Endothelin and prostaglandin H(2)/thromboxane A(2) enhance myogenic constriction in hypertension by increasing Ca(2+) sensitivity of arteriolar smooth muscle.

The myogenic response of skeletal muscle arterioles is enhanced in hypertension because of the release of endothelin (ET) and prostaglandin H(2) (PGH(2))/thromboxane A(2) (TxA(2)) from the endothelium. We hypothesized that ET and PGH(2)/TxA(2) modulate Ca(2+) signaling in arteriolar smooth muscle and thereby enhance myogenic constriction. Thus, simultaneous changes in intracellular Ca(2+) concentration in smooth muscle ([Ca(2+)](i)), measured by fura 2 microfluorometry (expressed as Ca(2+) fluorescence ratio [R(Ca)]), and diameter were obtained as a function of intraluminal pressure (P(i)) in isolated cannulated gracilis muscle arterioles (diameter approximately 120 micrometer) of normotensive Wistar rats (WR) and spontaneously hypertensive rats (SHR). In the absence of extracellular Ca(2+), increases in P(i) from 20 to 160 mm Hg increased the passive diameter of arterioles without changes in R(Ca). In the presence of extracellular Ca(2+) and endothelium, increases in P(i) elicited similar increases in R(Ca) (30+/-7% for control and 33+/-8% for SHR at 160 mm Hg) but a significantly (P<0.05) greater constriction of SHR arterioles compared with WR arterioles (at 160 mm Hg, 55+/-4% versus 38+/-2%, respectively, of passive diameter). In the absence of the endothelium, P(i)-induced changes in the R(Ca) and diameter of SHR and WR arterioles did not differ significantly. Also, a step increase in P(i) (from 80 to 140 mm Hg) elicited a similar increase in R(Ca) but greater constrictions in SHR versus WR arterioles. In the presence of the TxA(2) receptor inhibitor SQ29,548 and the ET(A) receptor inhibitor BQ123, there was no difference between responses of SHR and WR arterioles. In WR arterioles, increasing concentrations of KCl elicited a significant increase in R(Ca) (38+/-7% at 80 mmol/L) and completely constricted the arterioles. In contrast, constrictions to ET (52+/-7% at 3x10(-12) mol/L) and the TxA(2) agonist U46619 (40+/-8% at 3x10(-9) mol/L) were not accompanied by increases in R(Ca) at submaximal concentrations. Collectively, these findings suggest that in hypertension, endothelium-derived ET and PGH(2)/TxA(2) increase the Ca(2+) sensitivity of the contractile apparatus of arteriolar smooth muscle; thus, the similar increases in [Ca(2+)](i) in response to the elevation of intraluminal pressure elicit greater myogenic constriction.

Role of endothelium and vasoconstrictor prostanoids in norepinephrine-induced vasoconstriction in isolated rat common carotid arteries.

We investigated the role of endothelium and vasoconstrictor prostanoids in the norepinephrine (NE)-induced vasoconstriction of isolated rat common carotid arteries (CCAs). Isolated CCAs were cannulated with stainless steel cannulae by the cannula inserting method. NE was administered intra- or extraluminally by a single microinjection. For denudation of endothelium, the intimal surface was gently rubbed with a cotton pellet. NE produced dose-related vasoconstricting responses in isolated arteries with intact endothelium. These responses were attenuated by pretreatment with denudation, OKY046, a thromboxane A2 synthesis inhibitor or indomethacin. The residual responses after denudation were further blocked by OKY046 or indomethacin. All NE-induced responses were blocked by a single injection of prazosin. There were no significant differences in responses between intra- and extraluminal administration of NE. These results show that most vasoconstriction induced by NE via alpha1-adrenoceptors in CCAs is dependent on vasoconstrictor prostanoids distributed both in endothelium and vascular smooth muscle.

Prostaglandin endoperoxide-dependent vasospasm in bovine coronary arteries after nitration of prostacyclin synthase.

