Volume 122, Issue 5 p. 899-905
Free Access

Analysis of the pulmonary hypertensive effects of the isoprostane derivative, 8-iso-PGF2α, in the rat

Gareth W. John

Corresponding Author

Gareth W. John

Centre de Recherche Pierre Fabre, Division of Cardiovascular Diseases, Castres, France

Institut de Recherche Pierre Fabre, Division of Cardiovascular Diseases, 17, Avenue Jean Moulin, 81106 Castres Cédex, France.Search for more papers by this author
Jean-Pierre Valentin

Jean-Pierre Valentin

Centre de Recherche Pierre Fabre, Division of Cardiovascular Diseases, Castres, France

Search for more papers by this author
First published: 05 February 2009
Citations: 38

Abstract

  • 1

    We analysed the pulmonary hypertensive effects of the F2-isoprostane derivative, 8-iso-prostaglandin F (8-iso-PGF), in comparison with those of the high efficacy thromboxane A2/prostanoid (TP) receptor agonist, U-46619, in pentobarbitone-anaesthetized, open-chest rats (n=4–15 per group).

  • 2

    8-iso-PGF produced dose-dependent increases in mean pulmonary arterial pressure, with an ED50 of 39.0 (31.4–50.6) μg kg−1, i.v. (geometric mean with 95% confidence limits in parentheses) compared to 1.4 (1.1–2.3) μg kg−1, i.v., for U-46619. The maximum responses evoked by U-46619 and 8-iso-PGF were not statistically significantly different (21.0±1.0 and 25.8±1.9 mmHg at 10 μg kg−1 of U-46619 and 630 μg kg−1 of 8-iso-PGF, respectively).

  • 3

    The TP receptor antagonist, SQ 29,548 (0.63 mg kg−1, i.v. + 0.63 mg kg−1 h−1) fully antagonised both U-46619 and 8-iso-PGF-induced pulmonary hypertensive responses.

  • 4

    Further experiments were carried out to determine whether 8-iso-PGF antagonized the pulmonary hypertensive responses evoked by U-46619, or those induced by itself, as would be predicted for a partial agonist. However, ED10 or ED25 doses of 8-iso-PGF (10 or 20 μg kg−1, i.v.) failed to reduce the pulmonary hypertensive responses induced either by U-46619 or by itself.

  • 5

    The data suggest that in the pulmonary vascular bed of the rat, 8-iso-PGF acts as an agonist of high intrinsic activity at SQ 29,548-sensitive (probably TP) receptors.

British Journal of Pharmacology (1997) 122, 899–905; doi:10.1038/sj.bjp.0701441

Introduction

8-Iso-prostaglandin F2α (8-iso-PGF2α) is mainly produced in man and rats in vivo during oxidative stress by non-cyclo oxygenase-dependent, free radical catalyzed peroxidation of arachidonic acid-containing phospholipids in plasma and cell membranes as well as by oxidation of low density lipoprotein and plasma (Morrow et al., 1990; 1992a; Awad et al., 1993; Lynch et al., 1994; Moore et al., 1995a, b; Roberts & Morrow, 1995; Morrow & Roberts, 1996). Small quantities of 8-iso-PGF2α may also be generated by the cyclo-oxygenase pathway (Pratico et al., 1995; Delanty et al., 1996; Patrignani et al., 1996). A number of studies have shown 8-iso-PGF2α to be an extremely accurate marker of lipid peroxidation in animal models of oxidative stress (Morrow et al., 1990; Roberts & Morrow, 1995; Delanty et al., 1997). Measurement of 8-iso-PGF2α production could therefore be used to explore the role of free radicals and oxidant stress in the pathophysiology of several human diseases (Roberts & Morrow, 1995; Delanty et al., 1997).

