TACHYKININ NK1 RECEPTOR SUBTYPES IN THE RAT URINARY-BLADDER

1 Following the recent proposal that the selective agonist septide, ([ pGlu(6),Pro(9)]SP(6-11)), acts on a novel tachykinin receptor distinct from the 'classical' NK1 receptor, the aim of the study was to invest igate the possible heterogeneity of tachykinin NK1 receptors in the ra t urinary bladder. 2 The synthetic tachykinin receptor agonists, septi de (pD(2) 7.87) and [Sar(9)]substance P (SP) sulphone (pD(2) 7.64) pro duced concentration-dependent contractions of the rat isolated urinary bladder. 3 The NK1 receptor antagonists GR82,334, (+/-)-CP96,345, and RP67,580 competitively antagonized (slopes of Schild plot not signifi cantly different from unity) the response to septide with the rank ord er of potency (pK(B) values in parentheses): RP 67,580 (7.57)>GR 82,33 4 (7.01)>(+/-)-CP 96,345 (6.80). The same antagonists were significant ly less potent when tested against [Sar(9)]SP sulphone, while maintain ing the same rank order of potency: RP 67,580 (7.00)>GR 82,334 (5.93)> (+/-)-CP 96,345 (<6). The antagonists did not affect the concentration -response curve to bombesin. 4 To exclude the involvement of the NK2 r eceptor, a second series of experiments was performed in the presence of the potent nonpeptide NK2 receptor antagonist, SR 48,968. SR 48,968 (1 mu M) produced a rightward shift of the concentration-response cur ve to the NK2 receptor selective agonist, [beta Ala(8)]neurokinin A (N KA) (4-10). SR 48,968 did not significantly modify the response to SP, NKA, neurokinin B (NKB), neuropeptide K (NPK), neuropeptide gamma (NP gamma), SP(4-1l), SP(6-11), septide or [Sar(9)]SP sulphone. 5 In the absence or presence of SR 48,968, RP 67,580 antagonized in a competiti ve manner the response to septide, [Sar(9)]SP sulphone, SP(4-11) and S P(6-11): pK(B) values obtained in the absence and presence of SR 48,96 8 were not significantly different for any of these four agonists. 6 R P 67,580 antagonized the response to SP and NKA both in the absence an d presence of SR 48,968. In both cases, the slopes of the Schild plots were significantly different from unity. Mean dose-ratios produced by RP 67,580 in the presence of SR 48,968 were larger than those measure d without NK2 receptor blockade for both SP and NKA. 7 RP 67,580 (3 mu M) did not antagonize the response to NKB in the absence of SR 48,968 . In the presence of SR 48,968, RP 67,580 acted as a competitive antag onist of NKB-induced contractions with a pK(B) value (7.63) not signif icantly different from that measured towards septide. In the present o f SR 48,968, RP 67,580, GR 82,334 and (+/-)-CP 96,345 antagonized the response to NKB with a rank order of potency identical to that measure d towards septide or [Sar(9)]SP sulphone. 8 In the absence of SR 48,96 8, RP 67,580 (3 mu M) produced a small shift of the concentration-resp onse curve to neuropeptide K and was ineffective toward neuropeptide g amma. In the presence of SR 48,968 a clear shift of the curve to both agonists was observed. 9 These findings are compatible with the idea t hat a septide-sensitive tachykinin receptor may exist in the rat urina ry bladder. The septide-sensitive receptor is recognized by NK1 recept or antagonists with higher affinity than the 'classical' NK1 receptor recognized by [Sar(9)]SP sulphone. Our data suggest that NKB, after NK 2 receptor blockade, is a more suitable ligand than SP for activation of the 'septide-sensitive' receptor. While the final proof for-the exi stence of possible NK1 receptor subtypes must await confirmation at th e molecular level, the present findings provide strong pharmacological evidence that either NK1 receptor subtypes or a novel type of tachyki nin receptor exist in the rat urinary bladder.