Nonpeptide Angiotensin II Antagonist Losartan Inhibits Thromboxane A2-Induced Contractions in Canine Coronary Arteries1

  1. Ping Li,
  2. Carlos M. Ferrario and
  3. K. Bridget Brosnihan
  1. The Hypertension Center, Bowman Gray School of Medicine of Wake Forest University, Winston-Salem, North Carolina
     
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    Abstract

    We investigated the selectivity of a nonpeptide angiotensin II AT1 receptor antagonist losartan for the vascular thromboxane A2 (TxA2)/prostaglandin endoperoxide (PGH2) receptor in canine coronary arteries. Isometric tension was measured in canine coronary artery rings suspended in organ chambers perfused with 95% O2/5% CO2. The TxA2 analog, U46619, produced dose-dependent vasoconstriction in coronary rings (EC50, 10.6 ± 0.9 nmol/l). Pretreatment with losartan (10−8–10−5 mol/l) inhibited the contractile response of U46619 and shifted the concentration-response curve to the right in dose-dependent manner. The EC50 of U46619 was increased 3- and 13-fold in the presence of both 1 and 10 μmol/l of losartan without a change in maximal contraction. The selective TxA2/PGH2 receptor antagonist SQ29548 blocked U46619-induced contraction with greater potency than losartan in isolated coronary arteries. The active metabolite of losartan EXP3174 at 1 μmol/l did competitively block U46619-induced contractions in canine coronary rings. In contrast, the contractile responses produced by U46619 were unaffected by exposure to the nonpeptide AT1 receptor antagonist CV11974, the AT2 receptor antagonist PD123319 or the nonselective peptide angiotensin II antagonist Sar1Thr8-Ang II, each at 1 μmol/l concentration. These data indicate that losartan and its active metabolite EXP3174 are antagonists to the TxA2/PGH2 receptor in canine coronary arteries. The antagonistic effect of losartan and EXP3174 on the vascular TxA2/PGH2 receptor may contribute to the long-term blood pressure-lowering effects of angiotensin antagonists in hypertension.

    The renin-angiotensin system has been well recognized as an important contributor to the pathogenesis of hypertension, cardiac hypertrophy and vascular disease (Ferrarioet al., 1994). Losartan is a potent nonpeptide, selective Ang II AT1 receptor antagonist, which produces concentration-dependent inhibition of Ang II-induced vasoconstrictionin vivo and in vitro and displaces125I-Ang II binding in radioligand binding studies (Timmermans et al., 1993; Liu, 1993; Rhaleb et al., 1991). Losartan reduces blood pressure in human hypertensive subjects, as well as in animal models of hypertension, such as renal hypertensive rats, SHR and transgenic hypertensive rats (Townsend and Ford, 1996; Wong et al., 1990c; Moriguchi et al., 1994). Acute administration of losartan is highly effective in lowering blood pressure in SHR, a genetic model of hypertension in which plasma renin is not elevated (Ohlstein et al., 1992; Cachofeiroet al., 1995). However, it has been noted that in this same hypertensive model the peptide Ang II antagonist saralasin is not as effective as losartan, and angiotensin converting enzyme inhibitors are only modestly effective after acute administration (Pals et al., 1971; Sweet et al., 1981; Ohlstein et al., 1992). Recent studies suggest that the long-lasting antihypertensive effect of losartan may not be caused solely by Ang II receptor blockade. This was illustrated by Ohlstein et al.(1992) who showed that 48 h after the administration of losartan, blood pressure was still reduced in the presence of a normal pressor response to Ang I or II. Furthermore, Cachofeiro et al.(1995) reported that nitric oxide and prostaglandin production are involved in the prolonged blood pressure-lowering effects of losartan in SHR. Losartan has also been shown to attenuate catecholamine-induced contractions in aortic rings of SHR in an endothelium-dependent manner, stimulate vasodilatory prostaglandin release in cultured cells and at high doses bind to imidazoline/guanidinium receptors in rat forebrain cardiovascular areas (Maeso et al., 1995; Jaiswal et al., 1991; Li et al., 1996).

