Tachykinin receptors in the guinea‐pig renal pelvis: activation by exogenous and endogenous tachykinins

1 The contractile response to substance P, neurokinin A, selective agonists for the NK 1 , NK 2 and NK 3 tachykinin receptors and the activity of receptor‐selective antagonists has been investigated in circular muscle strips of the guinea‐pig isolated renal pelvis in the presence of indomethacin (3 μ m ). 2 Neurokinin A was the most potent agonist tested, being about 32 times more potent than substance P. The action of both substance P and neurokinin A was enhanced by peptidase inhibitors (bestatin, captopril and thiorphan, 1 μ m each). The selective NK 2 receptor agonist [βAla 8 ] neurokinin A (4–10), was slightly less potent and effective than neurokinin A itself. The selective NK 1 receptor agonist [Sar 9 ] substance P sulphone was effective at low (n m ) concentrations but its maximal effect did not exceed 30% of maximal response to substance P or neurokinin A. The NK 3 ‐selective agonist [MePhe 7 ] neurokinin B was effective only at high (μ m ) concentrations. 3 The pseudopeptide derivative of neurokinin A(4–10), MDL 28,564, displayed a clear‐cut agonist character, although it was less potent than neurokinin A. 4 The responses to roughly equieffective (25–35% of maximal response) concentrations of [βAla 8 ] neurokinin A (4–10), MDL 28,564 and [MePhe 7 ] neurokinin B were antagonized to a similar extent by MEN 10,376 (3 μ m ), a selective NK 2 tachykinin receptor antagonist, while the response to [Sar 9 ] substance P sulphone was unchanged. 5 The response to [Sar 9 ] substance P sulphone was inhibited by the NK 1 receptor‐selective antagonist, GR 82,334 (3 μ m ) while the response to [βAla 8 ] neurokinin A (4–10) was unchanged. 6 The selective NK 2 receptor antagonists MEN 10,376, L 659,877 and R 396 antagonized competitively the response to [βAla 8 ] neurokinin A (4–10) with the following rank order of potency (pA 2 values in parentheses): MEN 10,376 (7.41) > L 659,877 (7.15) > R 396 (6.43). MEN 10,376 and L 659,877 also competitively antagonized the response to neurokinin A, although with lower potency as compared to the selective NK 2 receptor agonist. 7 MEN 10,376, L 659,877 and R 396 reduced in a concentration‐dependent manner the contractile response produced by electrical field stimulation (1 Hz, 100 V, 0.25 ms pulse width, trains of 10 s). The rank order of potency of NK 2 receptor antagonists in blocking the response to electrical stimulation (MEN 10,376 > L 659,877 > R 396) closely mimicked their potency in antagonizing exogenous tachykinins. 8 The inhibitory effect of MEN 10,376 toward responses produced by electrical field stimulation was significantly reduced when tested in the presence of peptidase inhibitors, which increased significantly the response to nerve stimulation. 9 GR 82,334 (3 μ m ) did not significantly affect the response to nerve stimulation in untreated preparations and slightly reduced it in the presence of peptidase inhibitors. 10 We conclude that both NK 1 and NK 2 receptors mediate the contractile effect of tachykinins in the circular muscle of the guinea‐pig renal pelvis and that the response ascribable to NK 2 receptor stimulation is larger than that ascribed to NK 1 receptor stimulation. The NK 2 receptor in the guinea‐pig renal pelvis belongs to the same subtype previously identified in the rabbit pulmonary artery. NK 2 receptors play a dominant role in the physiological response determined by the release of endogenous tachykinins and a contribution of NK 1 receptors becomes evident after inhibition of peptide degradation.