8-iso-PGF2α-induces vasoconstriction in rat preglomerular vessels (Takahashi et al., 1992), in rat and rabbit isolated lungs (Banerjee et al., 1992; Kang et al., 1993), in porcine and bovine coronary arteries (Kromer & Tippins, 1996) and in rat and guinea-pig aorta (Zhang et al., 1996) which in each case were antagonized by thromboxane A2/prostanoid (TP) receptor antagonists. These findings strongly suggest that the vasoconstrictor effects of 8-iso-PGF2α are mediated by TP receptors. However the possibility that 8-iso-PGF2α elicits agonist responses via non-TP receptors has been raised (Fukunaga et al., 1993; Yura et al., 1995; Pratico et al., 1996).

Kromer and Tippins (1996) showed that 8-iso-PGF2α and the high efficacy TP receptor agonist, U-46619, evoked contractile responses in porcine coronary arteries which were antagonized by the TP receptor antagonist SQ 29,548. In addition, the contractile responses evoked by U-46619 were dose-dependently reduced by 8-iso-PGF2α, thus demonstrating that 8-iso-PGF2α acted as a partial agonist at TP receptors (Kromer & Tippins, 1996). Furthermore, 8-iso-PGF2α, in contrast to U-46619, failed to elicit agonist responses in ovine isolated coronary arteries, as TP receptor reserve was likely to be low in this preparation (Kromer & Tippins, 1996). Similarly, Morrow et al. (1992b) showed that 8-iso-PGF2α acted as a partial agonist at TP receptors in both rat and human platelets.

The thromboxane A2 (TxA2) mimetic, U-46619, induces dose-dependent increases in mean pulmonary arterial pressure (MPAP) and variable effects upon mean systemic arterial pressure (MSAP), depending upon the dose, all of which are amenable to blockade by the silent TP receptor antagonist, SQ 29,548 (Bertolino et al., 1995a, b; Valentin et al., 1996). A role for TxA2 and thus TP receptor activation in the development and/or maintainance of pulmonary hypertension in man has been proposed by the observation of elevated plasma levels of the stable TxA2 metabolite, TxB2 (Rubin, 1995). At the present time the respective pulmonary and systemic haemodynamic effects if 8-iso-PFG2α in vivo have not yet been studied.

We therefore determined whether 8-iso-PGF2α produced pulmonary hypertension in vivo and analysed its effects in comparison with those of the full agonist, U-46619. Experiments were carried out in anaesthetised, open-chest rats, a model we have previously shown to be highly sensitive to TP receptor agonists (Bertolino et al., 1995a, b; Valentin et al., 1996).

Methods

General procedure

In accordance with French law and the local ethical committee guidelines for animal research, male Sprague-Dawley rats (280–400g, OFA, Iffa-Credo, France) were housed in climate controlled conditions (21°C and 55% relative humidity with a 12 h light/dark cycle) and provided standard rat chow and water ad libitum. On the day of the experiment, animals were anaesthetized with an intraperitoneal injection of 60 mg kg−1 sodium pentobarbitone (Sanofi Laboratories, France) and placed on a heated table to maintain rectal temperature at 37±0.5°C. They were prepared for acute experimentation as previously described (Bertolino et al., 1995a, b; Valentin et al., 1996). Animals underwent tracheotomy and were mechanically ventilated (60 cycles min−1; 2.5 ml/cycle; Harvard apparatus, South Natick, MA) in order to maintain blood gases within the physiological range as previously described (Bertolino et al., 1995b). Catheters were inserted into a femoral vein and artery for infusing fluids and drugs and continuous measurement of arterial pressure (AP) via a Statham P10EZ pressure transducer (Viggo-Spectramed, Oxnard, CA) connected to a Gould amplifier (Gould Instruments, France) and a computerized data acquisition system (AcqKnowledge®, BIOPAC Systems Inc., Goleta, CA). A left thoracotomy was performed through the third intercostal space. The pulmonary artery was exposed and a curved 19-gauge needle, connected to a silastic tube (Dow Corning Corporation, Midland, MI), was inserted near the bifurcation of the artery from the right ventricle. Prompt return of arterial blood through the silastic tubing attached to the needle confirmed successful placement. The silastic catheter was secured to the exposed muscle layer of the animal, then the thorax was closed. PAP was recorded via a Statham pressure transducer connected to the computerized data acquisition system. Experiments were started 15–30 min after completion of surgical procedures.