    Recent studies have shown that losartan interacts with the TxA2/PGH2 receptor in human platelets and inhibits the TxA2 analog U46619-induced platelet aggregation and pulmonary hypertension in rats (Liu et al., 1992; Bertolino et al., 1994). Furthermore, TxA2and PGH2 are involved in Ang II-dependent hypertension by stimulating contraction of vascular smooth muscle by a common receptor (Lin et al., 1991, 1994). In the present study, we further investigated whether losartan and its active metabolite EXP3174 interact with the TxA2/PGH2 receptor and stimulate production of prostaglandin and nitric oxide in isolated canine coronary arteries.

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    Materials and Methods

    After approval by the Institutional Animal Care and Use Committee, 16 male dogs (15–25 kg b.wt.) were anesthetized with ketamine (15 mg/kg i.m.) and 2% halothane inhalation; the dogs were then sacrificed with sodium pentobarbital (50 mg/kg i.v.). The heart was harvested immediately and immersed on ice-cold modified Krebs-Henseleit buffer, whereas the left anterior descending coronary artery was carefully dissected free of fat and adhering connective tissues. The coronary artery was cut into 3-mm-long rings and suspended in organ chambers containing Krebs-Henseleit solution of the following composition (in mmol/l): NaCl, 118.3; KCl, 4.7; CaCl2, 2.5; MgSO4, 1.2; KH2PO4, 1.2; NaHCO3, 25; CaNa-ethylenediaminetetraacetic acid, 0.026; and glucose, 11. The Krebs’ solution was aerated with 95% O2 and 5% CO2 at 37°C (pH 7.4) and the rings were allowed to equilibrate for 60 min at 1g initial resting tension. Basic tension was increased individually in a step-by-step fashion until the optimal length-tension relationship was obtained by repeated exposure to 40 mmol/l KCl. In some rings, the endothelium was denuded by gentle mechanical rubbing with a stainless steel wire. Isometric tension of vascular rings was measured continuously with force-displacement transducer (FTO3, Grass Instruments Co., Quincy, MA) connected to a Grass polygraph. The integrity of functional endothelium of vascular rings was confirmed by the presence of acetylcholine-induced relaxation in preconstricted rings with 10−8 mol/l U46619 (more than 90% relaxation at 10−7 mol/l of acetylcholine) and absence of acetylcholine-induced relaxation in vessels after mechanical denudation of the vascular endothelium.

    Experimental protocol.

    Control cumulative concentration-contractile response curves for TxA2 analog U46619 (10−10–3 × 10−6 mol/l) were generated after 1 h equilibration in intact quiescent rings. Losartan (10−8–10−5 mol/l) was used to pretreat the coronary artery rings for 30 min, and the concentration-response curves for U46619 were then repeated. To determine whether losartan interacts with other vasoconstrictors, concentration-response curves with PDGF (10 and 20 ng/ml) and KCl (10–80 mmol/l) were also constructed in the absence and presence of losartan (10−6 mol/l) in isolated coronary vascular rings. Phenylephrine, arginine vasopressin and PGF2 α(each at concentrations ranging from 10−10 to 10−6 mol/l) were also tested. In addition, to compare the potency and selectivity of losartan on TxA2 receptor in coronary arteries, the potent, selective TxA2/PGH2 receptor antagonist SQ29548 (Ogletreeet al., 1985) was used to pretreat the tissues for 30 min, and then concentration-response curves for U46619 were determined.

