Evidence for partial agonist properties of daltroban (BM 13,505) at TP receptors in the anaesthetized open-chest rat.
We sought to determine whether the intrinsic pulmonary hypertensive activity of the purported thromboxane A2/prostanoid (TP) receptor antagonist, daltroban, was mediated by TP receptors, using the high efficacy TP receptor agonist, U-46619, and the silent TP receptor antagonist, SQ 29,548. In pentobarbitone-anesthetized, open-chest rats (n = 4-10 per group), non-cumulative injections of U-46619, dose-dependently increased mean pulmonary arterial pressure (MPAP) with an ED50 (geometric mean with 95% confidence limits in parentheses) of 1.4 (1.1-2.3) microg/kg i.v.. Daltroban increased MPAP in a bell-shaped manner, with an apparent ED50 [29 (21-35) microg/kg i.v.] being 21 fold less potent than that of U-46619. The maximal pulmonary hypertensive responses evoked by daltroban represented about half those induced by U-46619 (25.4+/-1.0 vs. 12.7+/-2 mmHg; P < 0.05 between groups). The TP receptor antagonist SQ 29,548 fully antagonized increases in MPAP evoked by equihypertensive doses of U-46619 (1.25 microg/kg) or daltroban (80 microg/kg). Further experiments were carried out to determine whether daltroban antagonized the pulmonary hypertensive responses evoked by the high efficacy agonist, U-46619, or by itself as receptor theory would predict for a partial agonist. Daltroban (10-2500 microg/kg) antagonized, although not fully, U-46619 (20 microg/kg)-evoked pulmonary hypertensive responses, since prominent intrinsic pulmonary hypertensive effects of daltroban were observed in the same range of doses. Furthermore, in contrast to U-46619 (1.25 microg/kg), daltroban (80 microg/kg) failed to evoke a second pulmonary hypertensive response following a previous injection, as would be expected for a partial agonist. Collectively, the results strongly suggest that daltroban behaves as a partial agonist at TP receptors in the pulmonary vascular bed of the rat in vivo.
Bertolino
F
F
Centre de Recherche Pierre Fabre, Division of Cardiovascular Diseases, Castres, France.
Valentin
J P
JP
Patoiseau
J F
JF
Rieu
J P
JP
Colpaert
F C
FC
John
G W
GW
D019796:15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid / Q000037:antagonists & inhibitors / Q000494:pharmacology; D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D019086:Bridged Bicyclo Compounds, Heterocyclic; D005231:Fatty Acids, Unsaturated; D006834:Hydrazines / Q000494:pharmacology; D006976:Hypertension, Pulmonary / Q000139:chemically induced / Q000503:physiopathology; D008297:Male; D010648:Phenylacetates / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D051381:Rats; D017482:Receptors, Thromboxane / Q000819:agonists*; D013449:Sulfonamides / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D013928:Thromboxane A2 / Q000037:antagonists & inhibitors
D019086:Bridged Bicyclo Compounds, Heterocyclic; D005231:Fatty Acids, Unsaturated; D006834:Hydrazines; D010648:Phenylacetates; D017482:Receptors, Thromboxane; D013449:Sulfonamides; D013928:Thromboxane A2; D019796:15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid; C045749:SQ 29548; C051762:daltroban