In the present study we used a bioassay to study the effects of peroxynitrite (ONOO-) on angiotensin II (A-II)-triggered tension in isolated bovine coronary arteries in order to show the consequences of the previously reported PGI2-synthase inhibition by ONOO- in this model. The following results were obtained: 1. 1 micromol L(-1) ONOO- impaired A-II-induced vasorelaxation and caused a second long lasting constriction phase. Indomethacin (10(-5)M) prevented both effects. U51605, a dual blocker of PGI2-synthase and thromboxane (TX)A2-synthase mimicked the effects of ONOO-. 2. The selective TXA2/prostaglandin endoperoxide (PGH2) receptor antagonist SQ29548 antagonized the second vasoconstriction phase after ONOO- -treatment. Since a generation of TXA2 and 8-iso-prostaglandin F2alpha could be excluded a direct action of unmetabolized PGH2 on the TXA2/PGH2 receptor was postulated. 3. ONOO- dose-dependently inhibited the conversion of 14C-PGH2 into 6-keto-PGF1alpha in isolated bovine coronary arteries with an IC50-value of 100 nM. 4. Immunoprecipitation of 3-nitrotyrosine-containing proteins with a monoclonal antibody revealed PGI2-synthase as the only nitrated protein in bovine coronary arteries treated with 1 micromol 1(-1) ONOO-. 5. Using immunohistochemistry a co-localization of PGI2-synthase and nitrotyrosine-containing proteins was clearly visible in both endothelial and vascular smooth muscle cells. We concluded that ONOO- not only eliminated the vasodilatory, growth-inhibiting, antithrombotic and antiadhesive effects of PGI2 but also allowed and promoted an action of the potent vasoconstrictor, prothrombotic agent, growth promoter, and leukocyte adherer, PGH2.

Interactions between nitric oxide and prostanoids in isolated perfused kidneys of the rat.

1. The present study was aimed to assess the interaction between nitric oxide (NO) and thromboxane (Tx) A2-prostaglandin (PG) H2 in single-pass perfused isolated kidneys of the rat. 2. Noradrenaline (NA, 63 and 110 nM) dose-dependently elevated the renal vascular resistance (RVR), the glomerular filtration rate (GFR) and the urinary excretion of sodium (UNa V). Infusion of N omega-nitro-L-arginine methyl ester (L-NAME, 100 microM), an inhibitor of NO synthesis, enhanced the effects of NA on RVR and on UNa V, but decreased those on GFR. The TxA2-PGH2 (TP) receptor blockade by GR32191B (10 microM) attenuated this potentiating effect of L-NAME. 3. When renal perfusion pressure was stepwise increased from 90 to 150 mmHg, L-NAME similarly decreased renal perfusion flow rate and GFR. 4. The venous excretion of TxB2 and 6-keto-PGF1 alpha was increased by L-NAME in baseline conditions as well as after NA or increasing renal perfusion pressure (RPP). 5. These results suggest that: (1) TxA2 and PGH2 play an important role in the overall effect of the renal prostanoids, (2) NO strongly interacts with the cyclo-oxygenase pathway and reduces the prostanoid synthesis in the kidney, and (3) the pressor effect of L-NAME partly relies upon the vasoconstrictor effect of TxA2 and PGH2.

Both nitric oxide and prostaglandin-mediated responses are impaired in skeletal muscle arterioles of hypertensive rats.

OBJECTIVE: To investigate the role played by endothelium-derived dilator factors in the regulation of peripheral vascular resistance by determining whether the dysfunction of the endothelium contributes to the reduced dilator responsiveness of skeletal muscle arterioles in hypertension. METHODS: The endothelial function of isolated, cannulated, pressurized (at 80 mmHg) gracilis muscle arterioles (45-50 microns diameter) of normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR) was compared by utilizing vasoactive agents of known action. RESULTS: Acetylcholine (ACh, 10(-9), 10(-8) and 5 x 10(-8) mol/l) and sodium nitroprusside (SNP, 10(-8), 10(-7) and 10(-6) mol/l) elicited similar dilations in arterioles of WKY rats and SHR. Substance P (10(-9), 10(-8) and 5 x 10(-8) mol/l) caused significantly less dilation (by approximately 70%) of SHR arterioles compared with WKY rat arterioles. The calcium ionophore A23187 (5 x 10(-8), 5 x 10(-7) and 10(-6) mol/l) elicited dilations in WKY rat arterioles (9.1 +/- 1.1, 24.0 +/- 1.5, and 39.0 +/- 3.4%, respectively), whereas it evoked constrictions (6.5 +/- 1.1, 14.9 +/- 1.5, and 25.5 +/- 1.6%, respectively) in SHR arterioles. Removal of endothelium, inhibition of prostaglandin synthesis (indomethacin) or blockade of prostaglandin H2 (PGH2) receptors (by SQ 29548) eliminated A23187-induced constrictions of SHR arterioles. The nitric oxide synthase blocker, NG-nitro-L-arginine elicited a significantly greater inhibition of substance P-induced dilations and a greater reduction in basal diameter of WKY rat arterioles than it did in those from SHR. CONCLUSIONS: These data suggest that, in SHR arterioles, the synthesis and/or action of nitric oxide is, or are, impaired and the metabolism of arachidonic acid is altered, resulting in an enhanced production of PGH2. The simultaneous dysfunction of these two dilator pathways of arteriolar endothelium could contribute significantly to the enhanced peripheral resistance observed in hypertension.