    To evaluate whether other nonpeptide Ang II receptor subtype antagonists interact on the TxA2/PGH2 receptors in coronary vessels, the selective Ang II AT1 receptor antagonist CV11974, an active form of TCV-116 (Brunner et al., 1994), the Ang II AT2 receptor antagonist PD123319 and the active metabolite of losartan, EXP3174 (all at 10−6 mol/l) were chosen to pretreat the rings for 30 min, and then concentration-response curves for U46619 were generated. The nonselective peptide Ang II antagonist Sar1Thr8-Ang II (10−6 mol/l) was also tested. The cyclo-oxygenase inhibitor indomethacin (10−5 mol/l) combined with losartan (10−6mol/l) for copretreatment of vascular rings was used to ascertain whether the production of vasoactive prostaglandins is involved in the interaction of losartan with U46619 in the isolated coronary arteries. In addition, to test whether the interaction of losartan with U46619 is related to the release of nitric oxide in vasculature, rings were pretreated with the nitric oxide synthase inhibitor, l-NAME (10−4 mol/l). The vascular rings were then preconstricted with either 40 mmol/l of KCl or 10 nmol/l of U46619 to reach a similar degree of stable contraction, and then losartan (10−10–10−5 mol/l) was cumulatively added to organ chambers. The antagonists tested had no effects on the basal vascular tone, except a minimal constriction induced byl-NAME. Each ring was used only once for the antagonist study. A 60-min incubation was allowed between observations.

    Chemicals.

    Losartan and EXP3174 were generous gifts from DuPont/Merck Company (Wilmington, DE). PD 123319 was generously supplied by Parke-Davis Inc. (Ann Arbor, MI) and CV-11974 was from Takeda Chemical Industries, LTD. (Osaka, Japan). l-NAME and SQ29548 were purchased from Research Biochemicals International (Natick, MA). Other chemicals were obtained from Sigma Chemical Co. (St. Louis, MO). Indomethacin and CV-11974 were dissolved in 0.2 N Na2CO3 solution and diluted with Krebs’ buffer. U46619 was prepared as stock in ethanol and diluted with Krebs’ buffer. The concentrations of drugs reported are at final concentration in organ chambers.

    Data and statistical analysis.

    The concentration of U46619 and losartan causing 50% of the maximal contraction (EC50) and the maximal relaxation (IC50) were calculated with use of a nonlinear regression sigmoid curve fitting program of PRISM (Graphpad Inc., San Diego, CA). The apparent dissociation constant (K B) was calculated with the equationK B = [B] 46 ([A′]/[A] −1)−1, where [B] is the concentration of the antagonist and [A] and [A′] are the EC50 values obtained in each artery before and after the addition of antagonist, respectively (Corriu et al., 1995). Data were expressed as mean ± S.E.M. One-way analysis of variance followed by Newman-Keuls multiple comparisons and Student’s t test for paired observations was used for statistical evaluation. P < .05 was considered statistically significant.

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    Results

    Effects of losartan on U46619-induced contraction in coronary artery rings.

    The TxA2 analog, U46619, caused concentration-dependent contractions in canine coronary artery rings. Figure 1 illustrates a typical response in which addition of 10−6 M losartan reversed the contraction of U46619 and pretreatment with losartan shifted the concentration-response curve of U46619 to the right in a concentration-dependent manner without a change in the maximum constriction. The EC50 of U46619 (10.2 ± 0.9 nmol/l) was shifted 3- and 13-fold by preincubation with 1 and 10 μmol/l of losartan, respectively (P < .001 as compared with control) (fig.2A, table 1). The apparent dissociation constant K B averaged 0.5 ± 0.1 and 0.8 ± 0.1 μmol/l in the presence of 1 and 10 μmol/l of losartan, respectively. Losartan at 10−4 M concentration completely reversed the constriction of 10 nM U46619 (Li, P., Ferrario, C. M. and Brosnihan, K. B., unpublished observations ). The potent, selective TxA2/PGH2 receptor antagonist SQ29548 markedly shifted the concentration-response curves of U46619 to the right [EC50, 10.6 ± 0.9 (control) vs.199.1 ± 17.7 and 617.3 ± 63.8 nmol/l at 0.01 and 0.1 μmol/l of SQ29548, respectively] without changing the maximum contraction (fig. 2B). Pretreatment with 1 μmol/l of SQ29548 abolished the contractile responses of U46619 at the concentrations tested (Li, P., Ferrario, C. M. and Brosnihan, K. B., unpublished observations). The potency of antagonism of 0.1 μmol/l SQ29548 was 31-fold greater than losartan at equimolar concentrations [EC50, 19.7 ± 2.8 (losartan) vs.617.3 ± 63.8 (SQ29548) nmol/l).

