Effects of losartan on contractile responses of conductance and resistance arteries from rats

Functional comparison of the antagonistic properties of some Angiotensin II type 1 receptor blockers on the contraction elicited by Angiotensin II and thromboxane A2 on human saphenous veins

We previously described on human vascular preparations that, in addition to its antagonistic properties on Angiotensin II type 1 (AT1) receptor, losartan could also inhibit the contraction elicited by the stable thromboxane A2 mimetic U46619. The present study was designed (1) to investigate, in human vascular preparations (the saphenous veins) whether these antagonistic properties on thromboxane A2/prostaglandin H2 (TP) receptor were shared by some other AT1 receptor antagonists (irbesartan and valsartan) and the active metabolite of losartan EXP3174, and (2) to compare their antagonistic properties on TP receptors to their antagonistic properties on AT1 receptors. In the presence of indomethacin (10 microM) and Nomega-nitro-L-arginine (100 microM), irbesartan, valsartan, and EXP3174 induced a rightward shift of U46619- and angiotensin II-induced contraction. The inhibitory effect of irbesartan, valsartan, and EXP3174 on U46619-induced contraction was significant from 100 microM while their inhibitory effect on the contraction elicited by angiotensin II was significant from 1 nM. With regard to the plasma therapeutic concentrations of irbesartan, valsartan, and EXP3174, these data suggest that TP receptor blockade does not account for the antihypertensive effects of these AT1 receptor blockers.

Angiotensin II AT1 receptor antagonists inhibit platelet adhesion and aggregation by nitric oxide release

This study investigated the process of nitric oxide (NO) release from platelets after stimulation with different angiotensin II type 1 (AT1)-receptor antagonists and its effect on platelet adhesion and aggregation. Angiotensin II AT1-receptor antagonist-stimulated NO release in platelets was compared with that in human umbilical vein endothelial cells by using a highly sensitive porphyrinic microsensor. In vitro and ex vivo effects of angiotensin II AT1-receptor antagonists on platelet adhesion to collagen and thromboxane A2 analog U46619-induced aggregation were evaluated. Losartan, EXP3174, and valsartan alone caused NO release from platelets and endothelial cells in a dose-dependent manner in the range of 0.01 to 100 micro mol/L, which was attenuated by NO synthase inhibitor N(G)-nitro-L-arginine methyl ester. The angiotensin II AT1-receptor antagonists had more than 70% greater potency in NO release in platelets than in endothelial cells. The degree of inhibition of platelet adhesion (collagen-stimulated) and aggregation (U46619-stimulated) elicited by losartan, EXP3174, and valsartan, either in vitro or ex vivo, closely correlated with the NO levels produced by each of these drugs alone. The inhibiting effects of angiotensin II AT1-receptor antagonists on collagen-stimulated adhesion and U46619-stimulated aggregation of platelets were significantly reduced by pretreatment with N(G)-nitro-L-arginine methyl ester. Neither the AT2 receptor antagonist PD123319, the cyclooxygenase synthase inhibitor indomethacin, nor the selective thromboxane A2/prostaglandin H2 receptor antagonist SQ29,548 had any effect on angiotensin II AT1-receptor antagonist-stimulated NO release in platelets and endothelial cells. The presented studies clearly indicate a crucial role of NO in the arterial antithrombotic effects of angiotensin II AT1-receptor antagonists.

Angiotensin II receptor antagonists role in arterial hypertension

Angiotensin II receptor blockers represent a class of effective and well tolerated orally active antihypertensive drugs. Activation of AT(1) receptors leads to vasoconstriction, stimulation of the release of catecholamines and antidiuretic hormone and promote growth of vascular and cardiac muscle. AT(1) receptor blockers antagonise all those effects. Losartan was the first drug of this class marketed, shortly followed by valsartan, irbesartan, telmisartan, candesartan, eprosartan and others on current investigation. All these drugs have the common properties of blockading the AT(1) receptor thereby relaxing vascular smooth muscle, increase salt excretion, decrease cellular hypertrophy and induce antihypertensive effect without modifying heart rate or cardiac output. Most of the AT(1) receptor blockers in use controlled blood pressure during the 24 h with a once-daily dose, without evidence of producing tolerance to the antihypertensive effect and being with low incidence of side effects even at long term use. Monotherapy in mild-to-moderate hypertension controls blood pressure in 40 to 50% of these patients; when a low dose of thiazide diuretic is added, 60-70% of patients are controlled. The efficacy is similar to angiotensin-converting enzyme (ACE) inhibitors, diuretics, calcium antagonists and beta-blocking agents. AT(1) receptor blockers are specially indicated in patients with hypertension who are being treated with ACE inhibitors and developed side effects such as, cough or angioedema. The final position in the antihypertensive therapy in this special population and other clinical situations, such as left ventricular hypertrophy, heart failure, diabetes mellitus and renal disease, has to be determined in large prospective clinical trials, some of which are now being conducted and seem promising.

