]-PGI2/GFP binding assay has been used to evaluate the molecular basis of aggregation inhibition for prostacyclin analogs and mimics, three PGE type structures, and PGD2. Antiaggregatory IC50s and radioligand binding IC50s correlate for PGE2, E1, and six PGI2 analogs. PGD2, and to a lesser extent 6-oxo-PGE1, display greater antiaggregatory potency than expected based on PGI2-binding site affinity data.

Desensitization of prostacyclin responsiveness in a neuronal hybrid cell line: selective loss of high affinity receptors

The binding of [3H]-iloprost (ZK36374) to NCB-20 membranes revealed a single population of high affinity receptors (KD = 9.55 nM, Bmax = 431 fmol mg-1 protein) and a low affinity, non-saturable binding component. Desensitization of prostacyclin-responsiveness of NCB-20 cells is induced by culture in the presence of the stable prostacyclin analogue carbacyclin. Desensitization is accompanied by an increase in the Kact value for prostacyclin (64.1 nM to 175 nM), and a reduction in the prostacyclin-dependent increase in adenylate cyclase activity (41.2 to 15.1 pmol cyclic AMP min-1 mg-1 protein). Desensitization is not accompanied by changes in the coupling of the catalytic (C) to the regulatory (Ns) subunit of adenylate cyclase. In addition, the physical identity of the receptor molecule (as characterized by its sensitivity to electron bombardment in the beam of a linear accelerator) is not changed by desensitization. Desensitization of prostacyclin-dependent activation of adenylate cyclase may be explained most simply by a loss of prostacyclin receptors. The anomalous increase in the Kact (concentration of prostaglandin giving half-maximum enzyme activation) for prostacyclin-stimulated adenylate cyclase was not accompanied by a substantial change in the KD of [3H]-iloprost binding, and is explained by a loss of spare receptors. Prostacyclin responsiveness in non-dividing cells may be restored after desensitization by prolonged culture (up to 48 h) in the absence of carbacyclin. Resensitization is accompanied by restoration of the high affinity Kact value (143 nM to 45.5 nM), and is dependent on de novo protein synthesis.

The use of a prostacyclin analogue, [3H]iloprost, for studying prostacyclin-binding sites on human platelets and neuronal hybrid cells

The stable prostacyclin analogue [3H]iloprost has been used for labelling prostacyclin-binding sites on human platelets and NCB-20 neuronal hybrid cells. The ligand-binding properties of the sites have been determined and correlate well with stimulation of cAMP synthesis in NCB-20 cells and inhibition of aggregation in human platelets.

Evidence for high affinity prostacyclin binding sites in vascular tissue: radioligand studies with a chemically stable analogue

Prostacyclin-specific binding sites are described in the muscularis of pig aorta using [3H]ZK 36374, a chemically stable prostacyclin analogue, as radioligand. Under standard incubation conditions [300 micrograms membrane protein in 350 microliter Tris buffer (50 mmoles/l., pH 7.4) containing 3 mM Ca2+ at 37 degrees for 10 min] both association and dissociation were complete within 30 sec, thus not allowing the determination of association or dissociation rate constants. The Scatchard plot was upward convex, whereas the Hill plot was linear, having a slope of 1.9. The equilibrium dissociation constant (KD) was 22.4 nmoles/l. and the specific binding was saturated at 360 fmoles [3H]ZK 36374/mg protein. The reversibility of binding was demonstrated by displacement of bound ligand with ZK 36374, its 5-(Z)-stereoisomer (ZK 36375), PGI2 and PGE1, but not with PGF2 alpha. The data suggest high affinity binding sites for ZK 36374 in the smooth muscle cells of pig aorta for which PGI2 may be the physiological ligand. They also demonstrate a possible co-operativity with two molecules binding simultaneously to two interacting sites.

Release of prostacyclin in vivo and its role in man

Concentrations of 6-oxo-prostaglandin F1 alpha (6-oxo-PGF1 alpha), the stable hydrolysis product of prostacyclin (PGI2) were determined in venous blood sampled from a forearm vein after dextrose (10% w/v) perfusion and after subsequent distension with physiological saline in seven healthy volunteers. 6-oxo-PGF1 alpha was measured by gas chromatography/negative ion chemical ionisation mass spectrometry. Local release of PGI2 was demonstrated in six volunteers: in three both stimuli were effective and in three only one stimulus was effective. In seven trained athletes studied before and after vigorous exercise baseline plasma concentrations of 6-oxo-PGF1 alpha were less than 1.3-3.2 pg/ml. The concentration increased after exercise in each subject (mean 10.9 pg/ml; range 5.5-18.2 pg/ml). Mild chemical and mechanical stimuli therefore can cause local production of PGI2 from human blood vessels in vivo.

Desensitization of prostacyclin receptors in a neuronal hybrid cell line

1 Prostacyclin and its stable analogue, carbacyclin, bind competitively to a single population of receptors, and activate adenylate cyclase of the NCB-20 neuronal somatic cell hybrid (Kact = 40.1 nM and 96.1 nM respectively). 2 Culture of NCB-20 cells in the presence of 1 microM carbacyclin for 4 to 16 h results in a progressive decrease in the prostacyclin-dependent activation of adenylate cyclase in cell homogenates with an increase at 16 h of the Kact from 64.1 nM to 174.0 nM and decrease in the maximum adenylate cyclase activation from 41.2 to 15.1 pmol cyclic AMP min-1 mg-1 protein. 3 The prediction that the apparent decrease in affinity in the prostacyclin-dependent activation of adenylate cyclase was secondary to a reduction in receptor numbers was tested directly by measuring binding of [3H]-prostacyclin to membranes of cells exposed to carbacyclin for 16 h. This showed an actual decrease in affinity of the prostacyclin-receptor interaction, as well as a decrease in the total receptor numbers. Thus prolonged exposure of NCB-20 cells to carbacyclin caused reductions in both receptor numbers and affinity, reflected by measurements both of binding and adenylate cyclase activation.

Prostacyclin is not a circulating hormone in man

A highly specific stable isotope dilution assay for plasma 6-oxo-prostaglandin F1 alpha has been developed. The method employs capillary column gas chromatography coupled with negative ion chemical ionisation mass spectrometry. The limit of sensitivity of the assay is 0.5 pg.m1(-1). Concentrations of 6-oxo-prostaglandin F1 alpha in the plasma of 20 healthy volunteers determined by this assay were all below 3 pg.m1(-1). The levels were much lower than any previously reported and confirms that prostacyclin is not a circulating hormone in man under normal physiological conditions.