Role of prostaglandins in acetylcholine-induced contraction of aorta from spontaneously hypertensive and Wistar-Kyoto rats.

Evidence in support of prostaglandin (PG) H2 as the endothelium-derived contracting factor released in response to acetylcholine in vessels from adult spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) is to a large degree indirect. Therefore, the purpose of the present study was to test the hypothesis that a prostaglandin or prostaglandins other than PGH2 may serve as the endothelium-derived contracting factor that mediates acetylcholine-induced contraction in these vessels. Acetylcholine-induced contraction of endothelium-intact aorta from 7- to 12-month-old SHR and WKY in the presence of the nitric oxide synthase inhibitor N omega-nitro-L-arginine was abolished by indomethacin and only partially decreased by the thromboxane (Tx) A2/PGH2 receptor antagonist SQ29548. Contraction induced by the TxA2/ PGH2 receptor agonist U46619 was abolished by SQ29548. These findings suggest that in endothelium-intact aorta from SHR and WKY, acetylcholine causes the release of a cyclooxygenase product other than PGH2 that induces contraction independently of TxA2/PGH2 receptor activation. To investigate which prostaglandin or prostaglandins could be responsible for the TxA2/PGH2 receptor-independent component, we challenged endothelium-denuded aorta from SHR and WKY with various prostaglandins in the presence of SQ29548. In SQ29548-treated aorta from 7- to 12-month-old rats, maximal contractions to PGF2 alpha, PGE2, and carbacyclin (a PGI2 analogue) were greater than the magnitude of acetylcholine-induced contraction. These findings suggest that PGF2 alpha, PGE2, and/or PGI2 could serve as mediators of the TxA2 receptor-independent component of the acetylcholine-induced contraction. However, in studies with SQ29548-treated aorta from 4- to 6-week-old SHR and WKY (an age at which acetylcholine-induced contraction is known to be absent), maximal contraction to PGF2 alpha and PGE2 was also greater or equivalent to that of SQ29548-treated aorta from 7- to 12-month-old rats, whereas carbacyclin induced negligible contraction. Thus, unlike PGE2 and PGF2 alpha, the age-dependent pattern of contraction induced by carbacyclin closely resembles the pattern induced by acetylcholine. We also measured the levels of PGI2 released in response to acetylcholine and found that they are sufficient to account for the TxA2 receptor-independent component of the acetylcholine-induced contraction. Thus, we propose that PGI2 released in response to acetylcholine may serve as the endothelium-derived contracting factor that elicits the TxA2/PGH2 receptor-independent and dependent components of the acetylcholine-induced contraction.

Prostaglandin H2/thromboxane A2 pathway in platelet aggregation and activity of Dahl salt-sensitive rat--a sulotroban study.