Inhibition of platelet activation in stroke-prone spontaneously hypertensive rats: comparison of losartan, candesartan, and valsartan

In vitro studies have suggested that losartan interacts with the thromboxane (TxA2)/ prostaglandin H2 (PGH2) receptor in human platelets, reducing TxA2-dependent platelet activation. The aim of this study was to evaluate the effect of different angiotensin II type 1 receptor antagonists in stroke-prone spontaneously hypertensive rats (SHRSP). The level of platelet activation was assessed by determining P-selectin expression in platelets by flow cytometry. The ex vivo adhesion of platelets was also analyzed. The number of platelets that expressed P-selectin in SPSHR was significantly increased (% P-selectin expression: WKY 4 +/- 0, 4%; SHRSP 15.5 +/- 0, 8% [n = 8], p < 0.05). In SHRSP receiving losartan (20 mg/kg body weight per day) the percentage of platelets expressing P-selectin fell to levels close to that observed in WKY. The number of platelets from SHRSP treated with valsartan and candesartan (20 mg/kg body weight per day for 14 days) that expressed P-selectin was not significantly different from those from untreated SPRHR. Only losartan treatment reduced ex vivo platelet adhesion to a synthetic surface. The antiplatelet effect of losartan does not appear to be related to the level of blood pressure reduction. In ex vivo experiments, losartan significantly reduced the binding of the radiolabeled TxA2 agonist U46619 to platelets obtained from SHRSP in a dose-dependent manner. Treatment with losartan reduced the number of activated platelets in SHRSP independently of its blood pressure effects. TxA2-receptor blockade is proposed as a mechanism by which losartan can prevent platelet activation.

Comparative study of the cardiovascular effects of losartan in normal and in water- and salt-depleted sheep

The cardiovascular effects of losartan, a non-peptidic angiotensin II (ANG II) receptor antagonist, were studied in sheep. Eight normotensive, conscious sheep were tested twice: first under normal conditions and second when subjected to water and electrolytic depletion (furosemide 5 mg/kg twice a day for 3 days). Intravenous injection of 30 mg/kg losartan lowered the mean arterial blood pressure (MABP) in both control and water- and electrolyte-depleted sheep alike. The maximal decrease in MABP was significantly greater in diuretic-treated sheep than in controls (20.0 +/- 2.7 vs 9.3 +/- 1.1 mmHg) and occurred earlier (8.0 +/- 3.3 min vs 12.1 +/- 2.9 min). The decrease in blood pressure was associated with tachycardia in both controls and diuretic-treated sheep (+5.5 +/- 1.8 vs +11.3 +/- 3.9 beats/min). The vasopressor response to 0.1 microg/kg ANG II administered 30 min after losartan was completely antagonized. Two hours after losartan administration, MABP was on the increase in all animals and ANG II receptor blockade was partially obliterated in control sheep. The more marked cardiovascular effects recorded in diuretic-treated sheep as compared to control animals were associated with an increased activation of the renin-angiotensin system (plasma renin concentration: 6.51 +/- 1.33 vs 1.42 +/- 0.37 ng angiotensin I/ml/hr).