Effect of U-46619 and 8-iso-PGF on haemodynamic parameters

The vehicle of SQ 29,548 was administered intravenously (Na2CO3, 2 mm; see below) to 32 rats. Five minutes after initiation of the infusion, they received 5 successive increasing doses of either U-46619 (0.16, 0.63, 2.5, 10 and 40 μg kg−1; n = 9), 8-iso-PGF2α (2.5, 10, 40, 160 and 630 μg kg−1; n = 8) or their respective vehicles (Na2CO3, 2 mm, n = 7; NaCl 0.9% plus ethanol, 10%; 5 injections; n = 8). Each dose of drug or vehicle was administered as a 1 ml kg−1 solution over 5 min when PAP had returned toward baseline values or had stabilized for serveral minutes.

In separate experiments PAP was determined in 32 open-chest anaesthetized rats which received a single intravenous injection of 8-iso-PGF2α, at one of the following doses: 2.5, 10, 40 or 160 μg kg−1 (n = 8 in each group). Each rat received one dose of 8-iso-PGF2α as a 1 ml kg−1 solution over 2 min.

Effect of TP receptor blockade on the responses to U-46619 and 8-iso-PGF

Three groups of 6 animals each received SQ 29,548 at the dose of 0.63 mg kg−1, i.v. + 0.63 mg kg−1 h−1 followed 5 min later by 5 successive increasing doses of either U-46619 (0.16, 0.63, 2.5, 10 and 40 μg kg−1), 8-iso-PGF2α (2.5, 10, 40, 160 and 630 μg kg−1) or the vehicles.

Effect of 8-iso-PGF on the responses elicited by U-46619

Six rats received 8-iso-PGF2α (approximate ED10; 10 μg kg−1 h−1) followed 5 min later by 5 successive increasing doses of U-46619 (0.16, 0.63, 2.5, 10 and 40 μg kg−1) whereas 12 rats received 8-iso-PGF2α (approximate ED25; 10 μg kg−1 + 10 μg kg−1 h−1) followed by either U-46619 (n = 6) or the vehicle (n = 6).

Drugs and solutions

SQ 29,548 ([1S-[1α,2α(5Z),3α,4α]]-7-[3-[[2-[(phenyl-amino)-carbonyl] hydrazino] methyl]-7-oxabicyclo [2.2.1]hept-2-yl]-5-heptenoic acid) and U-46619 (9, 11-dideoxy-9α-(methanoepoxy) PGF2α) were dissolved in Na2CO3 (2 mm). 8-Iso-PGF2α was dissolved in NaCl 0.9% plus ethanol (10%). All compounds were purchased from the Cayman Chemical Company (Ann Arbor, MI). Drugs were maintained on ice after dissolution and were injected in μg kg−1 base weight. U-46619 and 8-iso-PGF2α were administered as 1 ml kg−1 solutions over 5 min whereas SQ 29,548 or its vehicle was administered as a 1 ml kg−1 solution over 2 min followed by a constant infusion at a rate of 20 μl min−1 for 60 min.

Calculations and statistical analysis

Data are expressed as mean absolute maximal changes±s.e.-mean. One way analysis of variance followed by Dunnett's test was used to assess significance between groups (StatView®, Abacus Concepts Inc., Berkeley, CA). P<0.05 was considered the minimum level of significance. Dose-response curves were fitted by use of an operational sigmoid model (Marquardt, 1963); ED10,25 or 50 refers to the geometric mean agonist dose (with 95% confidence limits in parentheses) inducing 10, 25 or 50% of its maximal effect, respectively.

