Potent bombesin receptor antagonists distinguish receptor subtypes

To determine whether newly described bombesin (BN) receptor antagonists distinguish subtypes of BN receptors, we investigated their abilities to interact with BN receptors on esophageal muscle or pancreatic acinar tissue. For inhibition of binding of ¹²⁵I-[Tyr⁴]BN to rat pancreatic tissue, the relative potencies were [D-Phe⁶]BN-(6-13)ethyl ester (5 nM) > Ac-gastrin-releasing peptide (GRP)-(20-26)ethyl ester (17 nM) > [D-Phe⁶,Cpa¹⁴,ψ13-14]BN-(6-14) (40 nM) > [Leu¹⁴,ψ13-14]BN (0.43 μM) > [Tyr⁴,D-Phe¹²]BN = [D-Pro⁴,D-Trp^7,9,10]substance P (SP)-4-11 (13 μM) > [Leu¹⁴,ψ9,10]BN (32 μM) > [D-Arg¹,D-Trp^7,9,Leu¹¹[SP (70 μM). Each antagonist also inhibited binding of ¹²⁵I-Bolton-Hunter-neuromedin B to rat esophageal tissue, and the potency of each antagonist for each tracer was similar. In comparison to rat pancreas, [D-Phe⁶]BN-(6-13)ethyl ester, Ac-GRP-(20-26)ethyl ester, [D-Phe⁶,Cpa¹⁴,ψ13-14]BN-(6-14), [Leu¹⁴,ψ13-14]BN, and [Leu¹⁴,ψ9,10]BN had a 10,000-, 2,940-, 1,425-, 122-, and 4-fold, respectively, weaker affinity for BN receptors. In contrast [Tyr⁴,D-Phe¹²]BN, [D-Pro⁴,D-Trp^7,9,10]SP-4-11, and [D-Arg¹,D-Trp^7,9,Leu¹¹] SP had a 4-, 4-, and 9-fold, respectively, higher affinity compared with pancreatic tissue. Comparison of the activity of each peptide at inhibiting the ability of equipotent concentrations of BN or neuromedin B to stimulate contraction of rat esophageal muscle demonstrated that each peptide had the same relative potencies as for inhibiting binding. Each peptide also had the same relative inhibitory potencies for inhibiting BN-stimulating amylase release from dispersed pancreatic acini as for inhibiting binding to pancreatic tissue except for the two SP analogues, which had agonist activity in rat pancreas. These results demonstrate that different subtypes of BN receptors can be easily distinguished by these various classes of receptor antagonists.