Comparative effects of angiotensin II AT-1-type receptor antagonists in vitro on human platelet activation

A recent study has shown that losartan, an AT-1-receptor antagonist, interacts with thromboxane A2 (TxA2)/prostaglandin H2 (PGH2) receptors in human platelets. The aim of this study was to analyze the ability of different angiotensin II (Ang II) AT-1-receptor antagonists to inhibit TxA2-dependent human platelet activation. Platelets were obtained from healthy volunteers. Platelets were stimulated with the TxA2 analogue, U46619 (10(-6) M). U46619-stimulated platelet activation was significantly reduced by both losartan and irbesartan in a dose-dependent manner. Only maximal doses of valsartan (5 x 10(-6) M) and the main metabolite of losartan, EXP3174 (5 x 10(-6) M), reduced U46619-induced platelet activation. Whereas the active form of candesartan cilexetil (candesartan, CV-11974) failed to modify platelet activation involved by TxA2, telmisartan showed a higher effect than valsartan and EXP3174 but lower than either losartan and irbesartan. Losartan or irbesartan reduced the binding of [3H]-U46619 to platelets, an effect that was observed with lower ability with the other AT-1 antagonists. Although platelets expressed AT-1-type receptors, exogenous Ang II did not modify platelet activation. This effect was not modified by blocking the AT-2 receptor with PD123319. These results suggest that some AT-1-receptor antagonists reduce TxA2-dependent activation independent of Ang II involvement.

Nitric oxide mediates inhibitory effect of losartan on angiotensin-induced contractions in hamster but not rat aorta

We investigated a possible contribution of nitric oxide (NO) and prostaglandins to the inhibitory effect of losartan on contractions to Ang I (10(-6) M) and Ang II (10(-7) M) with or without L-NAME (10(-4) M) or indomethacin (10(-5) M) in the aorta of WKY, SHR and hamster (n=7 each). Rings of thoracic aorta (2-mm long) were placed in a myograph (5 ml). Endothelium-dependent vasodilations were evaluated with acetylcholine (10(-8) to 10(-6) M). After a 45-minute incubation with L-NAME under a resting tension of 2 g, only hamster aorta contracted (p<0.01). The SHR aorta showed impaired relaxations to acetylcholine compared with the WKY and hamster aorta (p<0.05). Despite the difference in the stimulated NO release, losartan completely abolished the responses to Ang I and Ang II both in WKY and SHR vessels irrespective of the presence of L-NAME. In contrast to the rat aorta, the inhibitory effect of losartan was attenuated in the presence of L-NAME in the hamster aorta (78% vs 99% inhibition, p<0.05). Indomethacin did not alter the effect of losartan in any vessels. Our results suggest that the presence of NO, particularly a basal secretion of NO, is necessary for the full expression of the inhibitory effect of losartan in the hamster, but not in WKY or SHR, aorta. Unlike NO, prostaglandins do not appear to play a role in the effect of losartan.

Losartan inhibits in vitro platelet activation: comparison with candesartan and valsartan

A recent study has shown that losartan, an AT(1)-receptor antagonist, interacts with thromboxane A(2) (TxA(2))/prostaglandin H(2) (PGH(2)) receptors in human platelets. The aim of the present study was to analyse the ability of different angiotensin II (Ang II) AT(1)-receptor antagonists to inhibit TxA(2)-dependent human platelet activation. Platelets were obtained from healthy volunteers and were stimulated with the thromboxane A(2) analogue, U46619 (10(-6) mol/L). U46619-stimulated platelet activation was significantly reduced by losartan in a dose-dependent manner. Only maximal doses of valsartan (5x10(-6) mol/L), reduced U46619-induced platelet activation. The active form of candesartan cilexetil, candesartan (CV-11974), failed to modify platelet activation. Losartan reduced the binding of [(3)H]-U46619 to platelets, an effect that was observed to a lesser extent with valsartan but not with CV-11974. These results suggest that, whilst some AT(1)-receptor antagonists reduce TxA(2)-dependent human platelet activation, it is not a feature common to all AT(1) antagonists.

[Effect of losartan on human platelet activation by thromboxane A2]

INTRODUCTION AND OBJECTIVES

Previous studies have demonstrated that losartan, an AT-1 receptor antagonist of angiotensin II (Ang II) could block the receptor of thromboxane A2 (TXA2) in the vascular wall. The aim of the present study was to assess the effect of losartan on human platelet activation.

MATERIALS AND METHODS

Platelets were obtained from 15 healthy men between the age 26 and 40. Platelet activation was measured by changes in the light transmission of platelet-rich plasma stimulated by a synthetic TXA2 analogue, U46619 (5 x 10(-6) mol/l).