    Figure 1
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    Figure 1

    Typical dose-response contractions of U46619 during control conditions and after pretreatment with 10−6 mol/l losartan. Figure in upper left-hand corner shows a contractile response to 10−8 mol/l U46619, which is reversed by the addition of losartan (10−6 mol/l).

    Figure 2
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    Figure 2

    (A) Average dose-response contraction curve to U46619 during control conditions (n = 12 dogs) and after pretreatment with increasing concentrations of losartan (10−8–10−5 mol/l) (n = 4–6 dogs). (B) Average dose-response contraction curve to U46619 during control conditions (n = 12 dogs) and after pretreatment with increasing concentrations of SQ29548 (10−8 and 10−7 mol/l) (n= 4–6 dogs). Values are mean ± S.E.M.

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    Table 1

    EC50 values of U46,619 in various concentrations of losartan-treated canine coronary rings (nmol/l)

    Selectivity of nonpeptide Ang II antagonists on the TxA2/PGH2receptor.

    Pretreatment with the Ang II AT1 receptor antagonist CV11974, the active form of TCV-116, or the AT2antagonist PD123319 for 30 min. did not change the concentration-dependent response curves to U46619 at 1 μmol/l [EC50, 10.6 ± 0.9 (control) vs.12.6 ± 1.7 (CV11974) and 12.9 ± 0.7 (PD123319) nmol/l, P > .05 as compared with control) (fig. 3, table2). In contrast, 1 μmol/l EXP3174, the active metabolite of losartan at the AT1 receptor, significantly shifted the concentration-response curve of U46619 to the right (P < .001 as compared with control). In addition, EXP3174 was more potent than losartan (EC50, 33.4 ± 4.6 vs.66.9 ± 9.2 nmol/l, losartan vs. EXP3174, P < .01). None of the nonpeptide Ang II receptor antagonists changed the maximum constriction response to U46619 (table 2). In addition, the nonselective peptide Ang II receptor antagonist Sar1Thr8-Ang II at 1 μmol/l did not affect the U46619-induced contraction of coronary arteries.

    Figure 3
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    Figure 3

    Average dose-response contraction curve to U46619 during control (n = 12 dogs) and after pretreatment with the Ang II AT1 receptor antagonist CV11974, the AT2 antagonist PD123319, the active metabolite of losartan EXP3174 and the nonselective peptide Ang II receptor antagonist Sar1Thr8-Ang II. All Ang II antagonists were used at 10−6 mol/l concentration. (n = 3–6 dogs). Values are mean ± S.E.M.

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    Table 2

    Effects of Ang II antagonists on the U46,619-induced contraction in canine coronary arteries

    Specificity of losartan for vasoconstrictor-induced contraction in coronary arteries.

    Losartan pretreatment (1 μmol/l) did not affect the coronary vasoconstrictor responses to either PDGF (fig.4) or the smooth muscle cell depolarizing agent KCl (fig. 5). Phenylephrine and arginine vasopressin showed minimal contractile responses in canine coronary arteries, and losartan did not change these contractile effects. However, in preliminary studies, losartan (10−5 mol/l) blocked the TxA2/PGH2 receptor agonist PGF2 α-induced dose-dependent vasoconstriction (EC50, 23.4 ± 3.4 vs. 205.1 ± 23.5 nmol/l; maximal contraction, 5.0 ± 0.2 vs. 4.9 ± 0.3 g without and with losartan, n = 2).