Results

Effects of U-46619 and 8-iso-PGF on MSAP, MPAP and heart rate

No significant change in MSAP, MPAP or heart rate (HR) was detected in time control, vehicle-treated rats (Figure 1; Tables 1 and 2). Typical recordings of PAP following intravenous administration of U-46619 or 8-iso-PGF2α are presented in Figure 1. MPAP increased promptly, within 2–3 min, after either U-46619 or 8-iso-PGF2α, then progressively returned to preinjection values within a few minutes at doses ≤2.5 and 160 μg kg−1, respectively. At higher doses, mortality of 5/9 and 9/9 rats at 10 and 40 μg kg−1 of U-46619 and of 2/8 at both 160 and 630 μg kg−1 of 8-iso-PGF2α was noted. The increases in MPAP induced by 8-iso-PGF2α were dose-dependent with an ED50 being 28 fold less potent than that of U-46619 (geometric mean agonist dose (with 95% confidence limits in parentheses): 39.0 (31.4–50.6) vs 1.4 (1.2–1.6) μg kg−1, respectively; Figure 2). The maximum responses did not differ significantly between 8-iso-PGF2α and U-46619 (21.0±1.0 and 25.8±1.9 mmHg at 10 μg kg−1 of U-46619 and 630 μg kg−1 of 8-iso-PGF2α respectively; P>0.05; Figure 2, Table 2).

Details are in the caption following the image

Typical recordings of pulmonary arterial pressure following injection of successive increasing doses of either U-46619 (0.16, 0.63, 2.5 and 10 μg kg−1, i.v.), 8-iso-PGF2α (2.5, 10, 40, 160 and 630 μg kg−1, i.v.) or equal volumes of the vehicle (1). Drugs were administered as 1 ml kg−1 solution over 5 min as indicated by the arrows. V indicates the initiation of SQ 29,548 vehicle administration.

Table 1. Baseline values for haemodynamic parameters
Groups Pre-treatment Dose (μg kg−1 i.v.+μg kg−1 h−1) Treatment Dose (μg kg−1 i.v.) n BW (g) MPAP (mmHg) MSAP (mmHg) HR (beats min−1)
1 Vehicle 1 ml kg−1+20 μl min−1 Vehicle 5x1 ml kg−1, i.v. 15 332±7 18.1±0.8 85±5 346±13
2 Vehicle 1 ml kg−1+20 μl min−1 U-46619 0.16 to 40 9 352±9 19.1±0.7 84±8 358±17
3 Vehicle 1 ml kg−1+20 μl min−1 8-Iso-PGF2α 2.5 to 630 8 351±6 18.8±0.7 88±5 347±16
4 SQ 29,548 630+630 Vehicle 5x1 ml kg−1, i.v. 6 370±6 19.4±0.8 87±5 346±13
5 SQ 29,548 630+630 U-46619 0.16 to 40 6 349±7 20.1±0.6 81±8 342±6
6 SQ 29,548 630+630 8-Iso-PGF2α 2.5 to 630 6 358±12 20.1±0.6 87±6 351±15
7 8-Iso-PGF2α 10+10 Vehicle 5x1 ml kg−1, i.v. 6 387±8 22.6±2.2 89±7 366±12
8 8-Iso-PGF2α 10+0 U-46619 0.16 to 40 6 362±8 20.1±0.6 87±7 364±13
9 8-Iso-PGF2α 10+10 U-46619 0.16 to 40 6 363±16 21.1±0.4 92±7 391±18
  • Values are mean±s.e.mean. Baseline values were obtained before treatment with U-46619, 8-iso-PGF2α or vehicle. n, number of rats; BW, body weight; MPAP, mean pulmonary arterial pressure; MSAP, mean systemic arterial pressure; HR, heart rate.
Table 2. Mean maximal changes in haemodynamic parameters
image
Details are in the caption following the image

Effect of TP receptor blockade on the mean maximal changes in mean pulmonary arterial pressure (MPAP) induced by U-46619 (n = 15) and 8-iso-PGF2α (n = 14). *P<0.05 between U-46619 or 8-iso-PGF2α in the absence (n = 17) vs presence of SQ 29,548 (n = 12). §P<0.05 vs vehicle group. †P<0.05 vs SQ 29,548 alone.

As shown in Table 2, over the same range of doses, U-46619-induced slight increases in MSAP at low doses and marked decreases at higher doses. A similar trend was observed following injection of 8-iso-PGF2α. Slight but not statistically significant increases in MSAP occurred at low doses (≤160 μg kg−1) and marked decreases at higher doses of 8-iso-PGF2α (Table 2).