RESULTS

The U46619-stimulated platelet aggregation was significantly inhibited by losartan in a dose-response manner. Only a high dose of EXP 3174 (5 10-5 mol/l), the in vivo active metabolite of losartan, was able to attenuate U46619-induced platelet activation. Captopril, an angiotensin I-converting inhibitor failed to modify U46619-induced platelet aggregation. Despite the platelets expressing AT-1 type receptors, of Ang II exogenous Ang II did not modify platelet aggregation induced by U46619. The binding of U46619 to platelets was competitively inhibited by losartan in dose-dependent manner. However, only a high dose of EXP 3174 reduced the binding of U46619. Captopril failed to modify the binding of U46619 to platelets.

CONCLUSIONS

Losartan decreased platelet aggregation by a TXA2-dependent mechanism. EXP 3174 showed a lesser potency than losartan to reduce TXA2-platelet activation. Captopril and exogenous angiotensin II had no effect on human platelet activation. These results suggest that losartan reduced TXA2-dependent platelet activation independently of the blockade of AT-1 receptors.

The past, present and future of hypertension management: a potential role for AT(1)-receptor antagonists

The benefits of effective antihypertensive treatment were established at least 40 years ago, but despite the availability of more reliable and better-tolerated antihypertensive drugs, the control of blood pressure remains poor in most patients. Long-term antihypertensive efficacy requires treatment that combines reliable 24-hour reduction of blood pressure with good tolerability to facilitate patient compliance. Treatment should also protect against target-organ damage. As the majority of negative cardiovascular effects of angiotensin II are mediated through the angiotensin II type 1 (AT(1)) receptor, specific blockade of this receptor is a rational approach for achieving these ideals. Several AT(1)-receptor blockers have been developed that combine antihypertensive efficacy and placebo-like tolerability - the latter being unique in the history of antihypertensive therapy. In experimental animals these drugs prevent or reverse target organ damage in the heart, the vasculature and the kidney. Ongoing large-scale outcome studies are now underway to establish the benefits of AT(1)-receptor blockade beyond blood pressure control.

Pharmacological characterization of EK112, a new combined angiotensin II and thromboxane A(2) receptor antagonist

The pharmacological characterization of EK112, a new combined angiotensin II and thromboxane A(2) receptor blocking agent, was examined in this study. EK112 was found to be a angiotensin II receptor antagonist, as revealed by its competitive antagonism of angiotensin II-induced smooth muscle contraction (pA(2) value of 7. 63 +/- 0.14) in rabbit aorta. It also had an angiotensin II blocking action in guinea pig ileum (pA(2) value of 7.87 +/- 0.67). Additionally, EK112 also possessed thromboxane A(2) receptor blocking activity, since it competitively antagonized aortic contractile responses elicited by U46619 and PGF(2alpha)(pK(B) values of 6.67 +/- 0.09 and 6.24 +/- 0.09, respectively) in rat. In contrast, EK112 did not affect the contractile responses to many other receptor agonists. EK112 did not mimic that of the angiotensin-converting enzyme (ACE) inhibitor, captopril, to enhance the muscle contraction elicited by bradykinin in guinea pig ileum, suggesting that EK112 did not inhibit ACE. Neither cyclic AMP nor cyclic GMP content in rat aortic rings was changed by EK112. These data demonstrate that EK112 is a selective antagonist of angiotensin II > thromboxane A(2) thromboxane A(2) receptor.

Antagonistic effects of losartan on thromboxane A2-receptors in human isolated gastroepiploic artery and saphenous vein

In addition to its AT1-receptor antagonist activity, losartan has been shown to antagonize thromboxane A2 (TXA2)-induced contraction of animal vessels. We investigated for the first time in human isolated gastroepiploic artery (GEA) and saphenous vein (SV) the TXA2/PGH2-receptor antagonist activity of losartan in the presence of indomethacin (1 microM) and N(omega)-nitro-L-arginine (100 microg). Losartan at concentrations of > or =1 microM on GEA and from 10 microM on SV significantly shifted U46619-induced contractions to the right. In addition, 100 microM losartan decreased by 34% the amplitude of the contraction to U46619 on both GEA and SV. The potency of losartan for the TXA2 receptor was 50- and 80-fold lower than that for the AT1 receptor on human GEA and SV, respectively. This inhibitory effect of losartan appeared selective for angiotensin II and TXA2-induced contractions because 100 microM losartan did not modify either endothelin-1- or KCl-induced contraction in human SV, although a reduction of norepinephrine- and 5-hydroxytryptamine-induced contraction was observed in human GEA and SV, respectively. In conclusion, losartan is an antagonist of TXA2 receptor on human GEA and SV. However, this antagonist activity occurred for a relative high dose of losartan, suggesting that it contributes at a low level, if any, to its antihypertensive effect.