    Figure 4
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    Figure 4

    Pretreatment with losartan (10−6mol/l) did not affect the coronary constrictor responses to PDGF (10 and 20 ng/ml). Values are mean ± S.E.M. Studies were conducted in four dogs.

    Figure 5
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    Figure 5

    Pretreatment with losartan (10−6mol/l) did not affect the coronary constrictor response to KCl (10– 80 mmol/l). Maximal contraction was 5.69 ± 0.26 vs.6.1 ± 0.28 g, control vs. losartan treated. Values are mean ± S.E.M. Studies were conducted in six dogs.

    Effects of losartan on the production of vasoactive prostaglandins and nitric oxide.

    Copreincubation of 10 μmol/l of the cyclo-oxygenase inhibitor indomethacin with 1 μmol/l of losartan for 30 min did not significantly shift the concentration-response curve of U46619 as compared with pretreatment with 1 μmol/l of losartan alone (fig. 6A). On the other hand, U46619-preconstricted rings (10 nmol/l) were dilated by losartan in a dose-dependent manner. Neither removal of the endothelium nor pretreatment with NO synthase inhibitor, l-NAME (10−4 mol/l), abolished the relaxation induced by losartan (fig. 6B). There were no differences in the IC50 of losartan-induced vasodilation [1.4 ± 0.2 (intact) vs. 1.3 ± .4 (denuded) and 1.1 ± .0.2 (l-NAME treated) μmol/l, P > .05]. On the other hand, in intact coronary rings preconstricted with 40 mmol/l KCl, losartan (10−10–10−5 mol/l) had no vasodilatory effects on the vascular rings (Li, P., Ferrario, C. M. and Brosnihan, K. B., unpublished observations).

    Figure 6
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    Figure 6

    (A) Effect of copretreatment with losartan and indomethacin on U46619 dose-response curve. There is no difference in coronary constrictor responses to U46619 (10−10–10−6 mol/l) when copretreated with losartan (10−6 mol/l) and the cyclo-oxygenease inhibitor indomethacin (10−5 mol/l) as compared with pretreatment with losartan (10−6 mol/l) alone. (B) Effect of endothelium denudation (denuded ED) or treatment withl-NAME on losartan dilation of coronary vessels preconstricted with 10 nM U46619. Studies were conducted in six to eight dogs. Values are mean ± S.E.M.

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    Discussion

    Losartan, the nonpeptide Ang II AT1 antagonist, inhibits TxA2 analog U46619-induced contractions of canine coronary arteries without changing the maximal contractile response. These findings are consistent with losartan acting as a competitive antagonist of the TxA2/PGH2 receptor in canine coronary arteries. The active metabolite of losartan, EXP3174, also antagonized the TxA2/PGH2 receptor-mediated contractions in coronary arteries. In contrast, two other AT1 receptor antagonists (CV11974 and Sarthran) and an AT2 antagonist (PD123319) did not interact with the TxA2/PGH2 receptor in coronary vessels. The significant shift in the dose-response curve produced by losartan was not mediated by release of either prostaglandins or nitric oxide from the coronary arteries, because the responses were not altered by preincubation with inhibitors of cyclo-oxygenase or nitric oxide synthase. These data suggest that both losartan and its active metabolite EXP3174 bind to both the AT1 and the TxA2/PGH2 receptors, a finding that calls attention to a separate and possibly complementary action of losartan potassium on proatherogenic events which result from platelet aggregation and plaque rupture (Berliner et al., 1995).

    In contrast to saralasin, the first characterized Ang II peptide antagonist (Pettinger et al., 1975), losartan is an orally active, nonpeptide AT1 antagonist possessing no intrinsic agonist effects. Losartan potassium blocks most of the known Ang II-induced vasoconstrictor, dipsogenic, aldosterone and catecholamine stimulatory responses (Timmermans et al., 1995; Chiuet al., 1991). In addition, losartan possesses significant antihypertensive activity in most species studied (Wong et al., 1990a; Ohlstein et al., 1992; Moriguchi et al., 1994). Preclinical research showed that losartan reverses cardiac hypertrophy and vascular hyperplasia (Dostal and Baker, 1992;Dahlof, 1993). EXP3174, the in vivo active metabolite of losartan, is approximately 10- to 15-fold more potent than losartan and has a longer plasma half-life (Wong et al., 1990b). Evidence suggests that EXP3174 makes a major contribution to the long-lasting blood pressure-lowering effects of losartan (Wong et al., 1990b; Tallant and Ferrario, 1996).