Slight increases in HR were detected at the highest, non lethal, doses of U-46619 and 8-iso-PGF2α (Table 2).

We next determained whether the pulmonary hypertensive activities of U-46619 and 8-iso-PGF2α were susceptible to blockade by the silent TP receptor antagonist SQ 29,548 (Bertolino et al., 1995a).

Effect of TP receptor blockade on the responses to U-46619 and 8-iso-PGF

As shown in Table 1, SQ 29,548 per se was devoid of any significant effect on MSAP, MPAP and HR compared to vehicle-infused animals.

As depicted in Figure 2, SQ 29,548 antagonized both U-46619 and 8-iso-PGF2α-induced pulmonary hypertension, although slight but statistically significant, increases in MPAP compared to vehicle were observed at the highest doses of U-46619 and 8-iso-PGF2α. Changes in MSAP evoked by U-46619 and 8-iso-PGF2α were fully blocked by SQ 29,548. Furthermore, no mortality was detected at the highest doses of U-46619 and 8-iso-PGF2α. These results strongly suggest that the pulmonary hypertensive responses evoked by U-46619 and 8-iso-PGF2α are mediated by TP receptors.

We next determined whether 8-iso-PGF2α pretreatment reduced pulmonary hypertension induced by either U-46619 or by itself as would be predicted by receptor theory for a partial agonist (Kenakin, 1993).

Effect of 8-iso-PGF2α on the responses elicited by U-46619 and by itself

Baseline MPAP values were slightly, but significantly, higher in rats pretreated with ED25 (10 μg kg−1 bolus + 10 μg kg−1 h−1), but not the ED10 (10 μg kg−1 h−1), of 8-iso-PGF2α compared to those receiving the vehicle (MPAP: 21.8±1.0 vs 18.5±0.4 mmHg in 8-iso-PGF2α (ED25) and vehicle-pretreated animals, respectively; n = 12 and 32, respectively; P<0.05 between groups). Neither MSAP or HR differed significantly between vehicle and 8-iso-PGF2α-pretreated groups (Table 1).

Pretreatment with 8-iso-PGF2α at a threshold pulmonary hypertensive dose (ED10) and at a dose equal to the ED25 did not antagonize the pulmonary hypertensive responses evoked by U-46619 (Figure 3; Table 3). In fact a propensity to potentiate pulmonary hypertensive responses to U-46619 was noted following pretreatment with the ED10 of 8-iso-PGF2α (Table 3). The maximal responses evoked by U-46619 in the presence of 8-iso-PGF2α were slightly higher than those evoked by U-46619 alone (21.0±1.0 vs 27.6±2.6 and 23.4±3.3 mmHg in the vehicle, ED10 and ED25 of 8-iso-PGF2α-pretreated animals; P<0.05 and P>0.05, respectively).

Details are in the caption following the image

Effect of pretreatment with 8-iso-PGF2α on the mean maximal changes in mean pulmonary arterial pressure (MPAP) induced by U-46619. 8-Iso-PGF2α was administered at 10 μg kg−1 h−1 (ED10; n = 6) or 10 μg kg−1 i.v. followed by 10 μg kg−1 h−1 (ED25; n = 6). The effects of U-46619 alone are also presented (n = 9). *P<0.05 between U-46619 alone vs in presence of high dose of 8-iso-PGF2α.

As shown for MPAP, increases in MSAP and HR evoked by U-44619 in the presence of the ED10 of 8-iso-PGF2α were slightly greater than those induced by U-46619 alone. Changes in MSAP and HR evoked by U-44619 in the presence of the ED25 of 8-iso-PGF2α were not significantly different from those induced by U-46619 alone.

In separate experiments, the administration of single doses of 8-iso-PGF2α was associated with increases in MPAP of 2.3±0.7, 5.5±1.2, 14.7±2.2 and 20.5±3.5 mmHg at 2.5, 10, 40 or 160 μg kg−1, i.v. These increases were not different from those observed when the same doses were administered successively (2.4±1.6, 2.5±0.4, 13.2±1.9 and 22.9±1.7 mmHg at 2.5, 10, 40 or 160 μg kg−1, i.v., respectively).