It has been postulated that the main proaggregatory effect of thromboxane A2 (TxA2) is mediated by an inhibition of adenylate cyclase/cAMP complex, an effect similar to that of Na+ ion. The possibility of a modulation of the effect of Na+ on in vivo and in vitro platelet function of Dahl salt-sensitive (Dahl-S) and Dahl salt-resistant (Dahl-R) rats was investigated. Sulotroban, a powerful antagonist of prostaglandin peroxides and TxA2 was administered at doses of 1, 3 and 10 micrograms/kg/min. In vivo platelet activity was estimated on the basis of bleeding time and thrombocytopenia produced by i.v. bolus injection of 100 micrograms/kg collagen, and by plasma measurement of the stable prostacyclin and TxA2 analogs. In vitro aggregation was induced by collagen at concentrations of 10, 20, 30 and 40 micrograms/ml. Main blood pressure was measured directly in common carotid artery. The results showed that sulotroban did not produce hypotensive effects but did increase the bleeding time almost 2-fold. Plasma TxA2 levels were significantly increased in both Dahl-S and Dahl-R rats. The inhibition of collagen-induced aggregation and thrombocytopenia by sulotroban was negligible on Dahl-R rats but significant in Dahl-S rats. The results suggested a blocking effect of sulotroban not only on stimulus/receptor TxA2 complex but also on the inhibitory unit of adenylate cyclase complex, confirming this pathway of TxA2-induced aggregation.

Long-term inhibition of NO synthesis promotes atherosclerosis in the hypercholesterolemic rabbit thoracic aorta. PGH2 does not contribute to impaired endothelium-dependent relaxation.

We examined whether prostaglandin (PG) H2, as an endothelium-dependent contracting factor, or the disturbed production of endothelium-derived relaxing factor, impairs endothelium-dependent relaxation and whether long-term inhibition of nitric oxide (NO) synthesis aggravates atherosclerosis in hypercholesterolemic rabbits. Male New Zealand White rabbits were fed one of the following diets: (1) standard chow; (2) 2% cholesterol-supplemented chow; (3) standard chow with 80 micrograms/mL N omega-nitro-L-arginine methylester (L-NAME), an NO synthetase inhibitor, in their drinking water; or (4) 2% cholesterol-supplemented chow with 80 or 160 micrograms/mL L-NAME in their drinking water. The rabbits were fed these diets for 8 or 12 weeks. Then aortic rings were obtained, and changes in isometric tension were recorded. Intimal atherosclerotic areas of the thoracic aortas were subsequently measured by planimetry. The cholesterol-supplemented diet significantly impaired endothelium-dependent aortic relaxation to acetylcholine. Pretreatment with the thromboxane A2/PGH2 receptor antagonist ONO-3708 did not reverse this impaired response. Vessels from both normocholesterolemic and hypercholesterolemic rabbits given L-NAME showed more impaired endothelium-dependent relaxation than those from their dietary counterparts not given L-NAME. Morphometric analysis revealed marked enlargement of intimal atherosclerotic areas in aortas from L-NAME-treated hypercholesterolemic rabbits compared with those from untreated hypercholesterolemic rabbits. These findings suggest that PGH2 does not contribute to impaired endothelium-dependent relaxation and that long-term administration of L-NAME promotes atherosclerosis by inhibition of NO synthesis in the hypercholesterolemic rabbit thoracic aorta.

Expression of prostaglandin H2-mediated mechanism of vascular contraction in hypertensive rats. Relation to lipoxygenase and prostacyclin synthase activities.

We tested the hypothesis that a prostanoid-mediated mechanism of vascular contraction is expressed in rats with aortic coarctation-induced hypertension. Rings of descending thoracic aorta taken from normotensive and hypertensive rats were contrasted in terms of constrictor responsiveness to arachidonic acid (AA), AA-induced release of eicosanoids, and ability to convert exogenous prostaglandin (PG) H2 to PGI2. AA (10(-8) to 10(-5) mol/L) increased isometric tension in aortic rings (bathed in Krebs' bicarbonate buffer) of hypertensive but not normotensive rats. AA (10(-5) mol/L) also elicited the release of PGI2, PGE2, thromboxane (TX) A2, and monohydroxyeicosatetraenoic acids (HETEs); this release from the aortic rings of hypertensive rats exceeded the corresponding release from the aortic rings of normotensive rats. However, the rate of conversion of exogenous PGH2 to PGI2 by aortic rings of hypertensive rats was < 50% the rate of conversion by aortic rings of normotensive rats. The constrictor effect of AA in aortic rings of hypertensive rats was abolished by an inhibitor of cyclooxygenase (indomethacin, 10 mumol/L) and a blocker of TXA2-PGH2 receptors (SQ29548, 1 mumol/L) but was not affected by an inhibitor of TXA2 synthesis (CGS13080, 10 mumol/L), suggesting mediation by PGH2. The lipoxygenase inhibitor baicalein (75 mumol/L) also attenuated the constrictor effect of AA in aortic rings of hypertensive rats while decreasing the associated release of HETEs and correcting the impairment in the conversion of PGH2 to PGI2.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelial dysfunction augments myogenic arteriolar constriction in hypertension.