    Several studies indicate that Ang II blockade may not fully account for the long-term antihypertensive and antiproliferative actions of losartan. Ohlstein et al.(1992) reported a recovery of the pressor responses to both Ang I and Ang II during chronic therapy with losartan. More recent studies indicate that several non-Ang II-related actions of losartan may be involved in its prolonged blood pressure-lowering effects. Those studies suggested that losartan may stimulate the production of vasodilator prostaglandins and nitric oxide (Jaiswal et al., 1991; Cachofeiro et al., 1995), interact with alpha-1 receptor (Maeso et al., 1995), and block the tachykinin (Picard et al., 1995), imidazoline/guanidinium (Li et al., 1996) and TxA2 receptors (Liu et al., 1992; Bertolinoet al., 1994; Corriu et al., 1995). In agreement with those observations, we showed that the blocking actions of losartan on the TxA2 receptor were quite specific, because administration of either the cyclo-oxygenase inhibitor, indomethacin, or a nitric oxide synthase inhibitor did not eliminate the antagonistic effect of losartan on the U46619-induced vasoconstriction. These results suggest that the competitive inhibitory effects of losartan on the TxA2 receptor are not mediated by stimulation of vasodilators, prostaglandins or nitric oxide. Furthermore, losartan did not inhibit coronary artery vasoconstriction induced with PDGF, KCl, phenylephrine and vasopressin. In the rat brain neither losartan nor EXP3174 at 10 μmol/l were shown to be potent antagonists at the imidazoline/guanidinium receptor sites (Li et al., 1996). Taken together, these findings are consistent with a specific effect of losartan on the TxA2/PGH2 receptor as reported in previous studies (Liu et al., 1992; Bertolino et al., 1994; Corriu et al., 1995).

    The structural requirements necessary for antagonism of the TxA2/PGH2 receptor are different than those involved in antagonism of the AT1 receptor, because we demonstrated that losartan, but not another AT1 receptor antagonist, CV 11974, or the nonselective angiotensin II antagonist Sar1Thr8-Ang II were effective in blocking U46619-induced vasoconstrictor responses. In agreement with other studies demonstrating that EXP3174 is more potent than losartan at the AT1 receptor (Timmermans et al., 1993; Wonget al., 1990b), our study did prove that EXP3174 is a more potent antagonist than losartan for U46619-induced contraction of canine coronary artery rings. These findings are consistent with studies which demonstrated that the metabolite acts as a selective, noncompetitive receptor antagonist which dissociates from its receptor slowly and is more potent than losartan (Sachinidis et al., 1993; Wong et al., 1990b). With use of rat denuded aorta and small mesentery vessels, Corriu et al. (1995) showed that losartan had an inhibiting effect on U46619-induced contractile response. In their studies, however, EXP3174 at 3, 10, and 30 μmol/l was without effect, which suggests that the structural requirements for this antagonistic action may be different for the TxA2/PGH2 receptor in the rat.

    Both losartan and EXP3174 have a benzylimidazole moiety, with EXP3174 differing from losartan only by its being a diacidic metabolite of losartan. CV11974 is also a biphenyl tetrazole; however, the imidazole ring is fused to another heterocyclic ring which possesses a carboxylic acid at position 7. Our studies suggest that the extra phenyl ring of CV11974 either reduces or prevents the binding of this AT1antagonist at the TxA2/PGH2 receptor. Thus, our studies indicate that differences in the structure-activity of AT1 receptor antagonists determine the ability of biphenyl tetrazole to bind to the TxA2 receptor.