Action of AT1 receptor antagonists on angiotensin II-induced tone in human isolated subcutaneous resistance arteries

1. Human isolated subcutaneous arteries were studied under isometric conditions in a myograph. 2. Addition of angiotensin II (AII) induced a concentration-dependent increase in tone in isolated arteries. The active metabolite of candesartan (CV 11974), losartan and the active metabolite of losartan, E-3174 antagonized AII-induced tone in a non-competitive manner, but the AT2 selective antagonist, PD123319, was without effect on responses to AII. The effects of candesartan, losartan and E-3174 were analysed using a classical model of non-competitive antagonism and a two-state receptor model. 3. Mechanical removal of the endothelium; pre-incubation with Nomega-nitro-L-arginine methyl ester hydrochloride (L-NAME); pre-incubation with indomethacin, a cyclo-oxygenase inhibitor; or pre-incubation with BQ 485, an endothelin antagonist; had no significant effect on contractions induced by AII. 4. Our results suggest AII contracts human isolated resistance arteries by an action on AT1 receptors and does not involve release of endothelial factors. Use of a two-state receptor model successfully described the action of the AT1 antagonists without sacrificing assumptions regarding the competitive nature of binding of these antagonists.

Effect of losartan on human platelet activation

OBJECTIVE

Previous studies have demonstrated that losartan can block the thromboxane A2 receptor on the vascular wall. The aim of the present study was to assess the effect of losartan on human platelet activation.

METHODS

Platelets were obtained from 15 healthy men, aged 26-40 years. Platelet activation was measured by changes in the light transmission of platelet-rich plasma stimulated by the thromboxane A2 analog U46619 (5 x 10(-6) mol/l) or ADP (10(-5) mol/l).

RESULTS

U46619-stimulated platelet aggregation was significantly inhibited by losartan in a dose-dependent manner. Only a high dose of EXP 3174 (5 x 10(-5) mol/l), the in vivo active metabolite of losartan, was able to attenuate U46619-induced platelet activation. Captopril, an angiotensin I converting inhibitor, failed to modify U46619-induced platelet aggregation. Furthermore, the binding of [3H]-U46619 to platelets was competitively inhibited by losartan, whereas only a high dose of EXP 3174 reduced the binding of [3H]-U46619. Captopril failed to modify the binding of [3H]-U46619 to platelets. Losartan also reduced the platelet activation induced by ADP (10(-5) mol/l), a platelet agonist partially dependent on thromboxane A2. In addition, when thromboxane A2 generation was blocked by aspirin, ADP-induced platelet aggregation was inhibited to a similar degree to the inhibition induced by losartan. Exogenous angiotensin II did not elicit any modification of either U46619- or ADP-stimulated platelet aggregation.

CONCLUSIONS

Losartan decreased platelet aggregation by a thromboxane A2-dependent mechanism. EXP 3174 was less potent than losartan in reducing thromboxane A2-dependent platelet activation. Captopril and exogenous angiotensin II had no effect on human platelet activation. These results suggest that losartan reduced thromboxane A2-dependent platelet activation independently of its effect on angiotensin II.

Inhibition of angiotensin II-induced contraction by losartan in human coronary arteries

The in vitro effects of angiotensin II (Ang II) in human vessels are not well studied. The development of specific Ang II-receptor antagonists has made it possible to delineate more carefully the receptor mechanisms involved. The objective of this study was twofold: to investigate the effect of Ang II on human coronary arteries and to study the effects of angiotensin II type 1 receptor blockade with losartan. The setting was contractile experiments with ring segments of coronary arteries. We observed that Ang II is a vasoconstrictor of human coronary arteries, with a pEC50 value of 9.26 +/- 0.22 and Emax of 68.7 +/- 9.61% of potassium-induced contraction. Losartan (10-100 nM) shifted the concentration-response curve of Ang II to the right, with pEC50 values of 7.64 +/- 0.10 and 7.00 +/- 0.15, respectively (p = 0.001), demonstrating the antagonistic properties of losartan. We also noted a decreased maximal response to Ang II after incubation of losartan, with Emax of 51.1 +/- 7.08% and 41.9 +/- 4.70% (p = 0.05), respectively. In conclusion, this is the first report describing the contractile effect of Ang II and the antagonizing effects of losartan in isolated human coronary arteries.