To elucidate the underlying reason or reasons for the increased peripheral resistance in hypertension, we investigated the pressure-diameter relation--the myogenic response--of isolated, cannulated arterioles (approximately 50 microns) of cremaster muscle of 12-week-old Wistar-Kyoto (WKY) rats, spontaneously hypertensive rats (SHR), and normal Wistar (NW) rats. All arterioles constricted in response to step increases in perfusion pressure from 20 to 160 mm Hg. This constriction was, however, significantly enhanced from 60 to 160 mm Hg in arterioles of SHR compared with NW or WKY rats. For example, at 80 and 140 mm Hg, respectively, the normalized diameter (expressed as a percentage of the corresponding passive diameter of arterioles of SHR) was 11.8% and 27.6% (P < .05) less compared with those of WKY rats. Endothelium removal eliminated the enhanced pressure-induced tone in SHR. Similarly, indomethacin (10(-5) mol/L, sufficient to block prostaglandin synthesis) or SQ 29,548 (10(-6) mol/L), a thromboxane A2-prostaglandin H2 receptor blocker that inhibited vasoconstriction to the thromboxane agonist U46619, attenuated the enhanced pressure-diameter curve and reversed the blunted dilation to arachidonic acid in SHR. In contrast, the thromboxane A2 synthesis inhibitor CGS 13,080 (5 x 10(-6) mol/L) did not affect the increased pressure-induced tone or the reduced dilation to arachidonic acid in SHR. Thus, the present findings suggest that in early hypertension pressure-induced arteriolar constriction is increased. This seems to be due to an enhanced production of endothelium-derived constrictor factors, primarily prostaglandin H2.

Role of prostaglandin H2 as an endothelium-derived contracting factor in diabetic state.

This study investigated the possible involvement of prostaglandin H2, an acetylcholine-induced endothelium-derived contracting factor in rat aorta, in the development of abnormality of the vasculature in diabetes. Rings of thoracic aorta were prepared from control Wistar-Kyoto and STZ-induced diabetic rats to examine the changes in isometric tension. In 10(-7) M norepinephrine-precontracted rings, acetylcholine induced relaxations, which were significantly impaired in diabetic rats. Inhibition of thromboxane A2-prostaglandin H2 receptors with ONO-3708 (10(-6) M) prevented the development of the impairment of relaxation in diabetic rats. Thromboxane A2 synthesis inhibition with OKY-046 (10(-5) M) did not affect the acetylcholine-induced relaxation in both control and diabetic rats. In aortic rings under resting tension, acetylcholine induced a contraction that was greater in diabetic than control rats, when the nitric oxide production was inhibited by NG-nitro-L-arginine methylester (10(-4) M). This acetylcholine-induced contraction was observed only in the rings with intact endothelium and was completely abolished by ONO-3708 (10(-6) M). The concentration of 6-keto-prostaglandin F1 alpha in the solution bathing diabetic rat aortic rings increased significantly after acetylcholine (10(-5) M) administration. Prostacyclin (10(-9)-10(-6) M) did not induce contractions at all. Prostacyclin is unlikely to mediate contractions because of its low contractile potency.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased platelet thromboxane A2/prostaglandin H2 receptors in patients with pregnancy induced hypertension.