These results demonstrate that 8-iso-PGF2α failed to attenuate the responses evoked by U-46619 or by itself suggesting that 8-iso-PGF2α does not act as an antagonist at TP receptors in the rat pulmonary vasculature at the doses employed.

Discussion

The results of the present study show that 8-iso-PGF2α evoked dose-dependent pulmonary hypertensive responses of similar amplitude, but lower potency, compared to those of the high efficacy TP receptor agonist U-46619. Both U-46619 and 8-iso-PGF2α-evoked responses were inhibited by the TP receptor antagonist, SQ 29,548. Furthermore, pretreatment with 8-iso-PGF2α failed to antagonize the pulmonary hypertensive responses evoked either by U-46619 or by the isoprostane itself. Collectively, these findings indicate that in the pulmonary vascular bed of the rat in vivo, 8-iso-PGF2α appears to act as an agonist of high intrinsic activity and moderate potency at TP receptors, but do not exclude the possibility that it may act as a partial TP receptor agonist in other organs and/or species.

Intrinsic activity of 8-iso-PGF

Systemically administered 8-iso-PGF2α dose-dependently increased MPAP, with similar maximum increases but lower potency compared to the TP receptor agonist, U-46619. Pulmonary hypertensive responses evoked by 8-iso-PGF2α have previously been shown in rabbit lungs perfused in situ (Banerjee et al., 1992) and in the rat isolated lung, in which bronchoconstriction was also seen (Kang et al., 1993). 8-iso-PGF2α was 28 fold less potent than U-46619 in inducing pulmonary hypertension in the present study. Interestingly, 8-iso-PGF2α was found to be equi (Crankshaw, 1995) or less (Kinsella et al., 1997; Kawikova et al., 1996; Kromer & Tippins, 1996; Zhang et al., 1996) potent than U-46619 in inducing functional responses such as calcium mobilization in TP receptor transfected HEK 293 cells (Kinsella et al., 1997), or in mediating contractions of (a) human and guinea-pig airways (Kawikova et al., 1996), (b) porcine and bovine coronary arteries (Kromer & Tippins, 1996), (c) rat and guinea-pig aorta (Zhang et al., 1996), and (d) non-pregnant human myometrium (Crankshaw, 1995). Finally, 8-iso-PGF2α in contrast to U-46619 failed to evoke contractions of ovine coronary arteries, but antagonized the contractile responses evoked by U-46619 (Kromer & Tippins, 1996). Although the plasma concentrations of U-46619 and 8-iso-PGF2α attained in the present experiments are likely to be well above the physiological range of TxA2 and 8-iso-PGF2α concentrations, the pharmacological effects observed appear to be relevant to pathological situations in which high local concentrations of TxA2 (Gresele et al., 1991), and possibly 8-iso-PGF2α could be produced.

In the present experiments, the efficacy of 8-iso-PGF2α, as assessed by the mean maximal increases in MPAP, was sufficient to raise MPAP to a similar extent to that of U-46619. In fact, 8-iso-PGF2α was found to possess similar intrinsic activity compared to U-46619 in rat aorta (Zhang et al., 1996) and in the human myometrium (Crankshaw, 1995) or lower intrinsic activity in guinea-pig aorta (Zhang et al., 1996) and bovine, porcine and ovine coronary arteries (Kromer & Tippins, 1996). Interestingly, we observed that 8-iso-PGF2α evoked slight increases in haematocrit in the rat but to a lesser extent than U-46619 (unpublished observation). Furthermore, the 8-iso-PGF2α-induced mortality was lower at doses producing pulmonary equihypertensive responses compared to U-46619.