Losartan inhibits thromboxane A2-induced platelet aggregation and vascular constriction in spontaneously hypertensive rats

Our recent studies have shown that the nonpeptide angiotensin II (Ang II) antagonist losartan interacts with thromboxane A2/prostaglandin H2 receptors and inhibits the thromboxane A2 (TxA2) analog U46619-induced vasoconstriction in canine coronary arteries. In this study, we further investigated whether losartan prevents TxA2-induced platelet aggregation and vasoconstriction in spontaneously hypertensive rats (SHRs). Pretreatment with losartan (10 microM) significantly reduced U46619-induced, concentration-dependent washed platelet aggregation. The inhibition is specific for losartan, because another Ang II AT1-receptor antagonist, CV11974 (10 microM), an active metabolite of TCV116, did not block the platelet aggregation caused by U46619. In addition, losartan (10 microM) augmented acetylcholine (ACH)-induced nitric oxide (NO)-dependent vasodilation and abolished the ACH-induced endothelium-derived contracting factor (EDCF)-mediated vasoconstriction in the aortic rings from adult SHRs. U46619 produced dose-dependent vasoconstriction in aortic vessels of SHRs, which was demonstrated to be blocked by the potent, selective TxA2/PGH2 receptor antagonist SQ29,548. Pretreatment with losartan (10(-6)-10(-5) M) inhibited the contractile response of U46619 and shifted the concentration-response curve to the right in a dose-dependent manner. The effective concentration at half maximal contraction (EC50) of U46619 was increased 2.5- and 7.6-fold in the presence of 1 and 10 microM losartan, respectively, without changes in maximal contraction. The active metabolite of losartan, EXP3174, at 1 microM also competitively inhibited U46619-induced contractions in aortic rings of SHRs. In contrast, neither the AT1-receptor antagonist CV11974, the AT2 antagonist PD123319, nor the angiotensin-converting enzyme inhibitor lisinopril, each at concentrations of 1 microM, had any effect on the U46619-induced constriction in aortic rings. In conclusion, losartan, acting as both AT1- and TxA2/PGH2-receptor antagonists, may enhance its therapeutic profile in the treatment of hypertension and cardiovascular disease.

Losartan reduces constrictor responses to endothelin-1 and the thromboxane A2 analogue in aortic rings from spontaneously hypertensive rats: role of nitric oxide

OBJECTIVE

Our study was designed to investigate whether angiotensin II subtype 1 (AT1) receptors are involved in the constrictor responses evoked by endothelin-1 and the thromboxane A2 analogue U46619 in aortic rings from spontaneously hypertensive rats (SHR), by studying the effect of the AT1 receptor antagonist losartan. In addition, since nitric oxide seems to participate in the mechanism of action of losartan, we studied the effect of the nitric oxide synthesis inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), on the action of losartan.

MATERIALS AND METHODS

Dose-response curves of either endothelin-1 (10(-10) to 10(-7) mol/l) or U46619 (10(-10) to 10(-6) mol/l) were studied in the presence or absence of losartan (10(-5) mol/l) in aortic rings from SHR. Likewise, similar experiments were done in aortic rings pretreated with the nitric oxide synthesis inhibitor, L-NAME (10(-4) mol/l).

RESULTS

Pre-incubation with losartan significantly reduced the contractile response to endothelin-1 compared with control rings, without modifying the value represented by 50% of the maximal response (pD2). The concentration-response curve to U46619 was shifted to the right in the presence of losartan, reducing the pD2 compared with control rings. The presence of captopril (10(-5) mol/l) in the incubation media did not alter the response to either endothelin-1 or U46619. The diminished response to both endothelin-1 and U46619 in the presence of losartan was reversed in L-NAME-pretreated rings.

CONCLUSIONS

Angiotensin II seems to participate in the vasoconstriction induced by both endothelin-1 and the thromboxane A2 analogue through the stimulation of AT1 receptors in SHR aortic rings, because losartan inhibited this effect. Moreover, nitric oxide appears to be involved in this action of losartan.