Pregnancy induced hypertension (PIH) is associated with a variety of disturbances in the hemostatic system including alterations in platelet function, thrombocytopenia, and an increase in platelet turnover. The density of platelet Thromboxane A2 (TXA2)/Prostaglandin H2 (PGH2) receptors was determined in patients with PIH and normal pregnant women, using [125I]-PTA-OH, a TXA2/PGH2 receptor antagonist. The number of platelet TXA2/PGH2 receptors significantly increased (p < 0.008) from 1734 +/- 370 sites/platelet (n = 8) in normal pregnant women to 3703 +/- 846 sites/platelet (n = 9) in patients with severe PIH. The sensitivity of platelets to the TXA2 mimetic U46619 was significantly (p < 0.0005) increased in platelets obtained from severe PIH patients (EC50 = 150 +/- 10nM, n = 3) compared to controls (EC50 = 290 +/- 60 nM, n = 5). These results indicate that an increased number of TXA2/PGH2 receptors as well as increased sensitivity to TXA2/PGH2 mimetics occurs in PIH. Collectively, these results provide further support for the notion that TXA2 and its receptor may play an important role in the pathophysiology of PIH.

Characterization by photoaffinity labelling of the human platelet thromboxane A2/prostaglandin H2 receptor: evidence for N-linked glycosylation.

Thromboxane A2 (TXA2) and prostaglandin H2 (PGH2) are potent proaggregatory and vasoconstrictor lipids acting through a receptor referred to as the TXA2/PGH2 receptor. The receptor was purified using a modification of a previously described method from human platelet membranes solubilized using the detergent (3-[(3-cholamidopropyl)-dimethylammonio]-1-propane-sulfonate (CHAPS) and a combination of affinity chromatography and wheat germ lectin chromatography. This procedure resulted in a 1075 +/- 375-fold purification and a specific activity of 1.45 +/- 0.55 nmol/mg protein (n = 5). Repeating these chromatography steps on this partially purified receptor resulted in a preparation with a specific activity of 21 +/- 3 nmol/mg protein (n = 5). This represents the theoretical specific activity if one assumes a molecular weight of 50,000 for the receptor. The fold purification was 11,750 +/- 1250 based on crude membranes and an overall yield of 24%. To further the characterization of this receptor, we synthesized a new radioiodinated photoaffinity probe, 7-[(1R,2S,3S,5R)-6,6-dimethyl-3-(4-azido-3-iodobenzenesulfonylamino++ + )- bicyclo[3.1.1]hept-2-yl]-5(Z)-heptenoic acid (I-SAP-N3). [125I]l-SAP-N3 irreversibly incorporated into the purified receptor yielding a single band by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) autoradiography and indicated a molecular weight for the receptor of 50-51 kDa. The incorporation of the ligand could be inhibited by a variety of TXA2/PGH2 analogues. In addition, photoaffinity labelling was inhibited in a stereoselective manner as demonstrated by the pair of enantiomers (d)- and (l)-S145. Digestion of photoaffinity labelled receptor with N-glycosidase F demonstrated the presence of at least two N-linked glycosylation sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Contraction of diabetic rabbit aorta caused by endothelium-derived PGH2-TxA2.

Endothelium-dependent relaxations and vasoactive prostanoid production caused by acetylcholine were determined in the aortas of rabbits with diabetes mellitus induced by alloxan. Aortas of diabetic rabbits, contracted submaximally by phenylephrine, showed significantly decreased endothelium-dependent relaxations induced by acetylcholine compared with the aortas of normal rabbits. Indomethacin, a cyclooxygenase inhibitor, and SQ 29548, a prostaglandin H2-thromboxane A2 (PGH2-TxA2) receptor antagonist, normalized the sensitivity of diabetic aortas to acetylcholine, whereas these agents had no effect on the response of normal aortas. The relaxations in response to a nonreceptor-mediated endothelium-dependent vasodilator, A23187, and an endothelium-independent vasodilator, sodium nitroprusside, were not different between normal and diabetic aortas. Acetylcholine also caused contractions of resting aortic rings with endothelium from diabetic, but not normal rabbits; these contractions were inhibited by indomethacin. Synthesis of TxA2, measured as immunoreactive TxB2, was significantly increased in diabetic aortic segments only when the endothelium was present. These results suggest that in the diabetic state, the endothelium releases a major vasoconstrictor cyclooxygenase product that either directly counteracts the relaxation caused by or selectively interferes with the release of endothelium-derived relaxing factor(s) induced by cholinergic receptor stimulation. The vasoconstrictor is most likely TxA2 or possibly its precursor, PGH2.

Human erythroleukemia cells express functional thromboxane A2/prostaglandin H2 receptors.