Inhibition of 8-iso-PGF and U-46619-induced responses by SQ 29,548

As the pulmonary hypertensive responses evoked by both 8-iso-PGF2α and U-46619 were antagonized by SQ 29,548, clearly the receptors mediating both U-46619 and 8-iso-PGF2α-induced increases in MPAP recognize SQ 29,548, which does not lend support to the involvement of different receptors in the responses mediated by 8-iso-PGF2α as claimed by some groups (Fukunaga et al., 1993; Yura et al., 1995; Pratico et al., 1996). Indeed, the present consensus is that constrictor responses evoked by 8-iso-PGF2α are mediated by TP receptors. 8-Iso-PGF2α induces smooth muscle constriction in a number of preparations which are fully prevented by TP receptor antagonists (Banerjee et al., 1992; Takahashi et al., 1992; Kang et al., 1993; Crankshaw, 1995; Kromer & Tippins, 1996; Zhang et al., 1996).

Effect of 8-iso-PGF on the responses to U-46619 and itself

A further series of experiments was performed to examine whether 8-iso-PGF2α pretreatment attenuated the pulmonary hypertensive responses evoked by U-46619 or itself as would be predicted by receptor theory for a partial agonist (Kenakin, 1993). Pretreatment of animals with 8-iso-PGF2α failed to reduce U-46619-induced pulmonary hypertension.

8-Iso-PGF2α failed to diminish its own pulmonary hypertensive responses as confirmed by similar increases in MPAP evoked by the successive administration of increasing doses of 8-iso-PGF2α and those observed when single doses were administered only once per animal. As pretreatment of animals with doses of 8-iso-PGF2α higher than the estimated ED10 or ED25 was not performed, we cannot exclude the possibility that antagonism of pulmonary hypertensive reponses evoked by U-46619 or by 8-iso-PGF2α itself could occur under these conditions, in line with partial agonist properties of 8-iso-PGF2α at TP receptors described elsewhere (Morrow et al., 1992b; Kromer & Tippins, 1996). However the fact that 8-iso-PGF2α failed to attenuate its own pulmonary hypertensive responses strongly suggests that 8-iso-PGF2α does not exert antagonist activity at pulmonary vascular TP receptors. This phenomenon can be explained when TP receptor reserve is taken into consideration.

The functional response to a particular agonist in a given preparation is dependent upon receptor density and coupling of these receptors to intracellular signalling (Kenakin, 1993). In theory, when receptor reserve is low, the response to a partial agonist will be reduced whereas a high efficacy agonist will still induce a maximal response. Thus in the pulmonary vasculature of the rat, TP receptor reserve is likely to be sufficiently high for 8-iso-PGF2α to elicit similar maximal responses to those evoked by the high efficacy agonist, U-46619 assuming that 8-iso-PGF2α-induced responses are also mediated by TP receptors. This is corroborated by the fact that the pulmonary vascular bed of the rat is highly responsive to TP receptor agonists (Bertolino et al., 1995a, b; Valentin et al., 1996). Similarly, TP receptor reserve is likely to be relatively low in preparations in which U-46619 but not 8-iso-PGF2α elicited agonist responses, such as ovine coronary arteries (Kromer & Tippins, 1996). We tentatively suggest that TP receptor reserve is the primary determinant of the degree of intrinsic activity exhibited by 8-iso-PGF2α in a given preparation.

Thus, in the rat pulmonary vasculature 8-iso-PGF2α appears to act as an agonist of high intrinsic activity and moderate potency at SQ 29,548-sensitive (probably TP) receptors. However, our data do not exclude, and are compatible with, the possibility that 8-iso-PGF2α acts as a partial agonist at TP receptors in other organs and/or species in which TP receptor reserve is relatively low.

In conclusion, in anaesthetized, open-chest rats, the F2-isoprostane derivative, 8-iso-PGF2α, elicited dose-dependent pulmonary hypertensive responses which were antagonized by the TP receptor antagonist SQ 29,548 strongly suggesting that these responses were mediated by TP receptors. Furthermore 8-iso-PGF2α failed to attenuate its own pulmonary hypertensive responses or those evoked by U-46619. Thus, in the rat pulmonary vasculature 8-iso-PGF2α appears to act as an agonist of high intrinsic activity and moderate potency at TP receptors.

The authors thank Anne-Marie Bessac and Myriam Maffre for excellent technical assistance.