Antagonism of substance P and related peptides by RP 67580 and CP-96,345 at tachykinin NK1 receptor sites, in the rat urinary bladder

Tonic contraction of rat urinary bladder was elicited in vitro and in vivo by substance P, two selective NK1 receptor agonists, septide ([pGlu6,Pro9]substance P-(6-11)) and [Sar9,Met(O2)11]substance P, and an NK2 agonist, [Lys5,MeLeu9,Nle10]neurokinin A-(4-10), but not by senktide (succinyl[Asp6,MePhe8]substance P-(6-11)), an NK3 agonist. Substance P only stimulated the NK1 receptors of smooth muscle. The non-peptide selective NK1 receptor antagonists, RP 67580 and CP-96,345, both inhibited substance P-induced contraction (pKB values 6.7 and 5.7; ED50 = 1.4 and 5.0 mg/kg i.v., respectively) and septide-induced contraction (pKB values 7.5 and 6.5; ED50 = 0.076 and 0.250 mg/kg i.v., respectively). Both antagonists, at lower doses, also inhibited substance P- and septide-induced plasma extravasation. That both antagonists blocked the effects of septide much more than the effects of substance P suggests the existence of an NK1 receptor subtype or isoform. Selective NK1 receptor antagonists, by blocking both spasm and plasma extravasation in the urinary bladder, would be useful for treating substance P-related motor disorders and cystitis.