Mechanism of endothelium-dependent relaxation induced by thrombin in the pig coronary artery

The present study describes the characterization of the binding properties and autoradiographic distribution of a new nonpeptide antagonist of neurotensin receptors, 3H]SR 142948A (2-5-(2,6-dimethoxyphenyl)-1-(4-(N-(3-dimethylaminopropyl)-N-methyl carbamoyl)-2-isopropylphenyl)-1H-pyrazole-3-carbonyl]-amino]-ad amantane-2-carboxylic acid, hydrochloride), in the rat brain. The binding of 3H]SR 142948A in brain membrane homogenates was specific, time-dependent, reversible and saturable. 3H]SR 142948A bound to an apparently homogeneous population of sites, with a Kd of 3.5 nM and a Bmax value of 508 fmol/mg of protein, which was 80% higher than that observed in saturation experiments with 3H]neurotensin. 3H]SR 142948A binding was inhibited by SR 142948A, the related nonpeptide receptor antagonist, SR 48692 (2-1-(7-chloroquinolin-4-yl)-5-(2,6-dimethoxyphenyl)-1H-pyrazole -3-carbonyl]amino]-adamantane-2-carboxylic acid) and neurotensin. Saturation and competition studies in the presence or absence of the histamine H1 receptor antagonist, levocabastine, revealed that 3H]SR 142948A bound with similar affinities to both the levocabastine-insensitive neurotensin NT1 receptors (20% of the total binding population) and the recently cloned levocabastine-sensitive neurotensin NT2 receptors (80% of the receptors) (Kd = 6.8 and 4.8 nM, respectively). The regional distribution of 3H]SR 142948A binding in the rat brain closely matched the distribution of 125I]neurotensin binding. In conclusion, these findings indicate that 3H]SR 142948A is a new potent antagonist radioligand which recognizes with high affinity both neurotensin NT1 and NT2 receptors and represents thus an excellent tool to study neurotensin receptors in the rat brain.