    Recent studies have linked TxA2 and PGH2 to mechanisms associated with renin-dependent and angiotensin-induced hypertension. The selective antagonist of the TxA2/PGH2 receptor SQ29548 was reported to elicit a 20 mm Hg decrease in blood pressure in rats with either aortic coarctation-induced hypertension or Ang II-induced hypertension (Linet al., 1991; Mistry and Nasjletti, 1988). In these experiments, administration of an angiotensin antagonist reduced blood pressure by 60 mm Hg. This suggests that in high renin models of hypertension, TxA2 contributes about 30% to the elevation of blood pressure. An increase in the renal excretion of TxA2 and its production in blood vessels was found in several models of hypertension, including renal hypertension and SHR (Lin et al., 1994; Konieczkowski et al., 1983). In addition, it has been shown that TxA2 and PGH2 are endothelium-derived contracting factors and that these factors contribute to the evolution of hypertension in SHR (Daiet al., 1992; Kato et al., 1990).

    In our study, losartan blocked the TxA2/PGH2receptor of canine coronary arteries as a competitive antagonist withK B values similar to those determined by Corriuet al. (1995) in rat aorta and mesenteric arteries. This indicates that the apparent affinity of losartan for the TxA2 receptor was approximately 1000-fold lower than that for the AT1 receptor (Rhaleb et al., 1991). Our studies agree with observations by Liu and colleagues (1992), who reported that losartan was a competitive ligand at human platelet TxA2 receptor with a K d value of 9.6 μM. An effect of losartan on TxA2 receptor cannot be completely excluded after in vivo administration of a large concentration (Wong et al., 1990a; Ohlstein et al., 1992; Cachofeiro et al., 1995; DeGraaff et al., 1993). In the rat circulation, the concentration of losartan was estimated to reach approximately 250 μmol/l after a 10 mg/kg i.v. injection (Liu et al., 1992; Corriu et al., 1995). In humans, the concentration of losartan and its active metabolite EXP3174 was 250–550 ng/ml and 500–800 ng/ml (approximately 1 μmol/l) after an oral therapeutic dose of losartan (80–120 mg oral dose) (Munafo et al., 1992). From both the rat and the human studies, we estimate that the circulating values obtained by others after losartan administration are consistent with the concentration range of the binding and functional inhibition constants at the TxA2/PGH2 receptor. These findings suggest that the antagonistic effect of losartan and its active metabolite EXP3174 on the vascular TxA2/PGH2 receptor may contribute to the prolonged blood pressure reduction during long-term antihypertensive treatment. Thus losartan with its actions on the TxA2/PGH2 receptor may have therapeutic advantages over a more selective AT1 angiotensin receptor antagonist in preventing the vasoconstrictor and platelet aggregating actions of TxA2.

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    Acknowledgment

    Authors thank Drs. Debra I. Diz and E. Ann Tallant, Ms. Carolyn Kiger and Alicia Jones for their kind assistance in this study.

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    Footnotes

    • Send reprint requests to: K. Bridget Brosnihan, Ph.D., Hypertension Center, The Bowman Gray School of Medicine, Medical Center Boulevard. Winston-Salem, NC 27157-1032.

    • ↵1 This work was supported in part by grant P01 HL 51952 from the National Heart, Lung, Blood Institute.

    • Abbreviations:
      TxA2
      thromboxane A2
      PGH2
      prostaglandin H2
      PGF2α
      prostaglandin F2α
      KCl
      potassium chloride
      SHR
      spontaneously hypertensive rat
      PDGF
      platelet-derived growth factor
      l-NAME
      Nω-nitro-l-arginine methyl ester
      U 46619
      5-heptenoic-7-[6-(3-hydroxy-1-octenyl)-2-oxabicyclo [2.2.1]hept-5-yl] acid
      Ang II
      angiotensin II
      • Received November 14, 1996.
      • Accepted February 3, 1997.
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