The human erythroleukemia (HEL) cell line is a cultured hematopoietic cell line reported to express platelet membrane glycoproteins and alpha-2 adrenergic receptors. The present studies were designed to determine if functional thromboxane A2 (TXA2)/prostaglandin H2 (PGH2) receptors exist in HEL cells. Radioligand binding assays were performed using [125I]PTA-OH, a TXA2/PGH2 receptor antagonist. Scatchard analysis revealed one class of binding sites for 1-PTA-OH with a Kd = 122 +/- 18 nM and maximum binding = 1.7 +/- 0.3 x 10(5) sites/cell. Competition for [125I]PTA-OH binding with the TXA2/PGH2 receptor agonists SQ26655 and U46619 revealed one class of binding sites for SQ26655 with a Kd = 17 nM and two classes of binding sites for for U46619 with a Kd = 45 nM for the high-affinity site and a Kd = 450 nM for the low-affinity site. Competition for [125I]PTA-OH by the steroisomeric TXA2/PGH2 receptor antagonists L657925 and L657926 revealed two classes of binding sites for the more potent L657925 with a Kd = 8 nM for the high-affinity site and a Kd = 400 nM for the low-affinity site whereas L657926 bound to one class of sites with a Kd = 380 nM. Stimulation of the TXA2/PGH2 receptor by SQ26655 and U46619 resulted in concentration-dependent increases in [Ca++], as measured by FURA-2 fluorescence, with EC50 values of 28 +/- 2 and 149 +/- 32 nM, respectively. I-PTA-OH, L657925 and L657926 antagonized this response to U46619 with IC50 values similar in rank potency to that seen in the binding studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of endoperoxides in arachidonic acid-induced vasoconstriction in the isolated perfused kidney of the rat.

1. Administration of arachidonic acid caused dose-dependent vasoconstriction in the isolated rat kidney perfused in situ with Krebs-Henseleit solution. 2. Inhibition of cyclo-oxygenase with indomethacin or meclofenamate reduced the renal vasoconstrictor effect of arachidonic acid. 3. The renal vasoconstrictor effect of arachidonic acid was unaffected by CGS-13080 at concentrations that effectively reduced thromboxane A2 (TxA2) synthesis by platelets and the kidney. 4. The endoperoxide/TxA2 receptor antagonist, SQ 29,548, abolished the renal vasoconstrictor effect of arachidonic acid and of U46619, an endoperoxide analogue. In contrast, SQ 29,548 did not affect the renal vasoconstrictor response to angiotensin II, prostaglandin E2 or F2 alpha. 5. These data suggest that the vasoconstrictor effect of arachidonic acid in the isolated kidney of the rat is mediated by its metabolites, including the prostaglandin endoperoxides.

Cooperative mediation by serotonin S2 and thromboxane A2/prostaglandin H2 receptor activation of cyclic flow variations in dogs with severe coronary artery stenoses.

We have reported previously that thromboxane A2/prostaglandin (PG)H2 and serotonin independently mediate the occurrence of cyclic flow variations (CFVs) in a canine preparation of severe coronary artery narrowing. This may be due to an effect of these substances on platelets and/or the vascular wall. We tested the hypothesis that there is a cooperative effect between thromboxane A2/PGH2 and serotonin receptor stimulation in the development of CFVs in this animal preparation. After placement of a hard plastic cylindrical constrictor around the left anterior descending coronary artery, CFVs develop and are characterized by repetitive cycles of declines in coronary blood flow and abrupt increases in flow. In a control group of dogs, CFV frequency (cycles/hour) and severity (lowest coronary blood flow just before its restoration) did not change significantly over a 3 hr interval. In a second group of dogs, CFVs were established after constrictor placement, abolished with the serotonin (5HT2) receptor antagonist ketanserin, and reestablished by the continuous infusion of serotonin into the left atrium. Serotonin-induced CFVs were then abolished with a thromboxane A2/PGH2 receptor antagonist, SQ29,548, or a thromboxane synthetase inhibitor, dazoxiben (UK37,248). The relative specificity of the respective antagonists, SQ29,548 and ketanserin, was determined in canine platelets and rat aortic vascular strips. No significant cross-reactivity between ketanserin and SQ29,548 was found. Thus, the data obtained in these studies demonstrate a cooperative interaction between thromboxane A2/PGH2 and serotonin S2 receptors that contributes to the development of CFVs in this experimental preparation.

Binding of thromboxane A2/prostaglandin H2 agonists to human platelets.

The competition of [125I]-9, 11 dimethylmethano-11, 12 methano-16-(3-iodo-4-hydroxyphenyl)-13, 14-dihydro-13-aza 15 alpha beta-omega-tetranor-thromboxane A2 ([125I]-PTA-OH), a thromboxane A2/prostaglandin H2 receptor antagonist, with a series of thromboxane A2/prostaglandin H2 (TXA2/PGH2) mimetics for binding to the putative TXA2/PGH2 receptor in washed human platelets was studied. The rank order potency for the series of mimetics to compete with [125I]-PTA-OH for binding was compared with their rank order potency for induction of platelet aggregation. The rank order potency for the mimetics to compete with [125I]-PTA-OH for binding was ONO-11113 greater than SQ-26655 greater than U44069 greater than U46619 = 9, 11-azo PGH2 greater than MB28767. This rank order potency was highly correlated with their rank order potency for inducing platelet aggregation (r = 0.992). Changes in the intra or extracellular concentrations of Na+ did not have a significant effect on the competition between U46619 and [125I]-PTA-OH for binding to the putative receptor. In summary, it appears that these TXA2/PGH2 mimetics activate human platelets through the putative TXA2/PGH2 receptor.

Effect of OKY-046, a thromboxane A2 synthetase inhibitor, on arachidonate-induced platelet aggregation: possible role of "prostaglandin H2 steal" mechanism.

To clarify the mode of action of a selective thromboxane A2 (TXA2) blockade in platelet reactivity, we examined the effect of (E)-3-[4-(1-imidazolylmethyl) phenyl]-2-propenoic acid hydrochloride (OKY-046), a potent TXA2 synthetase inhibitor, on human platelet aggregation induced by arachidonic acid (1 mM) in the absence and presence of aspirin-treated aortic microsomes containing prostacyclin (PGI2) synthetase activity ex vivo. The production of TXA2 and PGI2 in platelet rich plasma was determined by the amounts of their stable catabolites, TXB2 and 6-keto-PGF1 alpha respectively, measured by radioimmunoassay. In the absence of aortic microsomes, OKY-046 (greater than 10(-5) M) produced more than 90% inhibition of TXA2 production, whereas platelet aggregation was less inhibited, about 40% inhibition over control, by OKY-046 in that concentration. In the presence of aortic microsomes, the inhibitory effect of OKY-046 on platelet aggregation was markedly augmented in a dose-dependent manner in proportion to the increment of PGI2 production, which paralleled the OKY-046-induced inhibition of TXA2. These results suggest that a selective TXA2 blockade produces effects on platelet aggregation mainly in dual fashion in the presence of PGI2 synthetase: one is due to mere inhibition of TXA2 synthetase and the other is due to the enhancement of PGI2 production probably involving "prostaglandin H2 (PGH2) steal" mechanism, in which PGH2 accumulated in platelets is partly converted to a substrate of PGI2 synthetase in aortic microsomes to produce PGI2.

Arachidonic acid-induced platelet aggregation is mediated by a thromboxane A2/prostaglandin H2 receptor interaction.

The mechanism by which the active metabolites of arachidonic acid (AA), i.e., thromboxane A2 and/or prostaglandin H2 (TXA2/PGH2) induce platelet aggregation is not understood. Several reports have suggested that AA-stimulated aggregation is mediated by secreted ADP, whereas other studies have proposed that this response is ADP-independent. In the present report, we used the specific TXA2/PGH2 receptor antagonist, 13-azaprostanoic acid (13-APA), and the ADP antagonist, ATP, to examine the contribution of TXA2/PGH2 or secreted ADP to aggregation. We found that 13-APA, but not ATP, deaggregates platelets stimulated by AA or U46619 (a TXA2/PGH2 mimetic). In contrast, ADP-induced aggregation was reversed in response to ATP but not to 13-APA. These results suggest that TXA2/PGH2-stimulated aggregation is mediated through TXA2/PGH2 receptor occupation. Furthermore, secreted ADP does not appear to be required for maintenance of the AA-aggregation response.